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M.C.
White, Chem
Mechanism
Week of September th,
Ligand Exchange Mechanisms
Associative ligand substition is often called square planar substition because e, d square
planar complexes generally undergo ligand substitution via an associative mechanism the
MNu bond is formed before the MX bond breaks. The intermediate is e and therefore
provides a lower energy route to the product than a e intermediate formed via dissociative
substitution the MX bond is fully broken before the MNu bond begins to form. Analogous in
many ways to SN reactions.
Nu empty, nonbonding pz orbital can act as an acceptor orbital for the e density of the
incoming nucleophile Nu L L M L X L L M L X L eM NiII, PdII, PtII, RhI, IrI. e intermediates L
L M Nu X L L X eL M Nu L L M L Nu
Dissociative ligand substitution is most favored in coordinatively saturated e complexes e.g.
d tetrahedral, d octahedral. In the dissociative mechanism, the MX bond is fully broken
before the MNu bond forms thereby avoiding an energetically unfavorable eintermediate.
Analogous in many ways to SN reactions.
Note that in all ligand substition processes, there is no oxidation state change at the metal
center.
L L M L L eM RuII, CoIII, RhIII, IrIII X L X
L
L L Nu L M L Nu L eL
LMLLe
M.C. White, Chem
Structure amp Bonding
Week of September ,
MO Description of bonding in ML square planar
Metal ValenceOrbitals au e Rule
pz au eu px py au
LML
LL
Linear Combinations of Ligand Donor Orbitals
The square planar geometry is favored by d metals e.g. Ni II, Pd II, PtII, Ir I, RhI. A stable
electronic configuration is achieved at e, where all bonding and non bonding orbitals are
filled. Spinpaired compounds display diamagnetic behavoir i.e. weakly repelled by magnetic
fields and may be readily characterized by NMR.
eu
LUMO
ag
ag s
ag
dz
When combining orbitals, the resulting MOs must be symmetrically dispersed between
bonding and antibonding. y Thus, combining orbitals i.e. ags requires one of the orbitals to
be nonx bonding.
ag bg eg bg
n HOMO
eg bg
LL
LL
bg
dxy
n
bg
eg
dxz dyz
bg
dxy
In a square planar ligand field the degenerate d orbitals split into orbitals of ag, b g, eg, and
bg symmetries. The degenerate p orbitals split into orbitals of eu and au symmetries.
eu
ag
M.C. White, Chem
Mechanism
Week of September th,
Associative Substitutionthe nucleophile
Nu
Rate d PtCl k PtCl k NuPtCl dt k first order rate constant that arises from substition of leaving
group by solvent. k secondorder rate constant for bimolecular attack of Nu on metal complex.
Cl N
N PtII Cl
MeOH, rt
Cl N
N PtII Nu
Cl
e
e
Nu MeOH CH CO CO F
relative rate lt lt lt
Nu
relative rate
NN
Basicity of the incoming ligand nucleophile plays only a minor role in its reactivity for soft
metal centers. In general, the softest i.e. most polarizable nucleophiles react fastest with soft
metals like PtII via associative substitution. Steric hinderance at the nucleophile i.e. picoline
vs pyridine can retard the rate of substition.
NHN
CH OEtN
lt
N H ClNH
BrICHCN CH OP PhSPh P Et P
,.x.x.x.x.x.x
M.C. White, Chem
Mechanism
Week of September th,
Associative Substitution Sterics
Sterically shielding the positions above and below the plane of the square planar complex
can lead to significant decreases in the rates of associative substition.
EtP EtP
PtII Cl
, rt N k , M sec
EtP EtP
PtII Py
Cl
EtP EtP
PtII Cl
, rt N k M sec
EtP EtP
PtII Py
Cl
EtP EtP
PtII Cl
, rt N k M sec
EtP EtP
PtII Py
Cl
Pearson J. Chem. Soc. .
