Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C-H Functionalizations.

Marina Uzelac,Alan R Kennedy,Eva Hevia

doi:10.1021/acs.inorgchem.7b00549

Marina Uzelac, Alan R Kennedy + Show 1 more

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https://doi.org/10.1021/acs.inorgchem.7b00549

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Journal: Inorganic chemistry	Publication Date: May 9, 2017
Citations: 29	License type: cc-by

Affiliation: University of Strathclyde

Abstract

Merging two topical themes in main-group chemistry, namely, cooperative bimetallics and frustrated-Lewis-pair (FLP) activity, this Forum Article focuses on the cooperativity-induced outcomes observed when the tris(alkyl)gallium compound GaR3 (R = CH2SiMe3) is paired with the lithium amide LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) or the sterically hindered N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu). When some previously published work are drawn together with new results, unique tandem reactivities are presented that are driven by the steric mismatch between the individual reagents of these multicomponent reagents. Thus, the LiTMP/GaR3 combination, which on its own fails to form a cocomplex, functions as a highly regioselective base (LiTMP)/trap (GaR3) partnership for the metalation of N-heterocycles such as diazines, 1,3-benzoazoles, and 2-picolines in a trans-metal-trapping (TMT) process that stabilizes the emerging sensitive carbanions. Taking advantage of related steric incompatibility, a novel monometallic FLP system pairing GaR3 with ItBu has been developed for the activation of carbonyl compounds (via C=O insertion) and other molecules with acidic hydrogen atoms such as phenol and phenylacetylene. Shedding new light on how these non-cocomplexing partnerships operate and showcasing the potential of gallium reagents to engage in metalation reactions or FLP activations, areas where the use of this group 13 metal is scant, this Forum Article aims to stimulate more interest and activity toward the advancement of organogallium chemistry.

Highlights

Deprotonative metalation is one of the most useful and widely used synthetic tools to functionalize organic molecules by transforming a relatively inert C−H bond into a more polar metal−C bond.[1]
Mulvey’s structural and reactivity studies using amidoalkyl bimetallic combinations established the concept of alkali-metalmediated metalation, where combining an alkali metal with a less electropositive metal such as magnesium, zinc, manganese(II), or iron(II) can promote unprecedented regioselective magnesiation, zincation, manganation, or ferration of aromatic molecules usually inert toward single-metal magnesium,[7] zinc,[8] manganese(II),[9] or iron(II)[10] reagents
Demonstrating that GaR3 is a viable effective Lewis acid (LA) for promoting small-molecule-activation processes when paired with sterically demanding inert atmosphere because of their air sensitivity (ItBu), we have recently reported the reduction of aldehydes, by insertion into the C O functionality at the C2 carbene position, affording zwitterionic compounds such as ItBuCH(p-Br-C6H4)OGaR3 (16; Scheme 7i).28d Reflecting the cooperativity of the ItBu/GaR3 pair, Scheme 7

Summary

■ INTRODUCTION

Deprotonative metalation is one of the most useful and widely used synthetic tools to functionalize organic molecules by transforming a relatively inert C−H bond into a more polar (and reactive) metal−C bond.[1]. One R group on gallium acts as a base, to give IPr·GaR2(C CPh) (24), with IPr behaving as an ancillary ligand This reactivity is similar to that described previously by Mitzel and co-workers for the reaction of GaMe3 and 4-ethynyl-2,6-lutidine, where the metalation of terminal alkyne by GaMe3 occurs with concomitant release of methane.[48] Very recently, Uhl and co-workers demonstrated that gallium hydrazides can act as active Lewis pairs for the cooperative C−H bond activation of phenylacetylene[49] where the steric bulk of the tris(alkyl)gallium reagent dictates the extent of metalation.[50] Complexes 23 and 24 illustrate how the reactivity of these new NHC/Ga FLP systems can be finely tuned by small modifications on the steric bulk of the components, in this case the LB (Scheme 9). The most informative resonances in the 13C NMR spectrum are CNHC−Ga at 179.4 ppm, which is upfield-shifted compared to the free carbene, and the two alkynyl resonances at 106.9 ppm (Ga C CPh) and 112.5 ppm (GaC CPh) (see the Experimental section and Supporting Information for details)

■ CONCLUSION AND OUTLOOK

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES