Combining gold and photoredox catalysis: Visible light-mediated oxy-and aminoarylation of alkenes

Article abstract of DOI:10.1021/ja400311h

A room-temperature intramolecular oxy-and aminoarylation of alkenes with aryldiazonium salts has been developed using a novel gold and photoredox dual-catalytic system. The compatibility of these two catalytic modes has been established for the first time and demonstrates the potential of this system as a method to expand the scope of nucleophilic addition reactions to carbon-carbon multiple bonds.

Full text of DOI:10.1021/ja400311h

Communication
pubs.acs.org/JACS
Combining Gold and Photoredox Catalysis: Visible Light-Mediated
Oxy- and Aminoarylation of Alkenes
Basudev Sahoo, Matthew N. Hopkinson, and Frank Glorius*
NRW Graduate School of Chemistry, Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 40,
̈
̈
̈
48149 Munster, Germany
̈
S
* Supporting Information
Scheme 1. Gold-Catalyzed Addition to C−C Multiple Bonds
ABSTRACT: A room-temperature intramolecular oxy-
and aminoarylation of alkenes with aryldiazonium salts has
been developed using a novel gold and photoredox dual-
catalytic system. The compatibility of these two catalytic
modes has been established for the first time and
demonstrates the potential of this system as a method to
expand the scope of nucleophilic addition reactions to
carbon−carbon multiple bonds.
he selective functionalization of carbon−carbon multiple
Tbonds remains a highly attractive and widely applied
method for the preparation of complex organic molecules.
Nucleophilic addition processes catalyzed by gold complexes
have emerged as powerful tools for the direct functionalization of
alkynes and allenes, while alkenes have also been successfully
employed as substrates.1 In these transformations, Au acts as a
highly carbophilic π-Lewis acid, activating C−C multiple bonds
toward intra- or intermolecular nucleophiles with often
remarkable levels of chemo-, regio-, and/or stereocontrol. In
the vast majority of cases, however, nucleophilic attack onto the
π-system is followed by proto-deauration, leading to products of
hydrofunctionalization. In recent years, several approaches have
been developed which seek to expand the scope of these
transformations by extending the range of demetalation
pathways beyond protonation. Several publications have focused
on cascade nucleophilic addition−oxidative coupling processes
involving AuI/AuIII redox cycles facilitated by strong external
oxidants such as PhI(OAc)2 or Selectfluor (a trademark of Air
Products and Chemicals).2 Recently, an alternative strategy has
been disclosed whereby Au is used in combination with another
catalyst in a dual-metal system. Stoichiometric organogold
intermediates generated upon nucleophilic attack onto C−C
multiple bonds have been employed as coupling partners in
palladium-catalyzed cross-coupling processes, leading to prod-
ucts not accessible using either metal in isolation.3 The extension
of this methodology toward dual-catalytic systems, however, has
proved challenging.4
processes under mild conditions when performed in the presence
of visible light. In this paper, we report the successful
development of a dual Au and photoredox catalytic system
applicable to the intramolecular oxy- and aminoarylation of
alkenes with aryldiazonium salts.11−13 This process involves the
formation of new C−Nu and C−C bonds across the alkene and
occurs at room temperature upon irradiation with a simple
household light bulb (Scheme 1).
In a preliminary experiment, 4-penten-1-ol (1a) was reacted in
the presence of 4 equiv of phenyldiazonium tetrafluoroborate
(2a), which is known to act as a source of phenyl radicals in the
presence of [Ru(bpy)3](PF6)2 under visible light irradiation.14,15
Upon treatment with 5 mol% of this photoredox catalyst and 10
mol% of the Au(I) precatalyst Ph3PAuCl in degassed methanol
for 6 h in the presence of light from a 23 W fluorescent light bulb,
the tetrahydrofuran 3aa resulting from a cascade 5-exo-trig
cyclization−arylation process was observed as the major product
in 51% NMR yield. In contrast to previously reported Au-
catalyzed alkene oxyarylation processes, this dual-catalyzed
reaction proceeds at room temperature and does not require
stoichiometric amounts of a strong external oxidant.11 With the
aim of identifying a set of standard reaction conditions, an
optimization study was conducted (Table 1).16 The reaction
efficiency was found to be highly dependent on the Au catalyst
We sought to evaluate the viability of dual catalysis as a means
of expanding the scope of Au-catalyzed functionalization
reactions of C−C multiple bonds. In this regard, our attention
was drawn to the recent reports of dual-catalytic systems5
involving photoredox catalysis.6 In these transformations, the
combination of organo-,7 Lewis acid,8 or transition metal
catalysts such as palladium9 or copper10 with photoactive
complexes of ruthenium or iridium led to novel synthetic
Received: January 10, 2013
Published: April 3, 2013
© 2013 American Chemical Society
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Communication
Table 1. Optimization Studies
Table 2. Scope of Alkene Substrates
a
b
entry
[Au] catalyst
[Au]/[Ru] (mol%)
yield (%)
1
2
3
4
5
6
7
8
9
10
Ph3PAuCl
10/5
51
[dppm(AuCl)2]
AuCl3
10/5
22
10/5
trace
trace
trace
84
(pic)AuCl2
10/5
IPrAuCl
10/5
[Ph3PAu]NTf2
[Ph3PAu]NTf2
[Ph3PAu]NTf2
[Ph3PAu]NTf2
[Ph3PAu]NTf2
[Ph3PAu]NTf2
[Ph3PAu]NTf2

