Chapter 6: Oxidative Addition of Nonpolar Reagents
6.1. Generalmononuclear

A
Metal: two electron oxidation

LnMn+2

LnM + A-B

B

cf. Grignard reagent formation
Carbene insertion

dinuclear
LnMn+1-A + LnMn+1-B

LnMn-LnMn + A-B
2LnMn + A-A

Metal: one electron oxidation

2LnMn+1 -A

2oxidative ligation[LnMn+2 -H]+

LnMn + H+anionicno bond cleavageO
LnMn+2

LnMn + O2

O
dianionic

A-B bond < M-A bond + M-B bond
cf.
AOC
PMe3Ir
Me3PClBOC
PMe3 + A-B
IrMe3PClstrong metal-H bondweak metal-C bondweak C-X bondA-B

Hkcal/mol

Gkcal/mol

H-H-15-6CH3-H
CH3-CH3-2-4+3+6CH3-I-35-25* estimated values

Qualitative trends for oxidative addition
1.

2.

metal center :electron richelectron pooroxi. add. :fastslow

red. elim. :slow(partial positive charge at the metal during the process)fastLess hindered metal centers are favorable.
(increased steric hindrance after an oxi. add.)(red. elim. : More hindered metal centers are favorable)

3.

4.

for nonpolar reagents

Requires a site of unsaturation.
less than 16e at the metal center

Rate of ligand dissociation affects the overall rate.
slowerfasterbidentate

monodentate

strongly boundmonodentate

weakly boundmonodentate

6.2. Oxidative Addition of H2*

single metal centerside on > end on
cis addition (trans addition is "forbidden")"early" transition state

H2M

M-symmetry-symmetry
ex.
Ph2
PCOIr
PClPh2+ H-HH
Ph2H
PCOIr
PCl
Ph2

Ph2 H
PHIrPCl
Ph2 CO

side oncis additiontwo metal centersmost common with cobalt complex
ex.

HCo

CoHorCo H H Co

2 Co(CN)53- + H2ONHON

1/2 H2CoNO

L NO
H2 HCo(CN)53-HOO
HNNCoN L NOO
H

L= Py or PBu3

rate= k[H2][M]2
The transition state would be formed by combination of H2 and two metal centers.
H
Rh

O(CH2)6O

Rh

Rh

H
O(CH2)6O

Rh

Preorganization enhances the rate :103 faster than (TMP)Rh
J. Am. Chem. Soc. 1994, 116, 7897.
6.3. Oxidative Addition of Si-H
similar mechanism to H2 oxidative addition
stereochemistry at Si : retention
6.4. Oxidative Addition of C-H
non-radical processes
via alkane -complexes

is the most common

eary examples

Shilov's early example

R-H

K2PtCl4/ 100 oC
D2O/DOAc + HCl
H2PtCl6/ K2PtCl4110~ 170 oC /CF3CO2H

dehydrogenation

t-Bu

L2IrH5, 150 oC

R-DRCl, ROCOCF3t-Bu

intramolecular
general tendency: Ar > primary > methylene
H(Ph3P)IrCl(Ph3P)2(Cl)Ir
P
Ph2
Me2P
Me3P
CH2M
Me3P
H
PMe3(Me3P)4M

M= Fe, Ru, Os

cyclometallationdeactivation pathway
can be a stable precursor of active catalyst
can be a active spiecies (less common) (J. Am. Chem. Soc. 2003, 125, 14272.)
intermoleculararene > alkane (both kinetically and thermodyanamically)
The difference between M-aryl and M-alkyl is larger than H-aryl and H-alkyl.
Hlower steric hindrance
lower directinality of sp2

M
precoordination
Metal alkyl-hydride complexes are known for group 7-10. (Re, Fe, Rh, Ir, Pt...)2

The greater strength of M-C bonds of third-row transition metal makes the oxid. add. common.
Rh and Ir with Cp, anion ligand, PCP-pincer consititute major examples.
RhRh
Me3PRh

HHR

Me3P+H2HAdditions to group 8-10 metals with chelating alkylphosphines are known.
PMe2
Me2P
HFe
Me2PH
PMe2

PMe2
Me2P
Fe
Me2P
PMe2

h-80 oC

hexane

PMe2
Me2P
H
Fe
Me2P
PMe2

Alkane oxidative additionhigh selectivity at primary C-H (contrast with radical, metal carbene)
Cp*Cp*keq
Me3P

Ir H
Me P

Ir H

keq = 10.8

3

Mechanism
same orbital interaction as H2
LnM + R-CH3LnM

HCH2R-complexInorg. Chem. 1985, 23, 1986

HH

MLn

not directly observed but supported by several dataCH2RMC
trajectrory ofoxid. add.

