Determination of the Electrophilicity Parameters of Diethyl Benzylidenemalonates in Dimethyl Sulfoxide: Reference Electrophiles for Characterizing Strong Nucleophiles

The second‐order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a–i with the carbanions 2 a–e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic CC double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 °C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a–i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a–e. The electrophilicities E for compounds 1 a–i cover a range of six units (−17.7>E>−23.8) and correlate excellently with Hammett's substituent constants σp. The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a–i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16−23.8) und korreliert ausgezeichnet mit Hammett's Substituentenkonstanten σp. Die Titelverbindungen sind ca. zehn Größenordnungen weniger reaktiv als analog substituierte Benzyliden‐Meldrum‐Säuren, ihre cyclischen Analoga. Wegen ihrer geringen Reaktivität sind die Verbindungen 1 a–i passende Referenzelektrophile zur Bestimmung der Reaktivitäten hoch reaktiver Nucleophile, z. B. von Carbanionen mit 16