10/5
10/2.5
10/1
88 (79)
61
5/2.5
1/2.5
10/2.5
10/
/2.5
50
22
c
11
6
12
13
4
0
a
General conditions: 1a (0.2 mmol), [Au] catalyst, [Ru(bpy)3](PF6)2,
2a (0.8 mmol), degassed MeOH (2 mL), rt, 4−12 h, 23 W fluorescent
b
light bulb. Yield determined by 1H NMR using diethyl phthalate as an
c
internal standard. Isolated yields in parentheses. Reaction was
performed in the absence of light. dppm = bis(diphenylphosphanyl)-
methane, IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene, pic
= picolinato.
employed. Au(III) precatalysts such as AuCl3 or (pic)AuCl2 led
to only trace amounts of the oxyarylated product 3aa, while the
N-heterocyclic carbene-stabilized Au(I) complex IPrAuCl was
also unsuitable (Table 1, entries 1−5). The highest yield of 3aa
(84% NMR yield) was obtained using the Gagosz catalyst
[Ph3PAu]NTf2, which is thought to dissociate to afford a cationic
Au(I) species upon solvation (Table 1, entry 6). With the
optimal Au catalyst identified, the effect of changing the relative
loadings of each reactant and catalyst was investigated. A 10:2.5
mol% ratio of Au to Ru was found to give the highest yield of the
desired product (88% NMR yield, 79% isolated yield, Table 1,
entry 7), while lowering the equivalents of the aryldiazonium salt
2a or changing solvents from methanol had a detrimental effect
on the reaction efficiency. The dual-catalyzed nature of the
process was confirmed by control experiments. Performing the
reaction in the absence of light or in the absence of
[Ru(bpy)3](PF6)2 led to a dramatic reduction in the yield of
3aa (NMR yields not exceeding 6%), while no product was
observed in the absence of Au (Table 1, entries 11−13).
a
General conditions: 1b−j (0.2 mmol, 1 equiv), [Ph3PAu]NTf2 (10
mol%), [Ru(bpy)3](PF6)2 (2.5 mol%), 2b (4 equiv), degassed MeOH
b
(0.1 M), rt, 4−16 h, 23 W fluorescent light bulb. Isolated yields. dr
c
d
determined by 1H NMR. Reaction performed on a 0.4 mmol scale. 5
equiv of 2b used.
With a set of optimized reaction conditions in hand, we turned
our attention to an evaluation of the scope and limitations of the
dual-catalyzed process with a range of different alkenes and
aryldiazonium salts.17 The results of these studies are
summarized in Tables 2 and 3, respectively.
The reaction with (4-methylphenyl)diazonium salt 2b
proceeded readily with a range of substituted 4-penten-1-ol
derivatives 1a−g, delivering the corresponding tetrahydrofuran
products 3ab−gb in generally moderate to good yields (Table 2,
entries 1−7). In addition to tolerating substituents on the alkyl
tether, the reaction was also successful for substrates bearing
methyl groups on the alkene itself (Table 2, entries 4−7). In
contrast to previously reported Au-catalyzed alkene oxyarylation
processes,11 internal alkenes were also suitable substrates, with
(E)-4-hexen-1-ol (E)-1g and (Z)-4-hexen-1-ol (Z)-1g delivering
tetrahydrofurans ( )-(R,R)-3gb and ( )-(R,S)-3gb with high
diastereoselectivity in 59% and 56% yield, respectively (Table 2,
entries 6 and 7). In all cases, selective 5-exo-trig cyclization was
observed without contamination with products resulting from 6-
endo-trig cyclization. Similarly, 5-hexen-1-ol 1h, which possesses
an extra CH2 unit between the alkene and alcohol functionalities,
cyclized exclusively in a 6-exo-trig fashion to afford the
oxyarylated pyran product 3hb in a moderate 34% yield (Table
2, entry 8). In no case were 2-methyl-substituted heterocycles
resulting from conventional cyclization−protodeauration iso-
lated from the reaction mixture. The cascade nucleophilic
addition−arylation process was also successful with alkene
substrates 1i,j bearing a pendant nitrogen nucleophile. These
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Journal of the American Chemical Society
Communication
Table 3. Scope of Aryldiazonium Salts
Scheme 3. Mechanistic Hypothesis
a
General conditions: 1a (0.2 mmol, 1 equiv), [Ph3PAu]NTf2 (10 mol
%), [Ru(bpy)3](PF6)2 (2.5 mol%), 2a−h (4 equiv), degassed MeOH
(0.1 M), rt, 4−12 h, 23 W fluorescent light bulb. Isolated yields.
A tentative mechanistic hypothesis consistent with the above
observations is shown in Scheme 3. In this scenario, the cationic
Au(I) catalyst initially reacts with the alkene substrate to afford
the alkylgold(I) intermediate A resulting from anti-selective
