Searching for intermediates in Prins cyclisations: the 2-oxa-5-adamantyl carbocation.
The 2-oxa-5-adamantyl carbocation 4 is shown to be a viable intermediate in several S(N)1 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 (beta-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.
Alder
Roger W
RW
School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK BS8 1TS.
Carta
Fabrizio
F
Reed
Christopher A
CA
Stoyanova
Irina
I
Willis
Christine L
CL
D000218:Adamantane / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D003500:Cyclization; D008958:Models, Molecular; D015394:Molecular Structure; D011714:Pyrans / Q000138:chemical synthesis* / Q000737:chemistry; D013816:Thermodynamics
D011714:Pyrans; D000218:Adamantane