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.
School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK BS8 1TS.
Roger W
RW
Alder
Fabrizio
F
Carta
Christopher A
CA
Reed
Irina
I
Stoyanova
Christine L
CL
Willis
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