M.C. White, Chem
Mechanism
Week of September ,
Associative Substitution Sterics
as the steric bul k of the imine backbone increases, the aryl groups become more rigidly
locked perpendicular to the square plane making their ortho substituents more effective at
blocking the axial sites above and below the plane.
N Pd N CH BAr N Pd N CH N
associative second order rate constants for ethylene exchange were examined by HNMR in
CDCl at o C
BAr N Pd CH BAr
k too fast to measure even at o C.
k L/mol/sec
k L/mol/sec
Brookhart JACS .
BAr
N Pt N
CH R
Ruffo OM .
M.C. White, Chem
Mechanism
Week of September th,
Brookhart Polymerization Catalysts
BAr N Pd N CH N
BAr N Pd CH
Polymer Mw ,
BAr
Polymer Mw ,
BAr
R N Pd N R CH
BAr
RN
R N Pd
polymer propagation
CH Pd
CH
insertion
NR
NR
High Mw polymers
H
hydride elimination
R N Pd N R H
BAr
R N Pd N R H
BAr
R
associative displacement
BAr
RN
BAr
CH
CH
N Pd N R H
Pd N R H
Low Mw polymers
Brookhart JACS .
M.C. White/Q.Chen Chem
Mechanism
Week of September th,
to Cp via Slipped Cp Intermediate
Cp
PCH OC OC Re CO oC OC CO
slipped Cp
H
Cp
PCH
OC OC Re PCH OC CO Re
PCH
e
proposed quotslippedquot Cp intermediate e
e
Based on the observation that the rate of reaction of with PCH to form depends on both the
concentration of and PCH , an associative mechanism was proposed. To account for
associative substition at a formally coordinatively and electronically saturated center, the
authors propose an quotslippedquot Cp intermediate that forms concurrently with phosphine
attack.
Casey OM .
M.C. White/M.W. Kanan Chem
Mechanism
Week of September th,
to Ring Folding
WII CO CO WII CO CO
e
e
e complexes with cyclopentadienyl, aryl, indenyl ligands may undergo quotassociativequot
substitution avoiding an energetically unfavorable eintermediate via ligand rearrangement
from to or even cyclopentadienyl and indenyl. Haptotropic rearrangement may take the form
of ring quotslippagequot where the ring is acentrally bonded to the metal and its aromaticity
is disrupted or ring quotbendingquot where the conjugation of the system is broken.
. a nonbonding distance
.
Huttner J. Organometallic Chem. .
M.C. White, Chem
Mechanism
Week of September th,
Ligand Exchange Dissociative Mechanism
O RhI O erate of ethylene exchange via associative displacement at oC is sec slower at oC
erate of ethylene exchange via dissociative displacement at oC is x sec BDE kcal/mol E
RhI RhI
RhI
The ratedetermining step in a dissociative ligand substition pathway is breaking the ML bond.
Because of the late, productlike transition state for forming the coordinatively unsaturated
intermediate in such a process, the ML BDE is a good approximation of the activation
energyEA.
EA
BDE
rxn coordinate
PPh mmol . RhI RhI PPh RhI PPh .
k x sec . .
C a fold increase in the concentration of nucleophile does not significantly affect the rxn rate.
Results are consistent with a mechanism where the ratedetermining step is ethylene
dissociation and is not affected by the concentration of the nucleophile.
o
e
e
e
Cramer JACS .
rate d LRhC H k LRhCH dt
M.C. White, Chem
Mechanism
Week of September th,
Ligand dissociation sterics
P
Fe
tBu tBu tBu tBu P Pd P P Fe Fe P Pd P P tBu tBu Fe catalyst quotresting statequot
P quotineffective catalystquot L
tBu tBu dissociative L tBu tBu P Fe P tBu tBu Pd catalytically active species
The steric bulk of the bidentate phosphine ligand is thought to weaken the PdP bond, thereby
favoring ligand dissociation required to form the catalytically active species.
Cl
HN also aniline, piperidine
NaOtBu
NH
also aryl Br, I, OTs
Hartwig JACS .
