Screw Sense Selective Polymerization of Achiral Isocyanides Catalyzed by Optically Active Nickel(II) Complexes

Poly(isocyanidcs), ( R N = C < ) n, can be prepared from isocyanidcs, R N += C " , by the catalytic action of nickel(II) compounds. The main chain of these polymers is a rigid helix. This helical conformation results from a restricted rotation around the single bonds that connect the main-chain carbon atoms. Polymerization of achiral isocyanides generally gives a racemic mixture of leftand right-handed helices, whereas polymerization of optically activc isocyanides results in helices with an excess of one screw sense. We describe a procedure for obtaining poly(isocyanides) with predominantly one screw sense, starting from an achiral monomer. A catalyst is prepared by adding an optically activc amine to a tetrakis(isocyanide)nickcl(II) perchlorate complex. Polymerization of various achiral isocyanides with this catalyst yields optically active polymers with an enantiomeric excess up to 83%. In the presence of protonic acids, Lewis acids, or N i(I I ) salts as catalysts, isocyanides polymerize to give poly(isocyanides), also called poly(iminomethylenes) or poly(carbonimidoyls).1,2 Ni(II) salts are versatile catalysts and, in our opinion, the most suited for our experiments. Poly(isocyanides) are unusual polymers in Scheme I n Ni(II) ( R N = C < ) „ the sense that each atom of their main chain carries a side chain .1,2 This feature causes a restricted rotation around the single bonds that connect the main-chain carbon atoms. Two conformations are possible around the single bonds, viz. R and S .3 If the polymer is highly isotactic (meaning that the configuration is the same around all the single bonds), a stable helix will be the result.4 The helix is right handed (P) if the aforementioned configurations are all 5 and left-handed ( M ) if they are all R.5 Following initial suggestions by Millich, we established ex­ perimentally that polymers of isocyanides have a helical s truc­ tu re .6"9 The polymer of tert-butyl isocyanide was completely resolved into P and M screws, which were shown to have negative and positive optical rotations, respectively.7,8 In the past, several procedures for obtaining optically active poly(isocyanides) have been developed in our laboratory. One procedure involves the polymerization of optically active mono­ mers. When one enantiomer of a chiral isocyanide is polymerized, the resulting polymer will be a mixture of diastereoisomeric molecules having P and M screws. This mixture will contain an excess of one of the screw senses. This was observed for ap ­ proximately 20 different optically active isocyanides .10" 12 In another procedure, we prepared optically active polymers by specifically inhibiting the growth of one screw sense of a racemic pair of helices.5,13 Finally, in one case an optically active poly(isocyanide) was obtained by resolution. A convenient way of preparing optically active polymers is the use of chiral initiators (e.g., see ref 14). In this way a racemic mixture of optically active monomers can be polymerized stereoselectively.143' ^ This procedure was used by Yuki et al .14c~f to prepare stable helical polymers from bulky methacrylic acid esters and by Vogl141,m to prepare helical polymers from chloral. The resolution of poly(fer/-butyl isocyanide) indicates that polymerization of isocyanides proceeds stereoselectively with re­ spect to the screw sense. Therefore, it should be possible to obtain optically active polymers by using chiral catalysts. In the present paper we describe the synthesis of optically active poly (isocyanides) by using Ni(II) complexes of optically active amines as catalysts.15 The prevailing screw sense of the polymers is derived from CD R N H 2 1 R N H C H O 2 R N +