In this work, we present a powerful synthetic strategy for selectively converting phosphole oxides by reaction with sulfonyl isocyanates into the corresponding sulfonylimino phospholes, which show a decrease or increase in solid-state emission.

A full account of the Brønsted acid catalyzed, enantioselective synthesis of 4H-chromenes and 1H-xanthen-1ones from o-hydroxybenzyl alcohols and β-dicarbonyl compounds is provided. The central step of our strategy is the BINOL− phosphoric acid catalyzed, enantioselective cycloaddition of βdiketones, β-keto nitriles, and β-keto esters to in situ generated, hydrogen-bonded o-quinone methides. Upon acid-promoted dehydration, the desired products were obtained with generally excellent yields and enantioselectivity. Detailed mechanistic studies including online-NMR and ESI-MS measurements were conducted to identify relevant synthetic intermediates. A reversible formation of a dimer from the starting alcohol and the reactive o-quinone methide in an off-cycle equilibrium was observed, providing a reservoir from which the o-quinone methide can be regenerated and introduced into the catalytic cycle again. Reaction progress kinetic analysis was utilized to determine kinetic profiles and rate constants of the reaction uncovering o-quinone methide formation as the rate-limiting step. In combination with Hammett plots, these studies document the relationship between o-quinone methide stabilization by electronic effects provided by the substituents and the reaction rate of the described process. In addition, DFT calculations reveal a concerted yet highly asynchronous [4 + 2]-cycloaddition pathway and an attractive CH−π interaction between the catalyst's tBu group and the o-quinone methide as an important stereochemical control element.

An organocatalytic, highly enantioselective addition of cyclic enamides to in-situ-generated 3-methide-3H-pyrroles with subsequent cyclization and elimination has been developed. This protocol represents a novel and straightforward route toward polycyclic cyclopenta­[b]­pyrroles with high yields and excellent enantioselectivity. Central to the success is the formation of a chiral, hydrogen-bonded 3-methide-3H-pyrrole upon phosphoric-acid-catalyzed dehydration of the starting 1H-pyrrol-3-yl carbinol that reacts with the enamide in a stepwise cycloannulation process.

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