Rearrangements Photorearrangements

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. 

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... 

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. 

Photorearrangement of 4-/>-cyanophenyl-4-phenylcyclohexene (69) took place mainly byp-cyanophenyl migration/67,70,74 The conclusion could then be drawn that the rearranging excited state is not electron deficient at the /3-carbon atom, since one would not expect a cyanophenyl group to migrate to a positive carbon. The/3-carbon was proposed to have odd electron character ... 

S,y-Unsaturated ketones undergo a rearrangement that is formally like the di-77-methane photorearrangement. An example of this rearrangement is provided by the photolysis of l,2,4,4-tetraphenyl-3-butenone ... 

Propose an explanation of why the photorearrangement of compound (14) takes place via a hydrogen migration while the rearrangement of compound (30) takes place via a di-ir-methane reaction. 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

In acidic solution the rearrangement of a-santonin to lumisantonin and then ultimately to photosantonic acid is not as efficient as rearrangement to the hydroxy ketone, isophotosantonic lactone (5), shown on page 308. Fisch and Richards(6) found that the photorearrangements of a-santonin could be sensitized with benzophenone or Michler s ketone. Moreover, if the irradiation of a-santonin (3660 A) is carried out in piperylene as solvent, the photoreaction is completely quenched. This suggests that the rearrangements proceed via triplet states. 

They argue that this and related enone photorearrangements do not take place via an a-cleavage (8.67), inasmuch as bonding to the central carbon atom of the allylic radical (47) or the rearrangement of an allyl radical to a cyclopropyl radical is the least favorable path available to the radical species ... 

Enones undergo a variety of photorearrangements 333). In contrast to a,P-unsaturated ketones which react in many different ways, p,y-unsaturated ketones undergo the oxadi-7c-methane rearrangement in sensitized irradiations in complete analogy the products formed are cyclopropyl ketones... 

The kinetic isotope effect (KIE) produced in the photorearrangement of 4-me-thoxyphenyl acetate to 2-hydroxy-5-methoxyacetophenone has been measured by determining the isotope ratios before and after irradiation (k, and ka, respectively) in the starting material and the rearranged products. A value of KIE = kjk, different from unity would indicate that the rearrangement proceeds along an activation energy barrier. This is neither the case for the [48,49] nor for the case of isotope, both included in the carbonyl moiety [49]. In fact, the obtained values are KIE ( C) = 0.9988 0.0051 and KIE ( 0) = 1.0000 0.0023. 

It is typical that the photorearrangement 57 - 58 proceeds readily.37 This seems to rule out the triplet state of the rearranging molecule 57, reasoning that the ferrocene part of the molecule would very probably act as a triplet quencher,35 as is the case with other ferrocene derivatives structurally able to undergo triplet photoreactions.36 On the other hand, the reaction 55 - 56 cannot be sensitized with acetophenone33 (ET = 73.6 kcal mole-1 32ft). 

The sensitized photo-Fries rearrangement of 55 in benzene, toluene, and in concentrated polystyrene solution in dioxane is effectively quenched with biacetyl.4 This phenomenon must again be attributed to quenching of the aromatic energy donor, because in pure dioxane the photorearrangement of 55 is not influenced by biacetyl (vide supra). 

The observation that the yield of orrfto-rearranged product depends on the bulk of para-substituent is unique. It would be interesting to find out whether this also holds true with photorearrangement in a low viscosity solvent, e.g., in an experiment where only the substituent size would be changed preserving the electronic effect of the substituents. One can argue that the presented photorearrangement in polyethylene film is almost a photoreaction in the solid state. Such reactions occur with the minimum amount of atomic or... 

For the initial phase of the photo-Fries rearrangement, IA can be determined approximately by subtracting the absorption due to the strongest absorbing product of the reaction mixture, provided its concentration and molar extinction coefficient are known. By means of this initial rate approach Humphrey8 determined the quantum yields of the photorearrangement of 13 (913-14 = 0.14), of 14 (914-.18 = 0.056), and of poly-2,2-propanebis(4-phenyl carbonate) ([Pg.138]

The photorearrangement of 7V-aryl lactams 34 to the ortho-position offers an interesting possibility for the preparation of cyclic benzo-aza-ketones of type 35, whereas the rearrangement to the para-position should lead to para-cyclophane 144 formation. 7-, 8-, and 13-Membered ring jV-aryl lactams (34, n = 5, 6, and 11) were successfully rearranged in ethanol to 35 in chemical yields of 60, 83, and 80%,13 and in quantum yields of 0.071, 0.11, and 0.082, respectively.14... 

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