On the Possibility of Conversion of Alcohols to Ketones and Aldehydes by Phosphinoboranes R₂PBR′R′′: A Computational Study

Abstract

Can phosphinoboranes promote hydrogenation of carbonyl moieties? By means of B3LYP and MPW1K density functional calculations the likelihood of the oxidation of alcohols by phosphinoboranes R(2)PBR'(2) (1) was explored. As a proof-of-principle, a theoretical study that tests the reversibility of the alcohol oxidation is reported. The potential of 1 as a metal-free hydrogenation mediator is discussed for a series of hydrogen sources such as primary and secondary alcohols.

Dehydrogenation of amine–boranes with a frustrated Lewis pair

“…26,27 To our knowledge, however, no dehydrogenation reactions have been reported using these systems, 28 although calculations suggest such reactions should be possible. 29 Herein we report that the simple frustrated Lewis pair P t Bu 3 /B(C 6 F 5 ) 3 rapidly and cleanly dehydrogenates amine-boranes at room temperature, with concomitant formation of the phosphonium borohydride salt [ t Bu 3 PH][HB(C 6 F 5 ) 3 ] (Scheme 1).Initial experiments were carried out with Me 2 NHÁBH 3 , a common model for NH 3 ÁBH 3 which has desirable solubility properties and generally releases only 1 equivalent of H 2 . The frustrated Lewis pair P t Bu 3 /B(C 6 F 5 ) 3 was formed according to literature procedures in C 6 D 5 Cl, 22 and no discernable reaction was observed by NMR.…”

“…26,27 To our knowledge, however, no dehydrogenation reactions have been reported using these systems, 28 although calculations suggest such reactions should be possible. 29 Herein we report that the simple frustrated Lewis pair P t Bu 3 /B(C 6 F 5 ) 3 rapidly and cleanly dehydrogenates amine-boranes at room temperature, with concomitant formation of the phosphonium borohydride salt [ t Bu 3 PH][HB(C 6 F 5 ) 3 ] (Scheme 1).…”

Encumbering the intramolecular π donation by using a bridge: A strategy for designing metal-free compounds to hydrogen activation

“…At the current theoretical level, the predicted activation energies (∆E ≠ ) of 6 and 7 in the gas phase, 26.4 and 28.3 kcal/mol, respectively, are larger than 17.4 kcal/mol of 5. Note that the previous calculations at the MPW1K/6-31++G** and ONIOM2-MPW1K predicted the activation energy of 6 to be 28 [35] and 22 kcal/mol [44] , respectively. The predicted activation free energies (∆G ≠ ) of 6 and 7, 34.8 and 35.3 kcal/mol, respectively, are also larger than 26.1 kcal/mol of 5.…”

“…The combinations of various bases and acids, as well as the various coupling ways of the two components, have led to the various FLPs, which can mainly be categorized into P···B [9,11,14,[20][21][22], N···B [17][18][19], and stable carbene···B FLPs [15,16]. In addition to hydrogen activation, some FLPs have been demonstrated to be able to function as hydrogenation catalysts to reduce imines [17,[23][24][25][26]32], nitriles [23,24], enamines [17,26], and silyl enol ethers [27,32], experimentally, or imines [33,34] and ketones [35], computationally. On the other hand, we noted that, while R 2 PB(C 6 F 5 ) 2 has been demonstrated to be capable of activating hydrogen, the hydrogen activation ability of the compounds featured by a B-N bond, (C 6 F 5 ) 2 BR(R=NC 4 H 4 and NC 4 H 8 ), synthesized by groups who are active in metal-free hydrogen activation, was not reported [36].…”

“…These factors may include (1) finding the best match between the bridge length and the distance between donor and acceptor centers; (2) applying the approach to other combinations of electron donors and acceptors; and (3) finding optimal bulky groups for more realistic systems. Recently, the experimental [17,[23][24][25][26][27]32] and theoretical calculations [33][34][35] have shown some of the FLPs can be used as metal-free hydrogenation catalysts. According to the present study, we speculate that 5 could be a promising hydrogenation catalyst.…”