Intramolecular homolytic substitution chemistry: An ab initio study of 1,n-chalcogenyl group transfer and cyclization reactions in some omega-chalcogenylalkyl radicals

Ab initio calculations using a pseudopotential (DZP) basis set and with the inclusion of electron correlation (MP2) predict that intramolecular homoly tic substitution at the chalcogen atom in the 4-chalcogenyl-1-butyl (4), 5- chalcogenyl-1-pentyl (5), 6-chalcogenyl-1-hexyl (6), and 7-chalcogenyl-1-he ptyl radicals (7) proceed's preferentially with the degenerate translocatio n of the chalcogen-containing moiety for radicals 6 and 7 and with ring clo sure in the case of the lower homologues (4, 5). All reactions involving ho molytic substitution at the tellurium atom are predicted to proceed with th e involvement of [9-Te-3] hypervalent intermediates, while the analogous re actions involving sulfur and selenium are calculated to proceed without the involvement of intermediates at all levels of theory, except during the 1, 6-translocation of selanyl in which a shallow local minimum was located on the potential-energy surface at the MP2/DZP level of theory. Energy barrier s for ring-closure reactions of between 48.4 (cyclization of 4: E = Te) and 162.6 kJ.mol(-1) (cyclization of 5: E = S) are calculated and are expected to decrease significantly with the inclusion of better leaving groups. Ene rgy barriers for 1,n-translocation reactions of between 62.8 (1,7-tellanyl transfer) and 139.3 kJ.mol(-1) (1,5-sulfanyl transfer) are predicted at the MP2/DZP level of theory; these high energy barriers are presumably a conse quence of unfavorable factors associated with ring size and long carbon-cha lcogen separations in transition states and intermediates (9-13) which lead to significant deviations from the ideal arrangement of attacking and leav ing radicals preferred in homolytic substitution reactions at chalcogen. Th e dependence of transition-state energy on attack angle at chalcogen has be en explored for the attack of methyl radical at methanethiol. Attack angles of around 110 degrees are calculated to lead to increases in the energy ba rrier of about 140 kJ.mol(-1) when compared with the preferred (159.5 degre es) arrangement of attacking and leaving groups. The mechanistic implicatio ns of these predictions are discussed.