Nucleophilic reaction of fluoroolefins. V. Regioselectivity and stereochemistry in the reactios of 1-phenylpentafluoropropenes with lithium dialkylamides

Abstract

1-Phenylpentafluoropropenes 1 readily react with lithium dialkylamides to give, in most cases, mixtures of 1-aminosubstituted alkenes 2 and 2-amino-substituted alkenes 3 , with the latter being the favoured products. The reactions with bulky lithium diethylamide and lithium 2-methylpiperidinoamide gave exclusively 1-amino-substituted products 2 . The effect of the increased bulk of N-nucleophiles is opposite to that observed for the reactions of alkenes 1 with C-nucleophiles Increasing electronegativity of the phenyl ring substituents in alkenes 1 shifts the regioselectivity of the attack of lithium amides towards the C2 carbon atom. The E to Z isomer ratios of enamines 2 were found to be time dependent and the slow isomerisation of the kinetic isomers E to the thermodynamic isomers Z was observed, while the ratio of isomers of enamines 3 did not change with time. A concerted, single-step process is suggested for the reactions of alkenes 1 with lithium dialkylamides, and a tentative explanation of the different stereochemistry of enamines 2 and 3 is given.