The 2-Norbornyl Cation via the Fragmentations of <i>exo</i>- and <i>endo</i>-2-Norbornyloxychlorocarbenes: Distinction without Much Difference

Abstract

exo- and endo-2-norbornyloxychlorocarbenes (7) were generated photochemically from the corresponding diazirines (6). Both carbenes fragmented to [2-norbornyl cation (carbon monoxide) chloride] ion pairs in MeCN or 1,2-dichloroethane solutions. Products included exo-norbornyl chloride (8), endo-norbornyl chloride (9), norbornene (10), and nortricyclene (11). Fragmentation activation energies were very low (< approximately 4 kcal/mol) and, as a result, the (laser flash photolytic) rate constants for fragmentation were essentially identical for exo-7 and endo-7 ( approximately 5 x 10(5) s(-1) in MeCN). Due to chloride return within the ion pairs, product distributions from exo- and endo-7 differed, with more endo-chloride formed from the endo-carbene: the 8/9 product ratio in MeCN was approximately 41 from exo-7, but only 4.6 from 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). In MeOH/MeCN, up to 28% of exo-2-norbornyl methyl ether formed at the expense 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 and endo-7 product mixtures (accompanied by 26-32% of norbornene). Electronic structure calculations on the ground states and fragmentation transition states of exo-7 and endo-7 are presented.