Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos‐type ligands by chloride abstraction with GaCl 3 . The aryl carbenes react with PPh 3 and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N ‐oxide and intermolecular C−H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.

Nickel was identified as a catalyst for the cyclopropanation of unactivated olefins by using in situ generated lithiomethyl trimethylammonium triflate as a methylene donor. A mechanistic hypothesis is proposed in which the generation of a reactive nickel carbene explains several interesting observations. Additionally, our findings shed light on a report by Franzen and Wittig published in 1960 that had been retracted later owing to irreproducibility, and provide a rational basis for the systematic development of the reaction for preparative purposes as an alternative to diazomethane or Simmons-Smith conditions.

Chloromethylgold(I) complexes of phosphine, phosphite, and N‐heterocyclic carbene ligands are easily synthesized by reaction of trimethylsilyldiazomethane with the corresponding gold chloride precursors. Activation of these gold(I) carbenoids with a variety of chloride scavengers promotes reactivity typical of metallocarbenes in solution, namely homocoupling to ethylene, olefin cyclopropanation, and Buchner ring expansion of benzene.

Described here is a synthetic approach to access two of the most widely invoked cationic cobaltacycles in Cp*Co -catalyzed C-H functionalization reactions by C-H activation. The unique stabilizing capability of MeCN was used to surmount the previously proposed reversible nature of the C-H metalation step. Moreover, it is revealed the boosting effect of 1,1,1,3,3,3-hexafluoroisopropanol in the metalation step and in the reaction between N-pyrimidinylindole and diphenylacetylene under catalytic conditions.

Methylenation of electron-rich olefins is a highly challenging reaction, for which we have developed a new methodology exploiting Pd-catalysis and halomethylboronate reagents, the latter replacing diazomethane and zinc carbenoids as methylene donors. Optimization of the reaction for norbornene and extension to several other olefins are reported, with reasonable-to-excellent yields of cyclopropanes in combination with β-H elimination products. Several mechanisms are plausible for this methylenation reaction.

Lithiomethyl trimethylammonium triflate, prepared from tetramethylammonium triflate, cyclopropanates several styrenes and stilbenes with electron-donating and selected electron-withdrawing substituents efficiently. Kinetic data support a stepwise nucleophilic addition-ring closure mechanism for this methylenation.

Straightforward deprotonation of soluble tetramethylammonium salts with alkyllithium reagents gives lithiomethyl trimethylammonium reagents. Coordination of the Li cation is crucial to the stability of these 'N-C ylides'. These reagents were used to prepare epoxides, aziridines and allylic alcohols.

A case study in the (re)discovery of a non‐diazo, catalytic, electrophilic cyclopropanation reaction is presented to illustrate the role of mechanistic concepts in chemical innovation. The specific instance also highlights how specific personalities interacted in the report, retraction, and reconsideration of a novel methylene transfer reaction. The case study ends with a series of lessons learned, which generalize the findings from the specific case.