william a. goddard, iii, [email protected]
DESCRIPTION
Lecture 26, December 3, 2009. Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy. Course number: KAIST EEWS 80.502 Room E11-101 Hours: 0900-1030 Tuesday and Thursday. - PowerPoint PPT PresentationTRANSCRIPT
EEWS-90.502-Goddard-L15 1© copyright 2009 William A. Goddard III, all rights reserved
Nature of the Chemical Bond with applications to catalysis, materials
science, nanotechnology, surface science, bioinorganic chemistry, and energy
William A. Goddard, III, [email protected] Professor at EEWS-KAIST and
Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics,
California Institute of Technology
Course number: KAIST EEWS 80.502 Room E11-101Hours: 0900-1030 Tuesday and Thursday
Senior Assistant: Dr. Hyungjun Kim: [email protected] of Center for Materials Simulation and Design (CMSD)
Teaching Assistant: Ms. Ga In Lee: [email protected] assistant: Tod Pascal:[email protected]
Lecture 26, December 3, 2009
EEWS-90.502-Goddard-L15 2© copyright 2009 William A. Goddard III, all rights reserved
Schedule changesDec. 3, Thursday, 9am, L26, as scheduledDec. 7-10 wag in Pasadena; no lectures,Dec. 14, Monday, 2pm, L27, additional lecture, room 101Dec. 15, Final exam 9am-noon, room 101
EEWS-90.502-Goddard-L15 3© copyright 2009 William A. Goddard III, all rights reserved
Last time
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Bonding in metallic solids
Mosty of the systems discussed so far in this course have been covalent, with the number of bonds related to the number of valence electrons.
Thus we have discussed the bonding of molecules such as CH4, benzene, O2, and Ozone.. The solids such as diamond, silicon, GaAs, are generally insulators or semiconductors
We have also considered covalent bonds to metals such as FeH+, (PH3)2Pt(CH3)2, (bpym)Pt(Cl)(CH3), The Grubbs Ru catalysts
We have also discussed the bonding in ionic materials such as (NaCl)n, NaCl crystal, and BaTiO3, where the atoms are best modeled as ions with the bonding dominated by electrostatics
Next we consider the bonding in bulk metals, such as iron, Pt, Li, etc. where there is little connection between the number of bonds and the number of valence electrons.
EEWS-90.502-Goddard-L15 5© copyright 2009 William A. Goddard III, all rights reserved
Bringing atoms together to form the solidAs we bring atoms together to form the solid, the levels broaden into energy bands, which may overlap . Thus for Cu we obtain
Energy
Density states
Fermi energy (HOMO and LUMO
Thus we can obtain
systems with no band gap.
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Metals vs inulators
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conductivity
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The elements leading to metallic binding
There is not yet a conceptual description for metals of a quality comparable to that for non-metals. However there are some trends, as will be described
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Body centered cubic (bcc), A2
A2
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Face-centered cubic (fcc), A1
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Alternative view of fcc
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Closest packing layer
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Stacking of 2 closest packed layers
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Hexagaonal closest packed
(hcp) structure, A3
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Cubic closest packing
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Double hcp
The hexagonal lanthanides mostly exhibit a packing of closest packed layers in the sequence
ABAC ABAC ABAC
This is called the double hcp structure
EEWS-90.502-Goddard-L15 17© copyright 2009 William A. Goddard III, all rights reserved
bcc
hcp
fcc
mis
Structures of elemental metals
some correlation of structure with number of
valence electrons
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Binding in metalsLi has the bcc structure with 8 nearest neighbor atoms, but there is only one valence electron per atom. Similarly fcc and hcp have 12 nearest neighbor atoms, but Al has only three valence electrons per atom. Clearly the bonding is very different than covalentOne model (Pauling) resonating valence bondsProblem is energetics:Li2 bond energy = 24 kcal/mol 12 kcal/mol per valence electronCohesive energy of Li (energy to atomize the crystal is 37.7 kcal/mol per valence electron. Too much to explain with resonance
New paradigm: Interstitial electron model (IEM). Each valence electron localizes in a tetrahedron between four Li nuclei.
