Metal Carbene Complexes.ppt

Education

1. Metal-CarbeneComplexes
1)Introduction
2)TypesofmetalCarbenecomplexes
1)ShrockCarbenes
1)Synthesis
2)Reactivity
2)FischerCarbenes
1)Synthesis
2)Reactivity
3)N-heterocycliccarbenes
3)Applications

2. Metal-CarbeneComplexes
Metalcarbenecomplexisanorganometalliccompoundwith
divalentorganicligand
Carbenecomplexesforalmostalltransitionmetalshavebeen
reported.
Thetermcarbeneisusedformallyasmanyarenotderivedfrom
carbenesandalmostnoneexhibitthereactivityofcarbenes.

3. Metal-carbon multiple bonds
•Many transition metals form not only M-C single bonds but also
M=C and (more rare) even M≡C bonds.
•Complexes containing an M=C bond are called
carbenecomplexes
–The ligand without the metal would be a free carbene
•Complexes with M≡C bonds are called carbynecomplexes
•Both types of complexes tend to be reactive,
and useful in catalysis
•Fischer-typeand Schrock-typecarbene (and carbyne)
complexes can be distinguished

4. Schrock-type carbenecomplexes
•"Standard" M=C bond:
normal sand pbonds,
both polarized as in M
d+
-C
d-
•Carbenenot particularly stabilized,
has a triplet ground state
•Early/middle transition metal,
high oxidation state
–count carbeneas 2-
•Carbenecarbon is nucleophilic
•hydrogen and alkyl substituents on carbenoidcarbon
•Pi acceptor ligands
•Prototype:
–formed by "a-elimination"M s
pM Ta
5
Ta

5. Synthesis of Schrock carbenes
Usually from decomposition of alkyls
(a-elimination)
Variations:Ta
5
Np
3Ta Cp
2TaMe
3 Cp
2TaMe
2
+
Cp
2Ta
CH
2
Me
E
+
B
- Cp
2Ta
Me
Cp
2Ta
Me
Cp
2Ta
CH
2
H

6. Reactivity of Schrock carbenes
•With ketones: "Wittig"
•With electrophiles: attack on carbene carbon
Unhindered carbenes dimerize:
•With olefins: metallacycles
–Metathesis catalysis !
–"intermediate" carbenes are better for thisMC
OC
+
M
O
C
C
+ MC
CM
+
M
CM
C MC
CC
+
M
CC
C

7. Fischer-type carbenecomplexes
•Ligand best described as strong s-donor,
medium-weak p-acceptor
•Carbenehas singlet ground state
–because of p-donor substituents
•Metal: late, low oxidation state
–count carbeneas neutral ligand
•Carbenecarbon is electrophilic
•pi-donor substituents on the carbeneatom such as alkoxyand
alkylated amino groups.
•pi electron acceptor metal ligands
•Prototype:
–nucleophilic attack
at coordinated COp
s
M C p
sM
M (CO)
5W
OMe
Me
W(CO)
6
1) MeLi
2) MeI

8. Synthesis of Fischer carbenes
•Usually from CO or RNC complexes via
1)Nucleophilic attack on C
2)Electrophilic attack op O/N
•Also: spontaneous isomerization of vinyl, olefin of acetylene
complexes (Ru)MeI
MeLi
W(CO)
6
(CO)
5W
OMe
Me
(CO)
5W
O Li
Me

9. Reactivity of Fischer carbenes
•Decomposition:
a) Dimer of carbene (C=C)
b) Olefin, via isomerization of carbene
•Oxidation to ketone
•Nucleophilic substitution on carbene carbon

10. Applications
•heterogeneous catalysesfor alkene metathesis in the Shell
higher olefin process.
•A variety of related reactions are used to interconvert light
alkenes, e.g. butenes, propylene, and ethylene.
•Carbene-complexes are invoked as intermediates in the
Fischer-Tropschroute to hydrocarbons.
•A variety of soluble carbenereagents, especially the Grubbs'
and molybdenum-imidocatalysts have been applied to
laboratory-scale synthesis of natural products and materials
science.
•In the nucleophilic abstraction reaction, a methyl group can be
abstracted from a Fischer carbenefor further reaction.