M.C. White, Chem
Mechanism
Week of September th,
Ligand dissociation or hv
coordinatively and electronically unsaturated complexes capable of oxidatively adding into
unactivated CH bonds.
RhI OC e
CO
hv or CO OC
RhI OC e
RhIII
H
e
Lightpromoted ligand dissociation MCO hv bond order bond order MCO
H RhI OC proposed intermediate
Bergman JACS .
M.C. White, Chem
Mechanism
Week of September th,
Ligand dissociationweakly coordinating solvents
First generation Crabtree hydrogenation catalyst
PMePh Ir PMePh PF
cat. R
R
H, solvent S
A glimpse into the catalytic cycle
PF H IrIII S PMePh dissociative displacement Ph MeP IrIII S PF H R PMePh Ph MeP IrIII S
PMePh H H PF
H PhMeP
H
S
S
catalytically active species
R
Solvent S
O
Turnover Frequency TOF
CHCl
TOF mol reduced substrate/mol catalyst/h
Crabtree Acc Chem Res .
M.C. White, Chem
Mechanism
Week of September th,
Noncoordinating solvents no such thing
N N N H B N N Cl HC N Ir CH PMe
BArf
The first isolated chloromethanemetal complex. There are also similar complexes formed
with CHCl, and Cl CH that have been characterized by NMR.
Bergman JACS .
M.C. White, Chem
Mechanism
Week of September th,
Oxidative Addition/Reductive Elimination
Oxidative Addition OA metal mediated breaking of a substrate bond and formation of or new
ML bonds. OA requires removal of electrons from the metals d electron count. This is
reflected in a two unit increase in the metals oxidation state. The formation of or new ML
bonds is accompanied by an increase in the metals coordination number by or units
respectively. The latter results in a unit increase in the electron count of the metal complex
e.g. e to e. Currently, OA of low valent, electron rich metals to polar substrates is the best
way to form MC bonds within the context of a catalytic cycle. The term oxidative addition
confers no information about the mechanism of the reaction.
LMn
quotNuquot
oxidative addition
R X quotE quot
R
LMn
X
or
L Mn R
X
reductive elimination
Reductive elimination RE microscopic reverse of oxidative addition where two ML bonds are
broken to form one substrate bond. RE results in the addition of two electrons into the metal
d electron count. This is reflected in a two unit decrease in the metals oxidation state. The
breaking of ML bonds is accompanied by a decrease in the metals coordination number by
units. The result is a unit decrease in the electron count of the metal complex e.g. e to e. The
two ML bonds undergoing reductive elimination must be oriented cis to each other. Currently,
RE is the most common way to form CC bonds via transition metal complexes. General OA
Mechanisms Nucleophilic displacement generally for polar substrates Concerted generally
for nonpolar substrates
A B LxM
n
A B centered TS
LxM
n
A LxM B
n
LxM
n
A
LxMn X
A
X
LxMn A
X
cis addition
Radical both nonpolar and polar LxMn A C A LxMn A C LxMn C
M.C. White, Chem
Mechanism
Week of September th,
Oxidative Addition
Metal Complex electron rich metals in low oxidation states, with strong donor ligands and a
site of coordinative unsaturation.
d, tetrahedral, e gt d, ML, e gt d, ML, square planar, e e.g. Ni , Pd, Pt
tBu tBu tBu tBu P Fe P Pd
tBu tBu P Fe Fe Pd P tBu tBu
Cl Fe
tBu tBu P Pd P tBu tBu
Cl
PP
tBu tBu tBu tBu
OA
L
d, ML , square planar, e gt d, ML, octahedral, e e.g. RhI, IrI
Ir
I
H H IrIII PCy N
PCy N
PF
H
e eSubstrates two groups segregated into nonpolar and polar. Currently, the most facile way
to form CM bonds is with polar substrates e.g. alkyl, aryl, and vinyl halides. Nonpolar
substrates RH Polar substrates RX where X I, Br, Cl, OTf
H H, RSiH, R BH, RCHH, R H , H , , R
OH
HX, RCHX, X ,
X,R
OX
M.C. White/M.S. Taylor, Chem
Mechanism
Week of September th,
Transition Metal Oxidation States
Ti V Cr Mn Fe Co Ni Cu Zr Nb Mo Tc Ru Rh Pd Ag
Hf
Ta
W
Re
Os
Ir
Pt
Au
Observed positive oxidation state Most stable oxidation state aqueous solution
Mingos Essential Trends in Inorganic Chemistry Oxford University Press, .
