Synthesis and Reactivity of Fluorinated Triaryl Aluminum Complexes.

Darren M C Ould,Jamie L Carden,Rowan Page,Rebecca L Melen

doi:10.1021/acs.inorgchem.0c01076

Abstract

The addition of the Grignard 3,4,5-ArFMgBr to aluminum(III) chloride in ether generates the novel triarylalane Al(3,4,5-ArF)3·OEt2. Attempts to synthesize this alane via transmetalation from the parent borane with trimethylaluminum gave a dimeric structure with bridging methyl groups, a product of partial transmetalation. On the other hand, the novel alane Al(2,3,4-ArF)3 was synthesized from the parent borane and trimethylaluminum. Interestingly, the solid-state structure of Al(2,3,4-ArF)3 shows an extended chain structure resulting from neighboring Al···F contacts. Al(3,4,5-ArF)3·OEt2 was then found to be an effective catalyst for the hydroboration of carbonyls, imines, and alkynes with pinacolborane.

Highlights

Since the ground-breaking discovery of frustrated Lewis pairs (FLPs) in 2006,1 the use of triarylboranes to aid organic transformations has rapidly grown in the past 20 years.[2,3] Interestingly though, the use of the heavier aluminum analogues has received notably less attention
Al(C6F5)[3] has been applied extensively toward FLP chemistry, to undergo H2 activation and hydride transfer to alkenes,[13] C−H activation,[14,15] and the activation of CO2.16 a note of caution has always been present when working with Al(C6F5)[3]; Pohlmann and Brinckmann found that attempts to sublime the crude mixture of AlCl3 and the Grignard C6F5MgBr in ether led to an explosion, as did heating a solution of AlEt3 with B(C6F5)[3] to 70 °C.4
Structural refinement of the single-crystal data revealed that an aluminum dimer with bridging methyl groups and only one aryl group had formed (2), as opposed to the expected Al(3,4,5-ArF)[3] triarylalane (Scheme 1)

Summary

Introduction

Since the ground-breaking discovery of frustrated Lewis pairs (FLPs) in 2006,1 the use of triarylboranes to aid organic transformations has rapidly grown in the past 20 years.[2,3] Interestingly though, the use of the heavier aluminum analogues has received notably less attention. Pohlmann and Brinckmann successfully prepared tris(pentafluorophenyl)alane [Al(C6F5)3] as the Et2O adduct in 1965,4 but no further reports were made until the analogous THF adduct was generated in 1995.5 Since these reports there have been disputes in the literature regarding the Lewis acidity of Al(C6F5)[3] and whether it is a stronger Lewis acid than B(C6F5)[3], with claims by Lee et al and Stahl et al that the latter is the much stronger Lewis acid.[6,7] On the other hand, a number of experimental and computational observations Lcoenwtirsadacicidt .t8h−i1s0vAiewpoasnsidblseuepxppolratnaAtli(oCn6fFo5r)3thbeeidnigsatghreeesmtreonntgeinr the Lewis acidity of Al(C6F5)[3] is its tendency to form strong adducts with Lewis bases, which in turn quenches its reactivity It was not until 2016 that the unsolvated structure of Al(C6F5)[3] was reported by Chen et al, who achieved this by transmetalation from B(C6F5)[3] with AlEt3 in hexane.[11]. This was observed in the solid-state structure of the compound which exists as a dimer [Al(C6F5)3]2 that displayed close intermolecular Al···F interactions between the aluminum center of one molecule and an ortho-fluorine atom of a second molecule.[11]

Methods

Results

Conclusion