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

Abstract

HN(CH₂CH₂PR₂)₂-ligated copper borohydride complexes, (^RPN^HP)Cu(BH₄) (R = ⁱPr, Cy, ^tBu), which can be prepared from (^RPN^HP)CuBr and NaBH₄, are capable of catalyzing the hydrogenation of aldehydes in an alcoholic solvent. More active hydrogenation catalysts are (^RPN^HP)CuBr mixed with KO^tBu, 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 (^iPrPN^HP)CuBr with an NMe group results in an inferior catalyst but preserves some catalytic activity. The hexanuclear copper hydride cluster, (^iPrPN^HP)₃Cu₆H₆, 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 ^iPrPN^HP. The three catalytic systems likely share the same catalytically active species, which is proposed to be a mononuclear copper hydride (^RPN^HP)CuH with the NH group bound to copper.