Cross-Coupling Reactions of Aryldiazonium Salts with Allylsilanes under Merged Gold/Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1021/acs.orglett.7b01148

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.

Full text of DOI:10.1021/acs.orglett.7b01148

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/photo-
redox 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.
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
Hattention 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 difunctional-
ization 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 cross-
coupling 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).
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, sophis-
ticated ligand systems, or harsh reaction conditions. Among
other approaches, the work of Albini and co-workers
concerning the photosensitized-decomposition of benzenedia-
zonium 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 p-
chlorophenyldiazonium 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 Electron-
donating phosphine-ligated gold complexes gave comparable
results (entry 2); however, poor conversion was noted when
Received: April 15, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01148
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Letter
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
a
Catalysis
a
Table 1. Optimization Studies
a
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),
b
c
b
N2, rt, 3 h. Reaction kept for 8 h. Desired product was accompanied
by impurities and could not be isolated in pure form. Analytically pure
3ad′ was isolated after the hydrogenation reaction.17
entry
[Au] cat.
yield (%)
c
1
2
3
4
5
6
7
Ph3PAuNTf2
61 (54)
60 (52)
11
c
(p-OMeC6H4)3AuNTf2
c
(C6F5)3PAuNTf2
c
IPrAuNTf2
12
reactions. The ArN2BF4 bearing very strong electron-with-
drawing 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 substitu-
ents such as −OMe, −OPh, and 2,5-dimethoxyphenyldiazo-
nium 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 1-
allylnaphthalene (3ab) was obtained in 51% yield. Notably, 3-
allylquinoline (3ac) was also accessed, albeit in 27% yield. The
reaction also worked well with indolylaryldiazonium salts;
c
Ph3PAuOTf
38
c
Ph3PAuBF4
48
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
−
79 (69)
89 (80)
84 (77)
88 (79)
74 (66)
9
d
8
e
9
f
10
g
11
h
12
13
21
i
14
Ph3PAuCl
−
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. Yields are calculated by GC using 1,2-
dibromobutane as an internal standard. Isolated yields in parentheses.
Generated in situ by mixing 10 mol % of Ph3PAuCl and 12 mol % of
AgX (X = NTf2, OTf, BF4). 4 equiv of 2a was used. 8 equiv of 2a
was used. 2 mol % of [Ru(bpy)3(PF6)2] was used. 5 mol % of
Ph3PAuCl was used. In absence of [Ru(bpy)3(PF6)2]. Reaction kept
b
c
d
e
f
g
h
i
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
B
DOI: 10.1021/acs.orglett.7b01148
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Organic Letters
Letter
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′)
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.
Scheme 3. Effect of Counteranions of Diazonium Salt
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
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.
Scheme 6. 31P NMR Studies
Next, the effect of a silyl group on the cross-coupling was
investigated (Scheme 4). The electronic nature and the bulk of
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.
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
In conclusion, we have developed a facile method for cross-
coupling reactions of aryl diazonium salts with alkyltriallylsi-
lanes 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.
C
DOI: 10.1021/acs.orglett.7b01148
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Organic Letters
Letter
Ramamurthi, T. D.; Rominger, F. J. Organomet. Chem. 2009, 694, 592.
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(5) (a) Sahoo, B.; Hopkinson, M. N.; Glorius, F. J. Am. Chem. Soc.
2013, 135, 5505. For subsequent papers from the same research
group, see: (b) Hopkinson, M. N.; Sahoo, B.; Glorius, F. Adv. Synth.
Catal. 2014, 356, 2794. (c) Tlahuext-Aca, A.; Hopkinson, M. N.;
Garza-Sancheza, R. A.; Glorius, F. Chem. - Eur. J. 2016, 22, 5909.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01148.
■
S
1
Experimental procedures, analytical data, and H and
13NMR spectra of all newly synthesized products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: n.patil@ncl.res.in.
ORCID
(7) (a) Shu, X.-Z.; Zhang, M.; He, Y.; Frei, H.; Toste, F. D. J. Am.
Chem. Soc. 2014, 136, 5844. (b) Patil, D. V.; Yun, H.; Shin, S. Adv.
Synth. Catal. 2015, 357, 2622. (c) Um, J.; Yun, H.; Shin, S. Org. Lett.
Nitin T. Patil: 0000-0002-8372-2759
Notes
̀
2016, 18, 484. (d) Xia, Z.; Khaled, O.; Mouries-Mansuy, V.; Ollivier,
C.; Fensterbank, L. J. Org. Chem. 2016, 81, 7182. (e) Qu, C.; Zhang,
S.; Du, H.; Zhu, C. Chem. Commun. 2016, 52, 14400. (f) Alcaide, B.;
Almendros, O.; Busto, E.; Luna, A. Adv. Synth. Catal. 2016, 358, 1526.
(g) Li, H.; Shan, C.; Tung, C.-H.; Xu, Z. Chem. Sci. 2017, 8, 2610.
(8) He, Y.; Wu, H.; Toste, F. D. Chem. Sci. 2015, 6, 1194.
The authors declare no competing financial interest.
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.
(9) (a) Kim, S.; Rojas-Martin, J.; Toste, F. D. Chem. Sci. 2016, 7, 85.
(b) Tlahuext-Aca, A.; Hopkinson, M. N.; Sahoo, B.; Glorius, F. Chem.
Sci. 2016, 7, 89. (c) Cornilleau, T.; Hermange, P.; Fouquet, E. Chem.
Commun. 2016, 52, 10040. (d) Gauchot, V.; Lee, A.-L. Chem.
Commun. 2016, 52, 10163. (e) Alcaide, B.; Almendros, P.; Busto, E.;
́
Lazaro-Milla, C. J. Org. Chem. 2017, 82, 2177. (f) Gauchot, V.;
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D
DOI: 10.1021/acs.orglett.7b01148
Org. Lett. XXXX, XXX, XXX−XXX