Combining transition metal catalysis with radical chemistry: Dramatic acceleration of palladium-catalyzed C-H arylation with diaryliodonium salts

Article abstract of DOI:10.1002/adsc.201200738

This paper describes a photoredox palladium/iridium-catalyzed C-H arylation with diaryliodonium reagents. Details of the reaction optimization, substrate scope, and mechanism are presented along with a comparison to a related method in which aryldiazonium salts are used in place of diaryliodonium reagents. The unprecedentedly mild reaction conditions (25 C in methanol), the requirement for light and a photocatalyst, the inhibitory effect of radical scavengers, and the observed chemoselectivity trends are all consistent with a radical-mediated mechanism for this transformation. This stands in contrast to the analogous thermal reaction with diaryliodonium reagents that is believed to proceed via an ionic' 2e^- pathway and requires a much higher reaction temperature (100 C).

Full text of DOI:10.1002/adsc.201200738

FULL PAPERS
DOI: 10.1002/adsc.201200738
Combining Transition Metal Catalysis with Radical Chemistry:
À
Dramatic Acceleration of Palladium-Catalyzed C H Arylation
with Diaryliodonium Salts
Sharon R. Neufeldta and Melanie S. Sanforda,*
a
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, USA
Fax : (+1)-734-647-4865; phone: (+1)-734-615-0451; e-mail: mssanfor@umich.edu
Received: August 17, 2012; Published online: December 4, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200738.
Abstract: This paper describes a photoredox palladi- are all consistent with a radical-mediated mechanism
À
um/iridium-catalyzed C H arylation with diaryliodo- for this transformation. This stands in contrast to the
nium reagents. Details of the reaction optimization, analogous thermal reaction with diaryliodonium re-
substrate scope, and mechanism are presented along agents that is believed to proceed via an ꢀionicꢁ 2eÀ
with a comparison to a related method in which aryl- pathway and requires a much higher reaction tem-
diazonium salts are used in place of diaryliodonium perature (1008C).
reagents. The unprecedentedly mild reaction condi-
tions (258C in methanol), the requirement for light
and a photocatalyst, the inhibitory effect of radical Keywords: C H activation; diaryliodonium salts;
scavengers, and the observed chemoselectivity trends palladium; photochemistry; radicals
À
Introduction
24 h) in acetic acid.[6] We reasoned that the rates of
these reactions could potentially be enhanced by re-
The merger of transition metal catalysis with radical routing them through radical pathways in which Ar2I+
chemistry has emerged as a powerful strategy for is converted into ArC in situ [Scheme 1, (b)].
achieving high-yielding transformations under mild
Tentative support for the feasibility of this ap-
conditions.[1] The ability to reroute textbook metal- proach is provided by recent reports showing Pd-cata-
À
catalyzed reactions via alternative metal/radical-medi- lyzed C H arylation using ArC generated from aroyl
ated pathways can lead to improvements in rate, func- peroxides[7a] or aryldiazonium salts.[7b] We report
À
tional group tolerance, and/or substrate scope. Mano- herein that this strategy enables C H arylation with
likakes and Knochel recently reported a striking ex- Ar2I+ under extremely mild conditions (room temper-
ample of this strategy in the context of the Pd-cata- ature in MeOH). We also present evidence supporting
lyzed Kumada coupling.[2] They showed that the addi- the proposal that two fundamentally different mecha-
tion of i-PrI led to a remarkable rate acceleration in nisms are operating in the ionic [Scheme 1, (a)] versus
the coupling of aryl bromides with aryl-Grignard re-
agents.[2a] This rate enhancement was rationalized
based on a new ꢀradical-catalyzedꢁ mechanistic mani-
fold involving in situ generated alkyl and aryl radicals
as key intermediates.
We hypothesized that a similar strategy could be
À
used to accelerate Pd-catalyzed C H arylation reac-
tions with diaryliodonium reagents. As shown in
Scheme 1, (a) Pd
(OAc)2 is known to catalyze ligand-
À
directed C H arylation via an ꢀionicꢁ mechanism in-
volving 2eÀ oxidation of a palladacycle intermediate
by Ar2I+.[3–5] However, these transformations are ex-
tremely sluggish, typically requiring high tempera-
À
Scheme 1. Two different pathways for Pd-catalyzed C H ar-
tures (80–1108C) over extended periods of time (8– ylation with Ph2I+.