Kinetic selectivityfor primary C-H is unclear. (ex. 64 times faster than secondary)
Cp*Ir(PMe3): not selective
Cp*Re(PMe3)2, Cp*Rh(PMe3): highly selective
Methods to generate unsaturated intermediates
photolysis of M-CO or MH2 (Cp*Ir(CO)2)
thermolysis of alkyl hydride complex
abstraction of halide (addition of AgB(Arf)4)
protonation of alkyl complex
Application
synthesis of rhazinilam
MeOMeOONPhOMeO
TfOH
CH2CL2NPh

Me2Pt(SMe2)2NN
Pt N
Me MeNNOPh

MeO
CF3CH2OH
70 oC, 90%

N Pt MeNONPhN Pt H
NDinuclear activation of C-Hrare example
process is similar to H2 oxid. add.
ArAr
N
N Rh

+ CH4

N(TMP)Rh-Me + (TMP)Rh-H

N
Ar(no reaction is observed with benzene)Ar

6.5. Other than oxidative additiond 0 Metals cannot undergo oxidative addition.
-Bond metathesis or [2+2] reactions.
-bond metathesis
LnM-R + R'-H

LnM-R' + R-H

common among d 0 transition metal (lanthnide, actinide, main group compounds)
multi-step sequencerequires a site of unsaturation
The reactivity of -bond metathesis of H2, C-H, Ar-H parallels the reactivities of oxid. add.
basic
LnM-R

LnM-R + R'-HR'H

MLnRR'R H

H

increased acidity

lower stability than late metals
H
Cp2Zr

H2

LnM-R' + R-H

LnM R'(Cp2ZrH2)n +[CpRu(dmpe)(H2)]+ : pKa 17.6 in MeCN

Even late transition metals can undergo -bond metathesis in some cases.
More common with H2 than C-H heterolytic cleavage.
(Due to higher acidity and stability of dihydrogen complexes.)
The amount of M-H bonding is a one determining factor.
TpOCPhPhRu[Ru]TpOCHRu (CH CH Ph)
32PhPhoxidative hydrogen migration[2+2] additions with M=N or M=C complexes
ex. Ti, Zr, Ta, W
NHtBu
ZrCp2

NHtBu

benzene
ZrCp2

ZrCp2 NtBu

MeR-H

Ph

(tBu3SiNH)3Zr-R[tBu3SiN(H)]2Zr=NSitBu3
Ti=N complex
kinetics: primary > secondary
arene > alkane

thermodynamics: primary > secondary
aryl complex > alkyl complexcarbene complex
Cp*ONWCp*

SiMe4tBu

ONWtBuSiMe3

alkylidyne complex
PCHPhbenzene
N Ti
CH2SiMe3PP
N Ti
PCPhbenzene

PPiPr2Ph

N Ti
CHPhPN
PiPr2

6.6. Oxidative Addition of C-C
strained ring system

J. Am. Chem. Soc. 2002, 124, 13976
ligating grouppyridine[(C2H4)2RhCl]2

NR

N
Cl Rh
RpyO

O

exception: aryl nitrile
iPr iPr
PNNiCP
iPr iPr Ph

iPr iPrP CNNiP Ph
iPr iPr

PCP pincer complex
PtBu2PtBu2

CH2CH3Rh

CH2CH3Rh Cl

PtBu2

PtBu2

PtBu2

PtBu2

[(C2H4)2RhCl]2-complex
M-C(sp2) has higher stability than M-C(sp3).
rerative rate of C-H or C-C insertion depends on substrates and complex
Cp*

PMe3
Cp*Rh
HH

Cp*Rh(PMe3) +Me3PRh6.7. Oxidative Addition of E-E
Oxidative addition to Si-Si, B-B, Si-B, Sn-B is easier.
O(PPh3)2Pt(C2H4)

OOOO

B B+O

PPh3 O
B Pt B
PPh3 OChapter 7: Oxidative Addition of Polar Reagents
7.1. SN2 Pathways
Metal acts as a nucleophile.

