University of Groningen

The reactivity of rare-earth metallocenes towards alkynesQuiroga Norambuena, Victor

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Introduction

1

1. Introduction

1.1. Historical background

The lanthanides (Ln) consist of the 4f metals, ranging from lanthanum (La) to lutetium (Lu).1 The

group 3 metals scandium (Sc) and yttrium (Y) exhibit similar chemical behavior as the lanthanide elements. Therefore, the lanthanide and group 3 metals are often considered as part of the same group, termed the rare earth metals.2 The organometallic chemistry of the rare earth metals has been slow to develop relative to that of other metals. The first well-characterized organometallic complexes of the lanthanide and group 3 elements, the tris(cyclopentadienyl) derivatives Cp3Ln, were prepared in 1954 by Wilkinson and Birmingham.3 Despite this early discovery, further development of this new area of organometallic chemistry was hampered by the intrinsic instability of these organometallic compounds towards moisture and oxygen. Another reason for the slow initial development of the organometallic chemistry of the rare earth metals was the belief that these compounds were ionic and represented merely trivalent versions of alkali and alkaline earth metal organometallic species.

As a consequence, two decades of relative stagnation followed, until the availability of more sophisticated experimental and analytical techniques made the rigorous exclusion of traces of air and moisture during preparation and characterization possible in the 1970s. Further progress in the late 1970s and early 1980s demonstrated that many group 3 organometallics and organolanthanides display a rich and interesting chemistry, such as catalytic alkene hydrogenation4 and polymerization5 at very high rates6 and reactivity towards the C-H bonds of arenes, alkenes and alkanes.7 It became evident that the lanthanide and group 3 metals had the potential for some unique chemistry, distinct from anything possible with main-group or transition metals. In addition, the rare earth elements offer exceptional opportunities for tuning catalyst properties depending on ionic radii and ancillary ligand architecture. The low cost of the majority of the rare earth metals as compared to most transition metals and the lack of heavy-metal toxicity also contribute to the increasing attraction of catalysts based on group 3 and lanthanidemetals. The last two decades have witnessed a tremendous growth in research activities in this field, establishing that organo rare-earth metal complexes exhibit distinctive structural and physical properties and that many of them are highly active in variety of catalytic processes (Section 1.3).8

Up to the early 1970s organo rare-earth metal chemistry had been limited to π-bonded organometallic compounds, such as tris(cyclopentadienyl)3, tris(indenyl)9 and cyclooctatetraene10 complexes. In addition, a variety of homoleptic compounds, such as Li[LnPh4]11 and Sc(CCPh)3

11a, and some ill-defined carbyls prepared via metal vapor synthesis had been described as well.12 In the mid-1970s, Tsutsui and Ely synthesized a number of alkyl, aryl and alkynyl bis(cyclopentadienyl) derivatives Cp2LnR.13 Since then, bis(cyclopentadienyl) complexes (metallocenes or sandwich complexes) attracted the most attention in the organometallic chemistry of rare earth metals. Cyclopentadienyl ligands are ideally suited to the rare earth metals, as they are capable of ionic bonding and can readily be tuned sterically with a variety of substituents. Unsubstituted cyclopentadienyl ligands provide complexes that are insoluble in hydrocarbon solution and usually display low reactivity.14 The pentamethylcyclopentadienyl ligand (C5Me5, abbreviated by Cp*), on the other hand, can generally provide excellent solubility and stability for the rare earth metal ions and constitutes the most studied ancillary ligand in the organometallic chemistry of the rare earth metals (e.g. 1 and 2, Scheme 1-1).15 A large variety of metallocenes with different subsituents (e.g. 3) has been prepared in the last decades.16 The Cp rings have, furthermore, been connected with a bridging group to give an ansa system (e.g. 4, 5, 7, 8).17 Besides these ansa-metallocenes, half-metallocenes or half-sandwich complexes bearing mixed cyclopentadienyl-modentate-anionic ligands have also been explored, the most important example being the silylene-linked amido-cyclopentadienyl ligand (e.g. 6).18

Although cyclopentadienyl ligands have proven to be very suitable for preparing and examing well-defined, monomeric organometallic complexes and much elegant chemistry has been uncovered, generally two Cp donors are required in the cyclopentadienyl chemistry of rare earth metals, thereby limiting the chemistry to reactions involving one M-C bond in neutral complexes. Thus, recent work in this field has shown a departure from cyclopentadienyl chemistry, searching not only for different dianionic ligand environments, but also for mono-anionic and neutral ligand environments that allow the preparation of bis- and tris(alkyl) complexes.19 The

Chapter 1

2

latter two compounds are of interest, because of the potential for generating rare earth metal cations, related to the group 4 metal cations, which have been of fundamental importance in the field of olefin polymerization.20 The last 5-10 years has witnessed a large amount of new ligands for use with group 3 and lanthanide metals and many of them have provided complexes that are of interest as (pre)catalysts (Section 1.3).

1.2. General properties

The lanthanides have the general electronic configuration [Xe]4fn5d16s2 with n = 0 (La) to 14 (Lu). The 4f valence orbitals of the lanthanides do not protrude significantly beyond the filled 5s2 and 5p6 orbitals. As a result, 4f electrons are commonly thought to be unavailable for bonding and ligand field effects are practically absent.21 For this reason, the chemistry of the lanthanides is believed to be predominantly ionic and governed more by electrostatic factors and steric requirements than by filled orbital considerations.22 As a consequence, the chemistry, spectroscopy and magnetism of rare-earth metal ions differ considerably from d transition-metal ions. With no electronic preferences for particular geometries, irregular geometries are common. General principles of d-transition metal ligand bonding, such as σ-donor/π-acceptor interaction and the 18-electron rule are not observed in group 3 and lanthanide chemistry and examples of imido or alkylidene complexes remain scarce.23 Contrary to common belief, a more recent study showed that both covalent and ionic contributions play an important role in determining the molecular structure of rare-earth metal complexes.24 This finding suggested that the bonding description of rare-earth metal complexes lies somwhere on a continuum between purely ionic and covalent bonding.