Bonding like in Li2+, which is 33.7 kcal/mol per valence electron
EEWS-90.502-Goddard-L15 19© copyright 2009 William A. Goddard III, all rights reserved
GVB orbitals of ring M10 molecules
Get 10 valence electrons each localized in a bond midpoint
Calculations treated all 11 valence electrons of Cu, Ag, Au using effective core potential.
All electrons for H and Li
R=2 a0
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New
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Hypervalent compounds
It was quite a surprize to most chemists in 1962 when Neil bartlett reported the formation of a compound involving Xe-F bonds. But this was quickly folllowed by the synthesis of XeF4 (from Xe and F2 at high temperature and XeF2 in 1962 and later XeF6.Indeed Pauling had predicted in 1933 that XeF6 would be stable, but noone tried to make it.
Later compounds such as ClF3 and ClF5 were synthesized
These compounds violate simple octet rules and are call hypervalent
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Noble gas dimers
Recall from L17 that there is no chemical bonding in He2, Ne2 etc
This is explained in VB theory as due to repulsive Pauli repulsion from the overlap of doubly occupied orbitals
(g)2(u)2
It is explained in MO theory as due to filled bonding and antibonding orbitals
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Noble gas dimer positive ions
On the other hand the positive ions are strongly bound (L17)
This is explained in MO theory as due to one less antibonding electron than bonding, leading to a three electron bond for He2
+ of 2.5 eV, the same strength as the one electron bond of H2
+ (g)2(u)1
-
The VB explanation is a little less straightforward. Here we consider that there are two equivalent VB structures neither of which leads to much bonding, but superimposing them leads to resonance stabilization
Using (g) = L+R and (u)=L-R
Leads to (with negative sign
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Re-examine the bonding of HeH
Why not describe HeH as (g)2(u)1 where (g) = L+R and (u)=L-RWould this lead to bonding?The answer is no, as easily seen with the VB form where the right structure is 23.9 eV above the left. Thus the energy for the (g)2(u)1 state would be +12.0 – 2.5 = 9.5 eV unbound at R=∞Adding in ionic stabilization lowers the energy by 14.4/2.0 = 7.2 eV (too big because of shielding) , still unbound by 2.3 eV
-
He H He+H-
IP=+24.6 eV EA = 0.7 eV
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Examine the bonding of XeF
Xe Xe+
The energy to form Xe+ F- can be estimated from
Consider the energy to form the charge transfer complex
Using IP(Xe)=12.13eV, EA(F)=3.40eV, and R(IF)=1.98 A, we get
E(Xe+ F-)=1.45eV
Thus there is no covalent bond for XeF, which has a weak bond of ~ 0.1 eV and a long bond
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Examine the bonding in XeF2
We saw that the energy to form Xe+F-, now consider, the impact of putting a 2nd F on the back side of the Xe+ Xe+
Since Xe+ has a singly occupied pz orbital pointing directly at this 2nd F, we can now form a bond to it?How strong would the bond be?Probably the same as for IF, which is 2.88 eV.Thus we expect F--Xe+F- to have a bond strength of ~2.88 – 1.45 = 1.43 eV!Of course for FXeF we can also form an equivalent bond for F-Xe+--F. Thus we get a resonance
We will denote this 3 center – 4 electron charge transfer bond asFXeF
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Stability of XeF2
Ignoring resonance we predict that XeF2 is stable by 1.43 eV. In fact the experimental bond energy is 2.69 eV suggesting that the resonance energy is ~ 1.3 eV.
The XeF2 molecule is stable by 2.7 eV with respect to Xe + F2
But to assess where someone could make and store XeF2, say in a bottle, we have to consider other modes of decomposition.
The most likely might be that light or surfaces might generate F atoms, which could then decompose XeF2 by the chain reaction
XeF2 + F {XeF + F2} Xe + F2 + F
Since the bond energy of F2 is 1.6 eV, this reaction is endothermic by 2.7-1.6 = 1.1 eV, suggesting the XeF2 is relatively stable. Indeed it is used with F2 to synthesize XeF4 and XeF6.
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The VB analysis would indicate that the stability for XeF4 relative to XeF2 should be ~ 2.7 eV, but maybe a bit weaker due to the increased IP of the Xe due to the first hypervalent bond and because of some possible F---F steric interactions.