Adv. Synth. Catal. 2012, 354, 3517 – 3522
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Sharon R. Neufeldt and Melanie S. Sanford
FULL PAPERS
radical [Scheme 1, (b)] systems. Finally, we compare (entry 6).[11] Importantly, both of the metal catalysts,
this new transformation to a related method that uses as well as visible light, are critical for efficient room
[12]
aryl-N2+ as the arylating reagent.[7b]
temperature C H phenylation. Without any one of
these three components, only traces of product 1a
were formed (0–2%, entries 7–9).
À
Results and Discussion
A variety of aromatic substrates underwent room
temperature C H phenylation under these conditions
À
Ph2I+ can be converted to PhC under mild conditions (Table 2). In addition to pyrrolidinones 1 and 2, other
using visible light and a photocatalyst [RuACTHNUTRGNEUNG(bpy)3Cl2 or N-aryl amides were effective directing groups (en-
IrACHTUNGTRENNUNG(ppy)3; bpy=2,2’-bipyridine, ppy=cyclometalated tries 3 and 4). C-Aryl amides, such as benzamides 5
2-phenylpyridine].[8,9] Thus, we initiated investigations and 6, also underwent room temperature C H phe-
of Pd-catalyzed C H phenylation of 1 with [Ph2I]BF4 nylation, albeit in moderate yields (40% and 54%).
À
À
in combination with a photocatalyst. Visible light irra- The N,N-disubstituted analog 7 provided phenylated
diation was provided by a 26 W household fluorescent product 7a in poor yield (9%), suggesting that C H
lightbulb, and the reactions were set up on the bench arylation is facilitated by the presence at least one N
top with no precautions to exclude moisture or air. H bond in this substrate class. 2-Arylpyridines 8 and 9
À
À
Remarkably, the use of RuACTHNUTRGEN(UNG bpy)3Cl2 resulted in as well as ketoxime and aldoxime ethers 10 and 11
a modest yield (18%) of the desired arylated product were also good substrates. The ability to use oxime
1a after 15 h in MeOH at room temperature (Table 1, ethers as directing groups is particularly notable, as
entry 1). Evaluation of several different PdII catalysts these do not undergo C H arylation with diaryliodo-
À
revealed that Pd
erating 1a in 23% yield (entry 2). Ru
Ir(ppy)3 afforded comparable results (entries 2 and 3),
but the cationic photocatalyst Ir(ppy)2A
G
AHCTUNGTRENNUNG
(bpy)3Cl2 and reaction conditions.[3]
R
Diaryliodonium salts containing diverse arene sub-
(dtbbpy)PF6 stituents were evaluated in this photocatalytic C H
(dtbbpy=4,4’-di-tert-butyl-2,2’-bipyridine)[10] provided arylation. As shown in Table 3, the highest yields
a significant improvement (57% yield, entry 4). Re- were obtained with those bearing relatively electron
placing [Ph2I]BF4 with the corresponding triflate salt neutral substituents (e.g., p-Cl, p-Br, p-CH3, o-CH3,
led to a further enhancement (66% yield, entry 5). Fi- entries 4–7). Nonetheless, oxidants possessing more
nally, briefly sparging the mixture with N2 prior to the strongly electron-donating and electron-withdrawing
À
start of the reaction resulted in 94% yield of 1a substituents were also effective. For example, C H ar-
ylation with p-methoxyphenyl (entry 9) as well as
para-, meta-, and ortho-trifluoromethylphenyl (en-
tries 1–3) reagents proceeded in moderate to good
À
Table 1. Optimization of the room-temperature C H phe-
nylation reaction of 1 with Ph2I+.[a]
yields. Remarkably, even the highly sterically hin-
dered mesityl group could be transferred, albeit in
low yield (11%, entry 8). Notably, the analogous ther-
mal reaction of [Mes2I]OTf with 1 did not provide de-
tectable quantities of 1i (as determined by GC).