M.C. White, Chem
Mechanism
Week of September th,
OA Concerted centered nonpolar substrates
HHMH
complex
backbondinggtgt donation oxidative addition
MH
metal dihydride
complex intermolecular binding of a substrate via its bond to a metal complex. complexes are
thought to be along the pathway for oxidative addition of nonpolar substrates to low valent,
erich metal complexes. Analogous to the DewarChattDuncanson model for olefin
metalbonding, bonding is thought to occur via a way donoracceptor mechanism that involves
donation from the bonding electrons of the substrate to empty orbital of the metal and
backbonding from the metal to the orbitals of the substrate. These bonding principles have
been applied to nonpolar bonds such as HH, CH, SiH, BH and even CC bonds.
H L xM n H . LxM n
H LxMn H . complex
H H backbondinggtgt donation
LxMn
H gt. H metal dihydride
Concerted mechanism complex formation precedes an early little bond breaking, centered
transition state where strong backbonding results in oxidative addition of the bound substrate
to the metal. The concerted mechanism is thought to operate primarily for nonpolar
substrates i.e. HH,CH, SiH, BH with electron rich, low valent metals. The spectroscopic
identification of metal dihydrogen complexes with HH bond distances stretched between the
nonbonding . and dihydride extremes gt. provides strong support for this mechanism with H.
M.C. White/M.W. Kanan Chem
Mechanism
Week of September th,
Dihydrogen complexes
PR OC W OC PR H CO H
.
The first stable dihydrogen metal complex was isolated by Kubas. The lengthened HH bond .
is greater than the HH bond length in free H .. This is thought to arise from metal
backbonding into the HH orbital.
Kubas Acc. Chem. Res. .
M.C. White/M.W. Kanan Chem
Mechanism
Week of September th,
Dihydrogen complexes
.
H H N Os N H OAc
HHN
NH
The HH bond distance is thought to be a measure of the metals ability to backdonate its
electrons. Low oxidation state metal complexes such as OsHen OAc with strong and donor
ligands are very effective backbonders as evidenced by the very long HH bond in their MH
complex.
Taube JACS .
M.C. White, Chem
Mechanism
Week of September th,
spCH OA via complex intermediates
H H M C complex C
backbondinggtgt donation oxidative addition
M
Hydridoalkylmetal complex
regioselectivity sp CH gt o spCHgt o sp CH gtgtgt o sp CH. There is both a kinetic and
thermodynamic preference to form the least sterically hindered CM bond. Kinetic preference
activation barrier to complex formation is lower for less sterically hindered CH bonds and
bonds with more s character. Thermodynamic preference stronger CM bonds are formed see
Structure and Bonding, pg. .
Evidence in support of a complex intermediate
CD D C CD CD RhI OC eCO hv flash, Kr K CO OC CO v cm D RhI Kr OC CD DC CD RhI
DC OC DC CD RhIII D CD CD
CO v cm complex
CO v cm
Bergman JACS .
M.C. White, Chem
Mechanism
Week of September th,
spCH concerted vs. radical
crossover experiment evidence in support of a concerted mechanism.
IrI H C OC D IrI OC eOC CO hv CO OC IrI D OC IrIII D IrI H IrIII OC H
D complexes
D
Less than of the crossover products were observed by HNMR. This may be indicative of a
minor radical pathway.