There is a report that the bond energy is 6 eV, which seems too high, compared to our estimate of 5.4 eV.
XeF4
Putting 2 additional F to overlap the Xe py pair leads to the square planar structure, which allows 3 center – 4 electron charge transfer bonds in both the x and y directions, leading to a square planar structure
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XeF6
Since XeF4 still has a pz pair, we can form a third hypervalent bond in this direction to obtain an octahedral XeF6 molecule.
Here we expect a stability a little less than 8.1 eV.
Pauling in 1933 suggested that XeF6 would be stabile, 30 years in advance of the experiments.
He also suggested that XeF8 is stable. However this prediction is wrong
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Estimated stability of other Nobel gas fluorides (eV)
Using the same method as for XeF2, we can estimate the binding energies for the other Noble metals.
Here we see that KrF2 is predicted to be stable by 0.7 eV, which makes it susceptible to decomposition by F radicals
1.3 1.3 1.3 1.3 1.3 1.32.71.0 3.9-5.3-2.9 -0.1
RnF2 is quite stable, by 3.6 eV, but I do not know if it has been observed
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XeCl2
Since EA(Cl)=3.615 eV and R(XeCl+)=2.32A and De(XeCl+)=2.15eV, can estimate that XeCl2 is stable by 1.14 eV with respect to Xe + Cl2.
However since the bond energy of Cl2 is 2.48 eV, the energetics of the chain dempostion process are exothermic by 1.34 eV, suggesting at most a small barrier
Thus XeCl2 would be difficult to observe
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Halogen Fluorides, ClFn
The IP of ClF is 12.66 eV which compares well to the IP of 12.13 for Xe.
This suggests that the px and py pairs of Cl could be used to form hypervalent bonds leading to ClF3 and ClF5.
Indeed these estimates suggest that ClF3 and ClF5 are stable.
Indeed the experiment energy for ClF3 ClF +F2 is 2.6 eV, quite similar to XeF2.
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EEWS-90.502-Goddard-L15 49© copyright 2009 William A. Goddard III, all rights reserved
Julius Su and William A. Goddard IIIMaterials and Process Simulation Center,
California Institute of Technology
Origin of reactivity in the hypervalentreagent o-iodoxybenzoic acid (IBX)
EEWS-90.502-Goddard-L15 50© copyright 2009 William A. Goddard III, all rights reserved
Hypervalent iodine assumes many metallic personalities
IOH
O O
O
I
OAc
OAc
I
OH
OTs
I
O
Oxidations
Radicalcyclizations
CC bondformation
Electrophilicalkene activation
CrO3/H2SO4
Pd(OAc)2
SnBu3Cl
HgCl2
Can we understand iodine as we understand metals?Martin, J. C. organo-nonmetallic chemistry – Science 1983 221(4610):509-514
EEWS-90.502-Goddard-L15 51© copyright 2009 William A. Goddard III, all rights reserved
Practical benefits of hypervalent iodine reagents
Cheap, non-toxic, and environmentally friendly.
$/molI 2
Pd 1,300Os 2,300Ir 2,700Pt 2,400Rh 19,500
oral rat LD50 (mg/kg)
I2 14,000OsO4 162SeO2 68
Variants possible:water solublepolymer supportednon-explosive mixture
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S
SOH
S
SO
NH
O
N
O
OH
t-Bu
O
t-Bu
O
O O
CHO
IBX, a single reagent withmany roles 78%
IOH
O O
O
1.1 eq IBX
25oC
CHXH oxidationsphenols to quinones unsaturationallylic oxidationradical cyclization
What is the origin of its diverse reactivity?
85%
99% conv
Solvents are DMSO mixtures or CHCl3.Taken from Palmisano, Nicolaou, McFadden.
86%
85%
1 eq IBX
23oC
2 eq IBX
75oC
3 eq IBX
85oC
2 eq IBX
90oC
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Theoretical methods
Density functional theoryHarmonic frequenciesSingle point continuum solvent
Added d and f functions on INeeded for correct bond length andenergy, even for covalent iodine bonds
Alternative functionalMPW1PW91 for better descriptionof long range binding
mpwb
pw
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Validation against experiment and more accurate theory
-20
-10
0
10
20
30
40
50
60
70
IF ICl IBr I2 IOH ICH3 XeF+ XeF2 XeF4 XeF6 XeO XeO3 XeO4
reference
our method
Includes atom spin-orbit effects,zero point energy, and BSSE
Systematic underestimation of bond energies,but relative energies are accurate.
