Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation
作者： Roger W. Alder , Fabrizio Carta , Christopher A. Reed , Irina Stoyanova , Christine L. Willis
DOI: 10.1039/B921957A
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摘要: The 2-oxa-5-adamantyl carbocation 4 is shown to be a viable intermediate in several SN1 substitution reactions. However, attempts observe the formation of from various precursors by NMR methods (which succeed for 1-adamantyl cation 5) failed, suggesting that does not survive on longer timescales. DFT calculations suggest retro-Prins process (β-cleavage, Grob fragmentation) give enantiomeric (1R,5R)- and (1S,5S)-7-methylene-2-oxoniabicyclo[3.3.1]non-2-ene cations 22 only realistic unimolecular escape route 4. With 6-31G(d) basis set, B3LYP calculation predicts 11 kJ mol−1 more stable than 22, barrier separating calculated 15 mol−1, so rapid equilibration these species likely laboratory time scale. implications this study mechanism Prins cyclisation are discussed.
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