Carbocations as Lewis Acid Catalysts: Reactivity and Scope

Abstract

One class of potential Lewis acids that has received negligible attention as a catalyst is the carbocation. Here we show the potential of triarylmethylium ions as highly powerful Lewis acid catalysts for organic reactions. The Lewis acidity of the triarylmethylium ion can be easily tuned by variation of the electronic properties of the aromatic rings and the catalytic activity of the carbocation is shown to correlate directly to the level of stabilization of the empty p C -orbital at the cationic carbon. The versatility of triarylmethylium ions as efficient Lewis acid catalysts for organic reactions is demonstrated in Diels-Alder, aza-Diels-Alder, conjugate addition, halogenation, epoxide rearrangement and intramolecular hetro-ene reactions.

Direct Organocatalytic Oxo‐Metathesis, a trans‐Selective Carbocation‐Catalyzed Olefination of Aldehydes

“…[12,19] As expected, the oxo-metathesis became slower as the electron density of one of the aromatic rings increased (i.e., decreasing the Lewis acidity of the carbocation) on going from trityl to a mono-methyl-(4) to mono-methoxy-substituted (5) trityl cation ( Table 2, entries 9-12). Unfortunately, decreasing the Lewis acidity of the trityl ion did not prevent any side-reactions and had a negative effect on the yield.…”

“…[11][12][13][14][15][16] In fact, the application of carbocations as Lewis acid catalysts is poorly studied and we reasoned that the carbocation might have properties different to those of metal-or metalloid-based Lewis acid catalysts. With this in mind we set out to evaluate the trityl cation in the reaction reported by Bickelhaupt and co-workers.…”

Carbocation Catalysis: Oxa‐Diels–Alder Reactions of Unactivated Aldehydes and Simple Dienes

“…The trityl ion constitutes a rather unique center of Lewis acidity that can be easily tuned by alteration of the electronic properties of the pendant aromatic rings 14a,14b. As expected, the rate of reaction decreased as electron density of the aromatic ring increased (e.g., decreasing Lewis acidity of the carbocation), going from hydrogen to methoxy (Table 1, entry 4).…”

“…In the course of our work we found that the trityl ion [e.g. triphenylcarbenium tetrafluoroborate (TrBF 4 )] is highly efficient for the activation of α,β‐aldehydes and tosyl imines in the Diels–Alder reaction14a,14b and the aza‐Diels–Alder reaction,14b respectively (Scheme , a and b).…”

Synthesis of a Chiral Borate Counteranion, Its Trityl Salt, and Application Thereof in Lewis‐Acid Catalysis

“…Additionally, the enormous electrophilicity of these Lewis acids could be incompatible with the relatively electron‐rich naphthyl groups in [ 4 ] – . We therefore also considered testing the corresponding trityl salt [Ph 3 C] + · [ 4 ] – as catalyst in asymmetric Diels–Alder reactions . Franzén and coworkers had recently shown that trityl cations promote challenging Diels–Alder reactions of cyclohexa‐1,3‐diene,[17a], [17b] and the same group accomplished a related enantioselective Diels–Alder reaction of 2,3‐dimethylbuta‐1,3‐diene, directed by a chiral counteranion.…”