HC
D IrII OC H
OC
IrII
D OC
IrIII
H OC
IrIII
D
Bergman JACS .
D
M.C. White, Chem
Mechanism
Week of September th,
Agostic interactions intramolecular complex
HMCRR
An agostic interaction is generally defined as an intramolecular complex that forms between
a metal and a CH bond on one of its ligands. Brookhart JOMC
CO H RuII PhP mol O CO O O H O O O OEt RuPPh Toluene, reflux SiOEt O O O PPh Y O
OO
H Ru
PPh PPh
H Y OEt O PPh PPh OA O O O H PPh O OEt O RuII PPh PPh
The strategy of identifying substrates that can act as transient metal ligands has led to the
only synthetically useful examples of CHgt CM CH activation to date. Like all substrate
directed reactions, the scope of such processes is limited.
O O O OEt SiOEt O O O
Trost JACS
M.C. White, Chem
Mechanism
Week of September th,
OA spCspC
C CR M C R
complex
backbondinggtgt donation oxidative addition R
M
R
C
metal dialkyl complex
Even though BDEs of CC bonds are lower than those of analogous CH bonds e.g. CH CH
kcal/mol vs. CH H kcal/mol, transition metal mediated OAs into CC bonds are much more
rare than those for analogous CH bonds. Formation of the complex is kinetically disfavored
by steric repulsion between the metal complex and the carbon substituents and by the high
directionality of the spCspC bond that localizes the bonding orbital deep between the carbon
nuclei. Milstein and coworkers are able to overcome the kinetic barrier by approximating the
CC bond at the metal center.
PtBu C H RhCl oC, tol NEt
PtBu RhI Cl oC k x /sec NEt stable o C observed by H NMR
PtBu RhIII NEt only product observed no CH activation product Cl
Milstein JACS .
M.C. White, Chem
Mechanism
Week of September th,
OA spCspC
PtBu RhIII NEt only product observed no CH activation product Cl
Milstein OM .
M.C. White, Chem
Mechanism
Week of September th,
OA with CspX bonds aryl and vinyl halides
LxM n
X
oxidative addition
LxM n
X
R
LxM n
RX
oxidative addition
LxM n
X
Note retention of stereochemistry in OA to vinyl halides Rate of OA X IgtBrgtClgtgtF
Three main mechanisms to consider for this process . Concerted process with
unsymmetrical, minimallycharged, centered transition state . S NArlike with highly charged
transition state . Singleelectron transfer processes with oppositelycharged, radical
intermediates
Csp L xM n X
LxM n
X
X
LxMn
M.C. White, Chem
Mechanism
Week of September th,
Hammet plots linear free energy relationships a valuable mechanistic probe
See Carey and Sunberg Part A, rd Edition. pp .
Recall the effect of e withdrawing groups and e donating groups on the acidity of benzoic
acid resulting from stabilization and destabilization of the carboxylate anion, respectively
COH COH COH COH COH COH COH
MeOC CF COMe
Me Me OH
pKa
.
.
.
.
Substituent group
.
.
.
meta
......
para
. . . by definition . . .
COH
Define pKaH pKax
CF COMe Cl H OMe Me OH
X
Because these same substituents can often similarly stabilize or destabilize polar transtition
states for reactions involving aryl substrates, a linear relationship can exist between and
reaction rate for such processes. This type of quotlinear free energy relationshipquot can
lend valuable insight into the charge characteristics of the transition state for various
reactions.
M.C. White, Chem
Mechanism
Week of September th,
Hammet plots linear free energy relationships a valuable mechanistic probe
See Carey and Sunberg Part A, rd Edition. pp .
.
pNO mNO
.
pCl
log k/ko
pMe pOMe
H mNH slope . positive indicates buildup of negative charge in the TS
log k/ko
slope . negative indicates buildup of positive charge in the TS
...
.
..
pNH
.
