Regiochemistry in alkylation, acylation and methoxycarbonylation of alkali salts from 2-substituted alkenylpropanedinitriles

Abstract

Alkali salts (4⁻) of several 2-substituted alkenylpropanedinitriles are prepared, isolated and characterised by ¹H and ¹³C NMR spectroscopy. By treating the salt 4a⁻ with 4a a dimer, 2-dicyanomethylene-6-methyl-4,6-diphenyl-1,2,5,6-tetrahydronicotinonitrile 6, is formed, for which a single-crystal X-ray structure is presented. Alkylation of the ambident salts 4⁻ with MeX (X = I, Tf, or Br) leads to regioisomers, 1- and 3-alkylation, and comparison with O vs. C-alkylation of ketones is made. Double alkylation is also observed, 3-alkylation followed by 1-alkylation. Thus from 4a⁻ a mixture of (E)- and (Z)-2-methyl-2-(1-phenylprop-1-enyl)propanedinitrile 9b is formed, and the X-ray structure of the former is presented. Acylation of 4a⁻ with benzoyl chloride gives only the 3-regioisomer; using equimolar amounts of reactants gives a ring-closure product, 2-oxo-4,6-diphenyl-2H-pyran-3-carbonitrile 12, while excess of benzoyl chloride gives the double-acylation product, (Z)-4,4-dicyano-1,3-diphenylbuta-1,3-dienyl benzoate (Z)-13, for which the X-ray structure is presented. Acylation of 4a⁻ with acetyl chloride gives both regioisomers; the 1-acylated product evaded isolation, and the 3-acylation product reacted further to give the (E)- and (Z)-form of 4,4-dicyano-1-methyl-3-phenylbuta-1,3-dienyl acetate 15. The X-ray structure of the latter isomer, (Z)-15, is presented. Methoxycarbonylation of salts 4⁻ with methyl chloroformate gives both regioisomers, while use of methyl cyanoformate gives only the 3-regioisomer, in addition to a Michael adduct from reaction of the CN⁻ group with protonated salt 4⁻.