Tetrahedron Letters
Palladium/copper tandem catalysis for carbon–carbon triple bond
cleavage of diaryl acetylenes
Zhiwei Wang a, Wenyou Fan a, Guo-Jun Deng b, Wang Zhou a,
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a College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China
b College of Chemistry, Xiangtan University, Xiangtan 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A method for carbon–carbon triple bond cleavage based on palladium/copper tandem catalysis is devel-
oped. In this chemistry, anilines and diaryl acetylenes were converted into benzamides through cascade
transformations combining palladium-catalyzed hydroamination of alkyne with copper-catalyzed aero-
bic oxidative C–C bond cleavage.
Received 4 June 2015
Revised 22 July 2015
Accepted 10 August 2015
Available online xxxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Tandem reaction
C–C triple bond cleavage
Alkyne
Amide
Alkyne could be readily converted into many different com-
pounds, such as aldehydes,1 ketones,2 diketones,3 alkenes,4 and
triazoles,5 which makes this class of compounds widely used in
organic synthesis as versatile building blocks.6 Recently, the trans-
formations based on carbon–carbon triple bond cleavage have
attracted considerable attention. Among them, alkyne metathesis
is the one investigated mostly and deeply.7 However, other trans-
formations that relied on the catalytic cleavage of the carbon–car-
bon triple bond still remain in a few examples.8–11 As a result,
research on developing new methods for C„C bond cleavage will
no doubt enrich the potential application of alkyne in organic
synthesis.
Tandem reaction that enables successive transformations with-
out isolating relevant intermediates so as to minimize waste and
increase efficiency has emerged as a hot research topic.12 In
2002, Yamamoto reported a palladium-catalyzed intermolecular
hydroamination of alkynes (Scheme 1),13 in which the hydroami-
nation products, enamines, are unstable and tend to undergo
hydrolysis, giving ketones as the final products. Combining with
our recent work, a copper-catalyzed aerobic oxidative C–C bond
cleavage of unstrained ketones with air and amines,14,15 we envi-
sioned the palladium/copper tandem catalysis for carbon–carbon
triple bond cleavage of alkyne for the synthesis of amide.16
To test our hypothesis, the reaction of aniline (1a) and
diphenylacetlyene (2a) was carried out. After extensive screening
of reaction variables, the optimized conditions [PdCl2 (10 mol %),
CuCl2ꢀ2H2O (20 mol %), DPPE (10 mol %), 1,10-phenanthroline
monohydrate (40 mol %), AgOTf (20 mol %), H2O (5 equiv), and
dioxane (0.25 mL), 100 °C, air, 24 h] were obtained. Under these
conditions, carbon–carbon triple bond cleavage product 3aa was
isolated in 76% yield (Table 1, entry 1). Removing each of these
necessary conditions from the standard, such as PdCl2, CuCl2ꢀ2H2-
O, AgOTf, and H2O will lead to the poor yields of this transforma-
tion (entries 2–5). On the other hand, carried out without using
ligands, the transformation also became sluggish (entries 6 and
7). Compared with the negative impact of Cu(OAc)2 instead of
CuCl2, Pd(OAc)2 showed good catalytic effect instead of PdCl2,
giving 3aa in comparable yield (entries 8 and 9). Notably, it pro-
ceeded smoothly in an oxygen atmosphere, while the reaction
was completely thwarted in a nitrogen atmosphere. Moreover,
the amount of solvent has a significant influence on yield (entry
15).
With the optimized conditions in hand, the substrate scope of
amines was explored firstly. The transformation afforded products
with para-, meta-, ortho-methyl substituents in moderate yields
(Table 2, entries 2–5). Substrates with the halo group, such as flu-
oro, chloro, and bromo, were well-tolerated, giving the correspond-
ing amides in moderate to good yields (entries 7–12). Notably,
substrates with trifluoromethyl and cyano were also compatible
(entries 13 and 14). However, no desired product was observed
when 4-nitro aniline was employed (entry 15). In addition, the
transformation delivered N-(naphthalen-2-yl) benzamide in good
yield (entry 16).
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Corresponding author.
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