The 2-norbornyl cation via the fragmentations of exo- and endo-2-norbornyloxychlorocarbenes: Distinction without much difference

exo- and endo-2-norbornyloxychlorocarbenes (7) were generated photochemical ly from the corresponding diazirines (6). Both carbenes fragmented to [2-no rbomyl cation (carbon monoxide) chloride] ion pairs in MeCN or 1,2-dichloro ethane solutions. Products included exo-norbornyl chloride (8), endo-norbor nyl chloride (9), norbornene (10), and nortricyclene (11). Fragmentation ac tivation energies were very low (< similar to4 kcal/mol) and, as a result, the (laser flash photolytic) rate constants for fragmentation were essentia lly identical for exo-7 and endo-7 (similar to5 x 10(5) s(-1) in MeCN). Due to chloride return within the ion pairs, product distributions from exo- a nd endo-7 differed, with more endo-chloride formed from the endo-carbene: t he 8/9 product ratio in MeCN was similar to 41 from exo-7, but only 4.6 fro m endo-7. Norbornene, formed by proton transfer to Cl- within the ion pairs , was a major product in both cases (44% from exo-7 and 62% from endo-7). I n MeOH/MeCN, up to 28% of exo-2-norbornyl methyl ether formed at the expens e of some of the norbornene, but even in 100% MeOH, the norbornyl chloride products of ion pair return still accounted for 46% and 31% of the exo-7 an d endo-7 product mixtures (accompanied by 26-32% of norbornene). Electronic structure calculations on the ground states and fragmentation transition s tates of exo-7 and endo-7 are presented.