The direct alkylation of indoles with a wide variety of tertiary propargylic alcohols under Brønsted acid catalysis has been studied. A general and environmentally friendly method for the synthesis of C3-propargylated indoles with a quaternary carbon at the propargylic position has been developed. The reactions are highly regioselective regarding both the indole and the alkynol counterparts.Only with N-unsubstituted-2-arylindoles a competitive S N´ reaction takes place affording 3-dienyl or 3-allenylindoles depending on the alkynol moiety. Reactions have been carried out in air with undried solvent, and water was the only side product.____________ [a]
A sulfoxide-directed, metal-free ortho-propargylation of aromatics and heteroaromatics exploits intermolecular delivery of a propargyl nucleophile to sulfur followed by an intramolecular relay to carbon. The operationally simple cross-coupling procedure is general, regiospecific with regard to the propargyl nucleophile, and shows complete selectivity for products of ortho-propargylation over allenylation. The use of secondary propargyl silanes allows metal-free ortho-coupling to form carbon–carbon bonds between aromatic and heteroaromatic rings and secondary propargylic centres. The ‘safety-catch’ nature of the sulfoxide directing group is illustrated in a selective, iterative double cross-coupling process. The products of propargylation are versatile intermediates and they have been readily converted into substituted benzothiophenes.
Abstract.A general and efficient synthesis of 4,9-dihydro-1H-carbazoles from 3-allenylmethylindoles is reported. The process, catalyzed by a cationic gold(I) complex, involves a formal C2H bond activation of the indole unit by reaction with allenes. The nature of the substituents at the allylic as well as the terminal position of the allene moiety has a crucial effect on the regioselectivity of the cyclization, which is also influenced by the catalyst and the solvent employed. Moreover, some evidence of the 2 contribution of different reaction routes is provided which led us to propose a plausible multi-pathway mechanism consistent with all the results described.
SummaryThe selectivity of our previously described gold-catalyzed tandem reaction, 1,2-indole migration followed by aura-iso-Nazarov cyclization, of 3-propargylindoles bearing (hetero)aromatic substituents at both the propargylic and terminal positions, was reversed by the proper choice of the catalyst and the reaction conditions. Thus, 3-(inden-2-yl)indoles, derived from an aura-Nazarov cyclization (instead of an aura-iso-Nazarov cyclization), were obtained in moderate to good yields from a variety of 3-propargylindoles.
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