Electron-donating methoxy groups

Using the same reaction, Cole and Wan later photogenerated mcta-quinone methides 110-112 from the appropriate m-hydroxy-a-phenylstyrenes.78 The three were characterized by LFP, with 110 and 111 showing strong and sharp absorptions at 430 and 450 nm, respectively, similar to those observed for 104 and 105. The dimethoxy-susbstituted 112 showed a much broader absorption centered at 420 nm. Addition of the electron-donating methoxy groups stabilized the quinone methides the lifetimes of 110-112 in 1 1 aqueous acetonitrile were 5-200 times longer than that of 104. 

Theoretical and experimental studies revealed a mechanistic twist of the concerted [8+2] cycloaddition between dienylbenzo[c]furans and dimethyl acetylenedicarboxylate (DMAD). Thus, DFT calculation at the (U)-B3LYP/6-31+G(d) level had suggested a stepwise mechanism involving the formation of a zwitterionic intermediate for the [8+2] cycloaddition between DMAD and dienylbenzo[c]furans with electron-donating methoxy groups in the diene moiety. When no electron-donating substituents are present in the diene moiety of the dienylbenzo[c]furan, computational results indicated an alternative mechanism in which a [4+2] reaction occurs between the furan moiety and DMAD, and was followed by a [l,5]-vinyl shift <07JA10773>. 

S—C Bond heterolysis dominates the photochemical reactivity of the keto tosylate 263247. Irradiation in benzene gives p-toluenesulphonic acid in 74% yield. The other products formed from this reaction are the ketones 264 and 265. The reaction is presumed to proceed via the intermediate carbocation 266 formed by S—C bond fission. The tosylate 267 is more able to undergo intramolecular addition due to the electron-donating methoxy group and gives 268 and 269 in 24% and 23% yields. An analogous mechanism is involved in the conversion of the sulphonate 270 on irradiation in benzene into the two products 271 and 272 in a ratio of 4 1249. The formation of the major product 271 presumably involves the heterolytic fission of an O—C bond to afford a cation which... 

Other studies into the effect of substituents on conjugation as monitored by UV-vis have shown interesting trends. For instance, the effect of placing an electron donating methoxy group on a thienylene-phenylene based polymer showed a shift in from 406 nm for the nonsubstituted polymer to 446 nm. 

---