Mn

M(n+2) C

M C X

C XX

X M C

faster reaction in more polar solvents
inversion
kinetics are second orderrelative rate: Me > 1 > 2 >> 3 I > Br > Cl >>FOCL
IrX
L

CH3OCL
Ir
I
L
X

CH3OCL
Ir
L
X
I+ CH3I

Na2Fe[CO]4 : d10 "supernucleophile"
It can react with neopentyl chloride.
Precoordination of an anion to a neutral metal can accelerate the oxid. add.
(The origin of the effect was unclear...)1/2[M(CO)2I]2I-[M(CO)2I2]-MeI[M(CO)2(Me)I3]-LiX

M = Rh, Ir

MeI[M(CO)2I2X]2--[M(CO)2(Me)I2X]--I
7.2. Radical Pathway

weaker and more-hindered electrophiles
coordinatively saturated metal centers
metal complexes which are prone to undergo one-electron oxidation(most common with first low metals)(aryl sulfonates : non-radical pathway)
Inner-Sphere Electron Transfer
Mn + RX

Mn

X-R

coordination·Mn+1 + R·

R-Mn+2-X

solvent cageex. Ni(PEt3)4 + ArI

racemization occurr
Radical Chain Pathway
require coordinatively unsaturated metal center
initiator: trace impurities, trace O2, light induced radical etc.
initiation
initiator
In-X + R·
R-X
propagation
R· + Mn

R-Mn+1

R-Mn+1 + X-R

R-Mn+2-X + R·Cl

Me3P Ir PMe3 +COF
BrFOC
PMe3Ir
Me3PClBr

Outer-Sphere Electron Transfer
metal: coordinatively saturated
electrophile: suceptible to electron transfer
sterically hinderedweak C-X bond

LnM + RX

LnM+ + RXRX
LnM+ + R·

R· + X[LnMR]+
R
Ln-1M
X

[LnMR]+ + XR

-2PR3

Cp2Zr(PR3)2 + RX

Cp2Zr

Cp2ZrX + R·X

Atom Abstraction and Combination of the Resulting Radical with a Second Metal
facile one-electron oxidation
slow two-electron oxdation
generates the two products

M + X-R

Co(CN)53- + R-X

rds

X-M + R·
R-M

XCo(CN)53- + R·

fast

R· + Co(CN)53-

fast

R+M

Ligand: the bulkier the slower
Electrophile: RI > RBr >> RCl

rds

RCo(CN)53-

7.3. Concerted Oxidative Addition
oxid. add. to ArX
concerted vs radical
Ar
X

L2Pd(0)
14e

LnPd

X

concerted pathway

PdL2
16e
XIr

radical pathway

Ir
18e
palladium complex
typically occurs to a 14-electron complex
PPdP

bent into a less stableconformation

frontier orbitals are more available
more reactive than linear L2Pd

highly bulky phosphine ligands: via LPd (12e) or associative mechanism to L2Pd (14e)bite angle
small bite angle
faster at the step of C-X cleavage
less hindered (slower dissociaion of ligands)

large bite angle ligands
faster overall multistep
oxid. add. process

alkyl vs aryl phosphines
Alkyl phosphines are more reactive due to the higher electron donating nature.
(related carbenes also have high reactivity)
reaction rate of R-X: I > OTf > Br > Cl OTs(but depend on the identity of the metal center)
Added anion can influence the rate of oxid. add. by forming anionic complex.

+LOxidative Addition of Reagents with H-X Bonds of Medium Porality
via three-centered transition state or protonation of a basic metal center
protonation pathway
with hindered trialkyl phosphine or mixed alkyl arel phosphine ligands
three-centered transition state
PtBu2
Ir
PtBu2

PtBu2
Ir
PtBu2

R

PtBu2
Ir NH3
PtBu2

PtBu2
HIr
NH2
PtBu2

P-H and S-H bonds are more favorable substrate than N-H and O-H.
(weaker X-H bonds, more acidic, stronger M-S, M-P bond)Oxid. add. of aniline is more common than alkyl amines due to the acidity.
7.4. Dinuclear Oxidative Additions of Electrophilic A-B
Both homodinuclear and heterodinulear complexes are known.
Mechanism are closely related to those of mononuclear complexes.
(CO)5Mn-Me + I-Mn(CO)5(CO)5Mn-Mn(CO)5 + MeI
Me2Me2
P P CORh Mo COOCCORh
Me+ MeI

Me2HNSiCp
Me2Fe Zr N SiHOCOC
HN Si
Me2OPhCpOEt

OCOC

Me2 COIPMoPMe CO CO2OFePh

Me2HN Si
Me2EtO Zr N SiHHN Si
Me2early-late heterobimetallic

Summary
MeI : SN2 pathway
higher alkyl : radical pathway coordinatively saturated : outer-sphere electron transfer
coordinatively unsaturated: inner-sphere electron transfer
ArX : Many radical pathways are known, but Pd(0) tends to undergo concerted pathway.
HX : Three-centered transition state is more common.