We propose that the Ir/Pd-catalyzed photocatalytic
À
C H arylation proceeds via a fundamentally different
Entry PdII
Photocatalyst
X
Yield [%][b]
mechanism than the analogous thermal reaction, de-
spite the fact that the reagents and products are the
same in both processes. A first piece of evidence to
support this proposal is the reaction outcome in the
presence of free radical scavengers. As shown in
À
1
2
3
4
Pd
Pd
Pd
Pd
Pd
Pd
none
Pd
Pd
N
A
ACHTUNGTRENNUNG
BF4À 18
BF4À 23
BF4À 17
57
ACHTUNGTRENNUNG
A
E
G
À
Table 4, the thermal C H arylation reaction is not in-
5
(dtbbpy)PF6 OTfÀ 66
hibited by the addition of 25 mol% galvinoxyl or
100 mol% of TEMPO. With both radical scavengers,
the reactions proceed to complete conversion and
afford comparable yield (entries 1–3). In contrast, the
% conversion and the % yield of the photocatalytic
reaction are suppressed in a dose-dependent manner
by galvinoxyl and TEMPO (entries 4–8). These results
are consistent with the intermediacy of radicals in the
latter but not the former reaction.
6[c]
7[c]
8[c]
9[c,d]
A
CHTUNGTRENNUNG
A
U
2
2
0
A
2ACHTUNGTRENNUNG
[a]
General conditions: 1 (1 equiv.), PdII (0.10 equiv), photo-
catalyst (0.05 equiv.), [Ph2I]X (2 equiv.), MeOH (0.2M in
1), 26 W lightbulb, 15 h, room temperature.
Yields determined by GC.
Reaction mixture was degassed by sparging with N2 for
1 min.
[b]
[c]
In addition, the chemoselectivity of the reaction be-
tween 1 and unsymmetrical iodonium reagent 12 is
[d]
General conditions, but with no light.
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Combining Transition Metal Catalysis with Radical Chemistry:
Table 3. Scope of Ar2I+ salts for Pd/Ir-catalyzed C H aryla-
À
À
Table 2. Substrate scope for the Pd/Ir-catalyzed C H phe-
nylation reaction with Ph2I+.[a]
tion of 1.[a]
Entry Substrate
1
Product
Isolated yield [%]
81
Entry
Ar
Product
Isolated yield [%]
1
2
3
4
5
6
p-CF3C6H4
m-CF3C6H4
o-CF3C6H4
p-ClC6H4
p-BrC6H4
p-CH3C6H4
o-CH3C6H4
Mes
1b
1c
1d
1e
1f
1g
1h
1i
69
56
46
77
79
87
85
11
41
2
94
7
8[b]
9
p-CH3OC6H4
1j
3[b]
72
44
[a]
General conditions: 1 (1 equiv.), [Ar2I]BF4 (2 equiv.), Pd-
(NO3)2 (0.10 equiv.), Ir(ppy)2A(dtbbpy)PF6 (0.05 equiv.),
U
G
CHTUNGTRENNUNG
MeOH (0.2M in 1), 26 W lightbulb, 15 h, room tempera-
ture, degassed by sparging with N2.
OTf salt of oxidant.
4[b,c,d]
[b]
5
6
7
8
40
54
9
Table 4. Effect of radical scavengers on the Pd/Ir-catalyzed
C H arylation of 8.
À
Entry
Scavenger
Mol%
Conditions
Yield [%][c]
1
2
3
4
5
6
7
8
none
–
A
A
A
B
B
B
B
B
79Æ7
81Æ1
74Æ8
52Æ4
42Æ2
20Æ9
34Æ14
6Æ2
62
galvinoxyl
galvinoxyl
none
galvinoxyl
galvinoxyl
TEMPO
TEMPO
25
100
–
10
25
50
100
9
67
60
[a]
Thermal conditions A: 8 (1 equiv.), [Ph2I]BF4 (1.1 equiv.),
Pd(OAc)2 (0.10 equiv.), AcOH (0.12M in 8), 15 h, 1008C.
Photocatalytic conditions B: (1 equiv.), [Ph2I]BF4
(2 equiv.), Pd(NO3)2 (0.10 equiv.), Ir(ppy)2A(dtbbpy)PF6
10
AHCTUNGTRENNUNG
[b]
8
G
G
CHTUNGTRENNUNG
(0.05 equiv.), MeOH (0.2M in 8), 26 W lightbulb, 15 h,
room temperature, degassed by sparging with N2.
