Theoretical study on the mechanism for NH₃BH₃reduction of ketones and imines

Abstract

In spite of a potential hydrogen storage material, ammonia borane (AB) was recently found to be a good hydrogenation reagent. It can reduce certain ketones to alcohols or borate esters, and imines to amines. The mechanisms of these reactions are not fully understood yet, and have been systematically studied using high-level CCSD(T) calculations in this work. We have validated theoretically that the forming of alcohols and amines undergoes concerted double-hydrogen transfer (DHT) mechanism. Furthermore, we predicted that the DHT process is facile for more general ketones and imines. For the borate ester formation, we found a pretty high barrier for the experimentally derived stepwise mechanism. Alternatively, we propose that the reaction starts with the DHT process to form alcohol and NH 2 BH 2 , followed by alcoholysis of NH 2 BH 2 to form the first B-O bond. This mechanism is in good agreement with the current experimental facts, and also explains why ketone reduction affords different products at different conditions. For these reaction systems, the performances of M06-2x and MP2 (underestimate the barrier by 5-7 kcal/mol, but with right trends) are better than B3LYP and BLYP methods (underestimate the barrier by 0-5 kcal/mol).Keywords: reaction mechanism; ammonia borane; hydrogenation; ketone; amine IntroductionAmmonia borane (NH 3 BH 3 , AB for short) is attracting more and more attention because of its potential 19.6 wt% hydrogen storage capacity [1-6]. 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). Later on, they found that AB was also able to reduce polarised C=C bond to yield alkane [25], as well as reduce C=O bond to form B(OR) 3 complexes [28]. Stepwise pathways have been proposed for these two reactions based on the experimental observations. The proposed pathway for C=O reduction is shown in Scheme 2, it starts with the coordination of BH 3 to C=O bond, followed by B-H addition to afford the BH 2 (OR) intermediate. The next steps are the B-H addition to the second and the third ketone molecules.

A Concerted Transfer Hydrogenolysis: 1,3,2‐Diazaphospholene‐Catalyzed Hydrogenation of NN Bond with Ammonia–Borane

“…Importantly, the hydrogen transfer from AB to imines was proven to follow a concerted mechanism involving a six‐membered transition state (Scheme ) 6d. 7 An analogous mechanism was also predicted for the process of Meerwein–Pondorf–Verley reduction 8. Similarly, Manners et al.…”

Dehydrogenation of Amine–Boranes Using p‐Block Compounds

“…This mechanism has been contested, however. The group of Zhou and Fan performed a theoretical study on the mechanism of ketone reduction by NH 3 ⋅BH 3 , which suggested that ketones can also undergo a concerted double hydrogen transfer via transition state 10 , similar to imines (Scheme ) . This process was found to be lower in energy compared with the initially proposed hydroboration mechanism by Berke .…”

Real-Time Monitoring of Aqueous Organic Reduction Reactions Using Ex Situ Fiber Optic Raman Spectroscopy

“…Pathway C involves a concerted transfer of a hydride from DMAB to the carbonyl carbon atom . Theoretical calculations, for the hydroboration of carbonyls with ammonia borane in THF and methanol, have predicted a concerted double hydrogen atom transfer. , If this double hydrogen atom transfer was to occur with aqueous DMAB, we would expect to observe the cyclic dimer of dimethylamino-borane [(Me 2 N-BH 2 ) 2 ] in the 11 B NMR spectrum (δ 5, t, J = 114 Hz). , Additionally, this cyclic dimer has been shown to be an inactive reducing agent in the absence of a catalyst . Since the reduction of cyclohexanone with DMAB, in 3:1 stoichiometry, afforded cyclohexanol essentially quantitatively and no (Me 2 N-BH 2 ) 2 signal was observed in the 11 B NMR spectrum, it was concluded that the double hydrogen atom transfer was not a viable pathway for this aqueous reduction reaction.…”

“…55 Theoretical calculations, for the hydroboration of carbonyls with ammonia borane in THF and methanol, have predicted a concerted double hydrogen atom transfer. 61,62 If this double hydrogen atom transfer was to occur with aqueous DMAB, we would expect to observe the cyclic dimer of dimethylamino-borane [(Me 2 N-BH 2 ) 2 ] in the 11 B NMR spectrum (δ 5, t, J = 114 Hz). 63,64 Additionally, this cyclic dimer has been shown to be an inactive reducing agent in the absence of a catalyst.…”