Towards the planning and execution of an "absolute" asymmetric synthesis of chiral dimers and polymers with quantitative enantiomeric yield

Abstract

Abstract An “absolute”asymmetric synthesis of chiral dimers and polymers has been accomplished through a process of crystallisation of an achiral monomer in a chiral crystal, followed by a topochemical photopolymerisation. Once (2π+2π) photocycloadditon had been selected as a suitable reaction, we designed the chiral crystalline motifs needed for a reaction with quantitative enantiomeric yield. Unsymmetrically disubstituted dienes, translationally related in a chiral crystal, and where reaction occurs between non-equivalent double bonds, separated by 4.0a, fit the suggested model best. The execution of the synthesis involved the following steps. By applying some empirical rules of “crystal engineering”, we selected a family of chiral resolved molecules (which guarantee a chiral crystalline motif), among which R(-) or S(+) ethyl-2-cyano-3(p-sec-butyl-3′-E-propenoate)-phenyl-E-propenoate- 1 was found to fulfil all the stated requirements, yielding photodimers and polymers with quantitative diasterioisomeric yield. Finally, on the basis of the crystal structure of 1 we have designed at least two achiral molecules which pack in chiral structural motifs isomorphic with 1 . Irradiation of crystals grown from the melt, led to polymers with high, probably quantitative optical yields. A possible model for an amplification of the generated chirality will be discussed.