Disubstituted pyrroles donor

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

Electron-donor substituents on the vinyl group further enhance reactivity. Methyl 2-(l-trimethyl-silyloxyethenyl)-pyrrole-l-carboxylate reacts with methyl propynoate to give a 4,7-disubstituted indole (Equation (141)) <9lH(32)ll99>. 

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. 

---