Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of S_NAr Chemistry

Abstract

The reactivity of the electron-rich anionic Al I aluminyl compound K 2 [(NON)Al] 2 (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono-and disubstituted arenes is reported. C À H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical S N Ar chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C À H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene CÀH activation at a site which is neither ortho nor para to a methyl substituent.

Oxidative Addition of Hydridic, Protic, and Nonpolar E–H Bonds (E = Si, P, N, or O) to an Aluminyl Anion

“…The 1 H NMR spectra for complexes 1−4 show the expected peaks for the NON Dipp − ligand system, with additional signals for the organic components of the ER m groups (ER m = SiH 2 Ph, PHMes, NHDipp, or OAr). The AlH signals were not observed in the 1 H NMR spectra, most likely due to coupling with the quadrupolar 27 Al nuclei, a phenomenon noted in the neutral (hydrido)aluminum compounds, Al(BDI Dipp )(H)(ER m ). 26 The presence of Al−H bonds in 1−4 was however confirmed using IR spectroscopy, with Al−H stretches observed between 1680 and 1760 cm −1 .…”

A meta‐Selective C−H Alumination of Mono‐Substituted Benzene by Using An Alkyl‐Substituted Al Anion through Hydride‐Eliminating SNAr Reaction

“…In summary, we revealed the reactivity of alumanylpotassium 1 toward toluene to form CÀH cleaved product 2 with a perfect meta-selectivity. [23] A relatively small kinetic isotope effect (k H /k D = 1.51) suggested a non-linear transition state. DFT calculations suggested a two-step reaction mechanism and electronically controlled meta-selectivity arising from the electron-donating methyl group.…”

Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory

“…Catalyst IV was found to activate H 2 in benzene solution at ambient conditions or 2 atm of this gas in the solid state. Moreover, the activation of C–H bonds in benzene and formation of Al–Al and Al–Mg bonds were also observed. , Following this report, several stabilized aluminyl anions have been prepared to be subject to experimental and theoretical characterizations. − …”