A transition-metal-free synthesis of arylcarboxyamides from aryl diazonium salts and isocyanides

Article abstract of DOI:10.1021/ol4017244

A transition-metal-free carboxyamidation process, using aryl diazonium tetrafluoroborates and isocyanides under mild conditions, has been developed. This novel conversion was initiated by a base and solvent induced aryl radical, followed by radical addition to isocyanide and single electron transfer (SET) oxidation, affording the corresponding arylcarboxyamide upon hydration of the nitrilium intermediate.

Full text of DOI:10.1021/ol4017244

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
A Transition-Metal-Free Synthesis of
Arylcarboxyamides from Aryl Diazonium
Salts and Isocyanides
Zhonghua Xia and Qiang Zhu*
Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences,
190 Kaiyuan Avenue, Guangzhou 510530, China
zhu_qiang@gibh.ac.cn
Received June 19, 2013
ABSTRACT
A transition-metal-free carboxyamidation process, using aryl diazonium tetrafluoroborates and isocyanides under mild conditions, has been
developed. This novel conversion was initiated by a base and solvent induced aryl radical, followed by radical addition to isocyanide and single
electron transfer (SET) oxidation, affording the corresponding arylcarboxyamide upon hydration of the nitrilium intermediate.
The transition-metal-catalyzed carbonylation reaction
of aryl (pseudo) halides is a well-established approach
to various aryl carbonyl derivatives, depending on the
nucleophiles utilized.1 For example, when primary or
secondary amines are used as nucleophiles, corresponding
arylcarboxyamides are produced following CO insertion
and reductive elimination.2 Aryl diazonium salts3 are
recognized as more reactive electrophiles than correspond-
ing aryl iodides in palladium-catalyzed cross-coupling or
carbonylative coupling reactions,4 although the most well-
known transformations associated with aryl diazonium
salts are the Sandmeyer reaction5 and the Meerwein
arylation.6 Nucleophiles applied in palladium-catalyzed
carbonylative couplings using aryl diazonium salts as
electrophiles include water, hydrides, alcohols, acids, and
organometallic reagents to furnish aryl carboxylic acids,7
aldehydes,8 esters,9 mixed anhydrides,10 and ketones,11
respectively (Scheme 1). However, arylcarboxyamides
cannot be obtained by direct aminocarbonylation of
aryl diazonium salts in the presence of CO and amines or
anilines, because adducts between aryl diazonium salts and
amines or anilines will form quickly.12
Isocyanide contains a stable formal divalent carbon
which can act as both a nucleophile and an electrophile
in reactions such as the Passerini and Ugi multicomponent
reactions.13 The terminal carbon of isocyanide can also
be attacked by a radical species, generating an imidoyl
radical intermediate for subsequent reactions.14 In addition,
(7) Siegrist, U.; Rapold, T.; Blaser, H.-U. Org. Process Res. Dev.
2003, 7, 429.
(8) Kikukawa, K.; Totoki, T.; Wada, F.; Matsuda, T. J. Organomet.
Chem. 1984, 270, 283.
(9) Sengupta, S.; Sadhukhan, S. K.; Bhattacharyya, S.; Guha, J.
J. Chem. Soc., Perkin Trans. 1 1998, 407.
(1) For selected reviews on transition-metal-catalyzed carbonylation,
€
see: (a) Brennfuhrer, A.; Neumann, H.; Beller, M. Angew. Chem., Int.
Ed. 2009, 48, 4114. (b) Veige, A. S. Polyhedron. 2008, 27, 3177. (c)
Barnard, C. F. J. Organometallics 2008, 27, 5402.
(2) Wannberg, J.; Larhed, M. J. Org. Chem. 2003, 68, 5750. (b) Wu,
X.; Larhed, M. Org. Lett. 2005, 7, 3327.
(10) (a) Nagira, K.; Kikukawa, K.; Wada, F.; Matsuda, T. J. Org.
