E/Z Isomerization, Solvolysis, Addition, and Cycloaddition Reactions of (E)-tert-Butylketene Methyl tert-Butyldimethylsilyl Acetal

Article abstract of DOI:10.1021/jo00026a013

In the presence of catalytic amounts of CF3COCH3 or CF3COCF3, the silyl ketene acetal E-1 was isomerized into its Z isomer (Z/E ratio 90:10).For this novel E/Z isomerization a mechanism is proposed, in which addition and reelimination of the fluoro ketone, through a 1,4-dipolar intermediate operates.With the protic nucleophiles CH3OH, CF3CH2OH, or PhOH, the ketene acetal E-1 afforded the ortho esters 2 as addition products, while CH3CO2H, CF3CO2H, or H2O led to methyl pivalate as the solvolysis product.This chemistry is readily explained through protonation of the ketene acetal E-1 to generate the corresponding carbenium ion.At low temperature the reaction with TCNE gave the silylketene imine 3 as labile cycloadduct, which underwent on workup desilylation to give the TCNE-incorporated ester 6; the latter eliminated hydrogen cyanide at room temperature to give the ene ester 7.With MTAD the labile silyl ene product 4 was obtained initially, which underwent silyl migration to give N-silylated urazole 8; final desilylation led to the stable urazole 9.Also for the ene reactions of TCNE and MTAD with the silyl ketene acetal E-1, a 1,4-dipolar intermediate is proposed to intervene.