Effects of Solvent and Concentration on Periselectivity

SCHEME 4.9 Different mechanistic pathways associated with periselectivity. 

Returning to the effect of solvents, two reasons for the frequently observed dominance of [2 + 2] cycloadditions in nonpolar solvents can be proposed. Less polar solvents will be less effective in stabilizing the radical cation species, thus giving rise to formation of CIP with the reduced sensitizer and more likely and more exothermic BET. Additionally, forward electron transfer might not favorable in all cases. The thermodynamically disfavored ET in nonpolar solvents increases the probability of a direct photoexcitation of the substrate, leading to the cyclobutane adduct via the normal Woodward-Hoffman dictated excited-state process.  

2 Effect of Diene ienophile Redox Potentials on Periselectivity 

TABLE 4.5 Periselectivity in the Reaction of 35 with Varions Dieneophiles 

Although oxidation potentials do not precisely correlate with the observed trend in selectivity, vinyl phenyl sulfide derivatives incorporating -bromo and -ethyl moie- 

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