Chem. 1980, 45, 2365. (b) Kikukawa, K.; Kono, K.; Nagira, K.; Wada,
F.; Matsuda, T. Tetrahedron Lett. 1980, 21, 2877. (c) Kikukawa, K.;
Kono, K.; Nagira, K.; Wada, F.; Matsuda, T. J. Org. Chem. 1981, 46,
4413.
(11) (a) Kikukawa, K.; Kono, K.; Wada, F.; Matsuda, T. Chem. Lett.
1982, 35. (b) Kikukawa, K.; Idemoto, T.; Katayama, A.; Kono, K.;
Wada, F.; Matsuda, T. J. Chem. Soc., Perkin Trans. 1 1987, 1511. (c)
Andrus, M. B.; Ma, Y.; Zang, Y.; Song, C. Tetrahedron Lett. 2002, 43,
9137.
(3) (a) Lindsay, R. J. In Comprehensive Organic Chemistry; Barton,
S. D., Ollis, W. D., Sutherland, I. O., Eds.; Pergamon Press: Oxford, 1979;
Vol. 2, p 154. (b) Zollinger, H. Diazo Chemistry I: Aromatic and
Heteroaromatic Compounds; John Wiley & Sons: New York, 1994. (c)
The Chemistry of Diazonium and Diazo Groups; Patai, S., Ed.; Wiley:
New York, 1978.
~
(4) Roglans, A.; Pla-Quintana, A.; Moreno-Manas, M. Chem. Rev.
2006, 106, 4622.
(5) Hodgson, H. H. Chem. Rev. 1947, 40, 251.
(6) (a) Doyle, M. P.; Siegfried, B.; Elliott, R. C.; Dellaria, J. F. J. Org.
Chem. 1977, 42, 2431. (b) Raucher, S.; Koolpe, G. A. J. Org. Chem. 1983,
48, 2066.
(12) (a) White, E. H.; Baum, A. A.; Eitel, D. E. Org. Synth. 1968, 48,
102. (b) Baines, K. M.; Rourke, T. W.; Vaughan, K. J. Org. Chem. 1981,
46, 856. (c) Vyas, D. M.; Benigni, D.; Partyka, R. A.; Doyle, T. W.
J. Org. Chem. 1986, 51, 4307.
r
10.1021/ol4017244
XXXX American Chemical Society

Table 1. Optimization of the Reaction Conditiona
Scheme 1. Palladium-Catalyzed Carbonylative Couplings of
Aryl Diazonium Salts
promoter
(equiv)
2a
yield
(%)b
entry
(equiv)
solvent
1
Pd(OAc)2 (0.05)
none
2
2
2
2
2
2
2
2
2
2
2
2
3
3
5
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
acetone/H2O
CH3CN/H2O
DMSO/H2O
DMF/H2O
9
2
16
61
67
42
65
65
12
65
36
36
40
73
80
83
3
K2CO3 (1.1)
Cs2CO3 (1.1)
NaOAc (1.1)
t-BuOK (1.1)
Cp2Fe (0.1)
FeCl2 (0.1)
4
reactions using isocyanides as isoelectronic equivalents of
carbon monooxide in palladium-catalyzed isocyanide in-
sertion have grown rapidly in recent years.15 Based on our
continuing interest on isocyanide chemistry,16 we anticipate
that aryl diazonium salts could be used as electrophiles
in palladium-catalyzed isocyanide insertion, affording
arylcarboxyamide.17 Research following this hypothesis
revealed that aryl diazonium tetrafluoroborates can react
with isocyanides in the absence of palladium. This formal
aminocarbonylation process occurs in the absence of CO
and amines or anilines in aqueous media at 0 °C.18 Mecha-
nistic studies suggest that radical intermediates are involved
in the process.
