dehydrogenation of alkanes - the dong group at ut...
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Dehydrogenation of Alkanes
Zhiqian “Wallace” Wang02-08-2012
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Contents:
• Introduction• Dehydrogenation of Alkanes by Pincer
Iridium Complexes• Alkane Metathesis• Dehydrogenation Involving Heteroatoms• Conclusion
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C COH, NH2
O
COOROH
OH
O
FG Nu
En
Introduction: Dehydrogenation of Alkanes
H H
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Early Work: From 1970s
Crabtree:
Felkin:
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Dehydrogenation of Alkanes by Pincer Iridium Complexes
1996: Jensen
82 turnovers/h at 150oC, 12 turnovers/min at 200oCno observable decomposition over one week at 200oC
IrH2
PtBu2
PtBu2
3-H2
J. Am. Chem. Soc. 1997, 119, 840
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Angew. Chem. Int. Ed. 2001, 40, 3750 - 3781
J. AM. CHEM. SOC. 2004, 126, 1804-1811
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The first example of efficient acceptorless dehydrogenation
Goldman:
Chem. Commun. 1997, 2273Chem. Commun. 1999, 655
close to 1000 turnovers
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IrH2
PtBu2
PtBu2
3-H2
Within cycle
Out of cycle
Resting state
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Catalysts 3 and 5 show high kinetic selectivity for dehydrogenation of the terminal position of n-alkanes to give a-olefins. This regioselectivity holds great promise with respect to the formation of valuable a-olefins and products derived from them in tandem catalytic systems.
IrH2
PtBu2
PtBu2
3-H2
IrH4
PiPr2
PiPr25-H4 But --- isomerization
J. Am. Chem. Soc. 1999, 121, 4086
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Mechanism of (PCP)Ir-Catalyzed Acceptorless Dehydrogenation of Alkanes
Rate determine step : H2 loss
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A Combined Computational and Experimental Study
J. AM. CHEM. SOC. 2002, 124, 11404-11416
2002: Karsten Krogh-Jespersen* and Alan S. Goldman*
Overall Reaction:
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Ir(I) pathway is not accessible in the Cp*Ir case, and the alternative of ó-bond-metathesis by Cp*Ir(III) was calculated to be less favorable than the Ir(V) pathway
Cp*Ir(III) + C H Ir(V)
Bergman & Hall:
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high entropy under these conditions of the free H2 molecule
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A wide range of modified PCP pincer ligands with varying electronic and steric properties have been explored synthetically and computationally
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The p-methoxy-substituted complex (MeO-tBu4PCP)IrH2 (6a- H2) gave a 2-3-fold increase in turnovers for acceptorless dehydrogenation of cyclodecane as compared to 3-H2.
The sterically less bulky (MeO-Pr4PCP)IrH4(6b-H4) was extremely effective in the acceptorless dehydrogenation of cyclodecane, with a total of 3050 turnovers obtained after 72 h under reflux conditions (201 oC)
Thermally very stable: Complex 7a-H2 tolerated reaction temperatures up to 250 oC
Complex 7a-H2 significantly less reactive than 3-H2 under comparable conditions
(R4PCP)Ir
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One of the most notable modifications of the PCP ligand: Brookhart and Jensen
(R4POCOP)Ir
J. AM. CHEM. SOC. 2004, 126, 1804-1811
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(tBu4PCP)Ir reversibly reacts with TBE to give a vinylic C-H addition Product(tBu4POCOP)Ir forms a π-coordinated complex
TBE TBA
(POCOP)IrH2, -70oC
(PCP)IrH2, 50oC
Alkene hydrogenation by 9b-H2 is much more facile than by 3-H2
(R4PCP)Ir (R4POCOP)Ir
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Geometric Differences:
(POCOP)Ir is much less sterically hindered than that of (PCP)Ir
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Electronic Differences:
J. Am. Chem. Soc. 2004, 126, 13044
The iridium center in 9a iscalculated to be very slightly more electron-rich than that in 3
Ir
O
O
PtBu2
PtBu2
Ir
PtBu2
PtBu2
Geometric
Electronic
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Alkane Metathesis : disproportionation or molecular distribution
potential applications in fuel and bulk chemical production
Platinum/alumina : alkane tansfer-dehydrongenation catalystTungsten oxide/silica: metathesis of resulting olefins
1973 Burnett and Hunghes
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2006 Goldman and Brookhart
Science 2006, 312, 257
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How to avoid secondary metathesis, which decreases the overall selectivity
During n-octane metathesis using the two-pot apparatus, the only observed products of secondary metathesis were very small quantities of n-C15H32 and n-C16H34
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Dehydrogenation Involving Heteroatoms:
Kaska and Jensen
Goldman
Jensen
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Goldman
only stoichiometric reactions were observed
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Dehydrogenation of Amine-Boranes
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Dehydrogenation of C-N and C-O Linkages
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Conclusion
IrH2
PtBu2
PtBu2
n
X X
X X
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Thanks
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Ir
PtBu2
PtBu2
HH
Count valence electrons for each complex:
Ir
PtBu2
PtBu2
Ir
PtBu2
PtBu2
HCl
Ir
PtBu2
PtBu2
HH
3-H2
Why these reactions have such a selectivity?
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Propose a mechanism of following reaction: no need to explain selectivity of product a and b