50 or 40 kJ molϪ1 smaller than the activation energies of the
reactions of P with B–D or B–T, respectively. The reason for
that discrepancy lies in the different complexity of the reaction
sequences. The main part of the overall reaction is determined
for B–D and B–T by the reactions of the pure educts in reac-
tions (R4) and (R1). In the case of the reaction sequence with
CB, however, P is mainly consumed in reaction (R7), a reaction
with the intermediate TB–CB–T, which itself is formed in reac-
tions (R6) and (R8). Thus, the experimental activation energy is
a complex composite of the activation energy of reaction (R7)
with the reaction enthalpies of reaction (R6) and (R8). Since,
according to AM1 calculations, reaction (R6) is exothermic
with ∆H7 = Ϫ95 kJ molϪ1 and since reaction (R8) is almost
thermoneutral a rather small activation energy results for the
overall reaction of P with CB.
NMR spectra
Chemical shifts for NMR spectra are reported as δ in units of
parts per million (ppm) downfield from trimethylsilane (δ 0.0)
using either internal or external standards. All measurements
were carried out on a Bruker AMX 400 NMR-spectrometer
at room temperature. The substances were dissolved in
tetrahydrofuran-d8 (99.8% deuterated), the small remaining
amount of undeuterated tetrahydrofuran was used as standard
1
in the case of H measurements (δ 1.7, singlet and δ 3.6 ppm,
singlet). For the 11B measurements BF3ؒOEt3 in THF was used
as external standard (δ 0 ppm, singlet).
Reagents
Pinacolone (P, 2,2-dimethylbutan-3-one) (Aldrich, 98%) was
distilled under Ar into a Schlenk-flask containing 4 Å molecu-
lar sieves. THF (T) (Riedel-de-Haën, p.a., 99.9%, stabilized by
2,6-di-tert-butyl-4-methylphenol) was dried over Na–K pearls
and distilled under Ar. Borane–dimethyl sulfide (Aldrich) was
cleaned by vacuum transfer into a trap cooled by liquid nitro-
gen.23 Catecholborane (CB, 1,3,2-benzodioxaborole, Aldrich,
98%) was distilled at 50 ЊC (50 Torr) under Ar. Hereby it was
found that the commercial CB was contaminated with surpris-
ingly large (up to 30 wt%) and varying amounts of a white
solid material, which remained as residue after distillation. The
concentration of both the borane reagents was determined
repeatedly by gas volumetry. The handling of all reagents was
Conclusions
The kinetics of the reduction of the ketone pinacolone in THF
depends strongly on the borane reagent and its purity. Com-
mercial B–T and B–D both give rise to different kinetics and a
strongly enhanced reactivity towards the ketone compared with
the purified reagents. The main reactions of purified B–T and
B–D with P are first-order in P and in borane complex. The
reactivity of B–D is by a factor of four smaller than the reactiv-
ity of B–T. The comparison of the results of semi-empirical
calculations with the experimental findings suggests that the
principal reaction, the formation of the monoalkoxyborane
from ketone and B–D, proceeds as a sequence of three reaction
steps. The first step is probably the rate determining: the nucleo-
philic substitution of D in the complex B–D by the ketone P.
The reaction of the ketone with catecholborane proceeds much
slower with very strange reaction kinetics. The reaction order is
two in CB and zero in P. These results indicate that the reactiv-
ity of the monomeric CB towards ketones must be very small.
The complex kinetics can be successfully simulated by numer-
ical integration. The reactive species is a catecholborane dimer
being present in low concentrations. The zero-order kinetics in
ketone points to some autocatalytic character of the reaction.
In fact, the product TB reacts with the dimeric CB species to
form a complex, which rapidly converts the ketone to TB. The
results of this study will be valuable for the interpretation of the
kinetics of the oxazaborolidine catalyzed reduction of ketones
by B–D and CB.
1
carried out under Ar to prevent contact with water. H-NMR
spectrum of purified CB (400 MHz, THF-d8) [CB] = 0.27 mol
dmϪ3: δ 7.25 (m, 2 H), 7.08 (m, 2 H), 3.5–5.5 (q, 1 H).
[CB] = 6.54 mol dmϪ3: δ 7.25 (m, 2 H), 7.08 (m, 2 H), 6.97 (s),
3.5–5.5 (q, 1 H), 2.2–3.3 (q). 11B-NMR spectrum (128 MHz,
THF-d8) [CB] = 6.54 mol dmϪ3: δ 32.5–23.0 (d, 1 B), 21.52 (s),
Ϫ17.4–Ϫ21.0 (q).
Acknowledgements
Financial support by the Adolf Messer Stiftung and the Fonds
der Chemischen Industrie is gratefully acknowledged. We thank
the Chemetall GmbH for gifts of chemicals.
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