To understand some experimental data at odds with the computed mechanism of the CpCo(L2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.

Hexafluoroisopropanol (HFIP) is a solvent with unique properties that has recently gained attention for promoting a wide range of challenging chemical reactions. It was initially believed that HFIP was almost...

Scheme 11. Synthesis of butenolides and isocoumarins by gold-palladium dual catalysis; dba = trans,trans-dibenzylideneacetone.

Cyclopentenylidene gold complexes can easily be formed from vinyl allenes through a Nazarov-like mechanism. Such carbenes may transform in four different ways into polycyclic frameworks: electrophilic cyclopropanation, C-H insertion, C-C migration, or proton shift. We have studied the selectivity of these different pathways and used our findings for the expedient preparation of valuable complex molecules. An application to the total synthesis of a natural product, Delta(9(12))-capnellene, is presented. DFT computations were carried out to shed light on the mechanisms.

Activation of allenes toward nucleophilic attack by gold complexes ([Au], see scheme) often proceeds with axis‐to‐center chirality transfer. The stereochemical information is shown to be maintained in not only η2 allene complexes, but also in bent η1 allene complexes.

The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct "collapse" pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C sigma-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy surfaces are presented. The reactivity of triplet cobalt species was found kinetically disfavored over that of their singlet counterparts. Moreover, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet cobalt complexes cannot be ruled out since all unsaturated species appearing in this study were found to exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying minimum-energy crossing points (MECPs) of the two surfaces.

Aniline derivatives are frequently encountered in molecules of industrial relevance such as dyes or antioxidants, which make the development of synthetic methods for the functionalization of these privileged structures highly sought-after. A general protocol for the hydroarylation of electronically diverse alkenes with anilines would be ideal to provide densely functionalized compounds. Yet, this transformation has been underexplored compared to more traditional hydroarylation of unactivated alkenes because of the significant challenges associated with the control of the selectivity and its substrate tolerance. Herein, we describe a selective, versatile and user-friendly ortho-C-alkylation of anilines with alkenes that hinges on the beneficial combination of a Lewis acid (Ca(II)) and hexafluoroisopropanol as a solvent. This protocol allows for the extension of this transformation to highly deactivated styrenes and demonstrates a remarkable improved reactivity regarding aliphatic alkenes, styrene derivatives and dienes. In addition, DFT computations were performed which, combined with experimental observations, suggest a nearly concerted mechanism that impart the ortho-selectivity.

A direct Pd(II)-catalyzed kinetic resolution of heteroaryl-enabled sulfoximines through an ortho-C-H alkenylation/arylation of arenes has been developed for the first time. The coordination of sulfoximine pyridyl-motif and the chiral amino...