Table 1. Heck Reaction of Arenediazonium Salts 2 Using Pd(0) Complex 1a
entry
diazonium salt, 2
olefin
time (h)
product
yieldb (%)
recovery of 1b (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15c
16d
17c
18c
2a
2b
2c
2d
2e
2f
2g
2a
2b
2c
2d
2e
2f
3
3
3
3
3
3
3
4
4
4
4
4
4
4
5
5
5
5
1.25
0.75
1
1.50
1.25
0.75
0.75
3
2.75
4
3.75
6.5
3
6a
6b
6c
6d
6e
6f
6g
7a
7b
7c
7d
7e
7f
95
95
100
100
78
97
78
88
100
100
71
69
99
94
97
100
96
38
97
100
97
95
94
100
100
99
99
97
95
88
95
98
98
95
95
93
2g
2a
2b
2c
2d
2.5
4.25
3.5
4
7g
8a
8b
8c
8d
4
a Reactions were carried out under aerobic conditions. Molar ratio 2/olefin ca. 1:1.6, [2] ca. 0.023 M. b Isolated by column chromatography on silica gel.
c 10% of catalyst 1 was required. d 15% of catalyst 1 was required.
Another drawback in the Heck reaction is the need for
high-cost aryl iodides or bromides as the most used aryl
electrophile components (ArX). An interesting alternative to
aryl halides for Heck reactions has been the use of arene-
diazonium salts.5 Diazonium salts offer several advantages
over the aryl halides: economic advantage of anilines
compared to aryl bromides and iodides, short reaction times,
mild and aqueous reaction conditions without added base,
and superior reactivity of the diazonium nucleofuge (N2) over
the bromides and iodides.
recovered catalyst described to date. Thus, we want to present
here the efficiency and recovery of palladium(0) complex 1
as catalyst in several Heck reactions summarized in Table
1.
Initial screening of the reaction conditions with the
diazonium salt 2a and ethyl acrylate 3 revealed that the
arylation was achieved using 5 mol % of Pd(0) catalyst 1 in
ethanolic solution at room temperature. It has to be pointed
out that the reaction was practical and very fast under aerobic
conditions using commercial and not degassed solvent. Pd-
(0) complex 1 was almost quantitatively recovered (97%)
by column chromatography on silica gel. The synthetic
efficacy of this reaction was studied with a number of
arenediazonium salts 2a-g and the olefins ethyl acrylate, 3,
tert-butyl acrylate, 4, and styrene, 5. Table 1 shows that the
nature of the substituents did not exert significant effect on
the yields and recovery of the catalyst. Formation of nitro-
substituted derivatives 6b (entry 2), 7b (entry 9), and 8b
(entry 16), from the 4-nitro salt 2b, are also quite significant
since this salt failed in several Heck reactions with acrylates.5b,e
Surprisingly, 4-methoxydiazonium salt failed to participate
in this reaction. Steric effects of ortho or meta substituents
do not seem to affect the reactivity of arenediazonium salts
(entries 5 and 6 in the case of olefin 3; entries 12 and 13 in
the case of olefin 4). Only in the case of styrene 5 did steric
effects come into play causing lowering of yields (entry 18).
In addition, m-methylbenzenediazonium tetrafluoroborate 2e
To the best of our knowledge, there has been no general
study of Heck reactions of aryl diazonium salts with
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K.; Matsumoto, S.; Kikukawa, K.; Matsuda, T. Chem. Lett. 1988, 873. (d)
Yong, W.; Yi, P.; Zhuangyu, Z.; Hongwen, H. Synthesis 1991, 967. (e)
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