Degenerate .beta.-aryl rearrangements in photochemically generated triarylvinyl cations

Abstract

Irradiation of triphenyl-(la), tri-p-tolyl-(lb), or tri-p-anisylvinyl bromide (lc) in methanol gave the vinyl ethers 4 and the phenanthrenes 5. A study of [2-13C]-labeled bromides showed that /3-aryl rearrangement takes place in the photochemically generated vinyl cations 3*. Irradiation of triaryl[2-13C]vinyl bromides 1* in trifluoroethanol and analysis of the extent of scrambling from C-2 to C-l in the trifluoroethyl vinyl ethers 8 showed 38%, 72%, and 92% rearrangement of the 13C label for la*, lb*, and lc*, respectively. Similar irradiation of 1* in methanol resulted in no rearrangement in 4. The results are discussed in relation to the nature of the solvent, the stability of the triarylvinyl cations, and the migratory aptitudes of the aryl groups. The behavior of the photochemically and thermally generated ions was compared.

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

“…Are the Alkoxide Ions Reductant Species? The reported reaction of the vinyl bromide 24 in MeOH offers further evidence for a competition between photoinduced heterolysis and homolysis of a C−Br bond, since both solvolytic ( 25 ) and hydrodehalogenated ( 26 ) products were obtained at 350 nm (eq 16) 21a …”

“…Competition among Heterolysis, Homolysis, and S RN 1 with 1,2,2-Triphenylbromoethene (1). The triphenyl-substituted vinyl bromide 1 gives an “S N 1” solvolytic reaction in ethanol (eq 2); this occurs either, and more slowly, upon heating at high temperature or, and more easily, on irradiation with UV lamps. , Intermediacy of the α-phenyl substituted vinyl cation 1 + was proposed. Structurally similar compounds, where one or more of the phenyl substituents of 1 are replaced by anisyl groups (An), were also investigated (eq 3). ,3a On irradiation, solvolysis products were consistently observed, involving also those derived from rearrangements (e.g., 1,2-aryl shift) governed by the tendency to afford the more stabilized vinyl cation, , accompanied by lower amounts of hydrodehalogenated product(s) deriving from the vinyl radical intermediate (Scheme ).…”

Why Are Vinyl Cations Sluggish Electrophiles?

“…Vinyl cations 2 and 3 can be transiently generated in solvents of high ionizing power by heterolysis of precursor vinyl halides or pseudohalides 4-X and 5-X . ,,,, We have found that heterolyses of 4-OMs and 5-Br occur at convenient rates at 40 °C in 2,2,2-trifluoroethanol (TFE), a solvent of low nucleophilicity ( N 1 = 1.11) . When the vinyl cations are generated in aqueous TFE, the ketones 42 and 43 are formed quantitatively, as shown in Scheme a.…”

“…Although S N 1 reactions are usually accelerated when the polarity of the solvent is increased by salt additives, ,,, in certain cases, the opposite effect is observed. Common ion rate depression is a phenomenon whereby the rate of a solvolysis reaction k obs is slowed by addition of a salt containing the anion of the leaving group (X – ). ,,, It is observed when the recombination of the carbocation R + with X – is faster than addition of the solvent to R + (with first-order rate constant k solv ), that is, when k –1 [X – ] ≥ k solv (see Scheme , and eq , in which the full expression for k obs is given, and α = k –1 / k solv ). …”

“…What is the consequence of these considerations for vinyl cation chemistry? The very low S N 1 reactivities of vinyl halides and vinyl tosylates − have commonly been ascribed to low thermodynamic stabilities of vinyl cations due to the sp-hybridization of the carbenium center. , However, when one considers that the 1-phenylvinyl cation has a hydride affinity similar to that of the benzyl cation, − only 2.9 kcal mol –1 greater than that of the tert -butyl cation, the question arises whether the low solvolysis rates of vinyl halides are really mostly due to the low thermodynamic stabilities of dicoordinated carbenium ions. If instead high intrinsic barriers for the sp 2 ⇌ sp rehybridization were responsible for the slow solvolyses of vinyl derivatives, the discussion of Figure implies that the reverse reaction should also be slow, and vinyl cations should not be extraordinarily reactive intermediates but rather sluggish electrophiles.…”