GC calibrated yield reported as % yieldÆstandard devi-
ation.
11
57
[c]
[a]
General conditions: substrate (1 equiv.), [Ph2I]OTf
(2 equiv.), Pd(NO3)2 (0.10 equiv.), Ir(ppy)2A(dtbbpy)PF6
G
N
CHTUNGTRENNUNG
(0.05 equiv.), MeOH (0.2M in substrate), 26 W lightbulb,
15 h, room temperature, degassed by sparging with N2.
[Ph2I]BF4 was the oxidant.
With 1 equiv. MgO.
0.20 equiv. PdACHTUNGTRENNUNG(NO3)2.
highly dependent on the reaction conditions [Eq. (1)].
Under the thermal conditions, the less hindered
phenyl group is transferred selectively, providing
a 1:0.2 ratio of 1a:1d. In contrast, selective transfer of
the o-CF3C6H4 group occurs under the photocatalytic
[b]
[c]
[d]
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Sharon R. Neufeldt and Melanie S. Sanford
FULL PAPERS
1 and 11). However, for benzamide substrates 5–7
and ketoxime ether 10, the Pd/Ir/Ph2I+ system provid-
À
ed better yields of C H phenylation. Conversely, the
Pd/Ru/PhN2 system performed better for acetanilide
+
3, and it is effective for hydroxyl oxime 17, a substrate
class that undergoes decomposition in the Pd/Ir/Ph2I+
system. Overall, the room-temperature Pd/photocata-
+
+
À
lyzed C H arylation methods using Ar2I and ArN2
reagents are often complementary in terms of sub-
strate scope.[13]
conditions, affording a 1:3.6 ratio of 1a:1d. These dif-
ferences in chemoselectivity provide further support
for divergent mechanistic pathways.
À
Table 5. Comparison of Pd/Ir-catalyzed C H phenylation
with Ph2I+ vs. Pd/Ru-catalyzed C H phenylation with
À
+
PhN2 .
We preliminarily propose a reaction pathway that
merges PdII/IV and IrIII/IV catalytic cycles. As shown in
Scheme 2, ground state Ir3+ undergoes photoexcita-
tion by visible light (step i), and the resultant Ir3+*
complex can reduce Ar2I+ to generate ArC, ArI, and
Ir4+ (step ii).[8,9] Ar· could then enter the Pd catalytic
cycle by oxidizing cyclopalladated complex 13 (step
Substrate
Product
Yield [%]
Yield [%]
+[a,c]
with Ph2I+[a,b] with PhN2
1a
3a
89
91
89
69[d]
iii). Complex 14 could then be further oxidized to 15
3+
by Ir4+ (step iv), regenerating Ir . Finally, C Ar
À
bond-forming reductive elimination from 15 would
afford product 16 and regenerate Pd2+ (step v). Im-
portantly, a number of other pathways are also possi-
ble (for example, PdI/III catalysis) and cannot be dis-
tinguished based on the current data.
5a
6a
7a
54
52
11
25
38
8
We have recently reported a related room tempera-
À
ture photocatalytic C H arylation reaction that was
proposed to proceed via a mechanism similar to that
shown in Scheme 1.[7b] However, this previously re-
ported transformation used a different ArC precursor
(aryl diazonium salts),
a
different photocatalyst
(OAc)2] and
10a
11a
52
63
23
68
[Ru(bpy)3Cl2], a different Pd catalyst [PdCAHTUNGTRENNNUG
G
slightly different optimized reaction conditions. Thus,
a final set of studies was conducted to compare these
two processes.
As illustrated in Table 5, the performance of the
two systems is often comparable (e.g., for substrates
<1[e]
66
[a]
GC calibrated yield.
[b]
General conditions: substrate (1 equiv.), [Ph2I]OTf
(2 equiv.), Pd(NO3)2 (0.10 equiv.), Ir(ppy)2A(dtbbpy)PF6
A
R
CHTUNGTRENNUNG
(0.05 equiv.), MeOH (0.2M in substrate), 26 W lightbulb,
15 h, room temperature, degassed by sparging with N2.
