the presence of two equivalents of HBpin. The bis(borate
ester) showed poor solubility in C6D6 and crystallised over
the course of the reaction. An X-ray diffraction experiment
was performed on crystals of the (R,S)/(S,R) diastereomer,
the results of which demonstrated unambiguously that the
carbonyl double bonds had been reduced (see Supplementary
Informationw).
of these reactions and will report our findings in subsequent
publications.
Notes and references
y X-Ray diffraction data for 4. C61H75BMgN2O4, M = 935.35,
orthorhombic, Pbca, a = 19.2649(2) A, b = 20.4622(3) A, c =
26.9490(4) A, V = 10623.4(2) A3, Z = 8, r = 1.170 g.cm3, Temperature
150(2) K, R1 [I > 2s(I)] = 0.0543, wR2 [I > 2s(I)] = 0.1281,
R1 [all data] = 0.0893, wR2 [all data] = 0.1465, measured reflections =
115166, unique reflections = 9691, Rint = 0.0914.
Comparison of the reactivity of acetophenone and 20,40,60-
trimethylacetophenone once again highlighted the importance
of steric factors in these hydroboration reactions (Table 2,
entries 5 and 6). Reduction of aliphatic ketones such as
indanone and 5-hexen-2-one with HBpin also proceeded with
equal efficiency (Table 2, entries 7 and 8). In the case of the
latter enone substrate no hydroboration of the alkene moiety
was observed, an observation which is consistent with the
results of a recently reported attempt to carry out the calcium-
catalysed hydroboration of alkenes.11 In this latter case, any
observed reactivity of 1,1-diphenylethylene with HBpin was
reasoned to be a consequence of a group 2-centred redistribu-
tion to BH3 and uncatalysed olefin addition rather than an
alkaline earth-catalysed hydroboration process.11 For all of
the current cases, the reactions were readily scaled up to allow
isolation of the resulting primary and secondary alcohols in
good yields after acid hydrolysis. It is noteworthy that in many
cases the intermediate borate esters proved surprisingly air and
moisture stable and could only be hydrolysed under forcing
conditions (see Supplementary Informationw).
In summary we have demonstrated that the easily prepared
and inexpensive magnesium alkyl 1 may be applied to the
efficient hydroboration of a variety of aromatic and aliphatic
aldehydes and ketones under extremely mild conditions and at
low catalyst loadings. Catalytic turnover most likely proceeds
via formation of a catalytically active magnesium hydride
species, insertion of the carbonyl moiety into the Mg–H bond
and subsequent s-bond metathesis with pinacolborane. We
are continuing to study the scope, selectivity and mechanism
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Soc., 1939, 61, 673. For a general survey of progress in this area,
see: (b) Catalytic Heterofunctionalisation, ed. A. Togni and
H. Grutzmacher, Wiley-VCH, 2001.
¨
2 (a) J. F. Carpentier and V. Bette, Curr. Org. Chem., 2002, 6, 913;
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of aldehydes and ketones see: B. T. Cho, Tetrahedron, 2006,
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5 C. W. Lindsley and M. Dimare, Tetrahedron Lett., 1994, 35,
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6 L. Koren-Selfridge, H. N. Londino, J. K. Vellucci, B. J. Simmons,
C. P. Casey and T. B. Clark, Organometallics, 2009, 28, 2085.
7 For recent reviews of group 2 catalysis see: (a) A. G. M. Barrett,
M. R. Crimmin, M. S. Hill and P. A. Procopiou, Proc. R. Soc.
London, Ser. A, 2010, 466, 927; (b) S. Harder, Chem. Rev., 2010,
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8 M. Arrowsmith, M. S. Hill, T. Hadlington, G. Kociok-Kohn and
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C. Weetman, Organometallics, 2011, 30, 5556.
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10 J. Spielmann and S. Harder, Eur. J. Inorg. Chem., 2008, 1480.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4567–4569 4569