A practical one-step method for the exhaustive reduction of aromatic α,β-unsaturated esters is reported. Cu catalysis with NaBH₄ and LiCl in THF enables the sequential reduction of the double bond and the ester moiety, delivering fully saturated aryl propanols under mild conditions. The protocol relies on CuH-mediated conjugate (1,4) hydride addition and, critically, slow in situ generation of LiBH₄, which delays direct ester reduction of the starting cinnamate and suppresses formation of allylic alcohol byproducts. The exhaustive reduction method shows broad tolerance to electronic variation on the aryl ring, affording the desired saturated alcohols in good to excellent yields with complete selectivity across a range of cinnamate substrates. Halogenated derivatives are compatible and undergo reduction without detectable dehalogenation, and a (Z)-cinnamate is reduced with efficiency and selectivity comparable to those of the corresponding (E)-isomer. Limitations were observed for nitro- and phenol-substituted substrates, as well as for trisubstituted alkenes, which showed diminished performance and, in the latter case, increased formation of the allylic alcohol byproduct.
Mesa et al. (Fri,) studied this question.