B. Breit. D.A. Evans Introduction to Photochemistry Chem 206 Norrish-type I reactions (a-cleavage C/N bond cleavage: Aza-Fries rearrangement Barton,JAcS1985,107,3607. occurs from both recombination Chem. ber. 1969. 342 Regioselectivity of bond cleavage: Depends on relative stability of the two radicals formed Norrish-type lI reactions Acc. Chem. Res 1971, 4, 168 here: benzyl vs phenyl radical H Intermolecular (Reaction with solvent Reaction occurs readily if stabilized radicals are formed 是sH ket71963,1863. OHOH CIO and CIN bond cleavage: Photo-Fries rearrangement Pinacoh-type products Intramolecular(a)Photoenolizati ChaR H H-AbstrB. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Norrish-type I reactions ( -cleavage) O R R' hn n ® p* O R R' * occurs from both S 1 and T1 O R + R' Regioselectivity of bond cleavage: Depends on relative stability of the two radicals formed. O Barton, JACS 1985, 107, 3607. Ph hn O Ph here: benzyl vs. phenyl radical H O Ph Ph hn Ph Ph - CO Quinkert, TL 1963, 1863. C/O and C/N bond cleavage: Photo–Fries rearrangement O O R hn OH O R OH R O + 1 : 1 O O R phenoxy radical (resonance stabilized) solvent cage O O R O O R Reaction occurs readily if stabilized radicals are formed. Ph H O S products (decarbonylation, recombination, disproportionation) N Ph O hn N O H N H O N O Chem. Ber. 1969, 342. Norrish-type II reactions Photo-excitation of carbonyl is followed by hydrogen atom abstraction. Acc. Chem. Res. 1971, 4, 168. Intermolecular (Reaction with solvent) Ar O R hn O Ar R * S-H OH Ar R * + S Dim. OHOH Ar Ar R R OHOH Ar R R Ar Pinacol-type products Intramolecular (a) Photoenolization CH2R O Ph C O Ph H CHR OH Ph CHR OD Ph CHR O Ph hn * H-Abstr. D C/N bond cleavage: Aza–Fries rearrangement R H