Di-ir-methane rearrangements

The unobserved di-ir-methane rearrangement and the observed a + n hydrogen migration route are as follows ... 

When the central substituents are phenyl the di-ir-methane rearrangement does take place,<25)... 

Hixson(36) (also see Ref. 37) has determined the relative rate constants for the di-ir-methane rearrangement of a series of p-substituted 1,3-diphenyl-propenes (Table 8.5) ... 

A study on mechanistic aspects of di-ir-methane rearrangements has been published recently [72]. The kinetic modeling of temperature-dependent datasets from photoreactions of 1,3-diphenylpropene and several of its 3-substituted derivatives 127a-127d (structures 127 and 128) show that the singlet excited state decays via two inactivated processes, fluorescence and intersystem crossing, and two activated processes, trans-cis isomerization and phenyl-vinyl bridging. The latter activated process yields a biradical intermediate that partitions between forma-... 

The photochemistry of 1,4-unsaturated systems has been studied intensively over the last three decades, as it can be exemplified by reactions like the di-ir-methane rearrangement or the oxa-di-TT-methane rearrangement. In this context, Armesto and co-workers reported a novel l-aza-di-Tr-methane rearrangement of 1-substituted-1-aza-1,4-dienes promoted by DCA sensitization. Because of the... 

Di-ir-Methane Rearrangement of Barrelene, Benzobarrelene, Dibenzobarrelene, and Related Derivatives... 

DiaryIcyclohex-i-enones undergo a different photorearrangement to bicydo[3.1.0]hexan-2-ones, in which an aryl substituent migrates from C-4 to C-3 (4.84). This reaction finds a parallel in the di-ir-methane rearrangement of 3-phenylalkenes (see p. 54). It is usually efficient ( = 0.1-0.2), it occurs by way of the (n,ji ) triplet... 

Scheme 15 Enantioselective di-ir-methane rearrangement using the ionic chiral auxiliary method.

Scheme 30 Absolute asymmetric di-ir-methane rearrangement of achiral dibenzobar-relenes.

It has been known for over two decades that optically active products can be formed from achiral precursors without the intervention of preexisting optical activity. This was illustrated by Penzien and Schmidt when they showed that 4,4/-dimethylchalcone, although itself achiral, crystallizes spontaneously in a chiral space group [140]. When these crystals are treated with bromine vapor in a gas-solid reaction, a chiral dibromide is produced in 6% enantiomeric excess. Since then several research groups have carried out similar reactions wherein the achiral substrate crystallizes in a chiral space group [141-184]. Photolyses of these chiral crystals lead to optically active photoproducts. An example of these types of reactions is the di-ir-methane rearrangement of substituted dibenzobarra-lenes shown in Scheme 7 [151,178]. The dibenzobarrelene derivatives 16a and... 

The term di-ir-methane rearrangement is meant to describe the photoisomerization of 1,4-dienes (i.e. two TT-systems separated by a methane unit) leading to vinylcyclopropanes. The reaction can be generalized in schematic form as in equation (1). As will be discussed more specifically in the section dealing with mechanism, such a simplistic presentation is not intended as the actual reaction path, but it gready helps in predicting the reaction products. 

The chemistry of the oxa-di-ir-methane rearrangement is covered in Chapter 2.6, so that here, for the sake of completeness, only a few remarks on its more salient features are presented. 

In nitrogen systems, all three arrangements are in principle possible, i.e. the 1-, 2- and 3-aza-di- ir-methane rearrangements. Only the first of them has recently been investigated, while examples of the 2-and 3-aza-di- ir-methane rearrangement appear still missing from the literature. 

The first aza-di-ir-methane rearrangement to be reported is illustrated in equation (26). The product of the reaction is the imine and not the aziridine the latter could have formed upon rupture of the alternative bond of the cyclopropyldicarbinyl diradical intermediate. In this regard, the aza-di-ir-methane resembles the 1-oxa case, wiA reestablishment of the imine bond. 

The l-aza-di-TT-methane rearrangement depends on the type of substitution of the nitrogen atom. For example, aza-di-ir-methane rearrangement occurs with aryl-substituted imines and with oxime acetates, but not with the oxime (23) or the nitrile (24). ... 

The question arises whether there is any need for formation of the cyclopropyldicarbinyl diradical in the aryl di- ir-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- ir-methane product. In Scheme 3 these mechanistic alternatives are illustrated for ben-zonorbomadienes, which have been studied in great detail by Paquette et alP... 

A useful mechanistic probe for the diradical intermediates postulated in the di-ir-methane rearrangement entails generating them via authentic routes and elucidating their chemical behavior. One of the early examples concerns the cyclopropyldicaibinyl diradical postulated in the photoisomerization of bar-relene into semibullvalene by nitrogen extrusion from the appropriate azoalkane. Indeed, as shown in... 

In a similar way, the 1,3-diradicals postulated in the di-ir-methane rearrangement of several benzonor-bomadienes and related species have been produced via denitrogenation of the appropriate azoalkanes (27)-(30). The products obtained in the thermal and photochemical denitrogenation of (27), for example, are the norbomadiene and the tricyclic di- ir-methane product (occasionally other products not derived from denitrogenation have been observed). The latter hydrocarbon, which derives from direct collapse of the 1,3-diradical, is formed almost exclusively (equation 37). 

This photochemical transformation, shown in equation (42), is a process that occurs especially frequently in the oxa-di-ir-methane rearrangement. This isomerization may proceed via a Norrish type I reaction. 

Such processes have been encountered in the photolysis of highly aryl-substituted systems. Representatively, 1-phenylindane is formed in the photolysis of fra r-l,3-diphenylpropene (equation 44). To the best of our knowledge, no cases have as yet been reported in which radical-type hydrogen abstractions compete with the di-ir-methane rearrangement. 

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