Hydrogenation reactions catalyzed by HN(CH2CH2PR2)2-ligated copper complexes

HN(CH2CH2PR2)(2)-ligated copper borohydride complexes, ((R)(PNP)-P-H)Cu(BH4) (R = Pr-i, Cy, Bu-t), which can be prepared from ((R)(PNP)-P-H)CuBr and NaBH4, are capable of catalyzing the hydrogenation of aldehydes in an alcoholic solvent. More active hydrogenation catalysts are ((R)(PNP)-P-H)CuBr mixed with (KOBu)-Bu-t, allowing various aldehydes and ketones to be efficiently reduced to alcohols except those bearing a nitro, N-unprotected pyrrole, pyridine, or an ester group, or those prone to aldol condensation (e.g., 1-heptanal). Modifying the catalyst structure by replacing the NH group in ((PNP)-P-iPr-P-H)CuBr with an NMe group results in an inferior catalyst but preserves some catalytic activity. The hexanuclear copper hydride cluster, ((PNP)-P-iPr-P-H)(3)Cu6H6, is also competent in catalyzing the hydrogenation of aldehydes such as benzaldehyde and N-methyl-2-pyrrolecarboxaldehyde, albeit accompanied by decomposition pathways. The catalytic performance can be enhanced through the addition of a strong base or (PNP)-P-iPr-P-H. The three catalytic systems likely share the same catalytically active species, which is proposed to be a mononuclear copper hydride ((R)(PNP)-P-H)CuH with the NH group bound to copper.

Automated summary

This study identified a series of copper borohydride complexes capable of catalyzing the hydrogenation of aldehydes, with some catalyst mixtures showing higher efficiency, although certain functional groups may inhibit the reaction. Modifying the catalyst structure can impact its catalytic activity.