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Scheme 1 Iron-catalysed hydrogermylation of styrene.
The developed methodology was also applied to the hydrobora-
tion of alkynes. (Z)-Vinyl boronic esters 5a and 5b were stereo-
selectively synthesised in excellent yield within one hour, with
no observed anti-addition of pinacolborane, representing the
most active and stereoselective iron catalyst reported for the
hydroboration of alkynes.
Finally, the developed methodology was applied to the
hydrogermylation of styrene using commercially available
triethylgermanium hydride, giving the linear hydrogermylation
product 6 in 86% isolated yield and with complete control of
regiochemistry (Scheme 1). To the best of our knowledge, this is
the first example of an iron-catalysed alkene hydrogermylation,22
but more significantly illustrates the generality of this iron
catalyst in the activation of small molecules, and indicates the
potential for further synthetic applications.
In summary, we have reported a highly functional group
tolerant, operationally simple, chemo-, regio- and stereo-
selective iron-catalysed hydroboration of alkenes and alkynes,
which uses just 1 mol% iron catalyst [FeCl2 (1 mol%), ligand 1
(1 mol%)] and 1.1 equivalents of pinacolborane at room tem-
perature. All reagents used were commercially available, easy to
handle and store, and the active iron catalyst was generated
in situ. Terminal, 1,1- and 1,2-disubstituted aryl and alkyl
alkenes and alkynes bearing an unprecedented diversity of func-
tional groups were successfully hydroborated with excellent
control of all aspects of selectivity (chemo, regio and stereo-
chemistry). The methodology was shown to operate under
‘solvent-free’ conditions and on gram-scale, improving indus-
trial applicability and the ease of product isolation. Preliminary
mechanistic experiments suggest that an iron(I) catalyst may be
formed under the reaction conditions. The use of an in situ
generated iron catalyst greatly simplifies practical require-
ments, and should allow the non-expert to fully utilise this
synthetic methodology.
17 In situ complexation of FeCl2 and 1 in toluene or neat alkene not
possible due to the low solubility of the Fe(II) complex.
18 See ESI‡ for further details.
M.D.G. thanks the University of Edinburgh for the provision
of a studentship. S.P.T. thanks the Royal Society for generous
funding.
19 For recent examples of Fe(I) in cross-coupling reactions see:
¨
(a) J. Kleimark, A. Hedstrom, P.-F. Larsson, C. Johansson and P.-O.
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Norrby, ChemCatChem, 2009, 1, 152; (b) A. Hedstrom, U. Bollmann,
J. Bravidor and P.-O. Norrby, Chem.–Eur. J., 2011, 17, 11991;
¨
(c) J. Kleimark, P.-F. Larsson, P. Emany, A. Hedstrom and
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This journal is The Royal Society of Chemistry 2013