In an initial attempt, p-nitrophenyl diazonium tetra-
fluoroborate 1a and tert-butyl isocyanide 2a were used as
model substrates in the presence of Pd(OAc)2 (5 mol %) in
acetone/H2O (2.5:1) at 0 °C (entry 1, Table 1). As we
expected, the corresponding N-(tert-butyl)-4-nitrobenz-
amide 3a was isolated in 9% yield. However, a control
reaction without a palladium catalyst also produces 3a in
16% yield, indicating that 1a could react with 2a through a
different mechanism as we anticipated. Addition of base
significantly improves the yield of 3a (entries 3À6). In the
case of using cesium carbonate (1.1 equiv) as a base, the
5
6
7
8
9c
10
11
12
13
14d
15d
Cs2CO3 (1.1)
Cs2CO3 (1.1)
Cs2CO3 (1.1)
Cs2CO3 (1.1)
Cs2CO3 (1.1)
Cs2CO3 (1.1)
Cs2CO3 (1.1)
acetone/H2O
acetone/H2O
acetone/H2O
a Reaction conditions: All reactions were performed with 1a
(0.2 mmol) and 2a (2À5 equiv), in 0.4 mL of H2O and 1.0 mL of organic
solvent at 0 °C, in air, for 20 min. b Isolated yields of 3a. c The reaction
was carried out in darkness. d The reaction was carried out in argon.
desired product 3a was obtained in 67% yield. Interest-
ingly, ferrocene (10 mol %) is also a good catalyst in
promoting this transformation, which suggests that this
reaction might proceed through a radical mechanism
(entry 7).19 When the reaction was carried out in darkness,
a similar yield of 65% was obtained, ruling out the
possibility that the reaction was initiated by light induced
decomposition of aryl diazonium salt (entry 9).20 In addi-
tion, it was observed that the reaction was less efficient in
other organic solvents with water as a cosolvent (entries
10À12). Finally, the yield of 3a was improved to 80%
under the optimized reaction conditions involving 1.1
equiv of Cs2CO3 and 3 equiv of tert-butyl isocyanide 2a
in argon (entry 14). Notably, this palladium-free carboxy-
amidation of aryl diazonium tetrafluoroborate occurrs
rapidly at low temperature, providing a formal aminocar-
bonylation reaction of aryl diazonium salts in the absence
of CO and amines or anilines.
(13) For recent developments of the Passerini and Ugi reactions, see
€
selected reviews: (a) Domling, A.; Ugi, I. Angew. Chem., Int. Ed. 2000,
€
39, 3168. (b) Zhu, J. Eur. J. Org. Chem. 2003, 1133. (c) Domling, A.
Chem. Rev. 2006, 106, 17. (d) Lygin, A. V.; de Meijere, A. Angew. Chem.,
Int. Ed. 2010, 49, 9094. (e) Ruijter, E.; Scheffelaar, R.; Orru, R. V. A.
Angew. Chem., Int. Ed. 2011, 50, 6234.
(14) (a) Camaggi, C. M.; Leardini, R.; Nanni, D.; Zanardi, G.
Tetrahedron 1998, 54, 5587. (b) Lenoir, I.; Smith, M. L. J. Chem. Soc.,
Perkin Trans. 1 2000, 641. (c) Masui, H.; Fuse, S.; Takahashi, T. Org.
Lett. 2012, 14, 4090. (d) Tobisu, M.; Koh, K.; Furukawa, T.; Chatani, N.
Angew. Chem., Int. Ed. 2012, 51, 11363.
(15) (a) Qiu, G.; Ding, Q.; Wu, J. Chem. Soc. Rev. 2013, 42, 5257.
(b) Lang, S. Chem. Soc. Rev. 2013, 42, 4867.
A series of substituted aryl diazonium tetrafluorobo-
rates were reacted with isocyanide 2a under the optimized
reaction conditions (Figure 1). Aryl diazonium tetrafluo-
roborates bearing electron-withdrawing groups in the para
(16) (a) Liu, L.; Wang, Y.; Wang, H.; Peng, C.; Zhao, J.; Zhu, Q.
Tetrahedron Lett. 2009, 50, 6715. (b) Zhao, J.; Peng, C.; Liu, L.; Wang,
Y.; Zhu, Q. J. Org. Chem. 2010, 75, 7502. (c) Wang, Y.; Wang, H.; Peng,
J.; Zhu, Q. Org. Lett. 2011, 13, 4604. (d) Peng, J.; Liu, L.; Hu, Z.; Huang,
J.; Zhu, Q. Chem. Commun. 2012, 48, 3772. (e) Wang, Y.; Zhu, Q. Adv.
