Chapter 6: Oxidative Addition of Nonpolar Reagents
6.1. General
mononuclear
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
2
oxidative ligation
[LnMn+2 -H]+
LnMn + H+
anionic
no bond cleavage
O
LnMn+2
LnMn + O2
O
dianionic
A-B bond < M-A bond + M-B bond
cf.
A
OC
PMe3
Ir
Me3P
Cl
B
OC
PMe3 + A-B
Ir
Me3P
Cl
strong metal-H bond
weak metal-C bond
weak C-X bond
A-B
H
kcal/mol
G
kcal/mol
H-H
-15
-6
CH3-H
CH3-CH3
-2
-4
+3
+6
CH3-I
-35
-25
* estimated values
Qualitative trends for oxidative addition
1.
2.
metal center :
electron rich
electron poor
oxi. add. :
fast
slow
red. elim. :
slow
(partial positive charge at the metal during the process)
fast
Less 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.
slower
faster
bidentate
monodentate
strongly bound
monodentate
weakly bound
monodentate
6.2. Oxidative Addition of H2
*
single metal center
side on > end on
cis addition (trans addition is "forbidden")
"early" transition state
H2
M
M
-symmetry
-symmetry
ex.
Ph2
P
CO
Ir
P
Cl
Ph2
H
Ph2H
P
CO
Ir
P
Cl
Ph2
+ H-H
Ph2 H
P
H
Ir
P
Cl
Ph2 CO
side on
cis addition
two metal centers
most common with cobalt complex
ex.
H
Co
Co
H
or
Co H H Co
2 Co(CN)53- + H2
O
N
H
O
N
1/2 H2
Co
N
O
L N
O
H
2 HCo(CN)53-
H
O
O
H
N
N
Co
N L N
O
O
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/ K2PtCl4
110~ 170 oC /CF3CO2H
dehydrogenation
t-Bu
L2IrH5, 150 oC
R-D
RCl, ROCOCF3
t-Bu
intramolecular
general tendency: Ar > primary > methylene
H
(Ph3P)IrCl
(Ph3P)2(Cl)Ir
P
Ph2
Me2P
Me3P
CH2
M
Me3P
H
PMe3
(Me3P)4M
M= Fe, Ru, Os
cyclometallation
deactivation pathway
can be a stable precursor of active catalyst
can be a active spiecies (less common) (J. Am. Chem. Soc. 2003, 125, 14272.)
intermolecular
arene > alkane (both kinetically and thermodyanamically)
The difference between M-aryl and M-alkyl is larger than H-aryl and H-alkyl.
H
lower 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.
R
h
Rh
Me3P
Rh
H
H
R
Me3P
+H2
H
Additions to group 8-10 metals with chelating alkylphosphines are known.
PMe2
Me2P
H
Fe
Me2P
H
PMe2
PMe2
Me2P
Fe
Me2P
PMe2
h
-80 oC
hexane
PMe2
Me2P
H
Fe
Me2P
PMe2
Alkane oxidative addition
high selectivity at primary C-H (contrast with radical, metal carbene)
Cp*
Cp*
keq
Me3P
Ir H
Me3P
Ir H
keq = 10.8
Mechanism
same orbital interaction as H2
LnM + R-CH3
LnM
H
CH2R
-complex
Inorg. Chem. 1985, 23, 1986
H
H
MLn
not directly observed but supported by several data
CH2R
M
C
trajectrory of
oxid. 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
MeO
MeO
O
N
Ph
O
MeO
TfOH
CH2CL2
N
Ph
Me2Pt(SMe2)2
N
N
Pt N
Me Me
N
N
O
Ph
MeO
CF3CH2OH
70 oC, 90%
N Pt Me
N
O
N
Ph
N Pt H
N
Dinuclear activation of C-H
rare example
process is similar to H2 oxid. add.
Ar
Ar
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 addition
d 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 sequence
requires 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'-H
R'
H
MLn
R
R'
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.
Tp
OC
Ph
Ph
Ru
[Ru]
Tp
OC
H
Ru (CH CH Ph)
3
2
Ph
Ph
oxidative hydrogen migration
[2+2] additions with M=N or M=C complexes
ex. Ti, Zr, Ta, W
NHtBu
ZrCp2
NHtBu
benzene
ZrCp2
ZrCp2 NtBu
Me
R-H
Ph
(tBu3SiNH)3Zr-R
[tBu3SiN(H)]2Zr=NSitBu3
Ti=N complex
kinetics: primary > secondary
arene > alkane
thermodynamics: primary > secondary
aryl complex > alkyl complex
carbene complex
Cp*
ON
W
Cp*
SiMe4
tBu
ON
W
tBu
SiMe3
alkylidyne complex
P
CHPh
benzene
N Ti
CH2SiMe3
P
P
N Ti
P
CPh
benzene
P
PiPr2
Ph
N Ti
CHPh
P
N
PiPr2
6.6. Oxidative Addition of C-C
strained ring system
J. Am. Chem. Soc. 2002, 124, 13976
ligating group
pyridine
[(C2H4)2RhCl]2
N
R
N
Cl Rh
R
py
O
O
exception: aryl nitrile
iPr iPr
P
N
Ni
C
P
iPr iPr Ph
iPr iPr
P CN
Ni
P Ph
iPr iPr
PCP pincer complex
PtBu2
PtBu2
PtBu2
CH2CH3
Rh Cl
[(C2H4)2RhCl]2
Rh
CH2CH3
PtBu2
PtBu2
PtBu2
-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
H
H
Cp*Rh(PMe3) +
Me3P
Rh
6.7. Oxidative Addition of E-E
Oxidative addition to Si-Si, B-B, Si-B, Sn-B is easier.
O
(PPh3)2Pt(C2H4)
O
O
O
O
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 X
X
X M C
faster reaction in more polar solvents
inversion
kinetics are second order
relative rate: Me > 1 > 2 >> 3 I > Br > Cl >>F
OC
L
Ir
X
L
CH3
OC
L
Ir
I
L
X
CH3
OC
L
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]2
I-
[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 cage
ex. 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 +
CO
F
Br
F
OC
PMe3
Ir
Me3P
Cl
Br
Outer-Sphere Electron Transfer
LnM+ + RX
LnM + RX
metal: coordinatively saturated
electrophile: suceptible to electron transfer
sterically hindered
weak C-X bond
R· + X[LnMR]+
R
Ln-1M
X
RX
LnM+ + R·
[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
X
Ir
radical pathway
Ir
18e
palladium complex
typically occurs to a 14-electron complex
P
Pd
P
bent into a less stable
conformation
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
H
Ir
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 CO
Rh Mo CO
OC
CO
Rh
Me
+ MeI
Me2
HN
Si
Cp
Me2
Fe Zr N Si
H
OC
OC
HN Si
Me2
O
Ph
Cp
OEt
OC
OC
Me2 CO
I
P
Mo
P
Me2 CO CO
O
Fe
Ph
Me2
HN Si
Me2
EtO Zr N Si
H
HN Si
Me2
early-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.