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-C₆H₄-4-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(C₆H₄-4-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.