carbenes and olefin metathesis peter h.m. budzelaar
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Carbenes andOlefin Metathesis
Peter H.M. Budzelaar
M
C C
C M
C
C
C+
M C
C C+
Carbenes and Olefin Metathesis2
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 calledcarbene 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
Carbenes and Olefin Metathesis3
Schrock-type carbene complexes
• "Standard" M=C bond:normal and bonds,both polarized as in M+-C-
• Carbene not particularly stabilized,has a triplet ground state
• Early/middle transition metal,high oxidation state
– count carbene as 2-
• Carbene carbon is nucleophilic• Prototype:
– formed by "-elimination"
M
M
Ta5
Ta
Carbenes and Olefin Metathesis4
and bonds
bond bond
TaMe3(=CH2)
Carbenes and Olefin Metathesis5
Real M=C double bonds
1.94
2.25
2.17-2.22 Å
TaBz5
TaBz2(CHPh)(amide-carbene)
Carbenes and Olefin Metathesis6
Synthesis of Schrock carbenes
Usually from decomposition of alkyls(-elimination)
Variations:
Ta5
Np3Ta
Cp2TaMe3 Cp2TaMe2+ Cp2Ta
CH2
Me
E+ B-
Cp2TaMe
Cp2TaMe
Cp2TaCH2
H
Carbenes and Olefin Metathesis7
Reactivity of Schrock carbenes
• With ketones: "Wittig"
• With electrophiles: attack on carbene carbonUnhindered carbenes dimerize:
• With olefins: metallacycles– Metathesis catalysis !
– "intermediate" carbenes are better for this
M C
O C+
M
O
C
C+
M C
C M+
M
C M
C
M C
C C+
M
C C
C
Carbenes and Olefin Metathesis8
Fischer-type carbene complexes
• Ligand best described as strong -donor,medium-weak -acceptor
• Carbene has singlet ground state– because of -donor substituents
• Metal: late, low oxidation state– count carbene as neutral ligand
• Carbene carbon is electrophilic
• Prototype:– nucleophilic attack
at coordinated CO
M C
M
M
(CO)5W
OMe
Me
W(CO)61) MeLi
2) MeI
Carbenes and Olefin Metathesis9
donation, backdonation
C-O C→Mo-donation
Mo→C-backdonation
LUMO
Carbenes and Olefin Metathesis10
Good ligands, but no "real" M=C bond
2.19
2.00-2.03
Carbenes and Olefin Metathesis11
Synthesis of Fischer carbenes
• Usually from CO or RNC complexes via
1) Nucleophilic attack on C2) Electrophilic attack op O/N
• Also: spontaneous isomerization of vinyl, olefin of acetylene complexes (Ru)
MeI
MeLiW(CO)6
(CO)5W
OMe
Me
(CO)5W
O Li
Me
Carbenes and Olefin Metathesis12
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
Carbenes and Olefin Metathesis13
Arduengo carbenes
• Free, stable carbenes
• Good ligands, strong -donors• More reluctant to dissociate than phosphines• Shape very different from that of typical phosphines
• Easy and convenient synthesis from imidazolium salts
N NR R
Carbenes and Olefin Metathesis14
Olefin metathesis catalysis
MoR'O
R'O NAr
CMe2Ph
R' = (CF3)2CMe
Ar =
PCy3
Ru
PCy3
Ph
ClCl
Schrock catalyst.Highly activebut sensitive tofunctional groups.
Grubbs catalyst.Stable, but PCy3
must dissociatefor the reaction to start.Tolerates manyfunctional groups.
Carbenes and Olefin Metathesis15
Olefin metathesis catalysis
Driving force:
• Escape ofgaseous products
• Decrease ofring strain
+
"ROMP"
"ADMET"
M
C C
C M
C
C
C+
M C
C C+
Carbenes and Olefin Metathesis16
Olefin metathesis catalysis
For Schrock catalysts, the equilibrium between carbene and metallacyclobutane can be tuned by the choice of substituents (OR groups).
Carbenes and Olefin Metathesis17
Carbyne complexes
• Fischer:
• Schrock: synthesis mostly ad-hoc
Cp*TaBr2 Cp*Ta
P P
Cp*Ta
P PH
Na/Hgdmpe
M
OMe
Me
BX3M Me
+
Carbenes and Olefin Metathesis18
Alkyne metathesis catalysis
e.g. with
Very sensitive to functional groups.
Not (yet) very useful in organic synthesis.
M C
C C+
M
C
C
C+
M M
(tBuO)3W
Carbenes and Olefin Metathesis19
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