ORGANIC
LETTERS
2012
Vol. 14, No. 17
4540–4543
Silica Gel Promotes Reductions of
Aldehydes and Ketones by N‑Heterocyclic
Carbene Boranes
Tsuyoshi Taniguchi and Dennis P. Curran*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260,
United States
Received July 19, 2012
ABSTRACT
N-Heterocyclic carbene boranes (NHC-boranes) such as 1,3-dimethylimidazol-2-ylidine trihydridoborane (diMe-Imd-BH3) serve as practical
hydride donors for the reduction of aldehydes and ketones in the presence of silica gel. Primary and secondary alcohols are formed in good yields
under ambient conditions. Aldehydes are selectively reduced in the presence of ketones. One, two, or even all three of the boron hydrides can be
transferred. The process is attractive because all the components are stable and easy to handle and because both the reaction and isolation
procedures are convenient.
The chemistry of N-heterocyclic carbene boranes (NHC-
boranes) has expanded rapidly in recent years along two
lines.1 On the one hand, much new main group chemistry
has appeared with unusual (but often easily accessible)
NHC-boranes featured both in stable bonding patterns
and in highly reactive intermediates.2 On the other
hand, NHC-boranes are emerging as attractive reagents
for radical,3 pericyclic,4 and organometallic reactions5 and
as co-initiators for polymerizations.6
We overlooked the ionic chemistry of NHC-trihydrido-
boranes (NHC-BH3) for a time because they are so stable.
Unlike other common boron reductants (boranes or boro-
hydride salts7), NHC-trihydridoboranes are typically
white solids that are stable to air, water, and even chro-
matography.1 The carbene and borane are tightly bound
and resist dissociation.
(1) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank,
^
L.; Malacria, M.; Lacote, E. Angew. Chem., Int. Ed. 2011, 50, 10294–
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In 2009, we discovered that the relatively hindered
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene borane
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12337. (b) Braunschweig, H.; Chiu, C.-W.; Kupfer, T.; Radacki, K.
Inorg. Chem. 2011, 50, 4247–4249. (c) Monot, J.; Solovyev, A.; Bonin-
(6) (a) Tehfe, M.-A.; Monot, J.; Malacria, M.; Fensterbank, L.;
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(b) Lane, C. F. Aldrichimica Acta 2005, 38, 43–45. (c) Zhenjiang, L.
Synlett 2005, 182–183. (d) Chen, G. In Organometallics: Boron Com-
pounds; Kaufmann, D. E., Matteson, D. S., Eds.; Georg Thieme Verlag:
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(5) Monot, J.; Makhlouf Brahmi, M.; Ueng, S.-H.; Robert, C.;
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r
10.1021/ol302010f
Published on Web 08/16/2012
2012 American Chemical Society