The substituted methyl cinnamates 4 and 5 have provided a unique system for the study of various mechanistic aspects of the nucleophilic 1,4 addition of sodium borohydride to ,/3-unsaturated esters. Competitive rates of reduction for two sets of methyl a-phenyl-trores-cinnamates (4), para-substituted in the a and ß rings, respectively, correlate linearly with Hammett values. The similarity in pa (1.74) and ps (1.44) indicates that the transition state for hydride transfer occurs before significant change in geometry of the ,/3-unsaturated carbonyl system occurs. Competitive rate studies for methyl a-(para substituted phenyl)acrylates (2) and methyl of-phenyl-a's-and -trans-crotonates (14 and 15) are corroborated by the data obtained for the cinnamates.Carbon-carbon double bonds conjugated with strong anion-stabilizing groups (e.g., COR, CO2R, CN, SO2R, NO2) have occasionally been observed to undergo reduction with sodium borohydride.2-11 Although it is recognized that sodium borohydride exhibits nucleophilic behavior,3-4 little is known concerning the mechanism or even the general structural requirements for the occurrence of such reactions.

Aus den α‐Phenyl‐acrylaten (I), die nach bekannten Verfahren hergestellt werden, erhält man bei der Reduktion mit NaBH4 die Dihydroverbindungen (II), wobei die Kinetik dieser Reaktionen in Relation zu den Substituenten diskutiert wird.

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