D.B.G. Williams, M. Lawton / Journal of Molecular Catalysis A: Chemical 317 (2010) 68–71
71
4
. Conclusions
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The work discussed in this paper points away from direct acetate
5
abstraction from retinyl acetate by the Al entity as the mech-
anism of retinyl carbocation formation. Rather, it indicates that
induced Brønsted acidity through Al-bound water is causative. This
is despite the relatively high hydrolysis constant of Al(III) [23],
8
(
(
b) R. Dumeunier, I.E. Markó, Tetrahedron Lett. 45 (2004) 825;
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which would lead to the supposition that Al(OTf) readily hydroly-
3
(
c) S.V. Bhilare, N.B. Darvatkar, A.R. Deorukhkar, D.G. Raut, G.K. Trivedi, M.M.
ses to HOTf – but the experimental evidence [[1–4,6], present study]
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in the present instance, Al(OTf)3 is not only water tolerant but is
water-dependent for its activity. It is possible that other reactions
performed making use of metal triflates also proceed via this mech-
anism of induced water-derived Brønsted acidity (Lewis-assisted
Brønsted acidity [9,15]) acting as the active catalyst. The results
highlight the possibility that a variety of metal triflate-promoted
organic transformations that proceed via cationic intermediates
may be catalysed (or co-catalysed) by water-derived Lewis-assisted
Brønsted acidity rather than directly or exclusively by Lewis acidity.
Such (co)catalysis and activation of water is well-known in bio-
chemistry and is one of the activation mechanisms in Zn-containing
hydrolytic enzymes [24] and their synthetic biomimetics [25].
Finally, the discussion here and elsewhere [8] highlights the need to
carefully consider the role of hindered pyridine bases in assessing
the nature of the active catalyst and its mode of action.
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(
(
[
[
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[
[
[
[
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a˘ rca s¸ iu, R. Leu, A. Corma, J. Phys. Chem. B 106 (2002) 928.
Acknowledgements
[19] D. F
[
[
20] C.F. Barnasconi, D.F. Carré, J. Am. Chem. Soc. 101 (1978) 2707.
21] D.R. Burfield, K.-H. Lee, R.H. Smithers, J. Org. Chem. 42 (1977) 3060.
It has been shown, for example, that acetonitrile dried from various desiccants at
5% w/v loading contains residual water as follows: CaH2 (1900 ppm), 3 Å molecular
sieves (49 ppm), 4 Å molecular sieves (450 ppm).
22] L.H. Doerrer, M.L.H. Green, J. Chem. Soc., Dalton Trans. (1999) 4325.
23] (a) C.F. Baes Jr., R. Mesmer, The Hydrolysis of Cations, Wiley, New York,
1976;
We gratefully acknowledge funding from the University of
Johannesburg, Sasol Ltd., THRIP, and the NRF. We are indebted to
[
[
Appendix A. Supplementary data
(
b) K.B. Yatsimirksii, V.P. Vasil’ev, Instability Constants of Complex Compounds,
Pergamon, New York, 1960.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2009.10.023.
[
24] (a) G. Schürer, T. Clark, R. van Eldik, in: E. Rappoport, E. Marek (Eds.), The Chem-
istry of Organozinc Compounds, J. Wiley and Sons, 2006, pp. 1–29, chapter 1,
and references cited therein.;
(
b) B.L. Valle, D.S. Auld, Biochemistry 29 (1990) 5647, and references cited
therein;
c) J. Weston, Chem. Rev. 105 (2005) 2151, and references cited therein.
[25] P. Molenveld, J.F.J. Engbersen, D.N. Reinhoudt, Chem. Soc. Rev. 29 (2000) 75.
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