176
M. Heshmat et al.
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
H -
H +
+
50 < T (C) < 100
5 < P (bar) < 100
OR 2
B
R 2 O
C 6 F 5
C 6 F 5
C 6 F 5
H -
H + OR 2
B
R 2 O
C 6 F 5
C 6 F 5
C 6 F 5
H -
H + O
2
O
2
H - transfer
+
BCF
Scheme 5.8 The solvent-mediated FLP mechanism of carbonyl hydrogenation with H 2 in the
presence of B(C 6 F 5 ) 3 . OR 2 refers to the ethereal solvent molecule
due to the entropic penalty of formation of the LA
… LB molecular complex, the free
energy of association is slightly positive, i.e., 0–5 kcal mol
−1 [34, 37]. The endergonic molecular encounter complex formation of the Lewis acid and base points to
a low probability of the associated complex in solution.
When the substituent groups connected to the reactive centers are too bulky and
the repulsive interactions between the two components are too large, the distance
between the centers is long and consequently, the FLP is not reactive toward H 2
cleavage [34, 35]. On the other hand, the complexation between LA and LB can also
be the result of a covalent interaction between the LA/LB fragment. This leads to
the formation of a dative-bond (Lewis-adduct), e.g., in case of ethereal O-based LBs
with borane derivatives. These are called thermally hindered FLPs because the LA–
LB adduct requires a higher temperature to dissociate the LA–LB bond and allow
for the H 2 activation reaction [28–30].
Overall, the bulkiness, the deformation energy in the structures of LA/LB, and
the electrophilicity of the LA are the dominant factors that determine the type of the
initial encounter complex of LA/LB, being a VdW-adduct or a Lewis-adduct. The
interaction energies between the two LA/LB fragments in Lewis- and VdW-adducts
are typically within the same order of magnitude [32]. However, the distance between
the reactive centers can vary dramatically from 1.6 Å to 6.0 Å. The impact of structural
and electronic properties of LA and LB on the initial complexation has been the topic
of several studies. The results indicate that the most active LA/LB combinations for
M. Heshmat et al.
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
B
OR 2
C 6 F 5
C 6 F 5
C 6 F 5
H -
H +
+
50 < T (C) < 100
5 < P (bar) < 100
OR 2
B
R 2 O
C 6 F 5
C 6 F 5
C 6 F 5
H -
H + OR 2
B
R 2 O
C 6 F 5
C 6 F 5
C 6 F 5
H -
H + O
2
O
2
H - transfer
+
BCF
Scheme 5.8 The solvent-mediated FLP mechanism of carbonyl hydrogenation with H 2 in the
presence of B(C 6 F 5 ) 3 . OR 2 refers to the ethereal solvent molecule
due to the entropic penalty of formation of the LA
… LB molecular complex, the free
energy of association is slightly positive, i.e., 0–5 kcal mol
−1 [34, 37]. The endergonic molecular encounter complex formation of the Lewis acid and base points to
a low probability of the associated complex in solution.
When the substituent groups connected to the reactive centers are too bulky and
the repulsive interactions between the two components are too large, the distance
between the centers is long and consequently, the FLP is not reactive toward H 2
cleavage [34, 35]. On the other hand, the complexation between LA and LB can also
be the result of a covalent interaction between the LA/LB fragment. This leads to
the formation of a dative-bond (Lewis-adduct), e.g., in case of ethereal O-based LBs
with borane derivatives. These are called thermally hindered FLPs because the LA–
LB adduct requires a higher temperature to dissociate the LA–LB bond and allow
for the H 2 activation reaction [28–30].
Overall, the bulkiness, the deformation energy in the structures of LA/LB, and
the electrophilicity of the LA are the dominant factors that determine the type of the
initial encounter complex of LA/LB, being a VdW-adduct or a Lewis-adduct. The
interaction energies between the two LA/LB fragments in Lewis- and VdW-adducts
are typically within the same order of magnitude [32]. However, the distance between
the reactive centers can vary dramatically from 1.6 Å to 6.0 Å. The impact of structural
and electronic properties of LA and LB on the initial complexation has been the topic
of several studies. The results indicate that the most active LA/LB combinations for