An easy access to trisubstituted vinyl chlorides and improved synthesis of chloro/bromostilbenes

Abstract

The α-chlorophosphonates (OCH2CMe2CH2O)P(O)CHCl-C6H4-4-R [R=H (4), Me (5), OMe (6)], which are now readily accessible, react with ketones R'C(O)R' in the presence of NaH (without recourse to the more expensive t- BuLi) to afford trisubstituted vinyl halides R'C(R')=CCl(C6H4-4-R) in good yields. The corresponding α-bromophosphonates [R=H (7), Me (8)] failed to react with ketones and gave the symmetrical acetylenes 4-R-C6H4-C=C- C6H44-R as isolable products in low yield. We have found that K2CO3 in refluxing xylene is a good base for the synthesis of chlorostilbenes; using this base the bromostilbenes ArCH=CBr(C6H44-R) can be prepared in significantly higher yields than by using NaH. The stereochemistry of two of the trisubstituted vinyl chlorides is unambiguously proven by X-ray structure determination. Thus for (Cl)PhC=CPh(Me), the isomer with the upfield NMR shift for the CH3 protons and for (Cl)PhC=C(Ph)(C6H4-4-Me), the isomer with the downfield NMR shift for the -C6H4-4-CH3 protons have Z stereochemistry.