133628-01-0Relevant academic research and scientific papers
Photoreactivity of Some α-Arylvinyl Bromides in Acetic Acid. Selectivity toward Bromide versus Acetate Ions as a Mechanistic Probe
Van Ginkel, Frits I. M.,Cornelisse, Jan,Lodder, Gerrit
, p. 4261 - 4272 (2007/10/02)
The photochemical reactions of four α-anisyl-β,β-diarylvinyl bromides (1a-1d), three α-phenyl-β,β-diarylvinyl bromides (1e-1g), and 9-(α-bromo-p-methoxybenzylidene)anthrone (2) in acetic acid in the presence of sodium acetate and tetraethylammonium bromide (labeled with 82Br) have been studied quantitatively. Bromide exchange, acetate formation, E/Z isomerization, an anisyl 1,2-shift, stilbene-type cyclization to phenanthrenes, reductive debromination, and oxidation are observed as primary pathways. For all compounds 1, nucleophilic substitution, accompanied by E/Z isomerization in both starting material and product with 1b,c and 1f,g and an anisyl 1,2-shift in 1e but not in 1f,g, is quite efficient and by far the most important process. (Quantum yields range from 0.1 to 0.3 as compared to 0.01-0.02 for cyclization and 0.001-0.01 for reduction). The α-anisylvinyl bromide 2 is virtually inert for photosubstitution. The selectivity constants toward bromide and acetate ions, corrected for their temperature dependence, the amounts of E/Z isomerized starting material and product, the occurrence or nonoccurrence of an anisyl 1,2-shift, and the nature of the capturing nucleophile in the acetolysis are all in quantitative agreement with the corresponding data for the thermal reactions of 1 in the same medium. The results strongly support a mechanism for the nucleophilic vinylic photosubstitution reactions that involves the generation of a product-forming intermediate, which is exactly the same as the intermediate formed in the thermal reaction, a "cold" (thermally relaxed) linear free vinyl cation.
