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DOI: 10.1039/C6RA19064B (Communication) RSC Adv., 2016, 6, 77830-77833

Stable yet reactive cationic gold catalysts with carbon based counterions†

Xiaojun Zeng, Shiwen Liu and Bo Xu*
College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620, China. E-mail: bo.xu@dhu.edu.cn

Received 27th July 2016 , Accepted 11th August 2016

First published on 11th August 2016

Abstract

L–Au–[TsC(CN)2] are new cationic gold catalysts with a carbon based counterion, which are widely applicable for gold catalyzed reactions. For reactions which need highly reactive gold catalysts, a Lewis acid co-catalyst can be added to increase the reactivity of L–Au–[TsC(CN)2]. L–Au–[TsC(CN)2] also have advantages of easy reaction steps and high functional group tolerance.

Cationic gold catalysis is one of the most important additions to organic synthesis in recent years.1–3 Silver-mediated halogen abstraction is the most common method to generate cationic gold from a gold catalyst precursor (e.g., L–Au–Cl). Recently we found that an excess of silver almost always had deleterious effect on the reactivity of gold catalysts,4 possibly due to the high affinity of silver towards gold and halogens.5–9 In general, preformed gold catalysts (e.g. L–Au–NTf2)10 have better reactivity. Preformed cationic gold catalysts also have the advantage of simplified experimental procedures. But cationic gold catalysts with commonly used counterions like OTf− are not bench stable for long-time storage. So it is highly desired to develop stable yet reactive cationic gold catalysts.

Commonly used counterions can be categorized as oxygen-based, nitrogen-based, carbon-based and fluoro-metalloid based or fluoroaryl-metalloid based (Fig. 1). Among these counterions, the O-based counterions (e.g. TfO−) and fluorometal based (e.g. SbF6−) are common and relatively inexpensive. In recently years, it has been reported that the N-based counterion (e.g. NTf2−) is a superior counterion for many gold catalyzed reactions.10 But reactivity of catalysts containing carbon based counterions are rarely explored in gold catalysis or even transition metal catalysis in general. And most literature reports on preparation of carbon based counterions need complex synthetic procedure,11–16 which significantly limited their wider adoption. In our continuing effort to improve the efficiency of gold catalysis,4,17–23 we tried to explore the effect of carbon based counterions on the reactivity of gold catalysts. Herein, we are glad to report stable yet reactive cationic gold catalysts with carbon based counterion – TsC(CN)2−, and TsC(CN)2− based reagents can be made conveniently and inexpensively.


image file: c6ra19064b-f1.tif
Fig. 1 Common counterions used in synthesis.

In order to make a carbon based counterion less coordinating, the carbon has to be attached to 1–3 strong electron withdrawing groups (Scheme 1a). Among many possible electron withdrawing groups, for easy synthesis, we chose p-toluenesulfonyl and –CN groups.24,25 The potassium salt of [TsC(CN)2] can be prepared in one step in good yield from inexpensive p-toluenesulfonyl chloride and malononitrile (Scheme 1b). Both Ag[TsC(CN)2] and L–Au–[TsC(CN)2] can be made conveniently from K[TsC(CN)2]. We prepared the PPh3–Au–[TsC(CN)2] and JohnPhos–Au–[TsC(CN)2] in quantitative yield and both of them are bench stable at room temperature. It also should be noted that both Ag[TsC(CN)2] and L–Au–[TsC(CN)2] are not hygroscopic unlike many silver salts (e.g. AgOTf).


image file: c6ra19064b-s1.tif
Scheme 1 Synthesis of Ag[TsC(CN)2] and L–Au–[TsC(CN)2].

To assess the applicability and generality of our prepared catalysts. We began by investigating the most common type of gold-catalyzed reactions – addition of oxygen nucleophiles to C–C unsaturated compounds (Scheme 2). First, JohnPhos–Au–[TsC(CN)2] was very efficient in regioselective transformation of alkynes into cyclic acetals (Scheme 2a),26–30 and the intermolecular version also worked well but higher temperature was needed (Scheme 2b).30 Similarly, JohnPhos–Au–[TsC(CN)2] also worked very well in hydration of propargyl acetate 6 using 1% gold loading and at room temperature (Scheme 2c).31 JohnPhos–Au–[TsC(CN)2] was also able to catalyze intermolecular addition of N-hydroxyl benzotriazole 8 to an alkyne (Scheme 2d).32 We also compared the efficiency of JohnPhos–Au–[TsC(CN)2] and commonly used L–Au–OTf and L–Au–NTf2 system. In cycloisomerization of propargyl amide 11, reaction catalyzed by JohnPhos–Au–[TsC(CN)2] was slower (Scheme 2e) and in the oxygen transfer reaction,33 they have similar reaction rate (Scheme 2f).


image file: c6ra19064b-s2.tif
Scheme 2 Addition of oxygen nucleophiles to alkynes.

