Kinetics of Electrophilic Alkylations of Barbiturate and Thiobarbiturate Anions

5 years ago

Kinetics of Electrophilic Alkylations of Barbiturate and Thiobarbiturate Anions

Alexander Oehlke, Joachim Friedrich, Ivan Tchernook, Alexander Schade, Stefan Spange, Martin Breugst, Mirko Bauer

Second-order rate constants (k₂) of the reactions of various barbiturate anions such as the parent barbiturate, 1,3-dimethylbarbiturate, 2-thiobarbiturate, and 1,3-diethyl-2-thiobarbiturate with diarylcarbenium ions and Michael acceptors have been determined in dimethyl sulfoxide solution at 20 °C. The reactivity parameters N and s_N of the barbiturate anions were derived from the linear plots of log k₂ versus the electrophilicity parameters E of these reference electrophiles, according to the linear-free-energy relationship log k₂ (20 °C) = s_N (E + N). Several reactions of these nucleophiles with benzylidenemalononitriles and quinone methides proceeded with reversible formation of the new C–C-bond followed by rate-determining proton shift. No evidence for initial attack of the electrophiles at the enolate oxygens of these nucleophiles was found by the kinetic measurements, in line with quantum chemical DFT calculations, which showed that in all cases C-attack is kinetically and thermodynamically preferred over O-attack. The nucleophilic reactivities of barbiturate anions were compared with those of structurally related carbanions, e.g., Meldrum’s acid and dimedone anions.

Publisher URL: http://dx.doi.org/10.1021/acs.joc.7b01223

DOI: 10.1021/acs.joc.7b01223