Pyrrole photochemical reaction with alkenes

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

The UV irradiation of the 4,5-dihydro-1,3,5-oxazaphosph(V)olene 39 leads to a cyclic elimination. Analogously to the thermal [5 + 3 + 2] cyclo elimination, the photochemical reaction generated nitrile ylides reacted with alkynes and alkenes to yield the 2/f-pyrrole 40 and the pyrrol-l-ine 41, respectively (equation 17)71. 

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

The photochemical addition of alkenes to the pyrrole ring of indole to give cyclobutane products was first reported by Julian and Foster in 1973 [43], They found that both electron-rich and electron-poor alkenes add to indole, but only if an acyl group is present on the indole nitrogen atom. With monosubstituted alkenes the reaction was regioselective and, with the exception of vinyl acetate, the exo stereoisomer was produced as either the major or the only product (Scheme 15). A mechanism involving the indole triplet excited state was suggested, based on the observation that the reaction could be sensitized by acetophenone and quenched by naphthalene. 

Cantrell, T. S., Photochemical reactions of 2-acylthiophenes, -furans and -pyrroles with alkenes, J. Org. Chem., 39, 2242,1974. 

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