Ready; Catalysis
Organometallics
Organometallic Reaction Mechanisms
Ligand association/dissociation
CuBr + PBu3
insoluble
toluene
P
P
Pd
NOTE: No ox. state change
(PBu3)nCuBr
soluble
P
P
P
Pd
P
+
P(tBu)2
P
P
P(tBu)2
active catalyst
for cross coupling
inactive catalyst
P
=
Fe
P
Ligand Exchange:
Associative - common for 16e- complexes
rate = k[py][Pt]
rate will depend on nature (sterics, electronics) of Nu and MLn
Cl
Py
Py
+ Et3P
Pt
H
Cl
Et3P
PEt3
H
Pt
Et3P
Cl
Pt
H
PEt3
Et3P
Py
PEt3
H
Dissociative - common for 18 e- complexes
Rate will depend on nature of leaving L, sometimes on new L'
PF6-
H
R3P
S
Ir
S
18e-
H
PR3
H
-S
R3P
S
Ir
16e-
PF6H
PR3
R3P
S
Ir
18e-
H
PR3
Py
+ Cl-
PEt3
-L slow:
Rate = k[ML]
PF6-
H
+
Pt
Hydrogenation
+L' slow:
Rate = Kk+L[ML][L']
[L]
1
Ready; Catalysis
Organometallics: ligand exchange
log (krel)
Nu
+MeI
+Pt(Py)2Cl2
MeOH
0.00
0.00
AcO-
2.00
4.30
Et3N
3.07
6.66
Cl-
3.04
4.37
Py
3.19
5.23
I-
5.46
7.42
PhS-
7.23
9.92
Ph3P
8.99
7.00
12.00
log krel (rxn 2)
log
(krel)
10.00
8.00
6.00
4.00
2.00
0.00
0.00
2.00
4.00
6.00
8.00
10.00
log krel (rxn 1)
Exchange rates vary over 20 orders of magnitude
[M] L
[M] + L
From: David T. Richens, The Chemistry of Aqua Ions, 1997, Wiley (ISBN 0-471-97058-1); C/O Prof. Eric Meggers
2
Ready; Catalysis
Organometallics
Oxidative Addition and Reductive Elimination
X
LnM(n)
+
X
oxidative addition
Y
O.A. and R.E. involve 2e- change
at M, increase # ligands by 2
LnM(n+2)
reductive elimination
Y
"O.A" and "R.E." give NO information on mechanism: can be concerted 3-centered, SN2-like or radical
H
Ph3P
Ph3P
CO
Ir
Cl
H
H
H
PPh3
Ir
CO
Cl
Note cis product from
concerted OA
PPh3
H2
Ph3P
Cl
Ir
H3C
CH3I
CO
Ph3P
PPh3
Ir
Cl
I
CO
Ph3P
PPh3
H
CH3
CO
Ir
PPh3
I
EtBr
H2C
Ph3P
Ir
Cl
Ready; Catalysis
CH3
CH3
Br
Ph3P
CO
H
PPh3
Ir
Br
Radical chain mech.
CO
PPh3
Organometallics
Sterochemical Issues
R2
R2
R1
R1
X + LnPd(0)
R3
R3 X
cis addition, olefin stereochemistry
is maintained
X = I > Br ~ OTf >> Cl ~ OTs
OTBS
L
Pd
SnBu3 + I
CO2H
L
65%
cat. Pd(II)
OTBS
Smith, Jacs, 1998, 3935
CO2H
Vinyl and Aryl C-X much more reactive than alkyl C-X despite more e- rich alkyl C-X
Why? precoordination
R2
R1
X
R3 Pd
Precoordination with allylic substrated allows O.A. to moderately activated bonds:
Pd
Pd
Backside (SN2-like) attack at C
OR
OR
OR = OAc, OCO2R, OP(O)(OR)2, OTs, I, Br, Cl
Precoordination can even allow activation of C-H bonds in some cases:
N
N
OTf -
H 3C
H3C
N
N
Pt
N
Pt
N
H
NH
O
Rhazinilam
Sames, JACS, 2002, 6900
3
Ready; Catalysis
Organometallics
Oxidative Addition: Thermodynamics
Bond strength is reflected in ease of O.A., with exceptions
BDE (kcal/mol)
of Cleaved Bond
Rxn
Ir = Ir(I)
L
L
Ir
L
L
+
H3C
I
L
L
CH3
I
Ir
L
L
est ΔG for O.A.
