carbenes and olefin metathesis - university of manitobabudzelaa/chem4680/lectures-new/15... · 2...
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Carbenes andOlefin Metathesis
Peter H.M. Budzelaar
M
C C
C M
C
C
C+
M C
C C+
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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-typecarbene (and carbyne) complexes
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Carbenes and Olefin Metathesis3
Schrock-type carbene complexes
• "Standard" M=C bond:normal σ and π bonds,both polarized as in Mδ+-Cδ-
• 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 "α-elimination"
M σ
πM
Ta5
Ta
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Carbenes and Olefin Metathesis4
σ and π bonds
σ bond π bond
TaMe3(=CH2)
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Carbenes and Olefin Metathesis5
Real M=C double bonds
1.94
2.252.17-2.22 Å
TaBz5
TaBz2(CHPh)(amide-carbene)
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Carbenes and Olefin Metathesis6
Synthesis of Schrock carbenes
Usually from decomposition of alkyls(α-elimination)
Variations:Ta
5Np3Ta
Cp2TaMe3 Cp2TaMe2+ Cp2Ta
CH2
Me
E+ B-
Cp2TaMe
Cp2TaMe
Cp2TaCH2
H
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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
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Carbenes and Olefin Metathesis8
Fischer-type carbene complexes
• Ligand best described as strong σ-donor,medium-weak π-acceptor
• Carbene has singlet ground state– because of π-donor substituents
• Metal: late, low oxidation state– count carbene as neutral ligand
• Carbene carbon is electrophilic
• Prototype:– nucleophilic attack
at coordinated CO
πσ
M C
π
σM
M
(CO)5WOMe
MeW(CO)6
1) MeLi2) MeI
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Carbenes and Olefin Metathesis9
σ donation, π backdonation
C-O π C→Moσ-donation
Mo→Cπ-backdonation
LUMO
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Carbenes and Olefin Metathesis10
Good ligands, but no "real" M=C bond
2.19
2.00-2.03
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Carbenes and Olefin Metathesis11
Synthesis of Fischer carbenes
• Usually from CO or RNC complexes via1) Nucleophilic attack on C2) Electrophilic attack op O/N
• Also: spontaneous isomerization of vinyl, olefin of acetylene complexes (Ru)
MeI
MeLiW(CO)6
(CO)5WOMe
Me
(CO)5WO Li
Me
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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
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Carbenes and Olefin Metathesis13
Arduengo carbenes
• Free, stable carbenes
• Good ligands, strong σ-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
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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 PCy3must dissociatefor the reaction to start.Tolerates manyfunctional groups.
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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+
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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).
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Carbenes and Olefin Metathesis17
Carbyne complexes
• Fischer:
• Schrock: synthesis mostly ad-hoc
Cp*TaBr2 Cp*TaP P
Cp*TaP P
HNa/Hgdmpe
MOMe
Me
BX3 M Me+
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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
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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