CYCLOADDITION OF HETEROFUNCTIONALIZED ALLENES WITH TERT-BUTYLTHIOACRYLONITRILE - DETERMINATION OF ACTIVATION PARAMETERS FOR DIRADICAL INTERMEDIATE FORMATION AND THE DETECTION OF REVERSIBLE RING-CLOSURE

The cycloaddition reactions of 4-methylphenylthio-, methoxy-, 4-methoxyphenyl-, phenyl-, chloro- and cyanoallene with tert-butylthioacrylonitrile (BTA) in toluene-d8 solution were investigated. The relative reactivity sequence of these substituted allenes is 4-methylphenylthio approximately methoxy- > 4-methoxyphenyl- > phenyl- > chloro- > cyanoallene. Activation parameters were measured for diradical intermediate formation of BTA with 4-methylphenylthio-, methoxy- and 4-methoxyphenylallene, giving average values of E(a) and DELTA-S+/- of 14.2-16.8 kcal mol-1 and ca. -33 eu, respectively. The relative reactivity sequence is consistent with a FMO allene-HOMO, BTA-LUMO dominant interaction for diradical intermediate formation. However, the regioselectivity of attack on the substituted allene appears to be thermodynamically controlled. The formation of the diradical intermediates in these cycloaddition processes appears to be irreversible. However, the ring closure of the diradical intermediates formed from 4-methylphenylthio-, methoxy- and 4-methoxyphenylallene is reversible under the conditions of the kinetic experiments. The kinetically controlled ring closure of the diradical intermediates is allyl radical SOMO controlled, while the final cycloadduct distribution is thermodynamically controlled.