Heavy Atom Kinetic Isotope Effects in HCN Elimination from Some Tricyanides in Methanol [1]

Rates of H C N elimination from polycyanides N,N-dimethyl-4-(l ,2 ,2-tricyanoethyl)aniline (1), 9-cyano-9-dicyanomethyl fluorene (2), 1,1-diphenyl-1,2,2-tricyanoethane (3), and 2-phenyl-l,l,2-tricyanopropane (4) have been studied in methanol. Elimination from 1 occurs via (E 1 c B ) r , mechanism. On the other hand olefin formation from 2 4 has been shown to occur via (E l)anion pathway. Heavy atom kinetic isotope effects indicated that product stability is not the sole factor controlling the transition state geometries. Values of k1 2 /k1 4 were found to be in the order 2 > 3 > 4 > 1 which implied transition states with more carbanion-like structure in the opposite direction. Solvent isotope effects and enthalpies of activation were also determined and discussed in terms of transition states geometries.