Transfer Hydrogenation of Imines with Ammonia–Borane: A Concerted Double‐Hydrogen‐Transfer Reaction

Abstract

Direct transfer hydrogenation of imines was observed with ammonia–borane, which proceeded under mild conditions without the help of a catalyst. The results of deuterium kinetic isotope effects, Hammett correlations, and DFT calculations all support a concerted double‐hydrogen‐transfer mechanism (see scheme).

Theoretical study on the mechanism for NH3BH3reduction of ketones and imines

“…As reported by Becke and co-workers, the hydrogenation of imine with AB followed the DHT process, and the energy barrier was 16.9 kcal/mol lower than the stepwise process [26]. The DHT mechanism, the stepwise mechanisms discussed [26], and the stepwise mechanism similar to that shown in Scheme 3 for ketone were all re-evaluated here using benzylidene aniline as substrate, and the results are shown in Figure 8.…”

“…In the following when we mention stepwise pathway, we mean the new stepwise pathway as shown in Scheme 3. The stepwise pathways featured by direct B-H and N-H addition have not been re-evaluated here, since they have been found to have much higher barriers than the DHT pathway before [26]. As mentioned before, NH 3 BH 3 can decompose to NH 3 and BH 3 , which afford new possible pathways featured by NH 3 and BH 3 addition to ketone.…”

“…Previous theoretical work on AB and LiAB hydrogenation, including Berke's and ours work [26,40], were based on DFT calculations, and Berke used M06-2x while we used B3LYP functional. In the work of LiNH 2 BH 3 hydrogenation [7] we have validated the reliability of different DFT methods for the LiNH 2 BH 3 decompose step by comparing the results with CCSD(T) calculations, and we find that both B3LYP and M06-2x produce reliable results.…”

“…The DHT mechanism, the stepwise mechanisms discussed [26], and the stepwise mechanism similar to that shown in Scheme 3 for ketone were all re-evaluated here using benzylidene aniline as substrate, and the results are shown in Figure 8. The stepwise mechanism is the one similar to that shown in Scheme 3 for ketone, and its ratedetermined barrier is higher than that for the DHT process by 5.1 kcal/mol.…”

“…ABs have hydridic H(B) and protic H(N) co-existing in the molecule [7,8], and the dehydrogenation of AB can be carried out through thermal [9][10][11][12][13][14][15], acid-catalysed [16] or metal-catalysed [17][18][19][20][21][22][23][24] methods. In addition to being taken as potential hydrogen storage materials, recently it was found that AB is also a good hydrogenation reagent which is able to hydrogenate polar unsaturated bond under mild conditions [25][26][27][28].The pioneer work of this field was done by Berke and co-workers [26] who found that AB was able to reduce the C=N bond of aromatic imines to produce amines under mild conditions. Joint research between theory and experiment suggested that the reaction underwent concerted double hydrogen transfer (DHT) mechanism (Scheme 1).…”