56
-25
104
-6
88
+6
104
+8
H
L
L
L
L
Ir
Ir
L
L
L
L
+
+
H
H3C
H
CH3
L
L
Ir
L
L
L
H
L
CH3
Ir
L
CH3
L
H
L
L
Ir
L
L
+
H3C
H
L
L
Ir
L
CH3
L
M-H bonds are stronger than we might predict
O.A. to C-H and C-C remain very challenging, but could be valuable (more on this later in the course)
Ready; Catalysis
Organometallics
A picture of oxidative addition from calculations.
From CHNF, figure 5.2
4
Ready; Catalysis
Organometallics
Oxidative addition: reactivity trends
Reaction rate increases with electron density on Pd
1
OMe
0.5
y = -2.8259x - 0.4246
H
0
CH3
log[krel]
-0.5
F
-1
Cl
-1.5
CF3
-2
-2.5
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Hammett σ
Amatore, Organometallics, 1995, 1818
Ready; Catalysis
Organometallics
Oxidative addition: reactivity trends
Reaction rate decreases with electron density on ArX
2.5
NO2
2
y = 2.547x + 0.078
log(krel)
1.5
CO2Et
1
0.5
0
-0.5
-1
-0.4
CF3
Cl
H
tBu
CN
Me
OMe
-0.2
0
0.2
0.4
0.6
0.8
1
Hammett σ
Jutand, OM, 1995, 1810
5
Ready; Catalysis
Organometallics
Oxidative addition: reactivity trends
Kink indicates change in mechanism
Ready; Catalysis
Organometallics
Oxidative addition: reactivity trends
With sp3 electrophiles, SN2 pathway dominates with Pd(0)
Rxns show other traits of SN2 (solvent effects, leaving group trends)
From Fu, ACIEE, 2002, 3910
See also Fu, ACIEE, 2003, 5749
6
Ready; Catalysis
Organometallics: Oxidative addition
Direct single electron OA:
-3
R X + 2 [Co(CN)5]
R
+ X Co(CN)5]-3
R Co(CN)5]-3
X Co(CN)5]-3
+
X = halide
Net single electron OA
CpFeIIIX2(CO)2
X2
+
[CpFe(CO)2]2
CpFeI(CO2)
+
2 CpFeIIX(CO)2
Single electron OA in catalysis
O
O
N
N
Br
N
nC6H13
(iPrPyBox)
13 mol%
(±)-
+
Hex
ZnBr
NiBr 2-diglyme (10 mol%)
96% ee
89% yield
LnNi(0)
Br
n-Hex
NiLn
NiLn
+ LnNiIBr
achiral
Fu, JACS, 2005, 10482
Ready; Catalysis
Organometallics
Reductive elimination: reactivity trends (substrate)
R
O
PPh2
Fe
110 oC
Pd
R
O
PPh2
OMe
% Yield
R
OMe
0
H
CN
CF3
Me
N
PPh2
Fe
-15 oC
Pd
R
N
PPh2
>90
Me
Hartwig, JACS, 2003, 16347
OM, 2003, 2775
R
CF3
R
kobs
H
3
13
38
Me
OMe
7
Ready; Catalysis
Organometallics
Reductive elimination: reactivity trends (substrate + catalyst)
R
CN
2
O
P
110 oC
Pd
Fe
NC
O
P
OMe
OMe
R
% Yield
H
>90
CF3
95
kCF3/kH ~ 2
2
R
R
t-Bu2
P
Pd
O
OMe
O
110 oC
Fe
OMe
R
2
R'
H
Ph5
Ph5
R'
Ready; Catalysis
R
2-Me
2-Me
4-Me
% Yield
5-25
66
0
Organometallics
Oxidation-induced reductive elimination
H3C
CH3
CH3
PF6ClMg
MeO2C
Ce(NH4)2(NO3)6
MeO2C
Fe(CO)3+
CH3
H3C
69%
MeO2C
Fe
(CO)3
Fe
(CO)3+
CO2Me
17e- Fe(III)
18e- Fe(II)
H3C
CH3
ox. at M
CH3
H3C
CH3
H3C
CH3
ox. at L
~70%
-CO2, -HO-
H2O2
O
MeO2C
Fe
(CO)3
18e- Fe(II)
MeO2C
Fe (CO)2
-
18e Fe(II)
MeO2C
O
OH
Fe(CO)2
(1:1)
CO2Me
-
16e Fe(II)
Donaldson, OL, 2005, 2047
8
Ready; Catalysis
Organometallics
Ready; Catalysis
Organometallics
Oxidative addition: applications of Sn2-like OA and carbonylation
