Electrophilic Aromatic Substitution with Silicon Electrophiles: Catalytic Friedel–Crafts C−H Silylation

• Published in: Angewandte Chemie International Edition Vol. 56; no. 1; pp. 52 - 59
• Main Authors: Bähr, Susanne, Oestreich, Martin
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 02.01.2017
• Edition: International ed. in English
• Subjects: Aromatic compounds; Catalysis; cations; Chemical bonds; C−H functionalization; electrophilic aromatic substitution; homogeneous catalysis; Intermediates; Protonation; Protons; Silicon; Si−H activation; Substitution reactions; C−H functionalization; cations; homogeneous catalysis; Si−H activation; electrophilic aromatic substitution
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201608470

Electrophilic aromatic substitution is a fundamental reaction in synthetic chemistry. It converts C−H bonds of sufficiently nucleophilic arenes into C−X and C−C bonds using either stoichiometrically added or catalytically generated electrophiles. These reactions proceed through Wheland complexes, cationic intermediates that rearomatize by proton release. Hence, these high‐energy intermediates are nothing but protonated arenes and as such strong Brønsted acids. The formation of protons is an issue in those rare cases where the electrophilic aromatic substitution is reversible. This situation arises in the electrophilic silylation of C−H bonds as the energy of the intermediate Wheland complex is lowered by the β‐silicon effect. As a consequence, protonation of the silylated arene is facile, and the reverse reaction usually occurs to afford the desilylated arene. Several new approaches to overcome this inherent challenge of C−H silylation by SEAr were recently disclosed, and this Minireview summarizes this progress. No way back: Unlike Friedel–Crafts alkylation, the related silylation remained elusive for decades. While both are reversible, the ability of C−Si bonds to stabilize adjacent positive charges makes the corresponding Wheland complexes low‐energy intermediates, thereby favoring the reverse reaction. Recently developed methods that overcome this problem differ in the catalytic generation of the silicon electrophile but all involve the release of H2.