Ener
gy p
er b
ond
(kca
l/mol
)
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Nature of the hypervalent bond
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Multicenter bonds go beyond the covalent bond
H
H
H
H
H H
H
HH
H
HH
Multicenter 6c-6eDepends on out-of-plane bend too
Covalent 2c-2eStrength depends on length only
60o bend, 114 kcal/mol
bend (degrees)
ener
gy
Bond angles and torsions change multicenter bonds.Dijkstra, F. et al. bent benzene – Int J. Quan. Chem 1999 74:213-221
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Electron rich or poor multicenter bonds: allyl variations
Electrostatics makes charged bonds especially stiff.
H
H
H
HH
H
H
H
HH
H
H
H
HH
38 13 33Etwist(kcal/mol)
bonding
antibonding
nonbonding
MO stabilizations are similar but twisting localizes charge
Gobbi et al. allyl resonance – J. Am. Chem. Soc. 1994 116:9275-9286
HH
HH
H
H
H
HH
H
localized charge,less stable
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Hypervalent bonds as 3c-4e bonds
Resonance stabilized 180o pairs
Xe
F Xe F F Xe F
F
F
F Xe F
F
F
F
F+ F2
h
valence bond picture molecular orbital picture
Each lone pair can bondtwo fluorines (3c-4e bond)
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Valence bond explanation of hypervalent bond strengths
E = 2 • (½ covalent + ½ ionic) + resonance
covalent = D(XeF+) = 38 kcal/molionic = IP(Xe) – EA(F) – 1/R = +35 kcal/molresonance = E(180o) – E(90o) = 71 kcal/moltotal = –74 kcal/mol vs. –62 kcal/mol (expt)
Majority of energy comes from resonance stabilization
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I Xe
O
Xe
F
Fcovalent dative
hypervalent
Combining different ligand typescr
ysta
l str
uctu
res
Occupy octahedralpositions
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Lower symmetry structure becomes more stable.
High symmetry structure has 2nd order Jahn-Teller distortion
e
e*a LUMO
Higher order multicenter bonds as transition states
F I F
F F
FFI
3c4e 4c6e
‡
E = 15.5 kcal/mol
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Valence bond comparison of IF3 geometries
To a first approximation, both geometries have the same energy.
In both cases, E = 2 • covalent + ionic + resonance
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VB explanation of C2v vs. D3h energy difference
90o 120oangle bending
electrostatic repulsionbetween ligands
E(bend) E(elec) sum DFT
ClF3 4.0 12.3 16.3 16.7BrF3 5.0 12.0 17.0 17.2IF3 6.4 11.4 17.7 15.4
Accounts for allobserved energydifference
energies in kcal/mol
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Rules of hypervalent reactivity
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Folk notion of hypervalent bonds as weak and reactive
Xe FFEatom = 62 kcal/mol, or31 kcal/mol per bond
breaking in oxidation
creating anionic fragment
single electronacceptor
Proposed mechanisms focus on reactivity of hypervalent bonds
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Rule 1. Individual hypervalent bonds are strong
Xe FF FF Xe + F Xe F+ +
+51 kcal/mol +8 kcal/mol
3e- 2 center bond,resonance lost.
The first bond is strong,since breaking it createsunstable fragments.
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Rule 2: Bonds next to hypervalent bonds are weakened
Adjacent bond weakened byhypervalent hyperconjugation3e-, 2 center
bond isparticularlystable
44 kcal/mol 41 13
Hypervalent oxo bond isparticularly effective at weakening neighboring bond
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Rule 3. Twist switches hypervalent and covalent ligands
hypervalentbonds circled
Twist proceeds viaD3h transition state,E‡ = 15.5 kcal/mol
3c-4e 3c-4e4c-6e: promotion tohigher order hypervalentbond.