9
Ready; Catalysis
Organometallics
agostic interactions: stable intermediates on the way to α- or β-hydride elimination
• Stable interactions often found with electron-poor metals
• Especially common with do metals
• Computation with Ti(carbene) and W(carbyne) estimates BDE ≤ 10 kcal/mol (OM, 2006, 118)
Schrock, et al
Things to note
Ta(III) carbene (d2)
Small Ta-C-H angle (78o)
Long C-H bond (1.14A, average here is 1.085A)
i.e. weakening of C-H bond
Big Ta-C-C angle (170o)
Unrelated to agostic interactions:
Ethylene C s out of plane (average 0.33A out of 4H plane)
Long ethylene C-C distance (1.48 v. 1.34 when free)
Ready; Catalysis
Organometallics
Agostic interactions are likely unobserved intermediates in normal tranisition metal-catalyzed reactions:
L
Heck Rxn:
I
I
Pd(0)
Pd
+
LnPd(H)I +
H
base
Generation of Pd(0)
PdCl2
Et2Zn
H
H3C
-CH2CH2
PdII
transmetalation
L
LnPd(0)
L
likely intermediate in α-elimination:
+
H
Cp2ZrCl2
-CH3CH3
PdII H red. elim.
β-Hydride elim
α-agostic interactions can happen, too
lewis acid
H3C
Zr
Cp*TaClBn3
H
Ph
H
H
-PhCH3
LnTa
catalyst for ethylene polymerization
cation stabilized by agostic interaction
Ph
Cl
Ta
Cp*
Bn
Schrock, Accts, 1979, 98
10
Ready; Catalysis
Organometallics
Agostic interaction can be dynamic
Nolan, ACIEE, 2005, 2512
PF6
PF6
N
Ir
N
N
N
H2
N
H H
N
N
Ir
N
H
H
N
N
D
DD
PF6
C(CD3)3
N
N
Ir
D3C
D3C
'cyclometalated' Ir(III)bis(NHC)
Ready; Catalysis
D2
C(CD3)3
D D
D
DD
CD3
CD3
Organometallics
A C-H complex observed crystallographically :
Milstein, JACS, 1998, 12539
Reversible deprotonation with Et3N
to form Ar-Rh bond
11
Ready; Catalysis
Organometallics
Hydroformylation (7 bil Kg/yr)
cat. Rh(I)
H2/CO (syn gas)
R
R
H
+
linear
R
R
O
CHO
branched
H
HRh(CO)2L2
O
H
H
Rh
R
CO
O
HRh(CO)L2
L
L
R
H2
R
OC
L
Rh
H
R
L
Rh
O
CO
H
H
Rh(CO)2L2
R
CO
R
H
Rh(CO)L2
R
Ready; Catalysis
L
L
Rh(CO)L2
R
CHO
Organometallics
4-centered reactions:
R
2+2
RNH2 + R'
N
cat. Cp2Zr(NHR)2
R'
see Eur. JOC, 2003, 935
R'
R'
NR
-
ZrCp2
NR
ZrCp2
R'
R
Cp
R'
R
N
Zr
R'
N
Cp
Cp
Cp
R'
Zr
R'
R
R'
N
Cp
Cp
Zr
NHR
σ-bond metathesis
Cp*
Sc
-HCMe3
CH3
Cp*
CH3
Tilley, Jacs, 2003,7971
Cp*
Cp*
Sc
Cp*
H2C
Sc
Cp*
H
CH4
12
Ready; Catalysis
Organometallics
Transmetallation
R-M
+
R-M'
+ M'-X
M-X
Stoichiometric Use:
Great way to make Grinards, alkyl zincs, cuprates, stannanes etc. Esp useful on small scale
where O.A. to R-X not possible
NC
I
BuLi
NC
Li
MgBr2, -100 oC, 5min
NC
MgBr
3
3
3
Tet, 1996, 7201
In catalytic cycle:
TMS
Cl
+
TMS
MgBr
cat. NiCl2(DPPP)
Cl
Ln
TMS
via
Ln
Ni
-MgBrCl
Ni
-T.M. usually from more eletropositive M to more electronegative M
-Endothermic T.M.'s can be part of catalytic cycle if R-M' subsequently reacts
-Likely by associative mech for coordinatively unsaturated M
-Likely metathesis for coordinatively saturated
13