Activation of sodium borohydride via carbonyl reduction for the synthesis of amine- and phosphine-boranes

Abstract

A highly versatile synthesis of amine-boranes via carbonyl reduction by sodium borohydride is described.

Scalable, Green Synthesis of Heteroaromatic Amine-boranes

“…This mechanistic insight of CO 2 reduction and ligand exchange guided successful attempts to synthesize amineborane using other carbonyl sources. 37 Both aldehydes and ketones were found to be efficient activators of NaBH 4 , producing monoalkoxyborohydride intermediates which form amine-boranes after displacement of the alkoxy group. This observation raised the possibility of using alcohols to form the same/similar intermediates.…”

“…In the earlier mentioned work investigating the mechanism of amine-borane formation via CO 2 reduction by NaBH 4 , 37 it was determined that product formation proceeded by nucleophilic displacement of a ligand (Scheme 1B). It is proposed that the activators being studied here will promote the reaction by the same mechanism, with the displaced ligand determined by the activator in each case.…”

“…In a recent report, we described a protocol for the synthesis of amine-boranes using NaBH 4 , carbon dioxide (CO 2 ), and the chosen amine . During our re-examination of this long-known route − to amine-boranes, it was determined that the reaction facilitated by CO 2 was occurring through exchange of the formyl ligand of the intermediate monoformatoborohydride with the present amine.…”

“…Compound characterization is in accordance with previous reports. 37 4.6.4. N-Methylbenzylamine-borane (1d).…”

“…Many methods for the preparation of amine-boranes have been reported. These approaches can be divided into three main categories: (i) direct reaction of the amine with diborane generated either ex or in situ (Scheme A), (ii) nucleophilic displacement of a ligand/Lewis base from another borane complex (Scheme B), ,− and (iii) salt metathesis of a tetraalkylammonium or metal borohydride with the corresponding ammonium salt and tandem dehydrogenation (Scheme C). ,, …”