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IBX oxidation of alcohols
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NMR kinetics study of alcohol oxidation
IOH
O O
O
IO
O O
O
H2O
IOH
O O
R'R
H
HOR'
RH
O R'
R
fast slow
De Munari et al, kinetics study – J. Org. Chem. 1996 61(26):9272-9276
Fast ligand exchangefollowed by slow oxidation
Large alcohols oxidize fasterthan small alcohols
axial geometry
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DFT supports ligand exchange preequilibrium
0.00
1.00
2.00
3.00
4.00
5.00
0 0.02 0.04 0.06Experimental Keq
Cal
cula
ted
dG (k
cal/m
ol)
Low barrier pathway found,and good correlation between G (theory) and Keq (expt)
methanol, isopropanol, t-BuOHbenzyl alcohol, etc.
IOH2
O O
O
HOCH3
I OH2O O
O
HOCH3
IOH2
O O
OOCH3H
protonated IBX internal ligand rotationE‡ = 7.2 kcal/mol
swapped ligand
+
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-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 0.5 1
measured chemical shift (ppm)
calc
. N
MR
shi
eldi
ng
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
axial geometry equatorial geometry
Theoretical prediction of alcohol NMR shifts
Confirms that axial species predominates in solution
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An oxidation mechanism with a twist
IOH
O O
O
IO
O O
O
HH
H
IO
O O
OH
HH
IOH
O O
O
H
H
-H2O
+CH3OH
hypervalent twist
axial
equatorial
Complex must twist for oxidation to happen
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Twisting switches strong and weak bonds
We find activating reactivity by twisting to be a common theme
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IOH
O O
O
IO
O O
O
HH
H
IO
O O
OH
HH
IOH
O O
O
H
H
-H2O
+CH3OH
Twisting9.9 kcal/mol
Oxidation2.6 kcal/molLigand exchange
7.2 kcal/mol (with H+)
Hypervalent twisting is rate-limiting
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Size selectivity explained, and an improved reagent?
Large alcohols oxidize fasterbecause they twist more easily.
IOR
O O
OR' An ortho group shouldaccelerate twisting further.
repulsionrelieved
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B(Ome)2
B(OH)2
tBu
iPA
Et
Ph
Cl
F
Me
H
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
-7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0
best Large enough to favor twisted structure
O
I
OHO
O
R'
Medium-sized aliphatics (Me, Et, i-Pr) best
But not so large it hinders alcohol/water exchange
better alcohol/waterexchange
better twisting
Screening for an optimal ortho substituent
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et
ipaiso
neo2,4
ph
-1
0
1
2
3
4
5
6
-1 0 1 2
measured log(kalc/kMeOH)
pred
icte
d lo
g(ka
lc/k
MeO
H)
100xfasterrate!
OI O
HO
O
ligandexchange
twisting
rate-limiting OI O
HO
O
H3C
Predicted rate acceleration for ortho-methyl IBX
Up to 100x faster,then limited byligand exchange rate
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Other predictions
IO
O O
O nn Etwist
‡ Eelim‡
2 - 21.93 - 12.84 5.9 4.85 5.5 3.8
Minimum chain length needed for oxidation
group Gtwist‡
none 12.1o-CH3 9.7
o-F 12.1m-CH3 13.8
m-F 13.9
Meta substituents inhibittwisting, for unknown reasons
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Other reactions of IBX
EEWS-90.502-Goddard-L15 81© copyright 2009 William A. Goddard III, all rights reserved
More complex reactions of IBX
Can we explain mechanism and reagent scope?Nicolaou, K. C. et al, allylic oxidation – J. Am. Chem. Soc. 2001 123:2183-3185Nicolaou, K. C. et al, HIO3 reactivity – Angew. Chem. Int. Ed. 2002 41:1386-1388
H
O
O O
NH
O
N
O
OH O
IBX
HIO3•DMSO
EEWS-90.502-Goddard-L15 82© copyright 2009 William A. Goddard III, all rights reserved
Evidence for a single-electron-transfer mechanism
O
HPh
Ph Ph
Ph
O
HO
HPh
Ph
IBX
Radical clock moiety fragments:
Hammett plot mildly e deficient resonance-stabilized TS:
IBX-mediatedcyclization, = –1.4
But EA(IBX-OMe) = 14 kcal/mol,while IP(substrate) = 187 kcal/mol
Experiments suggest yes,but theory says no.
