B. Breit D. A. Evans Introduction to Photochemistry Chem 206 Cis-Trans Isomerization Cis trans olefins may be interconverted by either direct or sensitized irradiation sensitizer: benzene △Hmm2=-33 (T)-benzene ('S)-benzene trans-cyclohexene cis-cyclohexene Sensitized Photohydration t-BUOH/H2O sensitizer Review: Marshall Accounts Chem. Res 1969. 2. 33 ROH Ground state trans cyclohexene is now sufficiently basic to deprotonate alcohols HOH tBuOH Kropp, JACS 1966, 88 Marshall, JAcs 1966. 8 Marshall, JOC, 1971, 214)B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Hmm2 = – 33 kcal/mol trans-cyclohexene cis-cyclohexene Me Me Me Me O O hn benzene t-BuOH Me CH2 Me Me O O 85% yield Evans et. al. unpublished results, 1972 HO Me hn PhH O Me (Marshall, JOC, 1971, 214). Cis–Trans Isomerization Cis & trans olefins may be interconverted by either direct or sensitized irradiation C R H R H C R H C H R hn sensitizer 3 C H R C H R sensitizer: benzene ( 0S)benzene hn (1S)-benzene ( 3T)-benzene ( 0S)-olefin (0S)-benzene (3T)-olefin Me hn sensitizer Me (trans-cyclohexene) Me H Me H OR Sensitized Photohydration CH2 H Review: Marshall, Accounts Chem. Res 1969, 2, 33 Ground state trans cyclohexene is now sufficiently basic to deprotonate alcohols Me H Me H OR CH2 H Product Partitioning Me Me Me Me Me Me Me Me Me hn sensitizer HOH tBuOH 28% 72% 70% 30% Kropp, JACS 1966, 88, 4091 Marshall, JACS 1966, 88, 4092 Me Me Me hn xylene t-BuOH/H2O Me CH2 Me 50% Marshall Me Me Me Me Me Me –H+ ROH ROH RO