Communications
ability of the MnII aqua complex in the manganese cluster of
[8] a) A. Gansꢄuer, M. Pierobon, H. Bluhm, Angew. Chem. 1998,
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[9] For recent reviews, see: a) J. M. Cuerva, J. Justicia, J. L. Oller-
Lꢀpez, B. Bazdi, J. E. Oltra, Mini-Rev. Org. Chem. 2006, 3, 23 –
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photosystem II,[22] or the highly exothermic generation of
hydrogen by the reaction between sodium and water. In fact,
À
our calculations of the bond-dissociation energy of the O H
bond in the aqua complexes [Cp2ZrIII(Cl)OH2] and
[Cp2HfIII(Cl)OH2] provided values of only 25.5 and 13.7kcal
molÀ1, respectively. These low values strongly support the
hypothesis that hydrogen-atom transfer from water might be
mediated by different metals capable of single-electron
transfer.
In summary, we have presented theoretical and exper-
imental evidence that indicates that water can act as an
efficient hydrogen-atom source in radical reductions medi-
ated by TiIII complexes and, presumably, other metals that
react by single-electron transfer. Moreover, this property can
be exploited advantageously for the reductive opening of
epoxides to provide a mild, safe, inexpensive two-step
procedure (complementary to the hydroboration–oxidation
method) for the preparation from alkenes of anti-Markovni-
kov alcohols, even in deuterated form and/or enantioselec-
tively. We are currently studying the hydrogen-atom-transfer
ability of other aqua complexes with different transition
metals.
[10] The quest for novel hydrogen-atom donors for titanocene-
mediated reductive epoxide opening has been the focus of recent
research; see: A. Gansꢄuer, A. Barchuk, D. Fielenbach, Syn-
thesis 2004, 2567– 2573.
[11] The results depicted in Scheme 2 suggest that the formation of
the minor product 4 and an equimolar amount of 2 (6%) might
be the result of a radical–radical disproportionation process. In
this aspect, our reaction differs from the reduction of tertiary
radicals by SmI2, in which, in the absence of any proton source,
radical–radical disproportionation products were the main
components of the mixtures obtained; see: T. Nagashima, A.
Rivkin, D. P. Curran, Can. J. Chem. 2000, 78, 791 – 799.
[12] The relatively low deuterium incorporation in 14 compared with
that in 15 is presumably due to the increased steric hindrance
introduced in the radical 18 by the cis disposition of the C14
methyl group relative to the adjacent primary radical. This steric
hindrance impedes the corresponding radical coupling with
bulky [Cp2TiCl].
Received: March 3, 2006
Published online: July 19, 2006
[13] W. A. Pryor, K. G. Kneipp, J. Am. Chem. Soc. 1971, 93, 5584 –
5586.
Keywords: epoxides · hydrogen transfer · radicals · titanium ·
water
.
[14] Moreover, epoxide 1 was recovered unchanged after treatment
with H2O, MnCl2, and Mn dust in the absence of [Cp2TiCl2].
[15] D. A. Spiegel, K. B. Wiberg, L. N. Schacherer, M. R. Medeiros,
J. L. Wood, J. Am. Chem. Soc. 2005, 127, 12513 – 12515.
[16] Similar double-bond character was observed in the crystal
structure of [Cp2Ti(OH)Cl]; see: H. D. Gibson, Y. Ding, M. S.
Mashuta, J. F. Richardson, Acta Crystallogr. Sect. C 1996, 52,
559 – 560.
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[2] B. Ruscic, A. F. Wagner, L. B. Harding, R. L. Asher, D. Feller,
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[3] The single-electron-transfer reagent bis(cyclopentadienyl)-
titanium(III) chloride can be generated in situ by stirring
commercial [Cp2TiCl2] with Mn dust in THF, where it exists as
a mixture of the mononuclear [Cp2TiCl] and the dinuclear
[(Cp2TiCl)2] species; see: R. J. Enemærke, J. Larsen, T.
Skrydstrup, K. Daasbjerg, J. Am. Chem. Soc. 2004, 126, 7853 –
7864; for the sake of clarity, we represent this complex herein as
[Cp2TiCl].
[17] W. Carruthers, I. Coldham, Modern Methods of Organic Syn-
thesis, 4th ed., Cambridge University Press, Cambridge, 2004,
pp. 337– 346.
[18] We tested the reaction with different quantities of water and
found that for 2.2 equivalents of [Cp2TiCl] the best yields of the
alcohol were observed with 40 equivalents of H2O. Detailed
experimental procedures and spectroscopic data for all products
described herein can be found in the Supporting Information.
[19] Z.-X. Wang, Y. Shi, J. Org. Chem. 1998, 63, 3099 – 3104.
[20] Note that alkene epoxidation followed by [Cp2TiCl]/H2O-
mediated epoxide opening might provide a useful (and comple-
mentary) alternative to the hydroboration–oxidation method.
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Angew. Chem. Int. Ed. 2006, 45, 5522 –5526