Download Organometallic Chemistry between organic and inorganic

Carbenes and
Olefin Metathesis
M C
+
C C
M C
M
C C
C
+
C
C
Peter H.M. Budzelaar
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
carbene complexes
– The ligand without the metal would be a free carbene
• Complexes with M≡C bonds are called carbyne complexes
• Both types of complexes tend to be reactive,
and useful in catalysis
• We distinguish between Fischer-type and Schrock-type
carbene (and carbyne) complexes
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Carbenes and Olefin Metathesis
Schrock-type carbene complexes
• "Standard" M=C bond:
normal s and p bonds,
both polarized as in Md+-Cd• Carbene not particularly stabilized,
has a triplet ground state
• Early/middle transition metal,
high oxidation state
M
s
M
p
– count carbene as 2-
• Carbene carbon is nucleophilic
• Prototype:
– formed by "a-elimination"
Ta
3
Ta
5
Carbenes and Olefin Metathesis
s and p bonds
TaMe3(=CH2)
s bond
4
p bond
Carbenes and Olefin Metathesis
Real M=C double bonds
TaBz2(CHPh)(amide-carbene)
TaBz5
1.94
2.25
2.17-2.22 Å
5
Carbenes and Olefin Metathesis
Synthesis of Schrock carbenes
Usually from decomposition of alkyls
(a-elimination)
Ta
5
Np3Ta
Variations:
+
Cp2TaMe3
E
Cp2TaMe2+
CH2
-
B
Cp2Ta
Me
H
Cp2Ta
Cp2Ta
Me
6
Cp2Ta
Me
CH2
Carbenes and Olefin Metathesis
Reactivity of Schrock carbenes
• With ketones: "Wittig" M C
M
+
O
O C
+
C
C
• With electrophiles: attack on carbene carbon
Unhindered carbenes dimerize:
M C
+
C M
• With olefins: metallacycles M C
– Metathesis catalysis !
+
C C
M C
C M
M C
C C
– "intermediate" carbenes are better for this
7
Carbenes and Olefin Metathesis
Fischer-type carbene complexes
• Ligand best described as strong s-donor,
medium-weak p-acceptor
• Carbene has singlet ground state
– because of p-donor substituents
p
M
C
s
M
s
M
p
• Metal: late, low oxidation state
– count carbene as neutral ligand
• Carbene carbon is electrophilic
• Prototype:
– nucleophilic attack
at coordinated CO
OMe
(CO)5W
Me
8
1) MeLi
2) MeI
W(CO)6
Carbenes and Olefin Metathesis
s donation, p backdonation
C-O p
Mo→C
p-backdonation
9
C→Mo
s-donation
LUMO
Carbenes and Olefin Metathesis
Good ligands, but no "real" M=C bond
2.19
2.00-2.03
10
Carbenes and Olefin Metathesis
Synthesis of Fischer carbenes
•
Usually from CO or RNC complexes via
1) Nucleophilic attack on C
MeLi
W(CO)
(CO)5W
6
2) Electrophilic attack op O/N
MeI
O Li
Me
OMe
(CO)5W
Me
•
11
Also: spontaneous isomerization of vinyl, olefin of acetylene
complexes (Ru)
Carbenes and Olefin Metathesis
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
12
Carbenes and Olefin Metathesis
Arduengo carbenes
• Free, stable carbenes
R
N
N
R
• Good ligands, strong s-donors
• More reluctant to dissociate than phosphines
• Shape very different from that of typical phosphines
• Easy and convenient synthesis from imidazolium salts
13
Carbenes and Olefin Metathesis
Olefin metathesis catalysis
NAr
Mo
R'O
CMe2Ph
R'O
R' = (CF3)2CMe
PCy3
Ph
Cl Ru
Cl
PCy3
Ar =
Schrock catalyst.
Highly active
but sensitive to
functional groups.
14
Grubbs catalyst.
Stable, but PCy3
must dissociate
for the reaction to start.
Tolerates many
functional groups.
Carbenes and Olefin Metathesis
Olefin metathesis catalysis
M C
+
C C
M C
M
C C
C
+
C
C
Driving force:
• Escape of
gaseous products
+
"ADMET"
• Decrease of
ring strain
"ROMP"
15
Carbenes and Olefin Metathesis
Olefin metathesis catalysis
For Schrock catalysts, the equilibrium between carbene and
metallacyclobutane can be tuned by the choice of substituents
(OR groups).
16
Carbenes and Olefin Metathesis
Carbyne complexes
• Fischer:
OMe
M
BX3
+
M
Me
Me
• Schrock: synthesis mostly ad-hoc
Cp*TaBr2
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Na/Hg
dmpe
Cp*Ta
P
P
Cp*Ta H
P
P
Carbenes and Olefin Metathesis
Alkyne metathesis catalysis
e.g. with (tBuO)3W
M C
+
C C
M
M
M
C
+
C
C
Very sensitive to functional groups.
Not (yet) very useful in organic synthesis.
18
Carbenes and Olefin Metathesis
Carbene and carbenoid chemistry
• Both main group metals and transition metals can be used to
transfer carbenes to e.g. double bonds ("cyclopropanation").
• Main group metals: carbenoid reagents
– Simmons-Smith: IZnCH2I, EtZnCH2I
• Transition metals: with diazo compounds
– (RCOO)4Cu2 and (RCOO)4Rh2
– Mode of action obscure
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Carbenes and Olefin Metathesis