Di-rr-methane rearrangements

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Both reactions proceed via triplet excited species and, to some extent, are controlled by whether the ti-tt (path A) or n-rr states are involved. The di-rr-methane rearrangement pathway is restricted to 4-aryl- or 4-vinylcyclohexenones. At the most basic level of... 

While this scheme is useful in helping to predict products from di-rr-methane rearrangements, all evidence indicates that the structures drawn are not intermediates in the reaction. That is, they do not represent energy minima on the potential energy surface leading from the excited state of the reactant to the ground state of the product. 

The very efficient di-rr-methane rearrangement from the triplet state in this case is probably due to two factors (a) the energetics is favorable for cleavage of bond 3-4 and formation of bond 2-4 [see Eq. (8.26)] and (b) rotation about bond 1-2, cis-trans isomerization, should be restricted because of steric interactions. 

For more than 15 years, the di-rr-methane rearrangements were limited to... 

These results provide strong evidence in favor of the involvement of radical-anions in electron-donor-sensitized l-aza-di-rr-methane rearrangements of aza-dienes 101, 117, 118, and 119 and also in the di-rr-methane reaction of diene 124. These observations open new lines of research in an area in which, due to the large number of smdies carried out for more than 30 years, apparendy there were very few things that remained to be uncovered. Further studies are in progress to determine the scope and synthetic applications of these reactions. 

Ring contraction of cyclopentenone and cyclohexenone derivatives to cyclobutanes was observed during direct irradiation of various j5,y-unsaturated cyclohexenones. By way of a Nor-rish type I cleavage in n,7i excited singlet (5t) and n,7t excited triplet (T2) states, an acyl/allyl diradical is formed which recombines in the allyl position to form the product of a 1,3-acyl migration,102 while sensitized irradiation leads mainly to oxa-di-rr-methane rearrangement (see Section 4.2.3.). 

An unconcerted photoisomerization the (di-rr-methane rearrangement. This reaction is of interest in synthetic organic chemistry, because it can form the small, strained three-carbon cyclopropane ring. Molecules of this type are rather unstable thermally, and therefore are useful reactants for further (thermal) processes. 

One of the important photoreactions of C=C bonds is the di-rr-methane rearrangement. Because of space limitation, the azadi-7r-rncthane132 and the oxadi-7r-methane133 rearrangements are not reviewed in this chapter, and we briefly summarize the principles and typical examples of the di-7r-methane rearrangement which was recently reviewed by Zimmermen134, the principal researcher of the process135. 

The third set of molecules that bring out the uniqueness of zeolites in the context of asymmetric induction is of 2,4-cyclohexadienones (6,6-dimethyl-2,4-cyclohexadienones 38, and 2,2-dimethyl-1,2-dihydronaphthalenones 39) [279,282,287,289,293-295]. The basic chromophore in these molecules is the conjugated dienone that undergoes oxa-di-rr-methane rearrangement to give a bicyclic product (Scheme 21). According to the accepted mechanism, the chirality is introduced into the system at the first step, yielding the diradical intermediate... 

Dimethyl-2,4-cyclohexadienone photorearranges to bicyclic [3.1.0] product via oxa-di-rr-methane rearrangement (Scheme 41). This achiral molecule photorearranges in solution to the chiral bicyclic product as a racemic mixture. This molecule within NaY zeolites in the presence of chiral inductors yields products with respectable ee. The best numbers are 50% (ephedrine as a chiral inductor at — 55°C) and 28% (pseudoephedrine at room temperature) [294]. These numbers are significant when one recognizes that in solution only a racemic mixture is obtained. 

A minimal structural feature for di-rr-methane rearrangement of acyclic arylpropenes to occur in solution appears to be the presence of an additional phenyl group.A similar requisite seems valid also for cyclic 1,4-systems such as indenes, while the scope for 1,4-dihydronaphthalenes is uncertain. 

It is perhaps worth pointing out that the di-i -methane rearrangements of 1,4-cycloheptadienes produce bicyclo[4.1.0]alkenes which are present in several natural products (e.g. in carenes). However, the most striking utility is perhaps evident in polycyclic transformations. The di-rr-methane rearrangement remains the most efficient and effective metht for synthesizing semibullvalene and its benzo and dibenzo derivatives. It is, indeed, surprising that this synthetic protocol has found so little utilization, especially in the subsequent transformation of products into useful complex target molecules. 

Many photoreactions of aromatic compounds proceed by way of nonaromatic intermediates which are then either trapped inter- or intra-molecularly, or revert to starting materials. Such processes involving aromatic groups include the di-rr-methane rearrangement and photoenolization of aromatic carbonyl compounds. These particular reactions are considered fully elsewhere in this Volume, but examples of this type of photoisomerization from the current literature are given here for illustrative purposes. 

Latifolin (10), the major constituent of Dalbergia latifolia, gives tra 5 -l-(5-hydroxy-2,4-dimethoxyphenyl)-2-(2-hydroxyphenyl)cyclopropane (11) as the sole di-rr-methane rearrangement product on irradiation in benzene. ... 

Benzobarrelenes are an important class of molecule where the di-rr-methane rearrangement is operative. Clearly, within this class there is a possibility that benzo-vinyl interactions can be in competition with vinyl-vinyl processes. The direct irradiation (X > 330 nm) of the benzobanelene 313 in a variety of solvents (benzene, acetonitrile, methanol or hexane) affords two principal products 314 and 315 in a ratio of 1 1. The products are formed via a di-rr-methane process involving vinyl-vinyl bridging which... 

The question arises whether there is any need for formation of the cyclopropyldicarbinyl diradical in the aryl di- rr-methane rearrangement. In the event that this intermediate is indeed formed, one has to keep in mind that the aromaticity of the aryl moiety has to be sacrificed along the reaction coordinate. Alternatively, a 1,2-aryl shift with direct formation of the 1,3-diradical may operate, which irreversibly cyclizes to the di-rr-methane product. In Scheme 3 these mechanistic alternatives are illustrated for ben-zonorbomadienes, which have been studied in great detail by Paquette et... 

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