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Issue 7, 2010
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Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation

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Abstract

The 2-oxa-5-adamantyl carbocation 4 is shown to be a viable intermediate in several SN1 substitution reactions. However, attempts to observe the formation of 4 from various precursors by NMR methods (which succeed for the 1-adamantyl cation 5) failed, suggesting that 4 does not survive on longer timescales. DFT calculations suggest that the retro-Prins process (β-cleavage, Grob fragmentation) to give enantiomeric (1R,5R)- and (1S,5S)-7-methylene-2-oxoniabicyclo[3.3.1]non-2-ene cations 22 is the only realistic unimolecular escape route for 4. With the 6-31G(d) basis set, B3LYP calculation predicts that 4 is only 11 kJ mol−1 more stable than 22, and the barrier separating 4 and 22 is calculated to be only 15 kJ mol−1, so rapid equilibration of these species is likely on the laboratory time scale. The implications of this study for the mechanism of the Prins cyclisation are discussed.

Graphical abstract: Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation

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Publication details

The article was received on 21 Oct 2009, accepted on 21 Dec 2009 and first published on 28 Jan 2010


Article type: Paper
DOI: 10.1039/B921957A
Citation: Org. Biomol. Chem., 2010,8, 1551-1559
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    Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation

    R. W. Alder, F. Carta, C. A. Reed, I. Stoyanova and C. L. Willis, Org. Biomol. Chem., 2010, 8, 1551
    DOI: 10.1039/B921957A

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