ABSOLUTE RATE CONSTANTS FOR REACTIONS OF ALPHA-CARBETHOXY AND ALPHA-CYANO RADICALS

Absolute rate constants were measured for cyclization of alpha carbeth oxy- and alpha-cyano-substituted radicals and for reactions of these r adicals with Bu(3)SnH. 2-Pyridimethioneoxycarbonyl ester radical precu rsors were employed in direct, laser-flash kinetic studies of cyclizat ions of (.)CRX-(CH2)(m)-CH=CPh(2) (4a: R = H, X = CO(2)Et, m = 3; 4b: R = CO(2)Et, m = 4; 4c: R = CH3, X = CO(2)Et, m = 3; 4d: R = CH3, X = CN, M = 3) over the temperature range 0=60 degrees C. Radical 4a was p roduced in two conformations that cyclized faster than they equilibrat ed by rotation of the carbethoxy group. Radicals 4b,c apparently cycli zed slower than rotation of the carbethoxy group. At 20 degrees C, the rate constants were 5.4 x 10(7) (4a) 7 x 10(5) (4b), 3.3 x 10(5) (4c) , and 2.1 x 10(5) (4d) s(-1). The alpha-carbethoxy group had no kineti c effect on the cyclizations of the secondary radicals 4a,b, which occ urred with rate constants slightly greater than those of the analogous secondary alkyl radicals (i.e., 4: R = H, X = CH3). However, the cycl izations of the tertiary radicals 4c,d were substantially reduced in r ate in comparison to that of the analogous tertiary radical with incre ase in Delta G(double dagger) at 20 degrees C of 2.0-2.22 kcal/mol. Th e latter kinetic results are ascribed to a steric effect in radicals 4 c,d enforced by the structure of the delocalized radical system. Rate constants and Arrnenius functions for reactionss of 4b, 4c, and 4d wit h Bu3SnH were determined by indirect methods; at 20 degrees C, these h ydrogen atom transfer reactions occur with rate constants of 3 x 10(6) , 2 x 10(5), and 3 x 10(5) M(-1) s-1, respectively. The kinetic values for reactions with B3SnH can be incorporated into synthetic planning.