Asymmetric Polymerization of Isocyanide

SCHEME 72. Asymmetric polymerization of isocyanides. [G. Wulff, Angew. Chem., lnt. Ed. Engl., 28, 21 (1989). Reproduced by permission of Verlag Chemie.]... 

These experiments revealed the important stereochemical features of polyisocyanide. First, racemic polyisocyanides consisting of achiral repeating units exist as a 1 1 mixture of right- and left-handed helices. Second, the right- or left-handed helical conformation is stable enough to exist as either enantiomer at ambient temperature in solution, although the stability may depend upon the AT-substituent as well as the degree of polymerization. Based on these ideas, efficient systems for asymmetric polymerization of isocyanides have been further pursued. 

The following subsections describe the asymmetric polymerization of isocyanides using four classifications based upon the mechanism of the asymmetric induction. The first two subsections deal with homo- and copolymerization of chiral, non-racemic isocyanides, and asymmetric polymerization of achiral isocyanides by chiral enantiopure nickel complexes are described in the final two subsections. 

In Nolte and Drenth s nickel catalyzed system, the polymerization was believed to be initiated by a nucleophilic attack by the alcohol used as a solvent or the halide on the starting complex on the coordinated isocyanides. Successful asymmetric polymerization was achieved using a dicationic tetrakis(isocyanide)nickel(II) complex 46 with enantiopure primary amines, which served as a chiral nucleophile in the initial step (Scheme 35) [58, 59]. In a typical experiment, a catalyst prepared from (f-BuNC)4Ni(II)(C104)2 (46a) (1 mol%) and an optically active amine (1 mol%), was used for polymerization of isocyanides with, or without a solvent, such as n-hexane, in... 

The first attempts to synthesis asymmetric polyisocyanides are recent ones [30]. Asymmetric polymerization of racemic isocyanide (Xllla) with nickel chloride or nickel acetylacetonate catalyst was attempted in the presence of chiral solvents, (-)-borneol or (+)-sec-butyl-alcohol, or in the presence of (+)-nickel alaninate with the racemic a-phenyl-ethylisocyanide (Xlllb), but the polymers did not show any activity. 

The significant contribution by the Nolte and Drenth group in this area was that, for the first time, they demonstrated the existence of a non-racem-ic helical structure for poly(isocyanide)s. Optical resolution using a chiral HPLC technique or asymmetric polymerization led to the isolation of optically active polymers, whose chirality was supposed to be solely due to the main chain helicity. Their effort, in conjunction with that of Novak s and Takahashi s significant contributions to asymmetric polymerization, will be discussed in the next section. Non-asymmetric and asymmetric polymerizations will be described separately in the following sections. 

A mechanism for asymmetric induction has been discussed based on the merry-go-round mechanism proposed for Ni(II)-catalyzed polymerization, and discussed earlier. In the initial stages, a chiral amine attacks one of the four isocyanide ligands (C1) on the nickel to form a diaminocarbene complex I, whose structure has been elucidated in detail [60]. Among the possible conformations, the one that involves weak interaction between the Ph group and Ni was presumed to have the most favorable conformation (Scheme 37). The structure of the intermediate I has also been discussed in... 

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