Stereochemistry of the Vinylic S_RN1 Reaction of Triarylvinyl Halides. The Structure of the Intermediate α-Arylvinyl Radical

Abstract

Evidence for the intermediacy of a vinyl radical in the vinylic S(RN)1 reaction (S(RN)1(V)) of 2-anisyl-1,2-diphenylvinyl bromide 2 is obtained. The photostimulated S(RN)1(V) reaction of pinacolone enolate ion with (E)-2 and (Z)-2, which are used as stereoindicators, gives complete loss of the original stereochemistry of the two precursors in the substituted and hydrodehalogenated products; i.e., stereoconvergence is found. It is concluded that in the reaction of 2 a beta-substituted alpha-phenylvinyl radical is a reactive intermediate and that it has either a linear structure or an average linear structure due to a rapidly interconverting E,Z mixture of bent radicals. This conclusion is supported by comparing the stereochemical course of the S(RN)1(V) reaction with those of other reactions of the same precursor, which unambiguously give rise to the same alpha-phenylvinyl radical intermediate by alternative mechanisms. Among the reactions investigated, the hydrodehalogenation of precursor 2 by LAH appears to take place by an ET mechanism.

Single‐Electron‐Transfer‐Induced Coupling of Alkylzinc Reagents with Aryl Iodides

“…This result shows that, under the standard conditions, BuZnI · 2LiI shows no reactivity towards Ph · , which rather reacts with the solvents to give benzene ( 6o ) as the major product (Scheme ). No involvement of σ‐radical intermediates is supported by retention of the stereochemistry in the coupling of alkenyl iodides shown in Scheme …”

A Search for Unambiguous Vinylic SRN1 Reactions

“…Electrochemical measurements indicated that the reduction potentials became progressively more negative in the series (Table 2), both for the RX precursors and for their RH derivatives. Actually, we had already reported evidence of S RN 1 reactions with the pinacolone enolate ion for a monoanisyl derivative, (E)-or (Z)-An(Ph)Cϭ C(Br)Ph, [10] and for the bis(o-anisyl) derivative (o-Me-OC 6 H 4 ) 2 CϭC(Br)Ph. [11] When we now tested the tris(p-anisyl) derivative 18 (E p ϭ Ϫ2.14 V), it gave 32% conversion into the pinacolyl derivative 19 under photostimulation conditions (Scheme 6).…”

“…[2,10] Scheme 7. Y ϭ Me 3 CCOCH 2 Consistently with this, the formation of 22 was suppressed in the presence of the electron scavenger p-dinitrobenzene, while that of 21 was not suppressed.…”

Competition of Mechanisms in the Photochemical Cleavage of the C−X Bond of Aryl-Substituted Vinyl Halides

“…Evidence for the intermediacy of a vinyl radical in this nucleophilic substitution process was provided,17d in analogy with the intermediacy of aryl radical in the reaction of aryl halides . In keeping with the aromatic case, the vinylic S RN 1 process is likely to be initiated by photostimulated electron transfer from a nucleophile to the vinyl halide, forming an intermediate radical anion that then fragments to the vinyl radical.…”

“…We became interested in this dichotomy in the behavior of the vinyl halides in connection with our studies on the vinylic S RN 1 reaction, whose propagation cycle is reported in Scheme .

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