A comparison of the reactivities of CCO radicals and free carbon atoms with bicyclo[2.2.1]hepta-2,5-diene and the photochemical reactions of allene with alkene

Bicyclo[2.2.1]hepta-1,5-diene, 248 was reacted with arc-generated C and photochemically generated C2O from C3O2 and their reaction mechanisms were explored. These investigations demonstrate that, although similar products were generated in both C and C2O reaction with 248 by addition of a carbon atom, the two reactions are quite different. The difference arises from the amount of energy that each reagent brings to the system. As the higher energy species, C atoms are less discriminating than C2O, and generate energetic intermediates in which excess energy is more difficult to dissipate and causes extensive intramolecular rearrangements. The reactions of C2O with C=C double bonds are best rationalized as proceeding via a cyclopropylidene intermediate and provide a way to examine the chemistry of such species. C atoms exhibit far richer chemistry which requires the use of labeled atoms for its complete elucidation.;The stability of energetic cyclohexylidenes and cyclohexenes generated in deoxygenation of cyclohexanones, cyclohexene oxides and desulfuration of cyclohexene sulfide by C atoms were also investigated. These cyclohexylidene carbenes can either cleave or rearrange to the corresponding cyclohexenes with excess energy which enables them to undergo retro Diels-Alder reactions. Substitution on the cyclohexene stabilizes the intermediates and decreases the rate of cleavage. The amount of cleavage of the intermediates was found proportional to the size of the substituent groups. Condensation of the carbon vapor on the cold layer of the substrates provides lower energy reaction condition and reduces the cleavage rate of the intermediates than those formed by co-condensation of carbon with substrate. The amount of cleavage was proportional to the energy released by the reaction of its precursor.;The photolysis of allene with alkenes by a vycor-filtered medium pressure mercury arc was investigated. A stepwise ene reaction mechanism via a radical pair was proposed. Initial interaction of allene with the alkene double bond generates a pair of radicals through H transfer. This radical pair can combine to form alkyne, or diffuse from the cage and initiate chain reactions or dimerizations. As expected for a reaction involving a radical pair, oxygen blocks the process.