Nicolaou, K. C. et al, SET model – J. Am. Chem. Soc. 2002 124(10):2245-2258
EEWS-90.502-Goddard-L15 83© copyright 2009 William A. Goddard III, all rights reserved
Proposed mechanism for phenol oxidation
IO
O O
OI O
O O
OMe
OMeO
H
I OO O
OMeHOB
IO OHO
OMeO
B[2,3]firstoxidation
secondoxidation
variant trappedas Diels-Alderadduct
breaking bothhypervalent bonds at onceis okay
OH
MeO
O
MeO
O
IBX
Diels-Alder adduct strongly suggests [2,3] involved.Magdziak, D. et al, phenol oxidation – Org. Lett. 2002 4(2):285-288
EEWS-90.502-Goddard-L15 84© copyright 2009 William A. Goddard III, all rights reserved
With theory, IBX adducts readily undergo [2,3]
E‡ = 3.0 kcal/mol E‡ = 5.5 kcal/mol
Twisting and [2,3]rearrangement occursin one step.
Transition states:
[2,3]
twist
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OMe
Et
F
ClNO2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
-1 -0.5 0 0.5 1
Theoretical Hammett analysis of [2,3]/twist step
log (k/k0)
p+
RHO
experiment, THF -1.4theory, gas phase -1.7
theory, THF -2.1
typical SN2 -2.8
typical cation -4.5
Small negative equally well supports [2,3]/twist
Small negative mildly electron deficientresonance stabilized
IO
O O
O
R
IO
O O
O
R[2,3]/twist SET
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Proposed mechanisms for IBX reactions
Alcohol oxidation
, oxidation
Radical cyclization
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Can now explain most known IBX reactions
Key step is either twist + elimination or twist/[2,3]
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Reagent scope of IBX versus HIO3•DMSO explained
can twistno twist possible,only free rotation
can perform
HIO3 cannot twist, so is limited to [2,3]/twist reactions
alcohol oxidationallylic oxidationdehydrogenationradical cyclization
doesn’t need twist
dehydrogenationradical cyclization (predicted)
needstwist
EEWS-90.502-Goddard-L15 89© copyright 2009 William A. Goddard III, all rights reserved
Allylic oxidation
IO
O OOH
IO
O O+ OH
IBX may generate hydroxy radical in situ:
performsallylic oxidations?
stable, like nitroso
Otherwise, we still cannot explain this reactionNicolaou, K. C. et al, allylic oxidation – J. Am. Chem. Soc. 2001 123:2183-3185
Me CHO3 eq IBX
12 h/85oCDMSO
85% yield,similar results with25 substrates.
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A new IBX reaction?
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IBX as a generator of alkoxy radicals
BDE (kcal/mol)IBX-OMe (untwisted) 60IBX-OMe (twisted) 33HO-OH 49AcO-OAc 38HO-ONO 22
When no hydrogens ordouble bonds are available,
we predict thermal homolysiscan occur to generate radicals.
Homolysis only favored inthe twisted form of IBX.
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Propose experiments to detect or trap alkoxy radical
Spin trapping with PhNO, EPR detection IO
O O
OtBu
Catch radical in fastestmanner possible: I
O
O O
O
O O
(most direct)
Cyclization with substratefree from other side rxns:
HO O
Looking for way to exploit or clearly demonstrate this pathway
EEWS-90.502-Goddard-L15 93© copyright 2009 William A. Goddard III, all rights reserved
Anticancer warheads cause dsDNA damage
Bleomycin A2 Dynemicin
Propose one based on distortions of hypervalent bonds
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A warhead that releases alkoxy radicals when triggered
iminehydrolysis
cleavable tether
no hydrogensor unsaturation
+
Cleaving tether enables twistand homolysis
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Conclusions
Hypervalent bonds are strong, and weaken adjacent bonds
Twisting allows the interchange of hypervalent and adjacent bonds, and is the rate-limiting step in manyreactions.
In IBX reactions, -elimination or [2,3] rearrangementis the key step.
EEWS-90.502-Goddard-L15 96© copyright 2009 William A. Goddard III, all rights reserved
Acknowledgements
Prof. William A. Goddard III
Prof. Brian Stoltz
Ryan McFadden