Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 202

Scheme 6.14 Polymerization of organostannanes with classical metathesis catalysts.

6.4
Well-Defined Tungsten and Molybdenum Catalysts

The applicability of ADMET was greatly expanded by the development of well-de-
fined catalysts by R. R. Schrock and co-workers. These catalysts promote metath-
esis without added Lewis acid co-catalysts, and thus eliminate the vinyl addition
chemistry that was observed for the WCl 6 /EtAlCl 2 catalyst. Complexes 1 [23, 63]
and 2 [64, 65] have been used extensively for ADMET polymerizations. These com-
plexes are pseudotetragonal and the carbene fragment tends to point towards the
imido ligand ( syn -rotamer), although the opposite rotamer ( anti -rotamer) is also
observed.
This general type of metal carbenes is termed “Schrock carbenes”. These car-
benes differ from Fischer carbene complexes (those with

-hetero-substituents) in
a number of ways. Schrock carbenes are viewed as X 2 ligands with +2 charge,
whereas Fischer carbenes are considered neutral L ligands. Fischer carbenes are
electrophilic at the

-carbon and Schrock carbenes are nucleophilic. Thus com-
plexes 1 and 2 are formally W(VI) and Mo(VI) complexes [66].

There are a number of differences between the tungsten and molybdenum com-
plexes. The main generality drawn from the data is that the tungsten complexes
promote olefin metathesis quite a bit faster than the molybdenum complexes, but
the tungsten complexes are less tolerant of functionality. The tungstacyclobutane
is more stable than the molybdacyclobutane. Several tungstacyclobutanes have