Disubstituted pyrroles alkene

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. 

Secondary thioamides, precursors of N-protonated azomethine ylids, give A1-pyrrol ine derivatives (after elimination of thiol). When the dipolarophiles are dissymmetric alkenes, high regioselectivity is obtained. Reaction with alkynes leads to pyrrole derivatives in good to excellent yields and with ArCHO leads regioselectively to 2,5-disubstituted 2-oxazolidines in moderate yields.264... 

Pyrroles (e.g., 16, Scheme 17.3) were produced in the reaction of aminocar-bene complexes (e.g., 13) and a,p-unsaturated aldehydes in a net [3-1-2]-cyclocondensation process [4]. The mechanism proposed involves cyclopropanation to afford a donor-acceptor activated cyclopropane (14), followed by ring opening to afford enamine-aldehyde 15, which undergoes intramolecular condensation and tautomerization to afford the pyrrole product. The optimal conditions employ the use of molecular sieves as an additive. A variety of alkylcarbene complexes that feature both alkyl and aryl groups at nitrogen were employed successfully in this transformation. Several aldehydes featuring disubstituted and trisubstimted alkene groups were employed similarly. 

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