Introduction

Owing to the high redox potential of Au(I)/Au(III) couple, gold catalyzed coupling reactions usually require external excessive oxidants or strong electrophilic reagents [1,2,3,4]. Based on these means, gold-catalyzed cross-coupling have made great progress and witnessed rapid development in recent years [5,6,7,8,9]. However, external excessive oxidants or strong electrophilic reagents generally result in unfriendly atomic economy and poor functional group compatibility. Therefore, the development new catalytic model of gold needs to be further improved [10,11,12,13].

The strategies of the combination of photocatalysis and gold catalysis have provided a refreshing and powerful pattern in this research area [14,15,16], which can overcome the barrier with high oxidation energy barrier from monovalent gold to trivalent gold. Throughout gold photocatalysis during the past few years, most of them so far have involved a single electron transfer (SET) of dinuclear gold catalysts. Nevertheless, harnessing visible light to realize excited states of the substrates by energy transfer (EnT) has remained relatively underdeveloped in gold catalysis. In general, the gold-based complex can be converted to the corresponding excited-state gold complex after absorbing energy from visible light. The excited-state gold complex is able to undergo the EnT process by transferring its energy to the other ground-state species, causing the formation of triplet-state of the substrates [17]. In this short review, we will introduce three breakthrough cases [18,19,20] in gold catalysis based on EnT, enriching and improving the development of gold homogenous catalysis.

In 2019, the groups of Fensterbank [18] carried out an in-depth study on a dual catalysis transformation, which includes electrophilic mononuclear gold catalysis and iridium photosensitization, leading to C(sp2)–C(sp) cross-coupling reaction under blue LED (Scheme 1). Interestingly, the 2-(p-tolylethynyl)phenol (1) reacts with iodoethynyl benzene (2) smoothly under the reaction conditions that use Ir[dF(CF3)ppy]2(dtbbpy)PF6 (1 mol%), (p-CF3Ph)3PAuCl (5 mol%), 1,10-phenanthroline (10 mol%) as ligand with irradiation by blue LED. This strategy can be applied to achieve the synthesis of a series of benzofuran containing arylalkyne (4–6) or alkyl alkyne (7) in good yields.

Scheme 1
scheme 1

Alkynylative cyclization of o-alkylnylphenols with iodoalkynes

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As a plausible mechanism is shown in Scheme 1, [Ir-F]* (12) that is excited by blue LED firstly interacts with the compound of benzofuran vinylgold (8) intermediate by EnT, leading to excited-state vinylgold (9). The alkynyl iodine (2) may occur oxidation addition with the excited-sate vinygold intermediate (8) to achieve C(sp2)–C(sp) coupling to finally deliver the benzofuran products (3) after reductive elimination. Fensterbank’s group also made a detailed modeling studies in his paper to support the event of EnT, rather than a redox pathway.

The utility of the dinuclear gold(I) complex in EnT reactions was recently reported by Hashmi and co-workers [19]. They reported a significant advance in the field of carbo cyclization/borylation of alkynes under visible-light irradiation using aryl iodides (Scheme 2). Significantly, the carbo-cyclization/gem-diborylation products can be achieved in 90% yield by using 1.0 mol % [Au2(μ-dppm)2](OTf)2 as photocatalyst (Scheme 3). It represents an important breakthrough in the dinuclear gold chemistry beyond the single electron transfer mechanism.

Scheme 2
scheme 2

Carbo-cyclization/borylation of alkynes

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Scheme 3
scheme 3

Visible light-induced α‑C(sp3) − H acetalization

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With the standard reaction conditions in hand, as shown in Scheme 2, benzofuran-(16, 19)-, indole-(18, 21)-, and benzothiophene-(17)-based benzylic gemdiboronates are obtained in satisfying results. Furthermore, the terminal alkynes, dialkyl-substituted alkynes, and the benzene rings which contain carbonyl and bromine substituents give the products in excellent yields (78–88%). As a possible mechanism shown in Scheme 2, the ground-state gold catalyst Au2(μ-dppm)2(OTf)2 with Na2CO3 in MeCN under blue LEDs could induce the formation of an excited-state gold complex (22). Aryl iodide compounds then proceed the EnT process with it to generate a triplet-state aryl iodide (25). This complex (25) readily undergoes C-I homolysis, leading to the formation of aryl radical. Subsequently, a 5-exo-cyclization produces vinyl radical (27), which continues to form alkenyl Bpin (29). The intermediate (30) is converted to the final product by a second borylation and aromatization. After EnT, the excited-state gold complex goes back to the ground-state, completing the photocatalytic cycle of gold complexes. The success of EnT process of dinuclear gold complex would significantly promote its future development.

After Hashmi and Zhang’s pioneering work, they also applied this EnT strategy to realize α‑C(sp3) − H acetalization under visible-light irradiation [20]. With this protocol, they can realize the concise synthesis of acetals (35–37), thioacetals (38), and α-alkoxypyrrolidines (39–40) in good yields (up to 95%).

Based on the possible mechanism mentioned in Scheme 2, the ground-state dinuclear gold(I) complexes are photoexcited to its excited-state under the irradiation of blue LEDs, which can induce an EnT event for aryl iodides. The resulting triplet-state iodobenzene (41) undergoes a homolytic cleavage to generate the aryl radical (42), which can go through a HAT progress with α-C(sp3)-H. The produced alkyl radical (43) is proposed to combine with iodine radical, leading to the formation of complex (44 or 45). Finally, the SN2 nucleophilic substitution of intermediate (44 or 45) with alcohols affords the final products.

In summary, we have witnessed the important discovery of EnT events in homogeneous gold catalysis. This new catalytic model provides a new opportunity to realize gold-catalyzed organic transformations by means of photocatalysis besides the classical single electron transfer pathway. It is believed that there will be more examples in this field inspired the recent reports.