Lanthanides and group 3 metals have one common thermodynamically favorable oxidation state, which is the trivalent positive state (Ln3+). Cerium is unique in that it also possesses an accessible tetravalent oxidation state. Other readily accessible nontrivalent oxidation states are Sm2+ (4f6), Eu2+ (4f7) and Yb2+ (4f14). The inert-gas electronic configuration of the Ln(III) derivatives implies a similar chemical behavior, resembling in this sense broadly the behavior of the early d-block elements in their highest oxidation states. The preferred coordination numbers of the Ln(III) cations are in the range of 8-12; eight-coordination is typical for rare earth

Scheme 1-1. Examples of metallocenes, ansa-metallocenes and half-metallocenes in organo rare-earth metal chemistry.

E = N or CH

R* = (-)-methyl, (+)-neomenthyl or (-)-phenylmethyl

Ln E(SiMe3)2 Ln HH Ln

1 2 3

SiMe3

LnC

LnC

Me3Si

SiMe3

SiMe3

H3

H3

Achiral

Chiral

Si Ln E(SiMe3)2 SiN

E(SiMe3)2Ln

4 5 6

Ln(Me3Si)2E

Si

R*

Ln E(SiMe3)2

7 8

Si

R*

Ln E(SiMe3)2

Introduction

3

metal complexes. High coordination numbers are usually accomplished by forming oligomeric structures or highly solvated complexes. Since the reactivity of lanthanide complexes is correlated with the steric saturation at the metal center, both forms are undesirable for the preparation of highly reactive compounds.

The relatively high charge-to-ionic-radius ratio results in an electropositive metal center that behaves as a hard Lewis acid, according to Pearson´s hard-soft-acid-base (HSAB) principle.25 Based on their relative preferences for pyridine, Lappert suggested a relative Lewis acidity scale: Cp2ScMe > AlMe3 > Cp2YMe ≈ Cp2LnMe (here: Ln = large lanthanide elements).26 Accordingly, metal ions of the rare earth metals prefer coordination to hard ligands, such as O donors. Hence, the term oxophilic is often apllied to these ions. Marks’ series of bond energies for Cp*2Sm-X compounds illustrates that the bonding preferences are not as clear as the HSAB principle would have it: Cl > CCPh > Br > O(t-Bu) > S(n-Pr) > I > H > NMe2 > PEt2.27 Although the Ln-X bonds are thermodynamically stable, group 3 and lanthanide complexes display high kinetic lability, due to their typical high ligand exchange ability.28 The pronounced oxophilicity makes organometallic compounds of the rare earth metals very reactive towards water and air, resembling early d-block metals in this respect.

(1.1) L2Ln X XL2Ln+

(1.2)

+L2Ln X Y Z L2LnY

XZ

+L2Ln Y X Z

δ+

δ-

δ-

Due to the absence of energetically accessible oxidation states that allow two-electron redox

processes, oxidative addition and reductive elimination reactions are not observed for rare earth metals. Instead, the reactivity of the organometallic complexes having metals in the trivalent oxidation state is dominated by (i) simple Lewis acid/base-type interactions, (ii) insertion of unsaturated moieties into polar Ln-C and Ln-H bonds (Eq. 1.1), (iii) β-H and β-alkyl elimination and (iv) σ-bond metathesis processes (Eq. 1.2). The latter is a concerted, bimolecular reaction that can involve both polar (i.e. alkane C-H bonds) and nonpolar (i.e. H2) bonds and proceeds via a highly ordered, polarized four-centered transition state. Only Sm2+/Sm3+, Eu+/Eu2+, Yb+/Yb2+ and Ce3+/Ce4+ have more than one energetically accessible oxidation state and one-electron redox shuttles are observed for these metals.

The effective eight-coordinate ionic radius of Ln3+ gradually decreases from 1.16 Å for La3+ to 0.977 Å for Lu3+. This particularly feature is known as the lanthanide contraction. The gradual decrease in ionic radius and the limited radial extension of the valence orbitals are manifested in subtle differences in the formation, coordination geometry and reactivity of rare earth metal complexes, having similar chemical environments, but different metals. The intrinsic properties of the rare earth cations, as revealed by their oxophilicity, hard Lewis-acid character and large size, govern the reactivity of the compounds of the rare earth metals. Hence, parallels to the chemistry of aluminum, group 2 and group 4 elements and the actinides are often found.

The lanthanide ions exhibit three types of electronic transitions. The colors arising from forbidden 4f → 4f transitions show little variation with ligand substitutions. This feature complicates separation techniques. Instead, reactions and appropriate reagents must be chosen, so that one (major) product is formed which can then be purified by (fractional) crystallization or sublimation. In addition, most of the lanthanide ions are luminescent, either fluorescent (e.g. PrIII, NdIII, HoIII

, ErIII and YbIII) or phosphorescent (e.g. SmIII, EuIII, GdIII, TbIII, DyIII and TmIII). The ions LaIII and LuIII have no f-f transitions and are not luminescent. The paramagnetic rare earth metals have Laporte allowed 4f → 5d transitions and have intense, variable colors that change significantly with metal environment, a clear synthetic advantage. The third type of electronic transition displayed by lanthanide ions are charge-transfer transitions and both ligand-to-metal and metal-to-ligand transitions are allowed. Because their energies are high, however, these transitions are less widespread than encountered in d transition-metal chemistry. The technological importance of the lanthanide ions for magnetic and optical materials has long attracted interest in their physical properties and solid-state chemistry.29

NMR spectroscopy is the most important tool for solution structure determination in the organometallic chemistry of rare earth metals. Because the rare earth metals are paramagnetic for all configurations from 4f1 to 4f13, it is not surprising that many researchers have chosen to investigate the chemistry

