Alkenes, photochemically generated

Intermolecular addition of photochemically generated nitrenes and in particular acylnitrenes to alkenes provides a useful and widely applied route to aziridines.385 An analogous intramolecular photoreaction is thought to be involved in the conversion of the o-azidophenylethylfuran 461 into the pyrrolo[l,2-a]quinoline 462 as outlined in Scheme 13,386 and intramolecular addition to an azo group has been observed in the 8-azido-1-arylazonaphthalenes 463.387... 

The non-nitrogenous carbene precursor (102) was used for the photochemical generation of the carbene (103) without complications due to reactions of diazirine or diazo species. In the presence of alkenes, carbene (103) gave rise to cyclopropanes and in the absence of alkenes was proposed to undergo [1,2]-C shift to form (104), which suffered retro-Diels-Alder reaction to give a triene. 

More recent results support the view that Lewis acids, on the interaction with metal carbonyls, may facilitate dissociation of a ligand to leave a vacant site for alkene coordination.75 Photochemical generation of carbene species [Eq. (12.20)], the crucial step in photoinduced metathesis, is a complicated, multistep process 74,76,77... 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

The photochemical generation of hydroxy-o-quinodimethanes from o-alkylbenz-aldehydes may be followed by a thermal Diels-Alder reaction with alkenes (Scheme 9.33) [54]. This reaction is effective in both intermolecular and intramolecular modes, and has been applied to the synthesis of various natural products [55],... 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

This review article deals primarily with addition reactions of nucleophiles, electrophiles, and neutral radicals to photochemically generated radical ions of organic compounds and some organometallic compounds. Photocyclodimerizations of electron-rich alkenes, photo-Diels-Alder reactions between alkenes and alkadienes via dimer or heterodimer radical cations, and photocycloadditions via triplexes are also included. 

The photochemical generation and reactions of nitrile ylides have again been studied. Irradiation of azirine (55) in the presence of 1,4-naphthoquinone (56) gave the isoindoledione (57) via 1,3-dipolar addition of the transient nitrile ylide (SS)."" The kinetics of the reactions of nitrile ylides with electron-poor alkenes have been determined, with evidence being reported for the formation... 

The formation of aziridines by addition of photochemically generated nitrenes to alkenes has been extensively used in synthesis. The amino-acetal (119), for example, has been obtained in this way by photodecomposition of... 

Some arylcarbenes, prepared by photodecomposition of the corresponding diazo compounds, undergo intersystem crossing so effectively that the alkenes present are mainly attacked by triplet-excited carbenes. This type of reactivity has, in particular, been associated with nitro-substituted arylcarbenes. The reaction between photochemically generated 4-nitrophenyl-carbene and (Z)- and ( )-but-2-ene, which has been studied in particular detail, has shown that nonstereospecific carbene addition may become just as important as the stereospecific reaction. ... 

Many gew-diarylcyclopropanes have been synthesized photochemically from alkyl-substituted alkenes and diaryldiazomethanes. The outcome of the reaction is most sensitive to the alkene structure either cycloaddition to give cyclopropanes or abstraction-recombination to give alkenes can dominate. Thus, photochemically generated diphenylcarbene reacts only by cycloaddition with propene, but exclusively by abstraction-recombination with 2,3-dimethylbut-2-ene.39.90,9) alkyl-substituted alkenes undergo both types of reaction and, as a result, most 1,1-diaryIcyclopropanes are formed in moderate )deld. 

Numerous examples for the synthesis of cyclopropanes by [2-f 1] cycloaddition of photochemically generated acylcarbenes to alkenes, allenes, 1,3-dienes, and cycloheptatriene are given in Houben-Weyl Vol. 4/5b, pp 1158-1257 and in Vol. E19b, pp 1099-1107 and pp 1300-1303. 

Aside from these findings, the diastereoselective approach to alkenes by photochemically generated free carbenes cannot be controlled by experimental conditions. In contrast, the dia-stereomer ratio of the cyclopropanes formed can be varied within wide limits by conducting the cyclopropanation in the presence of an appropriate catalyst rather than photochemically (see Section 1.2.1.2.4.2.6.3.2.). 

Photochemically generated acylcarbenes do not only add to simple alkenes and arylalkenes, but also to functionalized systems such as enol ethers (see Houben-Weyl, Vol.4/3, p271), halogenated alkenes, and allyl-functionalized alkenes. As it seems, such reactions have been carried out only with alkoxycarbonylcarbenes (Table 6). 

Reactions of photochemically generated aryl carbenes with other species have also been studied. The mechanism of addition of carbenes to alkenes is of current interest. A non-concerted pathway with a reversible first step has been shown to be involved... 

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