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.

The C-H imidation of arenes and heteroarenes has been achieved via visible light induced photocatalysis. In the presence of an iridium(III) photoredox catalyst, the reaction of aromatic substrates with N-chlorophthalimide furnishes the N-aryl products at room temperature through a nitrogen-centered radical mediated aromatic substitution.

A rhodium-catalyzed oxygenative [2 + 2] cycloaddition of terminal alkynes and imines has been developed, which gives β-lactams as products with high trans diastereoselectivity. In the presence of a Rh(I) catalyst and 4-picoline N-oxide, a terminal alkyne is converted to a rhodium ketene species via oxidation of a vinylidene complex and subsequently undergoes a [2 + 2] cycloaddition with an imine to give rise to the 2-azetidinone ring system. Mechanistic studies suggest that the reaction proceeds through a metalloketene rather than free ketene intermediate. The new method taking advantage of catalytic generation of a ketene species directly from a terminal alkyne provides a novel and efficient entry to the Staudinger synthesis of β-lactams under mild conditions.

Nitrogen-Centered Radical-Mediated C-H Imidation of Arenes and Heteroarenes via Visible Light Induced Photocatalysis. -The photocatalytic imidation of simple arenes and pyridines is reported. The imidation of monosubstituted benzene derivatives leads to a mixture of all three possible isomeric products. -(KIM, H.; KIM, T.; LEE, D. G.; ROH, S. W.; LEE*, C.; Chem. Commun. (Cambridge) 50 (2014) 66, 9273-9276, http://dx.

Rhodium-Catalyzed Oxygenative [2 + 2] Cycloaddition of Terminal Alkynes and Imines for the Synthesis of -Lactams. -(KIM, I.; ROH, S. W.; LEE, D. G.; LEE*, C.; Org. Lett. 16 (2014) 9, 2482-2485, http://dx.