Photorearrangements

3 Photochemical Rearrangement Reactions in Synthetic Organic Chemistry 

This chapter has to do with different types of photoisomerization reactions. This includes the formation of constitutional, configurational, conformational and valence isomers. 

One important aspect of the photochemistry of aikenes 301,302) is the E—Z isomerization around the C—C double bond 303). This is also valid for cycloalkenes with the obvious exception of cyclopropenes304a,b) which exhibit a distinct photochemical behaviour, and cyclobutenes and cyclo-pentenes where the ring is to rigid to allow sufficient twisting of the double bond. 

The photochemical isomerization of E-stilbenes has been applied in the preparation of phenanthrenes, as Z-stilbenes undergo electrocyclie ring closure (cf. chapter 3.1.3) to dihydrophenanthrenes which in turn are easily oxidized to phenanthrenes (3.1) 305). This sequence has also been employed in the synthesis of benzoquinolines 306) or of benzoquinolizines (3.2) 307). 

Generally for simple alkenes the E-isomer is thermodynamically more stable than the Z-isomer. Photochemical isomerization is thus a powerful method to obtain Z-alkenes in good yields, as shown for p-ionol (3.3) 308). 

The photorearrangement of cyclohexenones to bicyclo[3.1.0]hexanones (type A process) has been extensively studied  

There are also numerous examples of a second rearrangement which occurs with 4-aryl or diarylcydohexenones  

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The photorearrangement of pyrazoles to imidazoles is probably analogous, proceeding via iminoylazirines (82AHC(30)239) indazoles similarly rearrange to benzimidazoles (67HCA2244). 3-Pyrazolin-5-ones (56) are photochemically converted into imidazolones (57) and open-chain products (58) (70AHC(ll)l). The 1,2- and 1,4-disubstituted imidazoles are interconverted photochemically. 

The reaction was conveniently carried out in DME in the presence of benzophenone, and the yields were in the range 20-30%. In ethanol solution, 1,3,5-trimethylpyrazole (177 R = R = R = Me, R = H) yielded (183) and (184) (69T3287). In contrast with the previously observed sensitized photorearrangements (67HCA2244, 67CC488), this last reaction was not sensitized by acetophenone or benzophenone. 

The evidence obtained clearly indicates that the above photorearrangements proceed by a mechanism involving a nitrile ylide intermediate since cycloadducts could be isolated when the irradiations were carried out in the presence of trapping agents. Intramolecular cycloaddition of the nitrile ylide followed by a 1,3-sigmatropic hydrogen shift of the initially formed five-membered ring readily accounts for the formation of the final product. 

Two further compounds are on the borderline of stability. The oxathiirane (14), obtained by photorearrangement of a thioketone 5-oxide, could be characterized by its UV spectrum (76JCS(P1)1404). Thiazirine (15) was stable only at 15 K (78JCS(P1)746). 

Oxazol-4(5ff)-one, 5-acetyl-5-methyl-synthesis, 6, 225 Oxazol-4(5ff)-one, 2-phenyl-photorearrangement, 6, 200 synthesis, 6, 225 Oxazol-5(2ff)-one, 2-acyl-2,4-disubstituted pyrolysis, 6, 200 Oxazol-5(2H)-one, allyl-photochemical rearrangement, 6, 200 Oxazol-5(2ff)-one, 2-arylmethylene-synthesis, 6, 227... 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

Ethenoanthracenes undergo the di-rc-methane photorearrangement. Analyze the substituent effects that are observed in this reaction. 

The photorearrangements of isoxazoles are well known reactions. Thus... 

These data demonstrate that a photorearrangement reaction of 1-alkoxy group actually occurs. We continue to speculate that any substituents at the 1 position of indoles may migrate to 3, 4, and/or 6 positions upon photoirradiation. 

Prolonged irradiation of 1-benzothiepins 3 results in the formation of the secondary photoproducts 4 via a diradical photorearrangement.12,95... 

Ethyl tram-5 a,9 a-dihydro-1 H- -benzazepine-1 -carboxy late (2), a photorearrangement product of ethyl 3,3a,7a,7b-tetrahydro-l//-cycIobut[6]indole-3-carboxylate (1), on treatment with tet-rachloro-l,2-benzoquinone in benzene undergoes oxidation to the l//-l-benzazepine 3.109... 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

Scheme 15 SET induced photorearrangement of phenylallylphosphites. Reprinted with permission from [43]. Copyright 2000 American Chemical Society...

During the last decade, Bentrude et al. [65] has shown that the triplet-sensitized photorearrangement of allylphosphites and analogs (Scheme 28) is a powerful method for preparation of alkylphosphonates from phosphites. Moderate to high yields were observed when triphenylene was used as photosensitizer [66,67]. 

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