Allenes are important building blocks in organic synthesis and have attracted much attention from researchers worldwide. So far, mono-and 1,3-disubstituted allenes can be prepared easily by applying the allenylation of terminal alkynes reaction of aldehydes. However, trisubstituted allenes have never been reported using this reaction due to the extremely low reactivity of ketones. Here, we report the one-pot synthesis of trisubstituted allenes from terminal alkynes and ketones utilizing cadmium iodide as the mediator. With this protocol, a series of different allenes, including 1,5-bisallenes and optically active allenols, which are especially important in synthetic chemistry, have been successfully prepared.

A Lewis acid that has received negligible attention as a catalyst is the carbocation. The carbocation is isoelectronic to boron and owes its Lewis acidity to a low‐lying empty pC orbital. In terms of reactivity and stability carbocations are very versatile Lewis acids, from the extremely unstable methylium cation to the water‐stable tris(N,N‐dimethylaniline) methylium ion (crystal violet). Although the Lewis acid properties of carbocations have been extensively studied since the discovery of the tropolium ion more than 130 years ago there is only a handful examples on the application of carbocations as Lewis acid catalysts. Herein, the research on triarylmethylium (trityl)‐cation catalysis is summarized. In light of the reports the trityl ion emerges as a highly efficient and highly versatile Lewis acid catalyst capable of catalyzing different classes of reactions often with high selectivity and low catalyst loadings (for some reactions down to ppm levels).

The mechanochemical, solvent‐free, highly regioselective, rhodium‐catalyzed C−H methylation of (hetero)arenes is reported. The reaction shows excellent functional‐group compatibility and is demonstrated to work for the late‐stage C−H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution‐based C−H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

4-Phenylphenyl-diphenylmethylium tetrafluoroborate catalyses a rare high yielding intramolecular aldehyde–olefin metathesis of enals under mild reaction conditions and low catalyst loading.