In general, we expect TsC(CN)2]− counterion to be more coordinating than commonly used weakly coordinating counterions like OTf− or NTf2−, so TsC(CN)2]− based gold catalyst will have slower reaction rate than OTf− or NTf2− based gold catalysts depending the type of reactions. In other word, L–Au–[TsC(CN)2] is less Lewis acidic than gold catalysts such as L–Au–OTf. This feature could potentially lead to different selectivity or increased functional group tolerance. For example, JohnPhos–Au–Cl/AgOTf catalyzed intramolecular addition of hydroxyl group to allene gave a mixture of 5-exo-trig and 6-exo-trig cyclization products 15 and 15′ (Scheme 3a),34 but JohnPhos–Au–[TsC(CN)2] only gave 5-exo-trig product 15 exclusively. Similarly, JohnPhos–Au–[TsC(CN)2] also worked well in cyclization of allenyl alcohol 16 with only one isomer isolated (Scheme 3b). The addition of a carboxylic acid to an alkyne could produce a useful vinyl acetate intermediate (Scheme 3c)23,35 – but JohnPhos–Au–Cl/AgOTf system produced a mixture of double bond migration products and the hydrolysis by-product, 2-octanone (Scheme 3c). On the other hand, JohnPhos–Au–[TsC(CN)2] gave only desired product 18 exclusively (Scheme 3c). The same approach was used successfully in the intramolecular version of the reaction (Scheme 3d).


image file: c6ra19064b-s3.tif
Scheme 3 Different selectivity and increased functional group tolerance of L–Au–[TsC(CN)2].

Then we moved our attention to gold catalyzed addition of nitrogen nucleophiles to alkynes/allenes (Scheme 4). In intermolecular hydroamination of alkyne 3,36,37 JohnPhos–Au–[TsC(CN)2] worked well, only 0.1% loading is needed (Scheme 4a). In intramolecular cyclization of allenyl tethered amine 23, JohnPhos–Au–[TsC(CN)2] had similar efficacy as JohnPhos–Au–Cl/AgOTf system (Scheme 4b) reported in literature,34 but JohnPhos–Au–[TsC(CN)2] had advantage of easy reaction setup.


image file: c6ra19064b-s4.tif
Scheme 4 Addition of N–H to alkyne/allenes.

Then we investigated the gold catalyzed carbon nucleophiles to alkynes/alkenes (Scheme 5). Carbon nucleophiles are generally weaker than O, N nucleophiles, so more reactive gold catalysts are needed. JohnPhos–Au–[TsC(CN)2] system worked well in the C–H addition to alkynes (Scheme 5a) in the presence of co-catalyst Ga(OTf)3,38 the reaction was very slow without Ga(OTf)3. We also evaluated the Conia-ene reaction of 27, (Scheme 5b),39 JohnPhos–Au–[TsC(CN)2]/Ga(OTf)3 system worked very well under similar condition. In hydroarylation of 29 (ref. 10) (Scheme 5c), the product, 2H-chromene 30, was obtained in 93% yield at room temperature using JohnPhos–Au–[TsC(CN)2]/Ga(OTf)3 system. Lafollée and Gandon reported the use of Cu(OTf)2 to activate L–AuCl directly in the intramolecular C–H addition of alkene40 (Scheme 5d); JohnPhos–Au–[TsC(CN)2]/Cu(OTf)2 system also worked well in this reaction at 0.1% gold loading.


image file: c6ra19064b-s5.tif
Scheme 5 Addition of C–H to alkynes/alkenes.

We also investigated other types of gold catalyzed reactions (Scheme 6b). In cyclization of propargylic tert-butylcarbonate 33, 4-methylene-1,3-dioxolan-2-one 34 was obtained in good yield using JohnPhos–Au–[TsC(CN)2]/Ga(OTf)3 (Scheme 6a).41 In the gold-catalyzed 1,3-transposition of ynones (Scheme 6b),42 good result was also achieved using JohnPhos–Au–[TsC(CN)2]/Ga(OTf)3 system.


image file: c6ra19064b-s6.tif
Scheme 6 Other examples of gold-catalyzed reactions.

Conclusions

In summary, L–Au–[TsC(CN)2] are widely applicable catalysts for gold catalyzed reactions. For reactions which need more active catalysts, a Lewis acid co-catalyst can be added. L–Au–[TsC(CN)2] also have advantages of easy reaction step and increased functional group tolerance.

Acknowledgements

We are grateful to the National Science Foundation of China for financial support (NSFC-21472018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, and NMR of compounds. See DOI: 10.1039/c6ra19064b
This journal is © The Royal Society of Chemistry 2016
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