Cross-Coupling Reactions of Aryldiazonium Salts with Allylsilanes

Letter pubs.acs.org/OrgLett

Cross-Coupling Reactions of Aryldiazonium Salts with Allylsilanes under Merged Gold/Visible-Light Photoredox Catalysis Manjur O. Akram,†,‡ Pramod S. Mali,† and Nitin T. Patil*,†,‡ †

Division of Organic Chemistry, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 025, India

‡

S Supporting Information *

ABSTRACT: A method for the cross-coupling reactions of aryldiazonium salts with trialkylallylsilanes via merged gold/photoredox catalysis is described. The reaction is proposed to proceed through a photoredox-promoted generation of an electrophilic arylgold(III) intermediate that undergoes transmetalation with allyltrimethylsilane to form allylarenes.

H

Scheme 1. Cross-Coupling Reaction of Aryl Diazonium Salts [ArN2X] with Organosilanes: Known and Present Work

omogeneous gold catalysis has received a great deal of attention over the past decade. In the majority of these transformations, the π-acidity of gold complexes triggers the activation of C−C multiple bonds such as alkenes, allenes, and alkynes, thereby favoring the attack of nucleophile.1 Unlike other late transition metals, gold(I) complexes are generally unreactive toward the oxidative addition of aryl and vinyl halides and pseudohalides, limiting the potential application of gold in the field of coupling reactions.2 Recent research revealed that the realization of reactivity similar to that of late transition metals is possible with two-electron redox processes wherein Au(III) intermediates are accessible.3 However, those reactions utilize sacrificial oxidants such as I3+ derivatives or F+ sources and often require harsh reaction conditions.4 Recently, Glorious and co-workers have pioneered an overall redox-neutral strategy for accessing Au(III) intermediates using merged gold/photoredox catalysis, effecting oxyarylation reaction of alkenes.5 Since then, the merged gold/photoredox protocol has emerged as an ecofriendly and powerful tool for accessing Au(III)-intermediates.6 Several other research groups including those of Toste, Fensterbank, Shin, Alcaide, and Zhu have exploited the potential of this reactivity for difunctionalization of C−C multiple bonds.7 The potential of merged gold/ photoredox catalysis could also be harnessed for C−P8 and C− C9 bond-forming processes as elegantly shown by the research groups of Toste, Glorious, Lee, Fouquet, and Alcaide. Interestingly, the research group of Hashmi and Shi independently developed photosensitizer-free approaches to access Au(III) intermediates for C−C coupling reactions.10 Inspired by the above-mentioned literature and especially by Toste’s report9a on the gold/photoredox-catalyzed crosscoupling reaction of alkynyltrimethylsilanes and aryldiazonium tetrafluoroborates (Scheme 1, path a), we envisioned that gold(III) complexes generated via photoredox catalysis might undergo transmetalation with allyltrialkylsilanes followed by reductive elimination to form a C(sp2)−C(sp3) linkage leading to allylarenes (path b). © 2017 American Chemical Society

Allylarenes represent an important class of substituted aromatic compounds11 which are traditionally accessed via (1) Friedel−Crafts allylation,12 (2) reaction of arylmetals (magnesium, copper etc.) with allylic halides13 or with π-allyl palladium complexes,14 and (3) palladium/copper-catalyzed reaction of aryl halides with allylmetals (magnesium, copper etc.), allylboronic acids, allylstannanes, or allylsilanes.15 Most of these approaches often require fluoride/base sources, sophisticated ligand systems, or harsh reaction conditions. Among other approaches, the work of Albini and co-workers concerning the photosensitized-decomposition of benzenediazonium salts to generate phenyl cation and subsequent allylation is particularly noteworthy.16 However, it is limited to 4-substituted diazonium salts and requires UV light for the sensitization. Therefore, we endeavored to investigate whether the proposed strategy (Scheme 1, path b) is efficient to access allylarenes with broad substrate scope. To this end, we explored the cross-coupling reaction of pchlorophenyldiazonium tetrafluoroborate (1a) with 2 equiv of allyltrimethylsilane (2a) in the presence of 10 mol % of Ph3PAuNTf2 and 5 mol % of [Ru(bpy)3(PF6)2] in degassed acetonitrile (0.2 M) under irradiation with 23 W fluorescent light (CFL). Gratifyingly, the desired product 3a was isolated in 54% yield (entry 1). The reaction efficiency was found to be highly dependent on the [Au] catalyst employed.17 Electrondonating phosphine-ligated gold complexes gave comparable results (entry 2); however, poor conversion was noted when Received: April 15, 2017 Published: June 7, 2017 3075

