DIFFERENCES IN REACTIVITY OF STABILIZED CARBANIONS WITH HALOARENES IN THE INITIATION AND PROPAGATION STEPS OF THE SRN1 MECHANISM IN DMSO

The photostimulated reaction of acetone (2), acetophenone (5), and anthrone (8) enolate ions with iodobenzene (1) in DMSO gave good yields of substitution products by the S(RN)1 mechanism. On the contrary, the monoanions of acetylacetone, diethyl malonate, and nitromethane (10) did not react under irradiation. However, the photostimulated reaction of 1 with 10 in the presence of 2 or pinacolone enolate ions gave 1-phenyl-2-nitroethane as a substitution product. The formation of this product can be explained if one assumes that the radical anion formed in the coupling reaction of phenyl radical with 10 fragments to give benzyl radical and nitrite ion. Benzyl radical then couples with 10 to give ultimately the observed product. From competition experiments the relative reactivity determined for the coupling reaction of different anions with phenyl radicals in the propagation cycle of the S(RN)1 mechanism is as follows: monoanion of beta dicarbonyl compounds (unreactive) << 2 (1.0) < 5 (7.5) < 8 (16.5). The difference in reactivity observed experimentally is explained on the basis of the pK(a) (HOMO energy) of the anions and on the change in the pi-energy that takes place in the coupling reaction. With reference to the initiation step (electron transfer from the carbanion to 1), the reactivity increases as the pK(a) of the conjugated acid of the nucleophile increases.