Chapter 1

4

of diamagnetic Sc3+, Y3+, La3+, Yb2+ and Lu3+. These nuclei are also accessible to direct observation by heteronuclear NMR spectroscopy.30 For yttrium complexes, the 89Y nucleus (100% abundance) has a spin of 1/2 and the coupling information available is highly informative. 89Y NMR spectroscopy has become a valuable diagnostic tool for the characterization of organoyttrium complexes, but the utility of this technique is confined to concentrated samples, due to long relaxation times and negative nOe effects.31 The 45Sc (I = 7/2, 100%), 175Lu (I = 7/2, 97.4%) and 139La (I = 7/2, 99.91%) nuclei have a significant quadrupole moment. As a result, the carbon atoms bonded directly to the metal are sometimes difficult to detect and α-protons in hydrocarbyl derivatives are broadened to varying degree. Although the 139La and 45Sc nuclei have high relative sensitivities and display a wide range of chemical shifts, their relatively large nuclear spins and quadrupole moments give rise to broad resonances. As a result, 139La and 45Sc NMR data are scarce in literature,32 while the use of 175Lu NMR spectroscopy has virtually completely been limited by its large quadrupole moment. 171Yb (I = 1/2, 14.27%) NMR spectroscopy, on the other hand, is well-developed and has become a useful method to study organoytterbium(II) complexes.33 It should also be noted that interpretable and diagnostic, albeit paramagnetically shifted, NMR spectra for Ce3+, Nd3+, Sm3+ and Sm2+ can be obtained.

1.3. Organo rare-earth metal complexes in homogeneous catalysis The first indication of catalytic activity of organolanthanides came from the observation that oxides,

halides or alkoxides in the presence of co-catalysts, such as lithium alkyls, aluminum alkyls or other hydrocarbyl transfer reagents catalyzed the cracking of hydrocarbons, the oligomerization of alkenes and the polymerization of alkenes and alkynes.34 Alkene and alkyne hydrogenation catalyst were prepared by co-condensing lanthanide metal vapors with unsaturated hydrocarbons.35 Later, when well-defined organolanthanide complexes became available, it was found that some of these complexes are also effective catalysts. For example, polymerization of ethene, propene and butadiene was observed for bis(cyclopentadienyl)lanthanide hydrocarbyls36, (cyclooctatetraenyl)cerium complexes and anionic tetra(allyl)lanthanide complexes.37 Organolanthanide hydrides38 and cyclopentadienyl ytterbium(II) complexes39 catalyzed the hydrogenation of alkenes and alkynes.

In the 1980s metallocene complexes of group 3 and lanthanide metals received much attention as alkene polymerization catalysts.40 This can be attributed to the intrinsic high polymerization activity of neutral alkyl and hydrido derivatives towards ethylene and the possibility to study the debated mechanism of Ziegler-Natta polymerization without the complication of ill-defined cocatalysts and possible ion pairing effects. Conventional rare-earth metallocene complexes are, however, inefficient catalysts for the polymerization of 1-alkenes higher than ethylene, due to rapid and irreversible alllylic C-H activation. Modifications of the ligand environment of these catalysts have been shown to suppress this side reaction.41 Other monomers, such as dienes42, styrene derivatives43, methacrylates44 and lactones45, have also been polymerized by metallocene complexes of the rare earth metals and their derivatives.46

The discovery that some organometallics of the rare earth metals are highly active as polymerization catalysts prompted interest in other catalytic applications, but efforts to apply these catalysts to organic synthesis have only recently begun.47 Most catalytic applications of the organometallic compounds of group 3 and lanthanide metals involve alkene transformations, such as hydrogenation,48 oligomerization,49 polymerization,50 cyclization,51 hydroamination,52 hydrosilylation,53 hydrophosphination54 and hydroboration.55 Moreover, these reactions have shown to proceed in some cases with high regio- and stereoselectivities, in combination with a high tolerance towards a wide variety of organic functional groups. Particularly impressive results have been obtained for the lanthanidocene-catalyzed hydroamination reaction, including regio-/diastereo-/enantioselective intramolecular cycloamination of aminoalkenes,56 aminoalkynes,57 aminoallenes58 and tandem bicyclizations of aminodienes, aminodiynes and aminoenynes.59

Recent research in this area has focused on extending the scope of catalytic reactions and on varying the bis(cyclopentadienyl) ligand coordinations sphere in order to achieve new catalytic applications. Especially variations of the cyclopentadienyl substituents or the use of ring-bridged cyclopentadienyl ligands has led to novel catalytic applications, such as oligomerization of 1-alkenes,60 isospecific polymerization of 1-alkenes,61 hydrocyclization of α,ω-dienes,62 C-C σ-bond activation63 and the hydromethylation of 1-alkenes.64 More recently, also the use of non-cyclopentadienyl supporting ligand environments has led to a wealth of catalytic applications, such as alkene polymerization,65 lactide polymerization,66 hydrosilylation67 and hydroamination.68 In some cases, improved catalytic behavior has been noted for cationic rare-earth metal complexes as compared

Introduction

5

to their neutral analogues,69 such as for ethylene polymerization,70 styrene polymerization,71 diene polymerization,72 hydroamination73 and 1-alkyne dimerization.74

1.4. Objective and overview Compared to alkenes, catalytic transformations of alkynes, mediated by rare earth metal compounds,

have received much less attention. Earlier it was observed that permethyllanthanidocene alkyl derivatives Cp*2LnCH(SiMe3)2 catalyze the unusual cyclodimerization of 1-methylalk-2-ynes CH3CCR (Eq. 1.3).75 However, the scope of substrates was believed to be confined to 1-methylalk-1-ynes with small alkyl groups and mechanistic details of this remarkable transformation were lacking. It was envisioned that the application of mono- and bis(propynyl)arenes in the catalytic cyclodimerization could provide synthetic access to novel and interesting materials. Therefore, a mechanistic study of the permethyllanthanocene-catalyzed cyclodimerization reaction of propynylarenes was initiated to determine the scope and mechanism of the catalytic cyclodimerization reaction of propynylaromatics, searching for selective catalytic pathways to the construction of a rare class of four-membered carbocycles and (cross-)conjugated polymers.

