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

Abstract

The photolysis of aryl halides causes homolysis of the carbon-halogen bond and formation of aryl radicals. In contrast, photolysis of vinyl halides can induce both heterolysis of the C-X bond, thereby generating vinyl cations, and homolysis, giving vinyl radicals. Examples of this competition among pathways is reported here for three vinylic precursors, namely, 1,2,2-triphenylbromoethene (1), 1-phenyl-2,2-bis(o-methoxyphenyl)-1-bromoethene (11), and β-bromostyrene (19). Incursion of the photoinduced S RN 1 process, through the intermediacy of the vinyl radical, is verified in the presence of reducing nucleophiles, such as the enolate ions of ketones, and in part with (EtO) 2 PO -. Conversely, incursion of the heterolytic path, and intermediacy of the vinyl cation, occurs in the presence of weak electron-donor anions, such as NO 2 -, N 3 -, and Cl -. The vinyl cation produced from 19, which is less stable than those derived from 1 and 11, gives phenylacetylene via an E1-type elimination. An estimate is provided for the intramolecular rate of interception of the vinyl cation derived from 11 by the ortho-methoxy groups of the β-o-anisyl substituents. Finally, evidence against a photoinduced electron transfer from ROions to vinyl halide 1 is presented.

A Search for Unambiguous Vinylic SRN1 Reactions

“…[2,11] The more reducing anions provided evidence for a photostimulated S RN 1 process in Me 2 SO, consistent with their redox potentials. [14] Conversely, anions that were not reducing enabled the photoheterolysis of the vinyl CϪX bond to take over, and cleavage products reasonably resulting from capture of the intermediate vinyl cation by water were obtained.…”

“…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). Scheme 6 This finding showed that vinyl halides endowed with redox potentials almost as negative as (or approaching) that of an aryl halide such as 1 (E p ϭ Ϫ2.4 V in Scheme 2) could react by the S RN 1 mechanism, but this was not always the main factor and the situation might be more complex.…”

Determination of Rate Constants for the Reaction of Aryl Radicals with Enolate Ions

“…Ϫ 1 H NMR (CDCl 3 ): δ ϭ 7.8Ϫ6.9 (m, 4 H, ArH), 5.9 (dd, 1 H, CHϭ), 5.2 (m, General Procedure of Calibration of a Radical Clock: Precursor 1 (0.9 mmol) was irradiated at 350 nm in benzene (1.8 mL) in the presence of Bu 3 SnH (0.8 mmol) and azobis(isobutyronitrile) (AIBN; 0.09 mmol) for 60 min. [13] The reaction flask was kept at 25°C by means of its glass jacket connected to an external thermostat. At the end, an internal standard was added (biphenyl), and direct analysis of the sample (no workup) by GC and GC-MS gave the molar amount of the open-chain reduction product 1H, along with that of the cyclised reduction product 2H.…”

“…Instead, some photohomolysis of the CϪI bond of 1 was observed, [13] with ensuing production of minor amounts of reduction products 1H and 2H. The lack of reactivity of NO 2…”

Reactivity of Aryl and Vinyl Radicals: Abstraction of Hydrogen Atom or Reaction with a Nucleophile

“…Scheme 6 In the presence of Bu 3 SnH, added as a H-atom donor, the rate of decay of the 300-nm species was observed to increase, thus supporting the radical nature of 6 · . [26] The rate of decay of 6 · was then measured in the presence of increasing amounts of Bu 3 SnH, and a plot of the observed rate constant (k obs , s Ϫ1 ) against [Bu 3 SnH] provided the value of 3.7·10 8  Ϫ1 ·s Ϫ1 for the H-abstraction process by 6 · from Bu 3 SnH (Figure 3). This value confirms the determination by Ingold et al [24] for H abstraction from Bu 3 SnH by vinyl radical CH 3 (CH 3 ) 5 CHϭCH 2 · remarkably well, and is only a factor of two lower than the corresponding k H value (7.8·10 8  Ϫ1 ·s Ϫ1 ) [15] for the phenyl-type radical 1 · .…”