Tetrahedron Letters
Rhodium(III)-catalyzed coupling of aromatic ketazines or oximes
with 2-vinyloxirane via C–H activation
Jing Wen a, An Wu b, Yuqin Miao a, Jin Zhu a,
⇑
a Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory
of Microstructures, Nanjing University, Nanjing 210093, China
b Kuang Yaming Honors School, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2015
Revised 27 July 2015
Accepted 10 August 2015
Available online 14 August 2015
Described herein is
a rhodium(III)-catalyzed coupling of aromatic ketazines or oximes with
2-vinyloxirane via directed C–H activation. This reaction proceeds efficiently under mild conditions with
a low catalyst loading, especially in conditions with room temperature in the absence of additives for
aromatic ketazines. A wide range of substituted substrates is supported and a possible mechanism is
proposed according to the experimental results of kinetic isotopic effect, reversibility studies, and
catalysis with rhodacycle intermediate c1.
Keywords:
Rhodium
Ó 2015 Elsevier Ltd. All rights reserved.
Ketazines
Oximes
2-Vinyloxirane
Methyl acrylate
C–H activation
Over the past decades, transition-metal-catalyzed directed C–H
bond activation, which occurs based on directing groups, has
emerged as a powerful tool for the functionalization of various are-
nes with advantages of step- and atom-economy, high selectivity,
and efficiency compared to traditional methods.1 Among them,
Rh-catalyzed directed sp2 C–H bond activations, based on directing
groups such as hydroxyl, carboxyl, amine, and so on, were broadly
exploited and used for their excellent catalysis and good tolerance
of functional groups.2 Recently, there are lots of literatures involv-
ing the Rh-catalyzed coupling of arenes with diverse coupling part-
ners with products in high yields via C–H activation based on
directing groups which contain ‘C@N’ as substructure.3 As a kind
of important intermediates in organic synthesis, ketazines and oxi-
mes, which are used in a wide range of agricultural chemicals,
medicines, and materials, have the generic ‘C@N’ as substructure
and it could direct Rh-catalyzed C–H bond activation.4 Therefore,
the method for the functionalization of ketazines and oximes via
Rh-catalyzed directed C–H bond activation would have realistic
significance compared with traditional methods.5 In addition, Li
et al. showed that 2-vinyloxirane coupled smoothly with various
substrates and an allylic alcohol fragment formed could be easily
derived.6 Under this, we carried out further research on the cou-
pling of aromatic ketazines or oximes with 2-vinyloxirane and
catalytic mechanism according to a range of mechanism experi-
ments. This study shows broadening of substituted substrates
under very mild conditions with [RhCp*(MeCN)3](SbF6)2 or [RhCp*-
Cl2]2 (Cp* = C5Me5) as catalysts, a possible mechanism that rhoda-
cycle c1 is the key intermediate in the catalytic cycle and a
kinetic test which identifies the rate-determining step for this
transformation.
With acetophenone azine (1a) as a model substrate, we initi-
ated our studies by examining the effects of various additives
(1.0 equiv) toward the reaction of acetophenone azine (1a,
1.0 equiv) and 2-vinyloxirane (2a, 1.2 equiv) in THF (2 ml) at
50 °C for 6 h, using [RhCp*Cl2]2 (3%)/AgSbF6 (12%) as the catalyst
system (Table 1, entries 1–6). It was found that the desired product
3a was barely observed in entries 1–6 with [RhCp*Cl2]2 as catalyst,
which revealed ineffectiveness of [RhCp*Cl2]2 toward the reaction
system. Using [RhCp*(MeCN)3](SbF6)2 as catalyst instead of [RhCp*-
Cl2]2, we subsequently carried out the reaction under various con-
ditions with additives, solvents, temperature, and time (Table 1,
entries 7–18). The results suggested that [RhCp*(MeCN)3](SbF6)2
as catalyst worked in the catalytic system with yields of product
3a more than 38% in all entries. The yields of 3a both in the pres-
ence of NaOAc and absence of additive, which were superior to
others, were more than 50% (Table 1, entries 8, 11). Among the
set of representative solvents, DCE was found to be optimal
(Table 1, entries 13, 16). Although there was no difference on yields
between the conditions with NaOAc as additive and no additive
⇑
Corresponding author.
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.