(1.3)

Cp*2LnCH(SiMe3)2R +

Ln = La, CeR = Me, Et, nPr

R

R

R

R

1-Alkynes are converted by permethylmetallocene alkyl derivatives Cp*2LnCH(SiMe3)2 to mixtures

of enyne dimers and higher oligomers (Eq. 1.4).76 From the desire to apply the rare-earth metallocene-catalyzed oligomerization of 1-alkynes to the synthesis of novel conjugated polymers, a mechanistic study was undertaken to assess the scope of variation in the aromatic moiety (e.g. substituents, heteroatoms) and determine the factors that govern the rate and selectivity of this catalytic transformation.

(1.4) Cp*2LnCH(SiMe3)2

RR

RR

R+ + higher oligomers

Ln = La, Ce, YR = Me, Et, nPr, tBu, Ph, SiMe3 In Chapter 2 the synthesis, structure and reactivity of lanthanidocene aryl-substituted

propargyl/allenyl complexes Cp*2LnCH2CCAr (Ln = Y, La; Ar = C6H5, C6H4Me-2, C6H3Me2-2,6, C6H3iPr2-2,6)

is described. Lanthanide propargyl/allenyls are presumed to be the active catalysts in the cyclodimerization reaction of 1-methylalk-2-ynes. Evidence for η3-bonding in these species was indicated by several spectroscopic techniques and this bonding description is also reflected by the observed reactivity, as these derivatives were found to be thermally robust, furnish acetylenic and allenylic quenching products upon reaction with protic acids and do not insert 1-alkenes into the metal-carbon bond. Sterically hindered 1-methylalk-2-ynes CH3CCAr undergo transmetalation with Cp*2LnCH2CCAr, while sterically less hindered 1-methylalk-2-ynes undergo catalytic cyclodimerization with Cp*2LaCH2CCAr. The reactivity of the Cp*2LnCH2CCAr complexes towards hard Lewis bases such as THF and pyridine has also been studied.

The cyclodimerization of 1-methylalk-2-ynes CH3CCAr catalyzed by Cp*2LaCH2CCAr is treated in Chapter 3. Factors governing the rate and selectivity, such as substrate (i.e. ortho-methyl substitution) and catalyst structure (i.e. ancillary ligation and metal ionic radius), have been investigated. A plausible mechanistic

Chapter 1

6

scenario is proposed to account for the observed products. Finally, the cyclodimerization reaction was investigated as a preprative route towards (cross-)conjugated polymers of 1,4-di(prop-1-ynyl)benzene and 2,5-n-hexyl-1,4-di(prop-1-ynyl)benzene.

The rare-earth metallocene-catalyzed oligomerization of phenylacetylene is discussed in Chapter 4. The effect of metal ion size, ancillary ligation and substrate concentration on the rate and selectivity has been investigated. Relatively high substrate-to-catalyst molar ratios were found to promote catalyst deactivation (via Cp* abstraction) and catalytic trimerization. Stoichiometric reactions established the identity of reaction intermediates observed during catalytic substrate conversion. A kinetic study in conjuction with in situ 1H NMR experiments provided evidence for a rapid pre-equilibrium of a monomeric alkynyl derivative with its base adduct of phenylacetylene. A plausible mechanistic scenario has been proposed to account for the observed reaction products and the observed kinetic behaviour.

The scope of the rare-earth metallocene-catalyzed oligomerization of aromatic 1-alkynes has been investigated in Chapter 5 on the basis of detailed studies of six representative substituted (hetero)aromatic 1-alkynes. The observed behavior in both catalytic and stoichiometric reactions has been rationalized in terms of substrate/product inhibition, electronic/steric 1-alkyne substituent effects, heteroatom-assisted carbon-carbon bond formation and catalyst deactivation. The relative importance of these effects was found to depend on the specific properties of the 1-alkyne. Appropriate 1-alkyne substitution was found to result in high catalytic rates and selectivities for (E)-but-1-en-3-yne formation. The nature of the 1-alkyne substituent effect in the dimerization of monomeric alkynyl derivatives Cp*2LaCCR to dinuclear butatrienediyl derivatives [(Cp*2La)2(µ-ηn:ηn-RC4R)] was also addressed.

The oligomerization of (hetero)aromatic diynes catalyzed by Cp*2LaCH(SiMe3)2 is discussed in Chapter 6. The effect of the (hetero)arene moiety, with and without solubilizing aliphatic substituents, on the regioregularity of the formed polymers has been investigated. The use of mono-ethynyl substrates to control the molecular weight of the polymers is discussed. The formed conjugated polymers and their physical properties have been studied by a variety of spectroscopic methods.

1.5. References and notes 1 The symbol Ln, not assigned to any particular element, is commonly used to designate the lanthanides

as a class. In this text, however, the symbol Ln is used to designate the rare earth metals. 2 The name rare earth metals is deprecated by IUPAC, since these metals are neither rare in

abundance, nor earths (which is an obsolete term for oxides). In fact, each is more common in the earth's crust than silver, gold or platinum, while cerium, yttrium, neodymium and lanthanum are more common than lead.

3 Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 6210. 4 For examples, see: (a) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1983,

105, 1401. (b) Ye, C.; Qian, C.; Yang, X. J. Organomet. Chem. 1991, 407, 329 and references therein.

5 For examples, see: (a) Ballard, D. G. H.; Courtis, A.; Holton, J.; McMeeking, J.; Pearce, R. J. Chem. Soc., Chem. Commun. 1978, 994. (b) Ballard, D. G. H.; Courtis, A.; Holton, J.; McMeeking, J.; Pearce, R. J. Chem. Soc., Chem. Commun. 1976, 1060. (c) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.

6 The highest turnover frequency in the hydrogenation of 1-hexene was found for [Cp*2La(µ-H)]2 (120000 h-1), comparing favorably to the activities of well-known transition-metal homogeneous catalysts, such as RhCl(PPh3)3 (3000 h-1) and [(COD)Ir(PCy3)]PF6 (6400 h-1) under similar reaction conditions (25 °C, 1 atm.), see: (a) Kobayashi, T; Sakakura, T.; Hayashi, T.; Yumura, M.; Tanaka, M. Chem.Lett. 1992, 1158. Even higher activities have been reported for silylene-bridged metallocene hydrides [{Me2Si(C5Me4)2}Ln(µ-H)], see: (b) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103.

