Hydrogenation of Multiple Bonds by Geminal Aminoborane‐Based Frustrated Lewis Pairs

Abstract

The hydrogenation reaction of multiple bonds that is mediated by geminal aminoborane-based frustrated Lewis pairs (FLPs) has been explored by means of density functional theory calculations. It was found that the release of the activated dihydrogen occurred in a concerted, yet highly asynchronous, manner. The physical factors that control the transformation were quantitatively described in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. This approach suggested a cooperative double hydrogen-transfer mechanism, which involves the initial migration of the protic (N)H followed by the nucleophilic attack of the (B)H hydride to the carbon atom of the multiple bond. The influence of both the substituents directly attached to the boron atom of the initial FLP and the nature of the multiple bond on the transformation was also investigated.

Influence of the Lewis Acid/Base Pairs on the Reactivity of Geminal E‐CH2‐E′ Frustrated Lewis Pairs

“…[12] This methodology has been particularly helpful very recently to understand the factorsc ontrolling both the H 2 activation and the subsequent dihydrogen release into multiple bonds mediatedb yg eminal B/N FLPs. [13,14] Indeed, by means of this state-of-the-art approach we have proposed an orbital-controlled mechanism,c omplementary to the traditional mechanisms suggested by Pµpai et al [15] and Grimme et al, [16] where the degree of charge-transferc ooperativity betweent he key donor-acceptor orbitali nteractions, that is, LP(N)!s * (H 2 )a nd s(H 2 )!p p (B), alongt he reaction coordinate constitutes as uitablei ndicator of the reaction barrier. [13] In addition, ac ooperative concerted, yet asynchronous, double hydrogen transferm echanism was also found for the subsequent hydrogenation of multiple bonds.…”

“…The interaction Δ E int (ζ) between the strained reactants can be further partitioned into chemically meaningful contributions by means of the EDA method . Thus, Δ E int (ζ) is decomposed into the following terms along the reaction coordinate [Eq.…”

“…The computed reactivity trends will be analyzed in detail by means of the activation strain model (ASM) of reactivity in combination with the energy decomposition analysis (EDA) method . This methodology has been particularly helpful very recently to understand the factors controlling both the H 2 activation and the subsequent dihydrogen release into multiple bonds mediated by geminal B/N FLPs . Indeed, by means of this state‐of‐the‐art approach we have proposed an orbital‐controlled mechanism, complementary to the traditional mechanisms suggested by Pápai et al .…”

“…and Grimme et al., where the degree of charge‐transfer cooperativity between the key donor–acceptor orbital interactions, that is, LP(N)→σ * (H 2 ) and σ(H 2 )→p π (B), along the reaction coordinate constitutes a suitable indicator of the reaction barrier . In addition, a cooperative concerted, yet asynchronous, double hydrogen transfer mechanism was also found for the subsequent hydrogenation of multiple bonds …”

“…[13] In addition, ac ooperative concerted, yet asynchronous, double hydrogen transferm echanism was also found for the subsequent hydrogenation of multiple bonds. [14] Theoretical Methods and Computational Details Geometry optimizations of the molecules were performed without symmetry constraints by using the Gaussian 09 suite of programs [17] employing the metahybrid M06-2X exchange-correlation functional [18] combined with the triple-z quality def2-TZVPP basis set. [19] Reactants and products were characterized by frequency calculations and have positive definite Hessian matrices.…”