Photoaddition of Water and Alcohols to 3-Nitrostyrenes. Structure-Reactivity and Solvent Effects

Article abstract of DOI:10.1021/jo00267a023

The photoadditions of water and several alcohols to the triplet excited states of 3-nitrostyrenes 1, 3-5, and 8 to give the corresponding anti-Markovnikov addition products are reported.Both 3- and 4-nitrostilbenes (6 and 7, respectively) do not undergo photoaddition on direct or sensitized irradiation in aqeous or alcohol solutions; only trans to cis photoisomerisation is observed.It is proposed that for the nitrostilbenes, efficient twisting of the alkene in the triplet excited state competes favorably with photoaddition.The efficient photoaddition of the water and methanol observed for 5-nitroindene (8)-the alkene moiety of which cannot attain an orthogonal ("twisted") state-is taken as additional evidence that these photoadditions probably occur via the planar triplet state and that twisting results in only deactivation to the ground state.The use of cosolvents (CH3CN and HCONH2) on several photoadditions is also reported.For example, use of CH3CN cosolvent in aqueous solution decreases the efficiency of photohydration in the parent 3-nitrostyrene (1) but is observed to enhance the efficiency of reaction (until about 40-70 mol percent CH3CN, depending on the substrate) for 3, 4, and 8.Quantum yields for photohydration and photoaddition of alcohols are reported for several systems.