7 For reviews, see: (a) Watson, P. L. In Selective Hydrocarbon Activation. Principles and Progress; Davies, J. A., Watson, P. L., Greenberg, A., Liebman, J. F., Eds.; VCH Publishers, Weinheim; 1990, Chapter 4, p. 78. (b) Rothwell, I. P. In Selective Hydrocarbon Activation. Principles and Progress; Davies, J. A., Watson, P. L., Greenberg, A., Liebman, J. F., Eds.; VCH Publishers, Weinheim; 1990, Chapter 3, p. 43. (c) Ref. 5b.

Introduction

7

8 For general reviews on organolanthanide chemistry, see: (a) Marks, T. J.; Ernst, R. D. In

Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 21. (b) Evans, W. J. Adv. Organomet. Chem. 1985, 24, 131. (c) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283. (d) Edelmann, F. T. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 4, Chapter 2. (e) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865. (f) Anwander, R.; Herrmann, W. A. Top. Curr. Chem. 1996, 179, 1. (g) Edelmann, F. T. Top. Curr. Chem. 1996, 179, 247. (h) Edelmann, F. T. In Metallocenes; Togni, A., Halterman, Ronald L., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, Chapter 2. (i) Chirik, P. J.; Bercaw, J. E. In Metallocenes; Togni, A., Halterman, Ronald L., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, Chapter 3. (j) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H, Chem. Rev. 2002, 102, 1851. (k) Arndt, S.; Okuda, J. Chem. Rev. 2002, 102, 1953. (l) Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187. (m) Inanaga, J.; Furuno, H.; Hayano, T. Chem. Rev. 2002, 102, 2211.

9 Tsutsui, M.; Gysling, H. J. J. Am. Chem. Soc. 1969, 91, 3175. 10 Hayes, R. G.; Thomas, J. L. J. Am. Chem. Soc. 1969, 91, 6880. 11 (a) Hart, F. A.; Saran, M. S. Chem. Commun. 1968, 1614. (b) Hart, F. A.; Massey, A. G.; Saran, M. S.

J. Organomet. Chem. 1970, 21, 147. (c) Cotton, S. A.; Hart, F. A.; Hursthouse, M. B.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1972, 1225.

12 For early reviews on organolanthanide chemistry, see: (a) Tsutsui, M.; Ely, N.; Dubois, R. Acc. Chem. Res. 1976, 9, 216. (b) Schumann, H, Angew. Chem. Int. Ed. Engl. 1984, 23, 474. (c) Evans, W. J. Adv. Organometallic Chem. 1985, 24, 131. (d) Watson, P. L.; Parshall Acc. Chem. Res 1985, 18, 51.

13 (a) Tsutsui, M.; Ely, N. J. Am. Chem. Soc. 1974, 96, 4042. (b) Tsutsui, M.; Ely, N. J. Am. Chem. Soc. 1975, 97, 3551. (c) Ely, M.; Tsutsui, M. J. Am. Chem. Soc. 1975, 97, 1280.

14 (a) Maginn, R. E.; Manastryrskyj, S.; Dubeck, M. J. Am. Chem. Soc. 1963, 85, 672. (b) Namy, J. L.; Girard, P.; Kagan, H. B. Caro, P. E. Nouv. J. Chim. 1981, 5, 479. (c) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291.

15 For Cp*2LnCH(SiMe3)2 complexes, see: (a) Sc: St. Clair, M.; Santarsiero, B. D.; Bercaw, J. E. Organometallics 1989, 8, 17. (b) Y: den Haan, K. H.; de Boer, J. L.; Teuben, J. H.; Spek, A. L.; Kojic,-Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726. (c) La, Sm, Nd, Lu: Ref. 41a. (d) Ce: Heeres, H. J.; Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1988, 7, 2495. For [Cp*2Ln(µ-H)]2 complexes, see: (h) Sc: Ref. 40b. (i) Y: den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (j) La, Nd, Lu: Ref. 41a. (k) Ce: Ref. 15d. (l) Sm: Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1983, 105, 1401.

16 For [(C5H4SiMe3)2Ln(µ-CH3)]2 complexes, see: (a) Y, Er: Ref. 5a. (b) Lu: Voskoboynikov, A. Z.; Parshina, I. N.; Shetstakova,A. K.; Butin, K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics 1997, 16, 4041. (c) Nd: Molander, G. A.; Dowdy, E. D.; Noll, B. C. Organometallics 1998, 17, 3734.

17 For Me2SiCp”2LnCH(SiMe3)2 complexes with Cp” = η5-C5Me4, see: (a) Sc: Ref. 49b. (b) Y: Coughlin, E. B.; Henling, L. M.; Bercaw, J. E. Inorg. Chim. Acta 1996, 242, 205. (c) Nd, Sm, Lu: Ref. 6b. For bis(indenyl)ethane-derived lanthanocenes, see: (d) Gilbert, A. T.; Davis, B. L.; Emge, T. J.; Broene, R. D. Organometallics 1999, 18, 2125. For Me2Si(Cp”)(CpR*)LnE(SiMe3)2 complexes with R* = (+)-neomenthyl, (-)-menthyl and E = N, CH, see: (d) Y, Nd, Sm, La, Lu: Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat, M.; Rheingold, A. L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10212. For Me2Si(OHF)(CpR*) complexes with OHF = η5-octahydrofluorenyl and R* = (–)-menthyl, see: (d) Y, Sm, Lu: Douglass, M. R.; Ogasawara, M.; Hong, S.; Metz, M. V.; Marks, T. J. Organometallics 2002, 21, 283.

18 For Me2SiCp”(tBuN)LnE(SiMe3)2 complexes, see: (a) Ln = Sc, E = CH: Ref. 41h. (b) Ln = Sm, Nd, Yb, Lu, E = N, CH: Tian, S.; Arredondo,V. M.; Stern, C. L.; Marks, T. J. Organometallics 1999, 18, 2568.

19 For reviews of non-cyclopentadienyl organometallic complexes of lanthanide and group 3 metals, see: (a) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. Rev. 2002, 102, 1851. (b) Piers, W. E.; Emslie, D. J. H. Coord. Chem. Rev. 2002, 233-234, 131. (c) Edelmann, F. T.; Angew. Chem., Int. Ed. Engl. 1995, 34, 2466.

