Imines, alternating copolymerization with

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

However, there are numerous reported instances of stereocontrol by a site-control mechanism involving chiral metal catalysts. That is, Nozaki and coworkers first illustrated the asymmetric alternating copolymerization of cyclohexene oxide and CO2 employing a chiral zinc catalyst derived from an amino alcohol (Fig. 2a) [13-16]. This was soon followed by studies of Coates and coworkers utilizing an imine-oxazoline zinc catalyst (Fig. 2b) [17]. Both investigations provided isotactic poly(cyclohexene carbonate) (Fig. 3) with enantiomeric excess of approximately 70%. 

Fujita [55, 57, 76-78] and coworkers developed bis(pyrrolide-imine)Ti complexes (named PI catalysts), which give highly alternating E-N copolymers. These group 4 transition metal [57, 76-79] catalysts with electronically flexible nonsymmetric [N N] chelate ligands displayed a marked tendency to produce E-N copolymers with a stereoirregular stmcture despite the C2-symmetric nature of the catalysts. The E-N copolymerization with PI catalysts is living [55]. 

Scheme 15 Alternating copolymerization of imines with carbon monoxide catalyzed by acylcobalt complex.

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