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A simple and efficient alkylation of aromatic and heteroaromatic compounds via the direct S N 1-type nucleophilic substitution of benzylic alcohols in the presence of catalytic amounts of the strong Bronsted acid o -benzenedisulfonimide under neat conditions is herein reported. A library of di- and triaryl (and heteroaryl) methanes was prepared in good yields and high regioselectivity. The observed reactivity was shown to be in agreement with Mayr's nucleophilicity and electrophilicity scales.

Diastereoselective SN1-type reactions have been known for many years and have been used in the synthesis of many natural products. It has recently been shown that chiral nucleophiles, obtained using organocatalytic methodologies, can react with highly reactive carbenium ions in a stereoselective fashion. Based on this, carbenium ions can be applied in asymmetric catalytic reactions. 1 Introduction 1.1 Reaction of Ferrocenyl Alcohols with Lewis Acids 2 Reaction of Alcohols ‘on Water’ 2.1 Stability of Carbenium Ions and the Mayr Scale 3 Organocatalytic Stereoselective SN1-Type Reactions of Aldehydes and Isolated Carbenium Ions 3.1 Attempts at the Related Asymmetric SN1-Type α-Alkylation of Ketones 3.2 Organocatalytic Stereoselective SN1-Type Reactions under Oxidative Conditions 4 Organocatalytic SN1-Type Reactions Involving Lewis Acids 5 Alkylation of Aldehydes Made Simple with the Benzodithiolylium Ion 6 Organocatalytic SN1-Type Reactions Involving Bronsted Acids 7 SN1-Type Reactions Promoted by Chiral Thioureas 8 Conclusion and Perspectives

Recently, the direct substitution of allylic, benzylic, and tertiary alcohols has been achieved via SN 1-type reactions with catalytic amounts of Brønsted or Lewis acids. When a new stereogenic center is formed most of these transformations produce the desired product as a racemate, as these reactions proceed through carbenium ions. The arsenal of activation modes available in organocatalysis can be used to set up suitable reaction conditions in which chiral nucleophiles (enamine catalysis) or chiral electrophiles (iminium catalysis, chiral counterion catalysis) can easily be generated. Recently, we have used stabilized carbenium ions, directly available or obtained from the corresponding alcohols, in new organocatalytic stereoselective SN 1-type reactions. The commercially available carbenium ion benzodithiolylium tetrafluoroborate can be used for the straightforward organocatalytic stereoselective alkylation of aldehydes. In this account we will illustrate the application of this methodology in the total synthesis of natural products and the preparation of valuable starting materials.

Best of both worlds: Organocatalytic SN1-type reactions promoted by indium(III) salts were performed with activated benzylic and benzhydrylic substrates. High ee values and moderate diastereomeric ratios were obtained. The presence of a 4-NMe2Ph group, which is useful for further transformations, is essential for obtaining high yields. The synergy of indium(III) salts with organocatalysis is a useful strategy for stereoselective transformations.

Benzylic carbocations, which are easily generated in situ from alcohols or acetates by Bronsted or Lewis acids, undergo selective facial discrimination in diastereoselective reactions (see scheme; FG=functional group). The A values are responsible for the facial selectivity. Catalytic amounts of various Lewis acids can be employed in discriminating one face of the carbocation.