178
M. Heshmat et al.
P, As, Sb (group 15 atoms) as the Lewis base and Y one of B, Al, Ga, In (Group 13
atoms) as the Lewis acid. They found that the barrier height increases when going
down in group 15 (N < P < As < Sb) regardless of the nature of the Y atom. Hence,
decreasing the basicity of the Me 2 X moiety (N > P > As > Sb) causes later transition
states with shorter Y
… H distances and thus higher barriers of H 2 activation. This
means that the deformation (strain) in the transitions state with respect to the initial
reactant state, which increases in the same order as the activation barrier (N < P < As
< Sb), is the reason for this trend. The influence of the Lewis acidity of the elements
of Group 13 shows the opposite trend, with the barrier decreasing from B to In, with
the exception of Al, which displayed the lowest H 2 activation barrier. The geminal
N/Al FLP was identified as the most active system for H 2 activation. The lowest
barrier for H 2 activation with Al correlates with the strongest interaction energy and
the lowest Pauli repulsion between FLP and H
… H fragments at the transition state.
The Pauli repulsion originates from the repulsion between the occupied orbitals of
the two fragments [49].
Recent investigations have shown that intramolecular FLPs can be more catalytically active than their intermolecular counterparts [50]. Heiden and coworkers
investigated the thermodynamics of hydrogen, hydride, and proton transfer from
P/B intramolecular and intermolecular FLPs. The aim was to determine whether
intramolecular or intermolecular FLPs are preferred in FLP-catalyzed hydrogenation reactions. Thermodynamic investigation of 22 intramolecular phosphoniumborohydrides and their corresponding intermolecular counterparts indicated that
an intramolecular phosphonium-borohydride is the preferred catalyst for reduction of imines and enamines. On the other hand, an intermolecular phosphoniumborohydride is the preferred catalyst for the reduction of ketones and aldehydes
[50].
5.3.2 Transition State and Reaction Kinetics
The mechanistic mode causing the polarization of the H–H bond has been investigated
in a number of theoretical studies leading initially to various proposed hypotheses.
The present consensus mechanism involves the heterolytic splitting of the H–H bond,
and two viable models have been proposed in the literature that aim to explain the
catalytic mode of action of the FLP, namely, the electron transfer (ET) and the electric
field (EF) models. In the ET model, which was proposed by Pápai and coworkers
[51–53], the HOMO of the Lewis base interacts with the σ* orbital of the H–H bond
and the LUMO of the Lewis acid interacts with the σ orbital of the H–H bond. As a
result, the H 2 bond is cleaved into H
+ and H
− . At the molecular scale, the activation
process is based on the interactions between the LB, H 2 , and LA molecules. Thus,
the electronic characteristics of the Lewis base and Lewis acid play crucial roles in
the reaction process. In the EF model, suggested by Grimme and coworkers [54],
the core principle is the polarization of the H 2 molecule by the electric field that is
created by the Lewis base and Lewis acid, causing an almost barrierless H 2 splitting.
M. Heshmat et al.
P, As, Sb (group 15 atoms) as the Lewis base and Y one of B, Al, Ga, In (Group 13
atoms) as the Lewis acid. They found that the barrier height increases when going
down in group 15 (N < P < As < Sb) regardless of the nature of the Y atom. Hence,
decreasing the basicity of the Me 2 X moiety (N > P > As > Sb) causes later transition
states with shorter Y
… H distances and thus higher barriers of H 2 activation. This
means that the deformation (strain) in the transitions state with respect to the initial
reactant state, which increases in the same order as the activation barrier (N < P < As
< Sb), is the reason for this trend. The influence of the Lewis acidity of the elements
of Group 13 shows the opposite trend, with the barrier decreasing from B to In, with
the exception of Al, which displayed the lowest H 2 activation barrier. The geminal
N/Al FLP was identified as the most active system for H 2 activation. The lowest
barrier for H 2 activation with Al correlates with the strongest interaction energy and
the lowest Pauli repulsion between FLP and H
… H fragments at the transition state.
The Pauli repulsion originates from the repulsion between the occupied orbitals of
the two fragments [49].
Recent investigations have shown that intramolecular FLPs can be more catalytically active than their intermolecular counterparts [50]. Heiden and coworkers
investigated the thermodynamics of hydrogen, hydride, and proton transfer from
P/B intramolecular and intermolecular FLPs. The aim was to determine whether
intramolecular or intermolecular FLPs are preferred in FLP-catalyzed hydrogenation reactions. Thermodynamic investigation of 22 intramolecular phosphoniumborohydrides and their corresponding intermolecular counterparts indicated that
an intramolecular phosphonium-borohydride is the preferred catalyst for reduction of imines and enamines. On the other hand, an intermolecular phosphoniumborohydride is the preferred catalyst for the reduction of ketones and aldehydes
[50].
5.3.2 Transition State and Reaction Kinetics
The mechanistic mode causing the polarization of the H–H bond has been investigated
in a number of theoretical studies leading initially to various proposed hypotheses.
The present consensus mechanism involves the heterolytic splitting of the H–H bond,
and two viable models have been proposed in the literature that aim to explain the
catalytic mode of action of the FLP, namely, the electron transfer (ET) and the electric
field (EF) models. In the ET model, which was proposed by Pápai and coworkers
[51–53], the HOMO of the Lewis base interacts with the σ* orbital of the H–H bond
and the LUMO of the Lewis acid interacts with the σ orbital of the H–H bond. As a
result, the H 2 bond is cleaved into H
+ and H
− . At the molecular scale, the activation
process is based on the interactions between the LB, H 2 , and LA molecules. Thus,
the electronic characteristics of the Lewis base and Lewis acid play crucial roles in
the reaction process. In the EF model, suggested by Grimme and coworkers [54],
the core principle is the polarization of the H 2 molecule by the electric field that is
created by the Lewis base and Lewis acid, causing an almost barrierless H 2 splitting.