Type A photorearrangement

Type A photorearrangement, 323, 329, 331 Type I cleavage of carbonyls, 135-171 application of extended Hiickel theory to, 179-181... 

Typical examples of circumambulatory rearrangements of bicyclo[3.1. Ojhexenyl cations are shown in Schemes 19 and 20. Swatton and Hart reported the isomerization shown in Scheme 19 in 1967 and proposed that the observed deuterium scrambling could be accounted for on the basis of a cyclopropyl walk reaction153. This circumambulation is comparable to that proposed by Zimmerman and Schuster as part of the sequence of reactions involved in the type A photorearrangement of 2,5-cyclohexadienones. ... 

Type A photorearrangements of a series of 2-methoxycarbonyl-, 2-cyano-, 2-methoxy-, and 2-methyl-substituted 4-methoxycarbonyl-4-methylcyclohexa-2,5-dienones gave 2-substituted phenols. It has been demonstrated that the regioselectivity of the photorearrangement is governed by electronic, rather than steric, effects. 

It is interesting that the 4,5-diphenylcyclohexenone (71) photorearrangement takes place 98.67, via a type A route and only 1.4% via a phenyl type... 

A number of examples of this type of photorearrangement have been observed with cyclic and bicyclic molecules. Some photoreactions involving cyclohexenones are as follows ... 

The photorearrangement of cyclohexenones to bicyclo[3.1.0]hexanones (type A process) has been extensively studied (55 65> ... 

Keywords A -n-methane and Norrish type II photorearrangement, chiral crystal... 

Pt A), 63-84 (c) Evans, S.V., Garcia-Garibay, M., Omkaram, N., Scheffer, J.R., Trotter, J., and Wireko, F. (1986) Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to the di-Jt-methane and Norrish type II photorearrangements. 

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a czs-diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid ... 

Cyclooctatetraene (COT) —> Semibullvalene (SB) Photorearrangement. Irradiation of COT yields semibullvalene [97], in spite of the fact that this photochemical reaction is forbidden by orbital conservation mles. The Longuet-Higgins loop for this system actually predicts that this should happen, although the reaction is phase preserving. (Fig. 42). This is another example of type C loop (Fig. 11). Only six of the eight electrons re-pair as COT transforms to SB. The reaction is made possible by the fact that COT valence isomerization, a phase-inverting reaction (four electron-pair Hiickel system), takes place simultaneously. One expects to produce in the reaction a COT isomer, that can be detected solely by proper substitution. 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

In 1966, Chapman and co-workers proposed a nitro-nitrite photorearrangement as an efficient primary photochemical process for nitroarenes in which the nitro group is out of the plane of the aromatic rings. This is followed by dissociation into NO and a phenoxy-type radical ultimately quinones and other oxy products are formed (Chapman et al., 1966). 

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