10.1021/ja00086a074
The study investigates a novel "umpolung" in C-C bond formation catalyzed by triphenylphosphine. The Michael addition reaction, where a nucleophile adds to an α,β-unsaturated carbonyl compound, is a fundamental synthetic reaction in organic chemistry. Typically, the γ-carbon in such compounds acts as a nucleophile due to conjugation with an electron-withdrawing group. However, this study demonstrates that triphenylphosphine can induce the γ-carbon to act as an electrophile, facilitating nucleophilic addition. The researchers used a mixture of methyl 2-butynoate and dimethyl malonate, with triphenylphosphine as a catalyst, acetic acid, and sodium acetate in toluene. They observed the formation of a 1:1 adduct, with yields varying based on the concentration of triphenylphosphine. The study explores the range of pronucleophiles that can participate in this reaction, finding that compounds with pKa < 16 work well, while introducing alkyl groups on the acidic carbon of the pronucleophile reduces yield. The study also examines the effects of different substituents on the acetylenic acceptor, such as esters, amides, and ketones, and proposes a mechanism where triphenylphosphine acts as a nucleophilic trigger, enabling unprecedented regioselectivity and atom economy in the addition process.