Pyrroles from nitrile ylides

The coupling of alkynes and iminocarbene complexes (e.g., 4, Scheme 17.2) affords pyrrole ring systems (e.g., 6) in a formal [3-1-2]-cycloaddition process [2,3]. The pyrroles obtained are analogous to those obtained from nitrile ylide cycloadditions. A mechanism was proposed involving a net metalla-[4- -2] cycloaddition to afford azametallacycle 7 followed by reductive elimination and alkene isomerization. The regiochemistry arises through a nonconcerted addition to the triple bond via... 

Furthermore, the chloro ester 1-Me also readily reacted with the nitrile ylide 37 at room temperature, however, the pyrrole derivative 40 was the only isolated product (Scheme 9) [26 a]. The latter was obviously formed from the primary cycloadduct 38 by a cyclopropylcarbinyl to homoallyl rearrangement [1]. 

Cyclization of the nitrile ylide generated from amide (150a) via an imidoyl chloride led to an 18% yield of a fused tricyclic pyrrole (Scheme 46).77 Similarly, the nitrile ylide derived from (150b) cyclized in 6% yield. 

Michael addition has been applied to the formation of cyclopropanic systems. Thus, the addition of phosphonate carbanions generated by LDA in THF, by NaH in THF, by thallium(I) ethoxide in refluxing THF, or by electrochemical technique to oc,P-unsaturated esters provides a preparation of substituted 2-(alkoxycarbonyl)cyclopropylphosphonates in moderate to good yields via a tandem Michael addition-cyclization sequence (Scheme 8.47). The cyclopropanation has also been achieved via oxidation with iodine in the presence of KF-AI2O3. Nitrile ylides prepared from acyl chlorides and diethyl isocyanomethylphosphonate in the presence of Et3N react with methyl acrylates by a 1,3-dipolar cycloaddition to give phosphoryl pyrrolines or pyrroles. ... 

The most important general method for preparing isopyrroles is by reaction of electrophiles with 1//-pyrroles. Other methods include cyclization of open-chain compounds, 1,3-dipolar cycloadditions using nitrile ylides, and preparations from pyrrolines. 

Fukushima and Ibata reported direct evidence for the formation of an acyl nitrile ylide intermediate during Rh2(OAc)4-catalyzed reactions of a-diazoaceto-phenones 133 with nitriles in the presence of dimethylacetylene dicarboxylate (DMAD). The authors isolated mixtures of both 5-aryl-2-phenyloxazoles 137 and tetrasubstituted pyrroles 138 (Scheme 1.37). They proposed that 137 arose from the acyl nitrile yhde 136 via 1,5-cyclization, whereas 1,3-diploar-cycloaddition of 136 with DMAD followed by a 1,5-hydrogen shift produced 138. The authors showed that 138 did not arise via a Diels-Alder reaction of 137 with DMAD. 

Irradiation of 3-vinyl-2/f-azirine (126) gives, among other products, the azatriene 128, which arises via intramolecular hydrogen transfer of vinylnitrile ylide 127. The pyrrole 129, which would have been derived from 1,5-dipolar cyclization of 127, was not found. When the reaction is run in the presence of acrylonitrile or methyl acrylate, the 1,3-dipolar cycloadducts 130 (X = CN or CO2CH3) are obtained in high yield, thus arguing for the intermediacy of the nitrile ylide. 

The N-unsubstituted azomethine ylides derived from the imines of a-amino nitriles are intramolecularly trapped by acetylenes to give pyrroles... 

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