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20 For reviews, see: (a) Kaminsky, W.; Sinn, H. Transition Metals and Organometallics as Catalysts for

Olefin Polymerization; Springer-Verlag: Berlin, 1988. (b) Keii, T.; Soga, K. Catalytic Olefin Polymerization; Elsevier: Amsterdam, 1990. (c) Guram, S. A.; Jordan, R. F. In Comprehensive Organometallic Chemistry II, Elsevier, Oxford; 1995, Vol. 4.; p. 589. (d) Britzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. Int. Ed. Engl. 1995, 34, 1143. (e) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem. Int. Ed. 1999, 38, 428. (f) Coates, G. W. Chem. Rev. 2000, 100, 1223.

21 For example, typical ligand field effects are a few hundred cm-1 for rare-earth metal complexes, whereas d transition metal complexes normally display values of 5 000-30 000 cm-1.29h

22 (a) Raymond, K. N.; Eigenbrot, C. W. Acc. Chem. Res. 1980, 13, 276. (b) Moeller, T. In Comprehensive Inorganic Chemistry, Trotman-Dickenson, A. F. et al. (Eds.); Pergamon Press: Oxford, 1973; Chapter 44.

23 For a review, see: (a) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 2387. For examples, see: (b) Aparna, K.; Ferguson, M.; Cavell, R. G. J. Am. Chem. Soc. 2000, 122, 726. (c) Cantat, T.; Jaroschik, F.; Nief, F.; Ricard, L.; Mézailles, N.; Le Floch, P. Chem Commun. 2005, 41, 5178. (d) Cantat, T.; Jaroschik, F.; Ricard, L.; Le Floch, P.; Nief, F.; Mézailles, N. Organometallics 2006, 25, 1329. (e) Dietrich, H. M.; Törnroos, K. W.; Anwander, R. J. Am. Chem. Soc. 2006, 128, 9298.

24 Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 2003, 42, 6682.

25 Lanthanide cations are located between Sr(II) and Ti(IV), see: (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. (b) Hard and Soft Acids and Bases; Pearson, R. G. (ed.); Dowden, Hutchinson and Ross, Stroudsberg, PA; 1973. (b) Fleming, I. Frontier Orbitals and Organic Chemical Reactions, Wiley-Interscience, London; 1976.

26 Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54.

27 Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 7844. 28 Even the thermodynamically very stable Ln-OR bond undergoes rapid ligand exchange reactions, see:

(a) Hubert-Pfalzgraf, L. G. New. J. Chem. 1987, 11, 163. (b) Hubert-Pfalzgraf, L. G. New J. Chem. 1995, 19. 727.

29 For reviews, see: (a) Carnall, W. T. In Handbook on the Physics and Chemistry of Rare Earths, Gschneidner, Jr., K. A., Eyring, L. (Eds.); North Holland Publishing Company: Amsterdam, 1979; Vol. 3, Ch. 24. (b) G. Blasse In Handbook on the Physics and Chemistry of Rare Earths, Gschneidner, Jr., K. A., Eyring, L. (Eds.); North Holland Publishing Company: Amsterdam, 1979; Vol. 4, Ch. 34. (c) Shionoya, S.; Yen, W. M. In Phosphor Handbook; Shionoya, S., Yen, W. M. (Eds.); CRC Press, Inc.: Boca Raton, Florida, 1999; Chapter 3. (d) Parker, D.; Williams, J. A. G. In Metal Ions in Biological Systems, Sigel, A; Sigel, H. (Eds.); Marcel Dekker Inc.: New York, 2003;Vol. 40. (e) Døssing, A. Eur. J. Inorg Chem. 2005, 1425. (f) Bünzli, J.-C. G.; Piguet, C. Chem. Soc. Rev. 2005, 34, 1048-1077. (g) Faulkner, S.; Pope, S. J. A.; Burton-Pye, B. P. Appl. Spectrosc. Rev. 2005, 40, 1. (h) Bünzli, J.-C. Acc. Chem. Res. 2006, 39, 53.

30 Solid state spectra have been obtained on paramagnetic compounds using nuclei with relatively low quadrupole moments. For general references, see: (a) Harris, R. K.; Mann, B. E. NMR and the Periodic Table, Academic Press, 1978. (b) Brevard, C.; Granger, P. Handbook of High Resolution Multinuclear NMR, Wiley, New York; 1981. (c) Fischer, R. D. In Fundamental and Technological Aspects of Organo-f-Element Chemistry; NATO ASI Series; Marks, T.J., Fragalà, I. L., Eds.; D. Reidel, Boston; 1985.

31 For examples, see: (a) Evans, W. J.; Meadows, J. H.; Kostka, A. G.; Closs, G. L. Organometallics 1985, 4, 324. (b) Schaverien, C. J.; Frijns, J. H. G.; Heeres, H. J.; van den Hende, J. R.; Teuben, J. H.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1991, 642. (c) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1994, 116, 2600.

32 For examples of 139La NMR spectroscopy, see: (a) Eggers, S. H.; Fischer, R. D. J. Organomet. Chem. 1986, 315, C61. (b) Eggers S. H.; Adam, M.; Haupt, E. T. K.; Fischer, R. D. Inorg. Chim. Acta 1987, 139, 315. (c) Adam, M.; Haupt, E. T. K.; Fischer, R. D Bull. Magn. Res. 1990, 12, 101. (d) Windisch, H.; Scholz, J.; Taube, R.; Wrackmeyer, B. J. Organomet. Chem. 1996, 520, 23. For examples of 45Sc NMR spectroscopy, see: (d) Mancini, M. Inorg. Chem. 1984, 23, 1072. (e) Pougeard, P.; Mancini, M.; Sayer, B. G.; McGlinchey, M. J. Inorg. Chem. 1985, 24, 93.

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33 For examples, see: (a) Avent, A. G.; Edelman, M. A.; Lappert, M. F.; Lawless, G. A. J. Am. Chem.

