Transcript 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
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Carbenes and Olefin Metathesis
Synthesis of Schrock carbenes
Usually from decomposition of alkyls
(a-elimination)
Ta
5
Np3Ta
Variations:
Cp2TaMe3
E+
Cp2TaMe2+
B-
CH2
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
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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
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1) MeLi
2) MeI
W(CO)6
Carbenes and Olefin Metathesis
s donation, p backdonation
C-O p
Mo→C
p-backdonation
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C→Mo
s-donation
LUMO
Carbenes and Olefin Metathesis
Good ligands, but no "real" M=C bond
2.19
2.00-2.03
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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
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2) Electrophilic attack op O/N
MeI
O Li
Me
OMe
(CO)5W
Me
•
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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
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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
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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.
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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"
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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).
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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.
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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