Synthesis and CD spectra of isocyanide polymers: some new aspects about their stereochemistry

Long-distance chirality transfer in polymerization of isocyanides bearing a remote chiral group

“…However, at this stage the role of this moiety in directing diastereoselective chain propagation is unclear. Also unknown is the ability of the tetrahydro [5]helicene moiety to adopt a stable chiral conformation, as suggested for the phenyl benzoate moiety in isocyanide A. Optical rotation of polyisocyanide 4 was not high, indicative of the co-existence of an opposite-handed helix. The majority of the polymer backbone was tentatively assigned to have an M helical conformation, based on the succession of the positive band at 330 nm to the negative band at 300 nm.11,12 These two bands were much stronger than those in the visible region and, therefore, should be accounted for mostly by the chirality of the polymer.…”

“…The chiral group was introduced as an N-substituent of the imide using (S)-( [ )-a-methylbenzylamine. The nitro group was reduced and then converted into the isocyanido group according to a known procedure (Scheme 1).10 Aromatization of 1 upon treatment with bromine in reÑuxing odichlorobenzene gave the corresponding [5]helicene analogue, which was used as a starting material for the synthesis of monomer 7.…”

“…These Cotton couplets indicated that this polymer had a prevailing one-handed helical backbone. Since the stereogenic centre in isocyanide 4 is located at the imide group with an S-conÐguration, this remote chiral group must impart its chirality to the reactive isocyanido group via the tetrahydro [5]helicene moiety during polymerization. However, at this stage the role of this moiety in directing diastereoselective chain propagation is unclear.…”

“…8 Considering the proposed mechanism of this novel long-distance asymmetric induction, it is intriguing for us to explore other chiral isocyanides having di †erent structural characterics. In connection with our work on [5]helicene-based chiral polymers derived from the nitro-anhydride 19 (Scheme 1), we realized the availability of chiral isocyanide monomers, such as 4 and 7 derived from 1, having a remote chiral centre. The stereogenic centre is separated from the polymerizable group by a rigid [5]helicene imide moiety in 7 or by a tetrahydro [5]helicene moiety in 4.…”

“…In connection with our work on [5]helicene-based chiral polymers derived from the nitro-anhydride 19 (Scheme 1), we realized the availability of chiral isocyanide monomers, such as 4 and 7 derived from 1, having a remote chiral centre. The stereogenic centre is separated from the polymerizable group by a rigid [5]helicene imide moiety in 7 or by a tetrahydro [5]helicene moiety in 4. In comparison with isocyanide A, these two monomers have rather short chiral tails but more rigid connecting moieties that may adopt chiral conformations.…”