Literature reports indicate either a linear or a bent
configuration for α-substituted vinyl radicals.
In order to get a better insight into this structural question,
calculations of the structure of some
α-substituted vinyl radicals were conducted by the DFT approach
[BLYP/6-31G(d,p) and B3LYP/6-311G(2d,2p)]. For vinyl radicals bearing σ-type
substituents (Me, SH, Cl, OH, F), the bent form
is found to be the minimum energy structure; the inversion barrier of
the E and Z forms is found
to markedly increase as the electronegativity of the group increases.
Vinyl radicals bearing π-type
substituents (CHCH2, CHO, CN,
C6H5) are found to be linear. The effect
of β-substituents is
much lower, and the β-fluoro- and β-ethenylvinyl radicals,
representative of σ- and π-type
substituents, are calculated to be bent. Comparison with other
literature calculations, obtained
by different methods for some similar vinylic radicals, supports our
computational results.
Experimental support was also sought, with particular regards to
the inversion barrier of the bent
radicals. Since a very high barrier is calculated for the
α-fluorovinyl radical, its inversion rate
could be slower than the “sampling” time of a proper chemical
reaction. Indeed, the α-fluorovinyl
radicals, generated from PhCHC(F)Br by reaction with
Bu3SnH/AIBN, retained the configuration
of their precursors in the overall hydrodebromination
process.
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.
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