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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.

The versatility of the trityl cation (TrBF4) as a highly efficient Lewis acid organocatalyst isdemonstrated in a light induced benzylic brominaion of alkyl-arenes under mild conditions. The reaction wasconducted at ambient temperature under common hood light (55 W fluorescent light) with catalyst loadingsdown to 2.0 mol% using N-bromosuccinimide (NBS) as the brominating agent. The protocol is applicable toan extensive number of substrates to give benzyl bromides in good to excellent yields. In contrast to mostpreviously reported strategies, this protocol does not require any radical initiator or extensive heating. Forelectron-rich alkyl-arenes, the trityl ion catalyzed bromination could be easily switched between benzylic sp3CH functionalization and arene sp2 CH functionalization by simply alternating the solvent. Thischemoselective switch allows for high substrate control and easy preparation of benzyl bromides andbromoarenes, respectively. The chemoselective switch was also applied in a one-pot reaction of 1-methylnaphthalene for direct introduction of both sp3 CBr and sp2 CBr functionality., QC 20180926

A highly efficient aldehyde-olefin metathesis catalysed by the carbocation, 4-phenylphenyl-diphenylmethylium ion, has been developed. This protocol is characterized by high yields, low catalyst loading (down to 2 mol%), good functional group compatibility and mild reaction conditions., QC 20190110

In recent years the carbocation has re-emerged as a highly efficient Lewis acid catalyst for a variety of organic transformations. However, the goal of asymmetric carbocation catalysis has so far been out of reach mainly as a result of difficulties associated with the preparation of stable chiral carbocations. Here, we describe developments towards asymmetric carbocation catalysis based on the concept of chiral-anion-directed catalysis. Chiral tritylium salts can be conveniently prepared in situ by mixing trityl chloride derivatives with chiral phosphonate, phosphoramide, bis(sulfonyl) amide, and bis(sulfuryl) amide silver or sodium salts. It is shown that the bis(sulfuryl) amide/tritylium ion salt catalyzes the Diels-Alder reaction with an up to 53 % enantiomeric excess., QC 20160513

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 p(C) 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 130years 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)., QC 20150825

The versatility of the trityl cation (TrBF4) as a highly efficient Lewis acid organocatalyst is demonstrated in the oxa-Diels-Alder reaction of various unactivated aromatic and aliphatic aldehydes and simple unactivated dienes, such as isoprene and 2,3-dimethylbutadiene. The transformation proceeds smoothly to give 3,6-dihydropyrane adducts in high to moderate yields with catalyst loadings down to 1.0 mol-% under mild reaction conditions. In contrast to most previously reported strategies, this protocol does not require substrate functional group activation, neither by electron-deficient aldehydes (2-oxo aldehydes) or electron-rich dienes (methoxy or amino-butadiene)., QC 20151211