The electronic and steric factors that favour the formation of 1,2,4-and 1,3,5-regioisomers in the intermolecular [2+2+2] cyclotrimerisation of terminal alkynes are not well understood. In this work, this problem was analysed from a theoretical and experimental point of view. Density functional theory (DFT) calculations of the [2+2+2] cyclotrimerisation of p-X-substituted phenylacetylenes (X = H, NO2, and NH2) catalysed by [Rh(BIPHEP)] + were carried out to determine the reaction mechanism in each case and analyse the effect that the electronic character of the substituents has on the regioselectivity. For the rate-determining step corresponding to the oxidative coupling leading to the rhodacyclopentadiene intermediate, we have taken into account two reaction pathways: the reaction pathway with the lowest energy barrier and the reaction pathway through the most stable transition state (Curtin-Hammett pathway). Our results show that the theoretical results conform experimental outcomes for different p-X-substituted phenylacetylenes (X = NO2, F, H, Me, t Bu, OMe, NMe2) only when the Curtin-Hammett reaction pathway is considered. A fairly good correlation has been obtained between the electronic nature of the substituents (as expressed by the Hammett σpara constant values) and the regioisomeric ratios experimentally obtained and computationally predicted.
Predictive catalysis was applied to the reaction of pyridinium 1,4‐zwitterionic thiolates with a copper carbene. Theoretical calculations were first performed to determine the suitability of their annulation reaction and to make predictions about the performance of different copper salts, diazo species, and pyridinium derivatives. With calculations in hand, the experimental conditions were optimized, the scope of the reaction was assessed, and the precision of the predictions was evaluated. Yields up to 90% were obtained for the synthesis of indolizine scaffolds in the reactivity of pyridinium 1,4‐zwitterionic thiolates with a metal carbene.
The trapping of the elusive vinylogous position of a vinyl carbene with an aliphatic C(sp3)−H bond has been achieved for the first time during a silver‐catalyzed carbene/alkyne metathesis (CAM) process. A Tpx‐containing silver complex first promotes the generation of a donor‐acceptor silver carbene which triggers CAM, generating a subsequent donor‐donor vinyl silver carbene species, which then undergoes a selective vinylogous C(sp3)−H bond insertion, leading to the synthesis of a new family of benzoazepines. Density functional theory (DFT) calculations unveil the reaction mechanism, which allows proposing that the C−H bond insertion reaction takes place in a stepwise manner, with the hydrogen shift being the rate determining step.
The selective assembly of the 1,4-oxathiin nucleus has been treated as a powerful strategy to access this scaffold present in molecules with very interesting properties. In this study, the chameleon-like reactivity of pyridinium 1,4-zwitterionic thiolates is exploited to assemble the 1,4-oxathiin core through a [3 + 3] annulation. The optimal annulation partner has been found to be the iodonium ylide of the cyclic 1,3-diketones. The developed protocol allows the synthesis of a variety of bicyclic 1,4-oxathiin derivatives under very mild conditions under copper(I) iodide catalysis. Access to benzoannulated 1,4-oxathiins has been achieved through iodine-mediated aromatization of the initially obtained bicyclic compounds.
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