Metal-Free Catalytic Reduction of α,β-Unsaturated Esters by 1,3,2-Diazaphospholene and Subsequent C–C Coupling with Nitriles

1,3,2-Diazaphospholene 1 catalyzes the conjugate transfer
hydrogenation as well as the 1,4-hydroboration of α,β-unsaturated
esters. The initial step for both processes involves a 1,4-hydrophosphination
of the α,β-unsaturated esters to afford a phosphinyl enol
ether. Subsequent cleavage of the P–O bond in the phosphinyl
enol ether by ammonia-borane (AB) generates an enol intermediate
which tautomerizes to saturated esters, while the P–O bond
cleavage by HBpin via a formal σ-bond metathesis affords boryl
enolate intermediate. The latter could undergo a further coupling
reaction with nitriles to form substituted amino diesters or 1,3-imino
esters, depending on α,β-unsaturated ester substrates.
These catalytic reactions can also be performed in a one-pot manner,
illustrating a protocol for metal-free catalytic C–C bond construction.