General conditions: substrate (1 equiv.), [PhN2]BF4
[c]
(4 equiv.), PdACTHUNTRGENN(UG OAc)2 (0.10 equiv.), RuAHCUTGNTREN(NUGN bpy)3Cl2·6H2O
(0.025 equiv.), MeOH (0.1M in substrate), 26 W light-
bulb, room temperature, 15 h, degassed by sparging with
N2.
[d]
[e]
[Ph2I]BF4 was the oxidant.
Product 17a was not detected by GC, and only trace
amounts of 17 and 3’-methylacetophenone were detect-
ed.
À
Scheme 2. Possible mechanism for the Pd/Ir-catalyzed C H
arylation with Ar2I+.
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Combining Transition Metal Catalysis with Radical Chemistry:
191–322; d) U. Jahn, Top. Curr. Chem. 2012, 320, 323–
451.
Conclusions
[2] a) G. Manolikakes, P. Knochel, Angew. Chem. 2009,
121, 211–215; Angew. Chem. Int. Ed. 2009, 48, 205–209;
b) For a related Negishi coupling see: M. Kienle, P.
Knochel, Org. Lett. 2010, 12, 2702–2705.
[3] a) D. Kalyani, N. R. Deprez, L. V. Desai, M. S. Sanford,
J. Am. Chem. Soc. 2005, 127, 7330–7331; b) N. R.
Deprez, M. S. Sanford, J. Am. Chem. Soc. 2009, 131,
In summary, this paper describes a photoredox Pd/Ir-
catalyzed C H arylation with diaryliodonium re-
À
agents. The unusually low reaction temperature, the
requirement for light and a photocatalyst, the inhibi-
tory effect of radical scavengers, and the observed
chemoselectivity trends are all consistent with a radi-
cal mechanism for this transformation. This stands in
contrast to the analogous thermal reaction that re-
quires dramatically higher temperature (1008C) and
is believed to proceed via an ꢀionicꢁ 2eÀ pathway. This
example adds to a growing body of work suggesting
that re-routing traditional metal-catalyzed transforma-
tions via radical pathways can offer major advantages
in terms of reaction rates, substrate scope, and func-
tional group tolerance.[1,2,14]
11234–11241.
+
À
[4] Other Pd-catalyzed C H arylations using Ar2I : a) O.
Daugulis, V. Zaitsev, Angew. Chem. 2005, 117, 4114–
4116; Angew. Chem. Int. Ed. 2005, 44, 4046–4048;
b) N. R. Deprez, D. Kalyani, A. Krause, M. S. Sanford,
J. Am. Chem. Soc. 2006, 128, 4972–4973; c) J. Spencer,
B. Z. Chowdhry, A. I. Mallet, R. P. Rathnam, T. Adatia,
A. Bashall, F. Rominger, Tetrahedron 2008, 64, 6082–
6089; d) B. Xiao, Y. Fu, J. Xu, T.-J. Gong, J.-J. Dai, J.
Yi, L. Liu, J. Am. Chem. Soc. 2010, 132, 468–469; e) A.
Wagner, M. S. Sanford, Org. Lett. 2011, 13, 288–291;
f) A. J. Hickman, M. S. Sanford, ACS Catal. 2011, 1,
Experimental Section
170–174.
+
À
[5] For Cu-catalyzed C H arylation reactions with Ar2I
À
Representative Procedure for C H Phenylation of
Substrate 1
see: a) R. J. Phipps, N. P. Grimster, M. J. Gaunt, J. Am.
Chem. Soc. 2008, 130, 8172–8174; b) R. J. Phipps, M. J.
Gaunt, Science 2009, 323, 1593–1597; c) C.-L. Ciana,
R. J. Phipps, J. R. Brandt, F.-M. Meyer, M. J. Gaunt,
Angew. Chem. 2011, 123, 478–482; Angew. Chem. Int.
Ed. 2011, 50, 458–462; d) H. A. Duong, R. E. Gilligan,
M. L. Cooke, R. J. Phipps, M. J. Gaunt, Angew. Chem.
2011, 123, 483–486; Angew. Chem. Int. Ed. 2011, 50,
463–466.
