Transition metal complex possessing a formal metal to carbon double bond are termed as
alkylidene or carbene complexes. There are primarily two types of metal carbenes: Fischer-type
and Schrock-type carbenes.
where X, Y = alkyl, aryl, H, or heteroatom (O, N, S, Halogen)
Schrock carbene:
Schrock carbene is nucleophilic and is named after the dicoverer Richard R. Schrock.
Characteristics:
sp hybridized and linear (triplet)
Carbon atom in carbene is nucleophilic (Electron rich)
Carbene-attacked by electrophiles
High oxidation state early transition metals such as Ti(IV) and Ta(V)
Substituents are Hydrogen, alkyl, cp, Cl etc.
Ligands are generally good σ or π donors
e.g.,
Bonding in such complexes can be viewed as the coupling of a triplet state metal and triplet
carbine.
Popular Schrock carbenes used in Olefin Metathesis:
Preparation:
The Petasis reagent is prepared by the salt reaction of methylmagnesium chloride or with
titanocene dichloride.
Cp2TiCl2 + 2 CH3MgCl → Cp2Ti(CH3)2 + 2 MgCl2
The Tebbe’s reagent is synthesized from diemethyltitanocene as follows,
Cp2Ti(CH3)2 + Al(CH3)2Cl → Cp2TiCH2 AlCl(CH3 )2 + CH4
Olefin Metathesis reaction:
General:
Olefin Metathesis reactions: