ChemComm
Communication
these five-membered rings are not aromatic but best described
as an isolated CQC double bond along with a carbenium ion
stabilised by two oxygen atoms.
NSERC of Canada is thanked for financial support. D.W.S. is
grateful for the award of a Canada Research Chair. M.M.H. is
grateful to the Fonds der Chemischen Industrie for a Chemiefonds
scholarship and the Studienstiftung des deutschen Volkes. We
thank Dr Matthias Rudolph (Heidelberg) for helpful discussions.
Notes and references
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2 (a) J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., Int.
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D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396; (d) M. A. Dureen,
C. C. Brown and D. W. Stephan, Organometallics, 2010, 29, 6422;
(e) C. Jiang, O. Blacque and H. Berke, Organometallics, 2010, 29, 125;
( f ) T. Voss, C. Chen, G. Kehr, E. Nauha, G. Erker and D. W. Stephan,
¨
Chem. – Eur. J., 2010, 16, 3005; (g) C. Chen, R. Frohlich, G. Kehr and
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G. Erker, Chem. Commun., 2010, 46, 3580; (h) R. Liedtke, R. Frohlich,
G. Kehr and G. Erker, Organometallics, 2011, 30, 5222; (i) T. Voss,
¨
T. Mahdi, E. Otten, R. Frohlich, G. Kehr, D. W. Stephan and G. Erker,
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3 R. L. Melen, M. M. Hansmann, A. J. Lough, A. S. K. Hashmi and
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Scheme 2 Proposed mechanisms of reactions of propargyl esters with
B(C6F5)3.
4 M. M. Hansmann, R. L. Melen, F. Rominger, A. S. K. Hashmi and
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5 H. Perst, Sci. Synth., 2001, 11, 13.
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Fig. 3 Possible delocalisation within the dioxolium ring.
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within the dioxolium ring. Bond orders and partial charges
indicated that the description as a carbenium ion stabilised by
two oxygen atoms along with an endocyclic CQC double bond is
appropriate (VII, Fig. 3). This view is consistent with the computed
partial positive charge of +0.82 at C(1) and partial negative charges
of À0.41 and À0.43 at the adjacent oxygen atoms. A second order
perturbation analysis of the NBO data reveal donation of the oxygen
lone pairs from O(1) and O(2) and the p-electron density from
the aryl substituent stabilise the carbocation centre at C(1). The
C(2)–C(3) bond within the ring is thus considered an isolated
double bond. Whilst the HOMO is based on the perfluoroaryl 13 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
14 (a) M. R. Caira and J. F. de Wet, Acta Crystallogr., 1981, B37, 709;
group, the LUMO is delocalised over the dioxolium and tolyl
groups. Notably the largest coefficient is at C(1) consistent with
(b) R. F. Childs, R. M. Orgias, C. J. L. Lock and M. Mahendran, Can.
the tendency for nucleophilic attack at this position.
The present reactivity of internal propargyl esters with B(C6F5)3
reveals a different reaction pathway to that observed for the analogous
J. Chem., 1993, 71, 836; (c) R. F. Childs, C. S. Frampton, G. J. Kang
and T. A. Wark, J. Am. Chem. Soc., 1994, 116, 8499.
15 (a) H. Paulsen and R. Dammeyer, Chem. Ber., 1973, 106, 2324;
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propargyl derivatives bearing a terminal alkyne fragment. The internal 16 See: P. Mauleon and F. D. Toste, Gold-Catalyzed Reactions of
Propargyl Esters, Propargyl Alcohols, and Related Compounds, in
alkyne derivatives give rise to stable dioxolium compounds affording
Modern Gold Catalyzed Synthesis, ed. A. S. K. Hashmi and F. D. Toste,
the first structural characterisation of such 1,3 dioxolium
2012, Wiley-VCH Verlag GmbH & Co, KGaA, Weinheim, Germany,
species. These data together with DFT calculations reveal that
and references therein.
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Chem. Commun., 2014, 50, 7243--7245 | 7245