With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis.

A series of robust iridium(I) complexes bearing a sterically encumbered N-heterocyclic carbene ligand, alongside a phosphine ligand, has been synthesised and investigated in hydrogen isotope exchange processes. These complexes have allowed isotope incorporation over a range of substrates with the use of practically convenient deuterium and tritium gas. Moreover, these active catalysts are capable of isotope incorporation to particularly high levels, whilst employing low catalyst loadings and in short reaction times. In addition to this, these new catalyst species have shown flexible levels of chemoselectivity, which can be altered by simple manipulation of preparative approaches. Furthermore, a number of industrially-relevant drug molecules have also been labelled, including the sulfonamide containing drug, Celecoxib. Alongside detailed NMR experiments, initial mechanistic investigations have also been performed, providing insight into both substrate binding energies, and, more importantly, relative energies of key steps in the mechanistic cycle as part of the overall exchange process.

/pdf/the-synthesis-of-highly-active-iridium-i-complexes-and-their-2rlbrto0vb.pdf

The Synthesis of Highly Active Iridium(I) Complexes and their Application in Catalytic Hydrogen Isotope Exchange

Bench-stable complexes of the type [Ir(COD)(NHC)Cl] (NHC = N-heterocyclic carbene) have been investigated within the field of hydrogen isotope exchange. By employing a sterically encumbered NHC within such complexes and catalyst loadings of only 5 mol%, moderate to high deuterium incorporations were achieved across a range of aromatic ketones and nitrogen-based heterocycles. The simple and synthetically accessible catalysts reported herein present alternatives to phosphine-based species and increase the available labelling systems with respect to established iridium-based isotope exchange methodologies.

/pdf/application-of-neutral-iridium-i-n-heterocyclic-carbene-4msld60b7r.pdf

Application of neutral iridium(I) N-heterocyclic carbene complexes in ortho-directed hydrogen isotope exchange†‡

A series of pincer complexes of Rh has been prepared and tested as catalysts for the dimerization of terminal alkynes. The pincers included aryl/bis(phosphinite) POCOP, aryl/bis(phosphine) PCP, and diarylamido/bis(phosphine) PNP ligands. RhI complexes of the general form (pincer)Rh(SiPr2) or (pincer)Rh(H2) were used as catalysts. In addition, the apparent donating ability of the pincer ligands was gauged through the carbonyl stretching frequencies in (pincer)Rh(CO) complexes by IR spectroscopy. All surveyed Rh complexes acted as catalysts for dimerization of 4-ethynyltoluene, 1-hexyne, or trimethysilylacetylene. The products were a mixture of E- and gem-enyne isomers, with small amounts of oligomers in some cases. The Z-enyne isomers were not observed except in two reactions. None of the catalysts showed useful selectivity for either the E- or the gem-enyne product. However, the POCOP-based catalysts bearing PiPr2 donor arms performed faster and possessed apparently greater longevity (up to 20 000 TON) th...

A Series of Pincer-Ligated Rhodium Complexes as Catalysts for the Dimerization of Terminal Alkynes

Stereoselective synthesis of β-fluoroenyne by the reaction of gem-difluoroalkenes with terminal alkynes

The development of an iridium-catalyzed regio- and stereoselective dimerization process has enabled the formation of (Z)-enyne products. More specifically, low catalyst loadings of an iridium(I) complex, featuring a bulky N-heterocyclic carbene–phosphine ligand combination, has been successfully employed in this selective head-to-head dimerization of terminal alkynes via C–H activation.

/pdf/z-selective-dimerization-of-aromatic-terminal-alkynes-v5tao6e305.pdf

Z-selective dimerization of aromatic terminal alkynes catalyzed by an iridium(I) n-heterocyclic carbene-phosphine system

The optimized conditions render possible the preparation of (Z)-enynes whose yield and selectivity depend on the substituents of the arene moiety.

(Z)-Selective Dimerization of Aromatic Terminal Alkynes Catalyzed by an Iridium(I)—N-Heterocyclic Carbene—Phosphine System.

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Z-selective dimerization of aromatic terminal alkynes catalyzed by an iridium(I) n-heterocyclic carbene-phosphine system

(Z)-Selective Dimerization of Aromatic Terminal Alkynes Catalyzed by an Iridium(I)—N-Heterocyclic Carbene—Phosphine System.