AN EFFICIENT SYNTHESIS OF SOME SUBSTITUTED VINYLIC CHLOROFORMATES - REACTION SCOPE AND LIMITATIONS

2,2-Dichlorovinyl chloroformate is isolated in 50% distilled yield when chloral is treated with phosgene and zinc dust in 2:1 methyl acetate/ether. The reaction has been extended to other a-chloro and a-bromo aldehydes and ketones in which both other a-positions are occupied by either halo and/or alkyl groups. The reaction fails with hydrogen in an a-position. Examples include the synthesis of Cl2C=C(Me)OC(=0)Cl in 23% yield, MeC(Cl)=CHOC(═O)Cl (56%, E:z = 1:1.1), C12c=c(c6h5)oc(=0)c1 (66%), and 2-methyl-l-cyclohexenyl chloroformate (68%). Similar treatment of a-halo esters gives only the C-acylated products expected from a Reformatsky type reaction, while ketenes are the well-known products from a-halo acid chlorides. However, acyl cyanides and acyl phosphonates, with leaving groups intermediate between fluoride and alkoxide, are converted to chloroformates; e.g., Me2C═C(CN)OC(═O)C1 in 67% yield and Me2C═C[P(O)(OMe)2]OC(═O)Cl in 83% yield. Carbonates and urethans from these chloroformates are of interest as monomers, pesticides, and chemical intermediates. © 1990, American Chemical Society. All rights reserved.