Activation of Nitriles by Metal Ligand Cooperation. Reversible Formation of Ketimido- and Enamido-Rhenium PNP Pincer Complexes and Relevance to Catalytic Design

The
dearomatized complex cis-[Re­(PNPtBu*)­(CO)2] (4) undergoes
cooperative activation of CN triple bonds of nitriles via
[1,3]-addition. Reversible C–C and Re–N bond formation
in 4 was investigated in a combined experimental and
computational study. The reversible formation of the ketimido complexes
(5–7) was observed. When nitriles
bearing an alpha methylene group are used, reversible formation of
the enamido complexes (8 and 9) takes place.
The reversibility of the activation of the nitriles in the resulting
ketimido compounds was demonstrated by the displacement of p-CF3-benzonitrile from cis-[Re­(PNPtBu–NCPhpCF3)­(CO)2] (6) upon addition of an
excess of benzonitrile and by the temperature-dependent [1,3]-addition
of pivalonitrile to complex 4. The reversible binding
of the nitrile in the enamido compound cis-[Re­(PNPtBu–HNCCHPh)­(CO)2] (9) was demonstrated via the displacement of benzyl
cyanide from 9 by CO. Computational studies suggest a
stepwise activation of the nitriles by 4, with remarkably
low activation barriers, involving precoordination of the nitrile
group to the Re­(I) center. The enamido complex 9 reacts
via β-carbon methylation to give the primary imino complex cis-[Re­(PNPtBu–HNCC­(Me)­Ph)­(CO)2]­OTf 11. Upon deprotonation of 11 and subsequent addition of benzyl cyanide, complex 9 is regenerated and the monomethylation product 2-phenylpropanenitrile
is released. Complexes 4 and 9 were found
to catalyze the Michael addition of benzyl cyanide derivatives to
α,β-unsaturated esters and carbonyls.