.
Angewandte
Communications
perchlorophenyl groups, Lewis acidity decreases as a result of
[
13]
increasing steric hindrance.
Significantly, and unlike 1a,
these boranes were also found to demonstrate appreciable
stability to air and moisture. Herein we describe investiga-
tions into the behavior of this family of boranes in the donor-
solvent THF, and report the ability of such solutions to
effectively catalyze the hydrogenation of even weakly basic
substrates, using an operationally simple method that does
not require the addition of an auxiliary Lewis base.
Although 1a binds strongly to THF, we envisioned that
the strength of this interaction might be reduced by increasing
steric bulk. Rational modification of the Lewis acid has been
shown to lead to improved functional-group tolerance in
Scheme 2. a) Reversible H activation by B(C Cl )(C F ) in THF and
b) potential hydride abstraction from THF, which is not observed.
2
6
5
6 5 2
[
10,14]
FLP-catalyzed hydrogenation reactions.
Thus B(C Cl )-
6 5
[
13]
(
C F ) (1b), though more electrophilic than 1a, is found to
instead as a result of hydride abstraction from the solvent can
6
5 2
1
1
bind the solvent only weakly when dissolved in neat THF. The
reversibility of the binding is clear from variable-temperature
be discounted based on the observation of the B borohy-
dride resonance signal as a doublet in both proteo and
deutero THF, as well as the lack of any reaction in the absence
(
VT) NMR analysis of THF solutions of 1b; below 08C the
1
1
B NMR shift remains constant at d = 3.8 ppm, consistent
with the four-coordinate 1b·THF adduct (c.f. d = 3.3 ppm for
of H (Scheme 2b). Conclusive evidence is provided by using
2
1
1
D in place of H , which replaces the B doublet at d =
2
2
[15]
1
a·THF in CD Cl ).
Upon warming, however, the reso-
nance signal moves progressively downfield, reaching d =
3.9 ppm at 608C, indicative of a shift in the equilibrium
À19.6 ppm with a singlet at the same shift, and a comparable
2
2
2
+
signal in the H spectrum diagnostic of [THF-D] , or a solvate
thereof (Figure 2).
2
towards free, uncoordinated 1b (c.f. d = 63.6 ppm for free 1b
in PhMe, see Supporting Information). A similar trend is
1
9
observed in the F NMR spectrum over the same temper-
ature range, with the para fluorine resonance signal shifting
from d = À158.0 ppm at 08C (Dd = 7.1 ppm) to d =
m,p
À153.3 ppm (Dd = 10.9 ppm) at 608C. The increased sep-
m,p
aration of the meta and para resonances is consistent with
a move away from four-coordinate and towards three-
[16]
coordinate boron (c.f. Ddm,p = 18.3 ppm for 1b in PhMe).
Based on these results the 1b/THF system can be considered
to be on the borderline between a classical and a frustrated
[
17]
Lewis pair.
THF solutions of B(C Cl ) (C F ) (1c), which is bulkier
6
5
2
6 5
still, show no sign of coordination at all at room temperature
1
1
(
B d = 63.5 ppm, c.f. d = 64.1 ppm in PhMe). Only upon
cooling to À408C do signals consistent with a THF adduct
1
9
become apparent in the F NMR (see Supporting Informa-
tion). We observed no evidence for adduct formation with
B(C Cl ) (1d) in THF between À1008C and 608C.
1
2
Figure 2. H and H NMR spectra of 1b in [D ]THF under H , and in
8
2
1
1
11
1
proteo THF under D , respectively (inset: B and B { H} spectra at
2
À258C).
6
5 3
Admission of H (4 bar) to a THF solution of 1b at room
2
temperature leads to immediate appearance of a resonance
1
signal at d = 11.19 ppm in the H NMR spectrum. Upon
Further evidence for H activation is provided by THF
2
1
cooling to À258C a new doublet (singlet in the H-decoupled
solutions of B(C Cl ) (1d). After heating to 608C for 1h
6
5 3
spectrum) can also be resolved at d = À19.6 ppm in the
under H2 (4 bar), new resonance signals can clearly be
1
1
11
B NMR spectrum (J = 90 Hz). The B NMR data is con-
observed at d = 11.34 ppm and d = À8.7 ppm (d, J =
[8c]
1
11
sistent with previous reports of the borohydride anion
while the new H NMR resonance lies within
the range reported for protonated THF. These results are
91 Hz) in the room temperature H and B NMR spectra,
respectively.
À
[18]
1
[
1b·H] ,
[
19]
Clearly H activation in this manner generates a substan-
2
therefore consistent with reversible H activation by an FLP-
tially acidic proton (the pK of protonated THF has been
2
a
[
22]
type mechanism, with THF acting as the Lewis base
measured as À2.05 in aqueous H SO ). Strong Brønsted
2
4
[
20]
[19b,c]
(
Scheme 2a). Although no resonance signals attributable
acids can initiate polymerization of THF,
as can strong
À
1
[23]
to [1b·H] are apparent in the H NMR spectrum, this can be
Lewis acids, including 1a. Nevertheless, during the course
of our studies no evidence for borane or proton-catalyzed
polymerization of THF was detected for solutions of 1a–d
attributed to line broadening as a result of the quadrupolar
1
0
11
B/ B nuclei, in addition to broadening arising from dynamic
dihydrogen bonding, which may be expected in the Brønsted
[
24]
under H , even after prolonged heating. Nor, during our
2
[
18,21]
À
acidic medium.
The possibility that [1b·H] is formed
subsequent investigations into catalytic hydrogenation, was
2
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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