Cl Cl
EtO
COEt HO
O OH EtO
COH
HH
OH
OH
X
HO X X X
HCl
XX
M.C. White, Chem
Mechanism
Week of September ,
Oxidative addition of aryl chlorides to tristriphenylphosphinenickel
pCN pCOPh pCOMe mCN .
log k/ko
mCOMe
mCl mMe H pMe
mOPh pCl
...
pOMe
pOPh
.
.
Cl LxMn
Csp X PPhNi X LxMn
X
X
or
PPhNi Cl
Foa and Cassar J. Chem. Soc Dalton .
M.C. White, Chem
Mechanism
Week of September th,
OA CspXalkyl, allyl, and benzyl halides
Nucleophilic displacement generally for polar substrates A L xM n X LxM n A X LxMn A X
quot Stereochemistry is the single most valuable type of mechanistic evidence in reactions
that make or break bonds to tetrahedral carbon.quot G.M. Whitesides JACS .
COMe PPhPd mol MeO C OAc MeO C inversion MeO C Na PhPPdII COMe OAc inversion
COMe COMe COMe
MeO C
net retention Trost Acc. Chem. Res. .
Mes P Pd P Ph OH Mes
II
OTf
Mes mol P Pd II P
Ph
OTf Ph H N NHPh
NH
Mes
Ozawa JACS .
M.C White/M.W. Kanan, Chem
Mechanism
Week of September th,
Transmetalation Definition and Utility
Definition the transfer of an organic group from one metal center to another. The process
involves no formal change in oxidation state for either metal.
LnM R LnM X LnM X LnM R
Transmetalation
Commonly used transmetalation reagents and their associated crosscoupling reaction
Transmetalation is often a reversible process, with the equilibrium favoring the more ionic MX
bond. Subsequent reactivity of one LnMR species can drive the equilibrium in one direction.
This is often exploited in crosscoupling reactions, where a transmetalated intermediate
undergoes a reductive elimination to generate a new organic product. Subsequent oxidative
additions generates a new substrate for transmetalation
reductive elimination R R R n Lr M X oxidative addition R n Ln M R
RM XM XM RM M is typically group Reagent LiR, MgXR RZrClCp RZnCl RCuLn RSnR
RBOR RBBN RSiR AlR, AlX R vinyl, aryl, allyl, alkyl vinyl, alkyl vinyl, aryl, alkyl alkynyl, aryl
vinyl, aryl, alkynl vinyl, aryl alkyl aryl, vinyl ,alkyl alkyl Xcoupling reaction Kumada Negishi
e.g. Sonagashira Stille Suzuki SuzukiMiyaura Hiyama
LrM X transmetalation LrM R
M Lr
n
In generalthe rates of transmetalation of R follow the order alkynylgt aryl,vinylgtalkyl
M Ni, Pd
RX
M.W. Kanan, Chem
Mechanism
Week of September th,
Transmetalation Mechanism
The mechanism for transmetalation is the leaststudied of the basic reaction steps. In a
simple picture, the metal accepting the R group is the electrophile and the MR bond being
transferred is the nucleophile. MR bond formation may or may not be simultaneous with MX
bond formation, depending on the nature of X and the actual complexes involved.
MX
RM
MM
RX
With this model, increasing the nucleophilicity of R by altering the ligands on M and
increasing the electrophilicity of M through its ligands will facilitate the transmetalation step.
For weakly nucleophilic transmetalation reagents, an added nucleophile or base often
facilitates the transmetalation.
RR
R R LnPd R
Transmetalation with the Suzuki coupling often requires added base
OR RO
B RO OR
B OR OR
LnPd
II
X
Suzuki Chem. Rev. . F is thought to activate the organosilicon reagent for transmetalation via
formation of a nucleophilic pentavalent silicate in a Hiyama coupling
R
SiRnFn
TBAF
R SiRnFn
R
R LnPd
LnPd X
R
Hiyama Tet. Lett. .
