The kinetics of alkyl radical ring closures at selenium: formation of selenane

Intramolecular homolytic substitution reactions of 5-(alkylseleno) pentyl radicals 4 have been investigated by competition kinetics as well as computational techniques. B3LYP/6-31G(d) calculations predict that cyclizations of radicals 4 proceed through unremarkable transition states 11 in which the attacking and leaving radicals follow trajectories that deviate some 10-20 degrees from co-linearity with distances in the expected 2.1-2.4 angstrom range. Competition kinetic experiments provide activation energies (Ea) that lie in the range: 20-36 kJ mol(-1), and depend on the nature of the leaving radical, while G3(MP2)-RAD calculations provide data that are in good agreement with those obtained experimentally. Values of log(A/s(-1)) lie in the expected range of similar to 9-11. These data provide rate constants for cyclization that span three orders of magnitude at 25 degrees, namely: 10(3)-10(6) s(-1). This work also provides valuable Arrhenius data for the benzyl-substituted system 4 (R = Bn) (k(c) = 5.8 x 10(4) s(-1) in benzene at 25 degrees) and is important because the benzyl radical has become the workhorse for radical ring closures at selenium.