Synth. Catal. 2012, 354, 1902. (f) Hu, Z.; Liang, D.; Zhao, J.; Huang, J.;
Zhu, Q. Chem. Commun. 2012, 48, 7371. (g) Peng, J.; Zhao, J.; Hu, Z.;
Liang, D.; Huang, J.; Zhu, Q. Org. Lett. 2012, 14, 4966.
(17) Palladium-catalyzed carboxyamidation of aryl bromides or
vinyl bromides with isocyanides was reported by Jiang’s group; see:
Jiang, H.; Liu, B.; Li, Y.; Wang, A.; Huang, H. Org. Lett. 2011, 13, 1028.
(18) While this manuscript was under preparation, a similar reaction
wss published; see: Basavanag, U. M. V.; Dos Santos, A.; El Kaim, L.;
(19) (a) Chernyak, N.; Buchwald, S. L. J. Am. Chem. Soc. 2012, 134,
12466. (b) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (b)
Beckwith, A. L. J.; Jackson, R. A.; Longmore, R. W. Aust. J. Chem.
1992, 45, 857. (c) Freidlina, R. K.; Kandror, I. I.; Gasanov, R. G.;
Kopylova, B. V.; Bragina, I. O.; Yashkina, L. V. Russ. Chem. Bull. 1984,
33, 758. (d) Kochi, J. K. J. Am. Chem. Soc. 1955, 77, 5090. (e) Kochi,
J. K. J. Am. Chem. Soc. 1955, 77, 5274.
(20) (a) Fagnoni, M.; Albini, A. Acc. Chem. Res. 2005, 38, 713. (b)
Ando, W. In The Chemistry of Diazonium and Diazo Groups; Patai, S.,
Ed.; Wiley: New York, 1978; Part 1, Chapter 9, p 341.
ꢀ
~
Gamez-Montano, R.; Grimaud, L. Angew. Chem., Int. Ed. 2013, 52,
7194.
B
Org. Lett., Vol. XX, No. XX, XXXX

position, such as CN, Cl, Br, COCH3, and CF3, afforded
correspondingamides3bÀ3ginmoderatetogood yields. It
is notable that iodides also survive the reaction conditions
(3fÀ3g), providing possibilities in diversification of the
products. The synthesis of these iodine-containing carboxy-
amides would be challenging by the existing transition-
metal-catalyzed methodologies. However, electron-neutral
or -rich substrates give the amidated products in much
lower yields even in the presence of 5 equiv of 2a (3hÀ3j),
indicating that the reaction is significantly affected by
the electronic density of the substrates. Meta NO2 or Cl
substituted aryl diazonium tetrafluoroborates reacted
smoothly with 2a, providing 3k and 3l in 72% and 61%
yields, respectively.
Figure 2. Scope of isocyanides. Reaction conditions: All reac-
tions were performed with 1 (0.2 mmol) and 2 (3 equiv), in
0.4 mL of H2O and 1.0 mL of acetone at 0 °C, in Ar, for 20 min;
isolated yields of 3. a The reaction was performed with 5 equiv
b
of 2. The reaction was performed with 3 equiv of 1, and 2
(0.2 mmol, 1 equiv).
Scheme 2. Two Possible Reaction Pathways
Figure 1. Scope of aryl diazonium salts. Reaction conditions: All
reactions were performed with 1 (0.2 mmol) and 2a (3 equiv), in
0.4 mL of H2O and 1.0 mL of acetone at 0 °C, in Ar, for 20 min;
isolated yields of 3. aThe reaction was performed with 5 equiv of 2a.