DOI: 10.1021/acs.orglett.7b01148 Org. Lett. 2017, 19, 3075−3078

Letter

Organic Letters electron-withdrawing phosphine was used as ligand on the Au center (entry 3). N-Heterocyclic carbene-stabilized Au(I) complex, i.e., IPrAuNTf2, was also found to be unsuitable for the present reaction (entry 4).7g Next, we decided to examine the role of counteranions in gold complexes. Accordingly, gold catalysts such as Ph3PAuOTf, Ph3PAuBF4, and Ph3PAuCl were examined (entries 5−7), out of which Ph3PAuCl catalyst was found to be the best, giving 3a in 69% yield. When the allyltrimethylsilane (2a) was used in excess (4 equiv), 3a was isolated in 80% yield (entry 8). Further increase in stoichiometry of 2a did not improve the yield of the reaction (entry 9). Pleasingly, when the catalyst loading of the Ru complex was lowered to 2 mol %, 3a was obtained in 79% yield (entry 10). On the other hand, lowering the catalyst loading of Ph3PAuCl caused a deleterious effect on the outcome of reactions (entry 11). The presence of both catalysts is essential; the absence of either of the catalysts led to poor conversion (entries 12 and 13). No product was observed when the reaction was performed in the dark, which clearly indicated the importance of light for the present transformation (entry 14). With the optimized reaction conditions in hand (Table 1, entry 10), we turned our attention to examine the scope and

Scheme 2. Cross-Coupling Reactions of ArN2BF4 with Allyltrialkylsilanes under Merged Gold/Photoredox Catalysisa

Table 1. Optimization Studiesa

a

entry 1 2 3 4 5 6 7 8d 9e 10f 11g 12h 13 14i

[Au] cat. c

Ph3PAuNTf2 (p-OMeC6H4)3AuNTf2c (C6F5)3PAuNTf2c IPrAuNTf2c Ph3PAuOTfc Ph3PAuBF4c Ph3PAuCl Ph3PAuCl Ph3PAuCl Ph3PAuCl Ph3PAuCl Ph3PAuCl − Ph3PAuCl

Reaction conditions: 0.20 mmol of 1, 0.80 mmol of 2a, 10 mol % of Ph3PAuCl, 2 mol % of [Ru(bpy)3](PF6)2, degassed MeCN (0.2 M), N2, rt, 3 h. bReaction kept for 8 h. cDesired product was accompanied by impurities and could not be isolated in pure form. Analytically pure 3ad′ was isolated after the hydrogenation reaction.17

yieldb (%) 61 (54) 60 (52) 11 12 38 48 79 (69) 89 (80) 84 (77) 88 (79) 74 (66) 9 21 −

reactions. The ArN2BF4 bearing very strong electron-withdrawing groups such as −NO2, −COMe, and −CO2Me provided 3d, 3e, and 3f in slightly better yields (63−77%). However, ArN2BF4 bearing electron-donating substituents such as −OMe and 3,4-methylenedioxy provided 3g (51%) and 3k (47%) in slightly lower yields. Very interestingly, the carboxylic acid groups were also tolerated, giving products 3h and 3i in 54 and 60% yields, respectively. Diazonium salts bearing ethynyl group provided the product 3j in 58% yield. It was noted that ortho-substitution in the aromatic ring of diazonium salts gave comparably lower yields, probably due to steric reasons. For example, ArN2BF4 containing halo groups such as −Cl and −Br provided the corresponding allylated products 3l and 3m in 55 and 53% yields, respectively. Electron-withdrawing substituents, such as −NO2, −CN, and −COMe, were also well tolerated to afford the corresponding products 3n, 3o, and 3p in 63, 73, and 55% yields, respectively. However, electron-donating substituents such as −OMe, −OPh, and 2,5-dimethoxyphenyldiazonium salts led to slightly poor conversion, i.e., 49 (3q), 41 (3r), and 42% (3s) yields, respectively. Conversely, the reaction proceeded smoothly irrespective of the electronic nature of the substituent at the meta-position, thus affording the allylarenes 3t−y in moderate to good yields (58−72%). Disubstitution on the aromatic ring of the ArN2BF4 was also well tolerated, giving products 3z and 3aa in 65 and 64% yields, respectively. Further, replacing the benzene ring with naphthalene backbone in the diazonium salts did not hamper the reaction, and the desired 1allylnaphthalene (3ab) was obtained in 51% yield. Notably, 3allylquinoline (3ac) was also accessed, albeit in 27% yield. The reaction also worked well with indolylaryldiazonium salts;

a

Reaction conditions: 0.20 mmol 1a, 0.40 mmol 2a, 10 mol % of [Au] cat., 5 mol % of [Ru(bpy)3(PF6)2], degassed MeCN (0.2 M), N2, 23 W CFL bulb, rt, 3 h. bYields are calculated by GC using 1,2dibromobutane as an internal standard. Isolated yields in parentheses. c Generated in situ by mixing 10 mol % of Ph3PAuCl and 12 mol % of AgX (X = NTf2, OTf, BF4). d4 equiv of 2a was used. e8 equiv of 2a was used. f2 mol % of [Ru(bpy)3(PF6)2] was used. g5 mol % of Ph3PAuCl was used. hIn absence of [Ru(bpy)3(PF6)2]. iReaction kept in dark. IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene.

