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