Gold‐Catalyzed Allylation of Aryl Boronic Acids: Accessing Cross‐Coupling Reactivity with Gold

Abstract

A sp 3 -sp 2 C-C cross-coupling reaction catalyzed by gold in the absence of a sacrificial oxidant is described. Vital to the success of this method is the implementation of a bimetallic catalyst bearing a bis(phosphino)amine ligand. A mechanistic hypothesis is presented, and observable transmetallation, C-Br oxidative addition, and C-C reductive elimination in a model gold complex are shown. We expect that this method will serve as a platform for the development of novel transformations involving redox-active gold catalysts. KeywordsGold Catalysis; C-C Coupling; Oxidative Addition; Bimetallic Catalysis; AllylationThe air-and water-stability of gold catalysts, coupled with their ability to promote complex transformations under mild conditions has attracted considerable interest from the academic community. 1 Despite the rapid pace of recent developments, the majority of gold-catalyzed processes rely on a select few reaction manifolds: (i) Lewis acid catalysis, (ii) π-activation, and (iii) the generation of carbenoid intermediates (Scheme 1A). 2 While these modes of reactivity have yielded important catalytic methodologies of broad scope and synthetic utility, 3 they are typified by catalytic cycles wherein gold maintains a +1 oxidation state, in stark contrast to the 2-electron redox cycles characteristic of late transition metal catalysis. 4 Indeed, access to Au III intermediates under catalytic conditions typically requires strong F + or I 3+ oxidants. 5,6 Despite this limitation, seminal work by Kochi and Schmidbaur has shown that Au I complexes oxidatively add alkyl halides, and are further competent to undergo C-C reductive elimination, furnishing formally cross-coupled products. 7 However, this mode of reactivity has not previously been realized in a catalytic fashion.

Rational development of catalytic Au(I)/Au(III) arylation involving mild oxidative addition of aryl halides

“…From practical and synthetic viewpoints, this transformation is complementary to the recently developed gold-catalyzed oxidative arylations. It involves aryl halides as electrophiles 54 , it tolerates electron-donating as well as electron-withdrawing substituents and it does not require an external oxidant to generate the key Au(III) catalytic intermediate 55 , 56 . Due to their ability to promote oxidative addition to gold and stabilize Au(III) species, hemilabile ligands such as Me-Dalphos hold great potential in gold-catalyzed arylation reactions.…”

Gold-Catalyzed Oxidative Coupling of Alkynes toward the Synthesis of Cyclic Conjugated Diynes

“…46 Several bis-gold catalysts were tested. Interestingly, while dppm, dppp and BINAP-bound gold complexes gave poor results similar to PPh 3 AuCl (entry 3), slightly improved yield (15%) of 2a was obtained using Xantphos[AuCl] 2 cat-1 (entry 4).…”

“…To improve the selectivity towards desired intramolecular cyclization over intermolecular polymerization, we wondered whether bis-gold complexes could improve the reaction performance through a faster transmetalation between two gold atoms in one catalyst. 46 Several bis-gold catalysts were tested. Interestingly, while dppm, dppp and BINAP-bound gold complexes gave poor results similar to PPh 3 AuCl (entry 3), slightly improved yield (15%) of 2a was obtained using Xantphos[AuCl] 2 cat-1 (entry 4).…”

Insights into the Mechanism of Gold(I) Oxidation with Aryldiazonium Salts

“…One of the earliest strategies encountered was the use of ab imetallic Au I -Au I complex, as its two-electron oxidation led to am ore energeticallya ccessible Au II -Au II complex, which facilitated for the first time ag old cross-coupling reaction in the absence of oxidants. [7] Later strategies relied on the design of specific ligands, which finely modulate the environment of Au I to access Au III . [8] Although interesting from af undamental viewpoint, this approachl ack translation into catalytic processes, as up to now it requiredt he presence of chelating atoms on the substrate being coupled, or the use of aryliodides as electrophiles.…”