M.W. Kanan, Chem
Mechanism
Week of September th,
Transmetalation with advanced intermediates
SuzukiMiyaura
OBn O BnO O BnO H H H O OTBS H BBN BnO THF, rt BnO H O H H O OTBS BBN OBn O
H
OBn H O O OBn BnO H
OBn O
H OTBS
TfO
O
O H H BnO
H
OHOHO
OBn
O
M aq. Cs CO, DMF PdPh, C
OBn
Synthetic studies on Ciguatoxin
Takakura ACIEE,
M.C. White/Q. Chen Chem
Mechanism
Week of September th,
Reductive Elimination
Reductive elimination is a key transformation in transition metal mediated catalysis, often
representing the product forming step in a catalytic cycle.
General trend for reductive elimination from d square planar complexes H M H gt M Csp H gt
M Csp
H gt M
H gt Csp
Csp M Csp
Best overlap
.
Orbitals with more s character are less directional and lead to better overlap in the transition
state for reductive elimination RE. Note cis orientation of the ligands is required for RE to
occur. Worst overlap
.
H
o.
H
o.
.
H
o.
.
H Pd
.o.
.
CH
.
CH
o.
Pd
Pd
Pd
.
H
metal dihydride
H
Transition state TS Calculated E . kcal/mol PdH bond is stretched only in TS
Pd
CH CH
. hydridoalkylmetal complex Transition state TS Calculated E . kcal/mol
o
.
Pd
CH CH
Transition state TS Calculated E . kcal/mol PdH bond is stretched in TS
metal dimethyl
Goddard JACS . Dedieu Chem. Rev. .
Computational studies suggest that the spherical symmetry of the s orbitals of H allows the
simultaneous breaking of the ML bonds while making the new bond of the product.
M.C. White Chem
Mechanism
Week of September th,
RE Bite Angle Effects
RE can be promoted by Increasing the bite angle of the ligand Increasing electrophilicity of
metal center e.g. acids Ligand dissociation
TMS P CHTMS Pd II P CN oC P Pd II P C N P CHTMS
P PdII
CN
k
Ph P
CHTMS PdII CN
Ph P Pd P Ph
II
CHTMS CN
O
Ph P Pd II CHTMS CN P Ph
bite angle
P Ph
O
bite angle o k . x
bite angle o k . x
bite angle o k . x
Large bite angles of diphosphines have been shown to enhance the rates of reductive
elimination from square planar complexes presumably by bringing the two departing ligands
closer together.
Moloy JACS .
M.C. White Chem
Mechanism
Week of September th,
RE Acid Effects
RE can be promoted by Increasing the bite angle of the ligand Increasing electrophilicity of
metal center e.g. acids Ligand dissociation
O Ni O I Bu PentZn FC O
II
O O mol
FC O O NiII O O Bu Pent Bu
mol
possible intermediate
yield, h w/out acid , h
Knochel ACIEE .
M.C. White, Chem
Mechanism
Week of September th,
Migratory Insertion/Deinsertion Alkyl, H
CosseeArlman Mechanism for alkyl migration to a coordinated olefin
CH Zr CH Zr C H Zr H
Brintzinger catalyst for Ziegler Natta polymerizations
no oxidation state change at M e from the electron count
The bonding electrons of the olefi n are used in bond formation with a Malkyl . Formation of
the new CC and MC bonds are thought to occur simultaneously with breaking of the bond
and alkylM bond through a centered concerted transition state. Migratory insertion of a
hydride into a coordinated olefin the microscopic reverse of hydride elimination is thought to
procede via the same mechanism. For metal alkyls, the equilibrium lies to the right, whereas
for metal hydrides it lies to the left.
Early TS Grubbs and Coates Acc. Chem Res. .
Late TS
M.C. White, Chem
Mechanism
Week of September th,
Hydride Elimination
A significant decomposition pathway for metal alkyls is hydride elimination which converts a
metal alkyl into a hydrido metal alkene complex.