cyclization. At this stage, aryl radicals generated upon Ru-
catalyzed photoredox decomposition of the diazonium salt could
react with this species to afford the Au(II) intermediate B bearing
both coupling partners.14,19 This unstable species would be
expected to rapidly donate an electron to RuIII, regenerating the
RuII photoredox catalyst and affording a highly electrophilic
Au(III) species, C. Reductive elimination at this stage would
deliver the product and regenerate the Au(I) catalyst.20 Although
further studies are required to determine whether a redox
mechanism of this type is operating in this system,21,22 the
implication that organic radicals generated via photoredox
catalysis could act as oxidants for Au(I) is an intriguing
possibility.23
In conclusion, a novel Au and visible light-mediated
photoredox dual-catalytic system has been applied to the
intramolecular oxy- and aminoarylation of alkenes with
aryldiazonium salts. This transformation gives access to arylated
heterocyclic compounds and occurs at room temperature under
irradiation from a simple household light bulb. Moreover, we
believe the apparent compatibility of Au and photoredox catalysis
demonstrated herein could lead to the development of a wide
range of novel, dual-catalyzed transformations of broad synthetic
appeal.
reactions delivered pyrrolidine products in good yields up to 84%
(Table 2, entries 9 and 10).
The oxyarylation process was also successful with a range of
diversely substituted aryldiazonium salts 2a−h. Under the
standard reaction conditions with 4-penten-1-ol (1a), arylated
tetrahydrofurans 3aa−ah were delivered in generally moderate
to good yields up to 83% (Table 3). Aryldiazonium salts bearing
an electron-withdrawing substituent were the most efficient
coupling partners, while comparatively electron-neutral groups
such as 4-methyl and 4-phenyl were also well tolerated. Notably,
halogenated substrates 2g,h also reacted successfully to afford the
corresponding products 3ag,ah possessing bromo or chloro
groups amenable to further functionalization.
Alongside scope and limitation studies, control experiments
were conducted to shed light on the reaction mechanism. In an
initial experiment, the effect of suspending the irradiation of light
during the reaction of alkene 1a and aryldiazonium salt 2a was
investigated. Whereas 40% conversion to product (NMR) was
observed after 20 min of visible light irradiation, the reaction shut
down once the light was switched off (1% conversion over 40
min stirring in the dark). Reapplying the light irradiation led to a
recovery of the catalytic activity, implying that the photoredox
and Au catalysts operate in tandem throughout the course of the
reaction.16 Insight into the reaction mechanism was also
provided by the stereochemical outcome of the reactions of
aryldiazonium salt 2a with deuterated alkene substrates D-(E)-
and D-(Z)-1i. These reactions proceeded in a diastereoselective
fashion to afford the corresponding pyrrolidines with stereo-
chemistry consistent with an overall trans-addition across the
alkene in each case (Scheme 2). Similar selectivity was observed
for internal alkene substrates (E)- and (Z)-1g (Table 2, entries 6
and 7). This observation is in line with previously reported Au-
catalyzed aminoarylation processes whereby an initial trans-
aminoauration is followed by oxidative arylation proceeding with
retention of stereochemistry.11,18
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data, and details of
optimization and control reactions. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
glorius@uni-muenster.de
■
Scheme 2. Aminoarylation of Deuterated Substrates
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank M. Suri for experimental assistance, Dr. K.
Bergander for NMR support, and Prof. Dr. A. Studer for helpful
■
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Journal of the American Chemical Society
Communication
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Council under the European Community’s Seventh Framework
Program (FP7 2007-2013)/ERC Grant agreement no. 25936,
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Alexander von Humboldt Foundation (M.N.H.).
Diamination: (j) Iglesias, A.; Muniz, K. Chem.−Eur. J. 2009, 15, 10563.
̃
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