Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2 / 0683 745

728 Tong-Ing Ho and Yuan L. Chow

O

+ H C (CH2 )10 CO2 H

(172)

The intramolecular photochemistry of the vinylogous amide 173 in tert-butyl alcohol yielded a retro-Mannich type reaction product 174 (equation 114)170.

Compared to amides, the photochemistry of thioamides was less studied. The 2- substituted aryl thioamide 175 undergoes photocyclization to quinoline derivatives (equation 115)171.

S

(175)

Thioamides 176 react photochemically with 2,3-dimethyl-2-butene in the absence of oxygen to give ketones (equation 116)172. In the absence of oxygen, the photoproducts of 176 include nitriles, 1,2,4-thiadiazole and isothiazoline (equation 117).

R = Me, C4 H9 , CH2 CH2 NMe2 , CH2 CH2 CH2 NMe2 , CH2 CH2 NMePh

The first example of a counterthermodynamic one-way E ! Z photoisomerization based upon intramolecular hydrogen bonding was reported for the N-methyl-3-(2-pyridyl)- propenamide systems.

B. Imides

There are several reviews available on the photochemistry of imides177. The photochemistry of N-alkylphthalimides (equation 122) has been extensively studied178. Photochemical cyclization, ˇ-cleavage and hydrogen atom transfer reactions are caused by intramolecular υ-H atom abstraction from the T1 ( Ł ) state of the phthalimide carbonyl group. The reaction is dependent on the solvent used. Benzazepinedione formation is observed in ethanol while ˇ-cleavage and H-transfer occur in acetone and acetonitrile. Both intermolecular and intramolecular electron transfers are important pathways for imides179. For example180, irradiation of N-allylphthalimide in methanol results in intramolecular electron transfer to yield the radical ion pair 177a followed by an anti-Markovnikov addition of the methanol to produce the biradical 177b. The cyclized products 178 and 179 are derived from the biradical intermediate (equation 123). The macrocyclic compound 180 was also synthesized using this methodology180 (equation 124). When a heteroatom such as nitrogen, oxygen or sulphur is introduced into the N-alkyl side chain of the phthalimide, photoinduced electron transfer to generate radical ion pairs becomes feasible181. Proton transfer from the methyl or methylene group adjacent to the heteroatom gives a diradical which will cyclize the products (equation 125). When the substituent is methylthio (MeS) rather than dimethylamino (NMe2), the chemical yield increases and medium to large rings containing 38 atoms can be achieved182.