limitations of the present cross-coupling reaction (Scheme 2). The reaction is found to be general irrespective of the substituents (electron-withdrawing/donating group) present in the aromatic rings of ArN2BF4. The reaction was well tolerated with ArN2BF4 containing −Br and −I substituents at the para position, giving 3b and 3c in 68 and 61% yields, respectively. These results should be of great importance because the resultant allylarenes can be further transformed to more functionalized scaffolds by metal-catalyzed cross-coupling 3076

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Organic Letters

Ru(III) species. This unstable Au(II) species is expected to undergo SET with Ru(III), affording a cationic Au(III)−aryl complex III with the regeneration of Ru(II). Alternatively, species II can also undergo a single-electron transfer with another equivalent of diazonium salt to yield the Au(III) species III and aryl radical which may trigger a radical-chain process as demonstrated by Glorious and co-workers.9b However, this possibility is ruled out based on a light on/off experiment which indicates continuous irradiation is essential.17 The allyltrimethylsilanes then undergo transmetalation19 with III to generate neutral Au(III) intermediate IV bearing both coupling partners, which eventually would undergo reductive elimination to afford the cross-coupled product. To gain further mechanistic insights, a few experiments were conducted. It was found that Ph3PAuCl was unreactive with allyltrimethylsilane 2a when stirred under light irradiation using a 23 W CFL bulb in the presence of a catalytic amount of [Ru(bpy)3(PF6)2] in CD3CN.17 This experiment rules out the possibility of Au(I)−allyl intermediate, indicating the oxidation of neutral Ph3PAuCl may be the first step. Next, to prove the intermediacy of Au(III)−Ar species III, the reaction employing 0.9 equiv of allyltrimethylsilane (2a) was monitored by 31P NMR (Scheme 6). After 1 h, the complete disappearance of 2a

however, the product (3ad) could not be isolated in pure form. When the crude reaction mixture was hydrogenated under Pd/ C conditions,17 product 3ad′ was isolated in pure form. Note that the reaction is not applicable for the substituted allyltrimethylsilanes such as crotyl(trimethyl)silane and cinnamyl(trimethyl)silane. Next, we became curious to know the role of counteranions associated with diazonium salts on the outcome of the reaction (Scheme 3). It was found that p-ClC6H4N2·O(CO)CF3 (1a′) Scheme 3. Effect of Counteranions of Diazonium Salt

and p-ClC6H4N2.PF6 (1a″) react reasonably well with 2a to give 3a in 68 and 49% yields, respectively. However, the reaction did not work in the case of p-ClC6H4N2·OTs (1a‴), clearly pointing out the crucial role of counteranions in aryldiazonium salts. Further studies are required to understand the precise role of counteranions. Next, the effect of a silyl group on the cross-coupling was investigated (Scheme 4). The electronic nature and the bulk of

Scheme 6. 31P NMR Studies

Scheme 4. Scope with Allylsilanes

was observed (1H NMR studies) with the appearance of a new signal (δ 22.9 ppm) which can account for species V, which was also detected by HRMS.17 However, this phosphonium species (V) was not observed in in the actual catalytic reaction since 2a was used in excess. The detection of species V [(Ph3P-Ar)+ where Ar = p-ClC6H4] thus clearly accounts for the intermediacy of Au(III) intermediate III.7a,9f Accordingly, it was believed that nucleophilic attack of the distal carbon of the C−C double bond in 2a would occur at the Au(III)−aryl intermediate (III), thereby rendering electron-donating arenes less efficient.9a The failure of substituted allylsilanes to undergo the present reaction could be attributed to steric factors.7a Note that the present reaction works only with allylsilanes, and simple terminal alkenes (for instance, 1-octene) are found to be inert under the established reactions conditions. This observation can be explained on the basis of the stability of the transient intermediate due to the β-silicon effect. In conclusion, we have developed a facile method for crosscoupling reactions of aryl diazonium salts with alkyltriallylsilanes under merged gold/photoredox catalysis involving Au(I)/ Au(III) redox cycle. The method does not require any sacrificial amount of base, proceeds under mild conditions, and shows excellent functional group tolerance.

the silicon atom were found to be very crucial for the successful outcome of the reactions. The allylsilanes 2b and 2f failed to give desired products, while allylsilanes 2c−e gave 3a in good to acceptable yields (61−73%). On the basis of various literature reports,5a,9a,f a plausible mechanism18 for the cross-coupling reaction is proposed in Scheme 5. At first, Ph3PAu(I)Cl reacts with the aryl radicals, generated upon Ru-catalyzed photoredox decomposition of the diazonium salt, to afford the Au(II) intermediate II along with Scheme 5. Proposed Catalytic Cycle

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01148. Experimental procedures, analytical data, and 1H and 13 NMR spectra of all newly synthesized products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nitin T. Patil: 0000-0002-8372-2759 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous financial support by the Board of Research in Nuclear Science (BRNS), Mumbai (Grant No. 37(2)/14/12/ 2015/BRNS/10549), is gratefully acknowledged. M.O.A. thanks CSIR for the award of a Senior Research Fellowship.

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DOI: 10.1021/acs.orglett.7b01148 Org. Lett. 2017, 19, 3075−3078