Persistent Bissilylated Arenium Ions

Abstract

A series of bissilylated arenium ions 1 with different substitution patterns on the aryl ring have been synthesized by hydride abstraction from 2-aryl-substituted 2,6-dimethyl-2,6-disilaheptanes (2) via transient silylium ions. The arenium ions have been identified by their characteristic NMR chemical shifts, (delta(29)Si=19.1-25.6, delta(13)C(ipso) =89.0-102.4, delta(13)C(ortho)=160.9-182.0, delta(13)C(meta)=132.5-146.9, delta(13)C(para)=150.2-169.9) supported by quantum mechanical calculations of structures, energies, and magnetic properties at the B3LYP/6-311G(d,p)//B3LYP/6-31G(d) + DeltaZPVE level of theory. The calculations clearly reveal the charge dispersing and stabilizing effect of the silyl substituents in arenium ions 1. The bissilylated benzenium ion 1a is more stable than the parent benzenium ion (C(6)H(7)(+)) by 37.6 kcalmol(-1). The synthesized arenium ions 1 are stable in solution at room temperature for periods ranging from a few hours to days. This unusual stability is attributed to: 1) the thermodynamic stabilization of the arenium ion by two beta-silyl substituents and 2) the essentially non-nucleophilic reaction conditions (the use of the weakly coordinating [B(C(6)F(5))(4)](-) anion and aromatic hydrocarbons as solvents). Addition of stronger nucleophiles than aromatic hydrocarbons (for example, acetonitrile) results in desilylation of the arenium ion 1 and recovery of the 2-aryl-2,6-disilaheptane moiety.