Soc. 1989, 111, 3423. (b) Recknagel, A.; Edelman, F. T. Angew. Chem., Int. Ed. Engl. 1991, 30, 693. (c) van den Hende, J. R.; Hitchcock, P. B.; Lappert, M. F.; Nile, T. A. J. Organomet. Chem. 1994, 472, 79.

34 For examples, see: (a) Shen, Z.; Farona, M. F. Polym. Bulletin 1983, 10, 298. (b) Shen, Z; Farona, M. F. J. Polym. Sci.: Polym. Chem. Ed. 1984, 22, 1009. (c) Zhiquan, S.; Mujie, Y.; Mingxiao, S.; Yiping, C. J. Polym. Sci.: Polym. Lett. Ed. 1982, 20, 417. For reviews, see: (a) Ref. 12a. (b) Bruzzone, M. In Fundamental and Technological Aspects of Organo-f-Element Chemistry; NATO ASI Series; Marks, T.J., Fragalà, I. L., Eds.; D. Reidel, Boston; 1985.

35 (a) Evans, W. J.; Coleson, K. M.; Engerer, S. C. Inorg. Chem. 1981, 20, 4320. (b) Evans, W. J.; Engerer, S. C.; Coleson, K. M. J. Am. Chem. Soc. 1981, 103, 6672.

36 (a) Ballard, D. G. H.; Courtis, A.; Holton, J.; McMeeking, J.; Pearce, R. J. Chem. Soc., Chem. Commun. 1978, 994. (b) Ballard, D. G. H.; Pearce, R. J. Chem. Soc., Chem. Commun. 1975, 621.

37 For examples, see: (a) Chigir, N. N.; Guzman, I. Sh.; Sharaev, O. K.; Tiniakova, E. I.; Dolgoplosk, B. A. Dokl. Akad. Nauk USSR, 1981, 263, 375. For reviews, see: (b) Mazzei, A. In Organometallics of the f-Elements; Marks, T. J, Fischer, R. D. (Eds.); D. Reidel Publishers, Boston; 1979.

38 (a) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1983, 105, 1401. (b) Evans, W. J. J. Organomet. Chem. 1983, 250, 217.

39 Zinnen, H. A.; Pluth, J. J.; Evans, W. J. J. Chem. Soc., Chem. Commun. 1980, 810. 40 For examples, see: (a) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337. (b) Thompson, M. E.; Baxter,

S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (c) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 1566. For reviews, see: (d) Burger, B. J.; Cotter, W. D.; Coughlin, E. B.; Chacon, S. T.; Hajela, S. H.; Herzog, T. A.; Kohn, R.; Mitchell, J.; Piers, W. E.; Shapiro, P. J.; Bercaw, J. E. In Ziegler Catalysts; Fink, G.; Mülhaupt R.; Brintzinger, H. H., Eds.; Springer Verlag: Berlin, 1995. (e) Gromada, J.; Carpentier, J.-F.; Motreux, S. Coord. Chem. Rev. 2004, 248, 397.

41 For examples, see: (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (b) Jeske, G.; Laurel, E.; Schock, P.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103. (c) Marks, T. J.; Mauermann, H. WO Patent 8605788 (Northwestern University, USA), 1986. (d) Wiesenfeldt, H.; Reinmuth, A.; Barsties, E.; Evertz, K.; Brintzinger, H.-H. J. Organomet. Chem. 1989, 369, 359. (e) Roll, W.; Brintzinger, H.-H.; Rieger, B.; Zolk, R. Angew. Chem. Int. Ed. Engl. 1990, 29, 279. (f) Shapiro, P. J.; Bunel, E. E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867. (g) Stern, D.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558. (h) Shapiro, P. J.; Cotter, W. D.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4623. (i) Hajela, S.; Bercaw, J. E. Organometallics 1994, 13, 1147. (j) Mitchell, J. P.; Hajela, S.; Brookhart, K.; Hardcastle, K. I.; Henling, H. M.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 1045. (k) Gilchrist, J. H.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 12021. (l) Ihara, E.; Nodono, M.; Katsura, K.; Adachi, Y.; Yasuda, M.; Yamagashira, H.; Hashimoto, H.; Kanehisa, N.; Kai, Y. Organometallics 1998, 17, 3945. (m) Ihara, E.; Yoshioka, S.; Furo, M.; Katsura, K.; Yasuda, M.; Mohri, S.; Kanehisa, M.; Kai, Y. Organometallics 2001, 20, 1752.

42 (a) Chen, W.; Xiao, S.; Wang, Y.; Yu, G. Kexue Tongbao 1984, 29, 892. (b) Qian, H.; Yu, G.; Chen, W. Gaofenzi Tongxun 1984, 3, 226. (c) Yu, G.; Chen. W.; Wang, Y. Kexue Tongbao 1984, 29, 421. (d) Taube, R.; Windisch, H. J. Organomet. Chem. 1994, 472, 71. (e) Barbier-Baudry, D.; Andre, N.; Dormond, A.; Pardes, C.; Richard, P.; Visseaux, M.; Zhu, C. J. Eur. J. Inorg. Chem. 1998, 1721. (e) Bonnet, F.; Barbier-Baudry, D.; Dormond, A.; Visseaux, M. Polymer Int. 2002, 51, 986. (f) Barbier-Baudry, D.; Bonnet, F.; Domenichini, B.; Dormond, A.; Visseaux, M. J. Organometl. Chem. 2002, 647, 167.

43 (a) Bogaert, S.; Carpentier, J.-F.; Chenal, T.; Mortreux, A.; Ricart, G.; Macromol. Chem. Phys. 2000, 201, 1813. (b) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J.-F. J. Am. Chem. Soc. 2004, 126, 12240.