Scheme 3. Radical Trapping by TEMPO
Other isocyanides were briefly investigated in reactions
with para NO2 or Cl substituted aryl diazonium tetrafluo-
roborates (Figure 2). Cyclohexyl, isopropyl, and R-methyl
benzyl isocyanides are all suitable carboxyamidation re-
agents in the reaction with aryl diazonium tetrafluoro-
borates, although they are less effective than tert-butyl
isocyanide 2a (3mÀ3r). Structurally more complicated
4-nitro-N-(tosylmethyl)benzamide 3s can also be pro-
duced in 64% yield under modified conditions. Unfortu-
nately, aryl isocyanide is less compatible with the current
protocol, affording corresponding amide 3t in low yield.
There are two possible reaction pathways involved in
this novel carboxyamidation process: ionic or radical
mechanism (Scheme 2). To gain insight into the reaction
mechanism, radical trapping experiments were performed
(Scheme 3). When 2,2,6,6-tetramethyl-1-piperidinoxyl
(TEMPO), a radical scavenger, is added to a reaction in
the absence of isocyanide 2a under otherwise identical
conditions, the arylate TEMPO 4 is isolated in 50% yield,
Org. Lett., Vol. XX, No. XX, XXXX
C

(Scheme 4). Initially, aryl radical A is formed through
single electron transfer (SET) from hydroxide and/or
acetone to aryl diazonium salt 1 followed by homolytic
dediazoniation. The resulting aryl radical A reacts with
isocyanide 2 to afford an imidoyl radical intermediate B
which is oxidized by aryl diazonium cation 1 via SET,
giving a nitrilium intermediate C and aryl radical A upon
dediazoniation. Finally, hydration and tautomerization of
the nitrilium intermediate affords the desired arylcarboxy-
amide 3.
Scheme 4. Proposed Reaction Mechanism
In summary, we have reported a transition-metal-free
arylation of isocyanides through aryl radical intermediates
initiated by base and solvent induced SET reductive
dediazoniation of aryl diazonium tetrafluoroborates. A
range of electron-deficient aryl diazonium tetrafluorobo-
rates react with alkyl isocyanides rapidly at low tempera-
ture, in the absence of transition-metal catalysts and
ligands. This carboxyamidation process provides a formal
aminocarbonylation reaction using aryl diazonium salts
as electrophiles in the absence of a palladium catalyst, CO,
and amines.
proving that an aryl radical is formed under the reaction
conditions. When both isocyanide 2a and TEMPO are pre-
sent, the TEMPO adduct 4 is formed in 27% yield together
with the amidated product 3a (27%). Although we did not
isolate the adduct 5 between the imidoyl radical intermediate
and TEMPO, a free radical mechanism is highly likely
involved in the reaction. However, the coexistence of an
ionic pathway cannot be ruled out at this stage.21
According to previous studies on aryl diazonium salts22
and the above results, a radical mechanism via hydroxide
and/or polar solvent induced dediazoniation is proposed
Acknowledgment. We are grateful to the National
Science Foundation of China (21072190 and 21272233)
for financial support of this work.
(21) (a) Zollinger, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 141. (b)
Gloor, B.; Kaul, B. L.; Zollinger, H. HCA 1972, 55, 1596. (c) kuokkanen,
T.; Haataja, A. Acta Chem. Scand. 1993, 47, 872.
(22) (a) Galli, C. Chem. Rev. 1988, 88, 765. (b) Gomberg, M.;
Bachmann, W. E. J. Am. Chem. Soc. 1924, 46, 2339. (c) Beadle, J. R.;
Korzeniowski, S. H.; Rosenberg, D. E.; Garcia-Slanga, B. J.; Gokel,
G. W. J. Org. Chem. 1984, 49, 1594. (d) Bunnett, J. F.; Yijima, C. J. Org.
Chem. 1977, 42, 639. (e) Zollinger, H. In The Chemistry of Triple-bonded
Functional Groups, Supplement C; Patai, S., Rappoport, Z., Eds.; Wiley:
New York, 1983; Part 1, Chapter 15, p 603.
Supporting Information Available. Experimental pro-
cedure; characterization data for all new compounds.
This material is free of charge via the Internet at http://
pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX