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

摘要

Absolute rate constants were measured for cyclization of alpha carbethoxy- and alpha-cyano-substituted radicals and for reactions of these radicals with Bu(3)SnH. 2-Pyridimethioneoxycarbonyl ester radical precursors were employed in direct, laser-flash kinetic studies of cyclizations 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 produced in two conformations that cyclized faster than they equilibrated by rotation of the carbethoxy group. Radicals 4b,c apparently cyclized 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 kinetic effect on the cyclizations of the secondary radicals 4a,b, which occurred with rate constants slightly greater than those of the analogous secondary alkyl radicals (i.e., 4: R = H, X = CH3). However, the cyclizations of the tertiary radicals 4c,d were substantially reduced in rate in comparison to that of the analogous tertiary radical with increase in Delta G(double dagger) at 20 degrees C of 2.0-2.22 kcal/mol. The latter kinetic results are ascribed to a steric effect in radicals 4c,d enforced by the structure of the delocalized radical system. Rate constants and Arrnenius functions for reactionss of 4b, 4c, and 4d with Bu3SnH were determined by indirect methods; at 20 degrees C, these hydrogen 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. [References: 65]