5-EXO-TRIG VERSUS 6-ENDO-TRIG CYCLIZATION OF ALK-5-ENOYL RADICALS - THE ROLE OF ONE-CARBON RING EXPANSION

Alk-5-enoyl radicals were made to cyclize in exo and endo modes to giv e the corresponding cycloketone radicals, which are related through on e-carbon ring expansion. Relative kinetic data were determined for the ring closure of the 2-methylhept-5-enoyl radical generated by the rea ction of the corresponding phenylseleno ester with Bu3SnH over the tem perature range 233-323 K, The conversion to absolute rates provided Ar rhenius expressions for the 5-exo-trig and 6-endo-trig cyclizations. A b initio and semiempirical (AM1) calculations were performed on the he x-5-enoyl and hept-5-enoyl radicals, respectively, and the outcomes ai ded in the rationalization of the preexponential factors and activatio n energies. Both 1,5- and 1,6-ring closure occur via a lower energy '' chairlike'' transition state. The observed high regioselectivity is du e to favorable entropic and enthalpic factors associated with the form ation of the smaller ring. The stereoselectivity was higher in the 1,6 -ring closure (70:30) than in the 1,5-ring closure (55:45), the trans isomer being predominant in both, For the one-carbon ring expansion st udies, the radicals of interest were obtained by deoxygenation of suit able alcohols via the O-phenyl thiocarbonates with (TMS)(3)-SiH. The o ne-carbon ring expansion in the cyclopentanone series for the secondar y alkyl radicals was studied over the temperature range 343-413 K by m eans of free-radical clock methodology and yielded the Arrhenius expre ssion. The rate constant was 4.2 x 10(3) s(-1) at room temperature and the reverse reaction (ring contraction) was found to be at least 10 t imes slower. Since the intermediacy of acyl radicals can be excluded, the reaction must occur via 3-membered cyclic intermediate radicals (o r transition states).