N-Phenylpyrrolidinone 1 (80.6 mg, 0.50 mmol, 1.0 equiv.),
[Ph2I]OTf (430 mg, 1.00 mmol, 2.0 equiv.), Ir
(dtbbpy)PF6 (22.8 mg, 0.025 mmol, 0.05 equiv.), and Pd-
(NO3)2·2H2O (13.3 mg, 0.05 mmol, 0.10 equiv.) were com-
ACHTUNGERTN(NUNG ppy)2
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
bined in MeOH (2.5 mL) in a 4 mL scintillation vial. The re-
action mixture was cooled in an ice bath (to prevent solvent
evaporation) and sparged with N2 using a submerged needle
for 10 min, and the vial was then immediately sealed with
a Teflon-lined cap. The vial was placed on a stir plate with
two 26 W compact fluorescent lightbulbs (one on either side
of the vial about 5–8 cm away), and the reaction mixture
was allowed to stir at room temperature for 15 h. The reac-
tion mixture was diluted with EtOAc (50 mL) and washed
with 10% aqueous Na2SO3 (2ꢃ25 mL) and brine (1ꢃ
25 mL). The combined aqueous layers were extracted with
EtOAc (3ꢃ10 mL), and the organic layers were then com-
bined, dried over MgSO4, filtered, concentrated, and puri-
fied by column chromatography on silica gel (Rf =0.17 in
20% hexanes/80% Et2O). Product 1a was obtained as a pale
yellow oil; yield: 96.3 mg (81%). 1H and 13C NMR data
matched those reported in the literature.[7b]
À
[6] Arylation of substrates containing activated C H
À
bonds, such as the electron-rich C H bonds in indole,
do not necessarily require forcing conditions (see ref.[4b]
for an example). For one rare instance of room temper-
ature ligand-directed arylation of unactivated aromatic
C H bonds with Ar2I , see ref. However, the latter
transformation requires added TfOH and has not been
demonstrated to be general for any directing groups
beyond phenol esters.
+
[4d]
À
[7] a) W.-Y. Yu, W. Sit, Z. Zhou, A. S. C. Chan, Org. Lett.
2009, 11, 3174–3177; b) D. Kalyani, K. B. McMurtrey,
S. R. Neufeldt, M. S. Sanford, J. Am. Chem. Soc. 2011,
133, 18566–18569.
[8] a) J. Lalevꢄe, N. Blanchard, M.-A. Tehfe, F. Morlet-
Savary, J. P. Fouassier, Macromolecules 2010, 43,
10191–10195; b) J. Lalevꢄe, N. Blanchard, M.-A. Tehfe,
M. Peter, F. Morlet-Savary, J. P. Fouassier, Macromol.
Rapid Commun. 2011, 32, 917–920.
Acknowledgements
[9] Photolysis of Ar2I+ under ultraviolet irradiation (248–
300 nm) in the presence or absence of organic triplet
sensitizers is also known; see: J. L. Dektar, N. P.
Hacker, J. Org. Chem. 1990, 55, 639–647.
This work was supported by the NIH NIGMS (GM073836).
[10] a) D. A. Nagib, M. E. Scott, D. W. C. MacMillan, J.
Am. Chem. Soc. 2009, 131, 10875–10877; b) A. G.
Condie, J. C. Gonzꢅlez-Gꢆmez, C. R. J. Stephenson, J.
Am. Chem. Soc. 2010, 132, 1464–1465.
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Adv. Synth. Catal. 2012, 354, 3517 – 3522
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Sharon R. Neufeldt and Melanie S. Sanford
FULL PAPERS
[12] Although one equivalent of H+ is generated over the
course of the reaction, the growing acid concentration
does not appear to influence the reaction rate. For ex-
ample, when 20 mol% TfOH is added to the reaction
of substrate 1, the GC calibrated yield of 1a after 2 h is
33%, compared to 32% in the absence of added TfOH.
[13] The comparative advantages/drawbacks of diaryliodo-
nium salts versus aryldiazonium salts is somewhat sub-
jective and has been the topic of a recent review: H.
Bonin, E. Fouquet, F.-X. Felpin, Adv. Synth. Catal.
2011, 353, 3063–3084.
[14] For another recent example from our group, see: Y. Ye,
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