B. Breit. D. A. Evans Introduction to Photochemistry Norrish-type ll reactions continued.gamma H Abstraction terno-Buchi Reaction Femptosecond Dynamics of Norrish Type-l Reaction: Nonconcerted Hydrogen Transfer and Diradical Intermediacy, Zewail et al., Angew. Chemie, Int. Ed 2000, 39, 260 Reviews: Scharf. ACIEE 1991 30. 477: Ynoue. Chem. Rev. 1992. 92. 741 ∵cH2OH Photochemical 2+2 cycloaddition of ketones/aldehydes with olefins lifetime: 900 ns e general reaction 76 + With some aliphatic aldehydes stereospecific reactions have been observed 991,93,1277T 63;,JAcs1972,94 人 Me Me I RXn may take place out of either singlet or triplet state I Fragmentation to enol is favored entrop I The decomposition of the diradical is essentially barrierless I Fragmentation to enol is favored entropically I Fagmentation similar to McLafferty Rearrangement in mass spectroscopy Remote hydrogen abstraction Aromatic aldehydes/ ketones show stereo-convergent behavi (a simple diastereoselectivity T. Bach, Liebigs Ann. 997.162 H(D) R TI 0 OTMS Breslow. Acc. Chem. Res. 1980. 13. 170 dr>9.1 ond rotation therme oduct also for r2B. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Norrish-type II reactions continued...gamma H Abstraction "Femptosecond Dynamics of Norrish Type-II Reaction: Nonconcerted Hydrogen Transfer and Diradical Intermediiacy", Zewail et. al., Angew. Chemie, Int. Ed. 2000, 39, 260 Me O Me OH +76 +37 Me OH +26 OH Me lifetime: 900 ns ■ Rxn may take place out of either singlet or triplet state ■ Fragmentation to enol is favored entropically ■ The decomposition of the diradical is essentially barrierless ■ Fragmentation to enol is favored entropically ■ Fagmentation similar to McLafferty Rearrangement in mass spectroscopy Me Me O barrier +88 Remote hydrogen abstraction O Me H Me H R O C H2 O hn O Me H Me R O C H2 OH H(D) O Me H Me R O C H2 OH H(D) Breslow, Acc. Chem. Res. 1980, 13, 170. H H H hn Paterno-Büchi Reaction Photochemical [2+2] cycloaddition of ketones/aldehydes with olefins: The general reaction: O R + hn O R + * aliphatic via S1 aromatic via T1 O R triplet or singlet diradical general: via most stable diradical. O R Oxetane With some aliphatic aldehydes stereospecific reactions have been observed: O Me * S 1 O O Me Me Me Me H Me H Me O Me via singlet￾diradical bond formation faster than rotation! Me Me JACS 1991, 93, 1277; TL 1964, 1425; JACS 1968, 90, 6863; JACS 1972, 94, 8761. Aromatic aldehydes/ketones show stereo-convergent behavior: T. Bach, Liebigs Ann. 1997, 1627. (a) simple diastereoselectivity R 2 R 1 OTMS R 1 OTMS R 2 O Ph * T 1 + O Ph * T 1 + Ar O R 1 OTMS R 2 life time long enough to undergo bond rotation O R 2 OTMS R 1 Ph T 1 ISC to S1 recomb. thermodyn. most stable Ar product formed H H R 2 R 1 O OTMS (Main product also for R2 = H) dr > 9:1 Reviews: Scharf, ACIEE 1991, 30, 477; Ynoue, Chem. Rev. 1992, 92, 741. H H H H CH2