Rearrangements di-7r-methane

The di-7r-methane rearrangement has been studied in a sufficient number of cases to develop some of the patterns regarding substituent effects. When the central sf carbon is unsubstituted, the di-7i-methane mechanism becomes less favorable. The case of 1,1,5,5-tetraphenyl-l,4-pentadiene is illustrative. Although one of the products has the expected structure for a product of the di-7t-methane rearrangement, labeling with deuterium proves that an alternative mechanism operates ... 

A phenyl group may be substituted for one of the vinyl groups 1,2 in the di-7r-methane rearrangement as follows ... 

In most of the acyclic examples of the di-7r-methane rearrangement studied, there has been methyl substitution on the central (C-3) atom. We should expect that electron-withdrawing substituents (relative to —CH3) on C-3 would lower the energy of the basis orbitals on C-3 and slow the transformation shown in the diagrams for schemes (3) and (4). 

However, Zimmerman has shown that (15) is not formed via a di-7r-methane rearrangement by the photolysis of the dideuterio compound (17) ... 

This example illustrates the effect of rather small electronic changes on the di-7r-methane rearrangement. 

The absence of exocyclic n bonds in the reactants [(8.38) and (8.39)] eliminates the possibility of efficient cis-trans isomerization about that bond and allows the triplet di-7r-methane rearrangement to become important. 

Asymmetric induction in the di-7r-methane rearrangement is also of interest and studies on this have examined the influence of chiral esters. Thus the irradiation of 370 yields... 

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 di-7r-methane rearrangement, takes place on structures that possess two -groups connected with a single carbon (the central carbon), upon direct and/or sensitized irradiation of a C=C double bond chromophore, affording vinylcyclopropane products. Zimmerman proposed a stepwise cyclization of the excited --system via a 1,4-diradical of... 

Interestingly, much attention has been paid to the di-7r-methane rearrangement in solid state chemistry146. 

Synthetic aspects of the di-7r-methane rearrangement have been reviewed.273 An aryl version of the cyclopropyl-7i-methane rearrangement has been reported 274 see (226) — (227). A model study of the mechanism of the di-7r-methane and lumiketone... 

In another example, direct irradiation of compound 38 yields cyclopropane 39 and the thiopyrane derivative 40, arising from secondary photolysis of 39 (Sch. 13). The formation of 39 is rationalized based on a di-7r-methane rearrangement involving the participation of two benzo [6] thiophene rings present in 38 [29]. 

In summary, di-7r-methane rearrangements have proved to be very general for a large number of 1,4-unsaturated systems affording the... 

For four of the reactions shown in Table 4 (entries 1,5,11, and 12), absolute configuration correlations were established between the reactant and its solid-state photoproduct. Such correlations represent one of the most powerful methods available to the organic chemist for elucidating reaction mechanisms. In the Norrish/Yang type II reaction (entries 1 and 5), for example, it allows an unequivocal determination of which y-hydrogen is abstracted, and for the di-7r-methane rearrangement of dibenzobarrelene derivatives (entries 11 and 12), it tells us precisely which atoms are involved in the formation of the initial cyclopropyldi-carbinyl diradical. 

A particularly intriguing set of reactions involving bicycling steps as well as a di-7r-methane rearrangement and a 1,4- (2,3) fragmentation is outlined in Scheme 12 Here there is a superficial paradox. Thus, diradical D is formed in two ways. It formed from the benzo di-7i reactant (labeled Di-7i ) as well as the benzo bicyclic reactant labeled Bicyclic in Scheme 12. However, diradical D reacts differently depending on its mode of formation. 

Photochemical procedures for the preparation of electrophilic cyclopropanes are relatively important. Appropriately substituted electron-deficient olefins on photolysis undergo a di-7r-methane rearrangement to give the corresponding cyclopropane derivatives. For example, l,l-dicyano-2-methyl-3-arylpropenes (455) provide the dicyanocyclo-propanes (456), presumably via the mechanism of equation 154 . ... 

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