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-typecarbene (and carbyne) complexes

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

Carbenes and Olefin Metathesis4

σ and π bonds

σ bond π bond

TaMe3(=CH2)

Carbenes and Olefin Metathesis5

Real M=C double bonds

1.94

2.252.17-2.22 Å

TaBz5

TaBz2(CHPh)(amide-carbene)

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

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 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

Carbenes and Olefin Metathesis9

σ donation, π backdonation

C-O π C→Moσ-donation

Mo→Cπ-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 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

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 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

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.

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*TaP P

Cp*TaP P

HNa/Hgdmpe

MOMe

Me

BX3 M 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