BAr CH Pd II N R H BAr CH PdII N R H
RN
R
R N Pd N R H
BAr
hydride elimination
N
CH
donative agostic interaction
concerted transition state
hydride elimination can occur when
cis to the alkyl group there exists is a site of coordinative unsaturation on the metal which
corresponds to a site of electronic unsaturation empty metal orbital. the MCCH unit can take
up a coplanar conformation which brings the hydrogen in close enough proximity to the metal
to form an agostic interaction. the metal is electrophilic resulting in an agostic interaction that
is primarily electron donative in nature i.e. donationgtgtbackbonding.
HMC
donation gtgt backbonding
complex
M.C. White, Chem
Mechanism
Week of September th,
Wacker Oxidation
R R PdCl / O HCl HO PdCl CuCl ClPd CuCl Pd HCl H Pd Cl OH O R OH R HClPd R H OH R
R OH HO Binding specificity terminal olefins Regioselectivity o carbon Remote functionality
tolerated
protonolysis
OH R
ClPd
hydride elimination
Commericial production of acetaldehyde
M.C. White Chem
Mechanism
Week of September th,
Hydride Elimination
Computational studies suggest that the higher energy of the NiII vacant d orbital . hartree
with respect to that of the analogous PdII complex . hartree results in a weaker donative
agostic interaction with the CH bond. The energetically optimized geometries of the agostic
complexes show a greater lengthening of the CH bond in the PdII complex than in the NiII
complex, indicative of greater donation in the former. These computational results are
consistent the experimentally observed greater stability of Ni alkyls towards hydride
elimination that Pd alkyls and can be rationalized based on the greater electronegativity of
PdII vs NiII as reflected in their respective second ionization potentials.
. H NiII H second ionization potential NiII . eV recall sp CH is . PH
. H Pd II H second ionization potential PdII . eV PH
Morokuma JACS .
M.C. White, Chem
Mechanism
Week of September th,
Migratory Insertion/Deinsertion CO
Mechanism for CO insertion via alkyl migration to coordinated CO
O CO . o H C . o Pd II PH H .
o
H C O C .o Pd II PH H Pd II PH H
CHC
alkyl groups migrate preferentially over hydrides
Computational studies suggest that in the TS, the methyl group has moved up half way
towards the carbonyl.
no oxidation state change at M e from the electron count
Morokuma JACS .
Experimental evidence also suggests that carbonyl insertion occurs via alkyl migration not
CO migration
hydride elimination
RP RP
Pd II
CO
PR
RP OC
Pd II
CO insertion
RP O C
Pd II H
RP O C
Pd II H
reductive elimination H
O
cisdiethyl complex
RP
Pd II PR
CO
PR
RP
Pd II CO
CO insertion
RP
Pd
II
reductive elimination C O
O
transdiethyl complex Yamamoto Chem. Lett. .
M.C. White, Chem
Mechanism
Week of September th,
Migratory Insertion/Deinsertion CO
RP PtII OC monomer R ROC Cl RP Pt
II
Cl Pt Cl dimer
II
Cl
COR
no dimer observed
Ph lt
Me lt CHPh lt Me lt Ph lt Et
Anderson Acc. Chem. Res. .
Electron donating substituents on aryl R groups promote migrations whereas electron
withdrawing substituents inhibit them. R Monomer Dimer
N
MeO
Me
Cl
NC
Cross J. Chem. Soc., Dalton Trans. , .
M.C. White Chem
Mechanism
Week of September th,
Heck Arylation
OMe I MeO N O O OMe PdPPh cat. NEt, CHCN o C MeO N O PPh I Pd MeO H PPh OMe
NCOMe migratory insertion MeO reasonable intermediates O MeO I PPh Pd OMe O NCOMe
associative displacement oxidative addition OMe I PdL O OMe NCOMe NCOMe
MeO O
N
O
N
hydride elimination
MeO MeO OMe MeO MeO N O yield FR Danishefsky JACS . O NCO Me O N O NCOMe
OCONH OMe