44 (a) Yasuda, H.; Furo, M.; Yamaoto, H. Macromolecules 1992, 25, 5115. (b) Jiang, T.; Shen, Q.; Lin, Y.; Jns, S. J. Organomet. Chem. 1993, 450, 121. (c) Yasuda, H.; Yamamoto, H.; Yamashita, M.; Yokota, K.; Nakamura, A, Miyake, S.; Kai, Y. Kanchisa, N. Macromolecules 1993, 26, 7134. (d) Yasuda, H.; Yamamoto, H.; Yokota, K.; Miyake, S.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 4908. (e) Simpson, C. K.; White, R. E.; Carlson, C. N.; Wrobleski, D. A.; Kuehl, C. J.; Croce, T. A.;

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Steele, I. M.; Scott, B. L.; Young Jr., V. G.; Hanusa, T. P.; Sattelberger, A. P.; John, K. D. Organometallics 2005, 24, 3685.

45 (a) Yasuda, H. Chem Abstr. 1994, 120, 135475a. (b) Evans, W. J.; Katsumata, H. Macromolecules 1994, 27, 2330. (c) Onozawa, S.; Sakakura, T.; Tanaka, M. Chem. Lett. 1994, 531.

46 For reviews, see: (a) Yasuda, H. Prog. Polym. Sci. 2000, 25, 573. (b) Hou, Z.; Wakatsuki, Y. Coord. Chem. Rev. 2002, 231, 1. (c) Yasuda, H. J. J. Organomet. Chem. 2002, 647, 128.

47 For reviews, see: (a) Molander, G. A. Chemtracts: Org.Chem. 1998, 18, 237. (b) Molander, G. A.; Dowdy, E. D. Top. Organomet. Chem. 2000, 2, 119. (c) Molander, G. A.; Romero, J. A. C. Chem. Rev. 2002, 102, 2161.

48 (a) Molander, G. A.; Hoberg, J. O. J. Am. Chem. Soc. 1992, 114, 3123. (b) Molander, G. A.; Hoberg, J. O. J. Org. Chem. 1992, 57, 3266. (c) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagné, M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241. (d) Haar, C. M.; Stern, C. L.; Marks, T. J. Organometalllics 1996, 15, 1765. (e) Roesky, P. W.; Denninger, U.; Stern, C. L.; Marks, T. J. Organometallics. 1997, 16, 4486. (f) Obora, Y.; Ohta, T.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 3745.

49 (a) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337. (b) Piers, W. E.; Shapiro, P. J.; Bunel, E. E. Synlett 1990, 74. (c) Kretschmer, W. P.; Troyanov, S. I.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 284.

50 (a) Olonde, X.; Mortreux, A.; Petit, F.; Bujadoux, K. J. Mol. Catal.1993, 82, 75. (b) Mauermann, J.; Swepston, P. N.; Marks, T. J. J. Am. Chem. Soc. 1985, 4, 200. (c) Jeske, G. ; Lauke, H. ; Mauermann, H. ; Swepston, P. N. ; Schumann, H. ; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (d) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276. (e) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (f) Evans, W. J. ; Bloom, I. ; Hunter, W. E. ; Atwood, J. L. J. Am. Chem. Soc. 1983, 105, 1401.

51 (a) Molander, G. A.; Hoberg, J. O. J. Am. Chem. Soc. 1992, 14, 3123. (b) Piers, W. E.; Shapiro, P. J.; Bunel, E. E. Synlett 1990, 74.

52 For a review of the organolanthanide-catalyzed hydroamination reaction, see: Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673.

53 (a) Sakakura, T.; Lautenschlager, H.; Tanaka, M. J. Chem. Soc., Chem. Commun. 1991, 40. (b) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 6347. (c) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 7157. (d) Molander, G. A.; Corrette, C. P. Organometallics 1998, 17, 5504. (e) Schumann, H.; Keitsch, M. R.; Winterfeld, J.; Mühle, S.; Molander, G. A. J. Organomet. Chem. 1998, 559, 181.

54 (a) Douglass, M. R.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 1824. (b) Douglass, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2001, 123, 10221.

55 (a) Harrison, K. N. ; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 9220. (b) Bijpost, E.; Duchateau, R.; Teuben, J. H. J. Nol. Catal. 1995, 95, 121. (c) Molander, G. A.; Pfeiffer, D. Org. Lett. 2001, 3, 361.

56 (a) Gagné, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 4108. (b) Gagné, M. R.; Nolan, S. P.; Marks, T. J. Organometallics 1990, 9, 1716 (c) Gagné, M. R.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275. (d) Giardello, M. A.; Conticello, V. P.; Brard, L. ; Gagné, M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241.

57 (a) Li, Y.; Fu, P.-F.; Marks, T. J. Organometallics 1994, 13, 439. (b) Li, Y.; Marks, T. J. Organometallics 1996, 118, 9295.

58 (a) Arredondo, V. M.; McDonald, F. E. Marks, T. J. J. Am. Chem. Soc. 1998, 120, 4871. (b) Arredondo, V. M.; McDonald, F. E.; Marks, T. J. Organometallics 1999, 18, 1949.

59 (a) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 707. (b) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1757. (c) Hong, S.; Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 15878.

60 Shapiro, P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867. 61 (a) Gagné, M. R.; Brard, L.; Conticello, V. P.; Giardello, M. A.; Stern, C. L.; Marks, T. J.

Organometallics 1992, 11, 2003. (b) Coughlin, E. B.; Bercaw, J. E. J. Am. Chem. Soc. 1992, 114, 7606. (c) Mitchell, J. P.; Hajela, S.; Brookhart, S. K.; Hardcastle, K. I. Henling, L. M. Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 1045.

62 Piers, W. E.; Shapiro, P. J.; Bunel, E. E. Synlett 1990, 74. 63 Bunel, E. E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 976. 64 Fontaine, F.-G.; Tilley, T. D. Organometallics 2005, 24, 4340.

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65 (a) Piers, W. E.; Hayes, P. G.; McDonald, R. J. Am. Chem. Soc. 2002, 124, 2132. (b) Hajela, S.;

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66 Giesbrecht, G. R.; Whitener, G. D.; Arnold, J. J. Chem. Soc., Dalton Trans. 2001, 923. 67 Gountchev, T. I.; Tilley, T. D. Organometallics 1999, 18, 5661. 68 (a) Hultsch, K. C.; Hampel, F.; Wagner, Th. Organometallics 2004, 23, 2601. (b) Bürgstein, M. R.;

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