Copper-catalyzed direct benzylation or allylation of 1,3-azoles with N -tosylhydrazones

Article abstract of DOI:10.1021/ja111249p

Cu-Catalyzed cross-coupling of N-tosylhydrazones with 1,3-azoles leads to the direct C-H benzylation or allylation. Cu carbene migratory insertion is proposed to play the key role in this transformation.

Full text of DOI:10.1021/ja111249p

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pubs.acs.org/JACS
Copper-Catalyzed Direct Benzylation or Allylation of 1,3-Azoles with
N-Tosylhydrazones
Xia Zhao, Guojiao Wu, Yan Zhang, and Jianbo Wang*
Beijing National Laboratory of Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China
S Supporting Information
b
Scheme 1. Cu-Catalyzed Reaction of 1,3-Azole with
N-Tosylhydrazone
ABSTRACT: Cu-Catalyzed cross-coupling of N-tosylhydra-
zones with 1,3-azoles leads to the direct C-H benzylation or
allylation. Cu carbene migratory insertion is proposed to play
the key role in this transformation.
1,3-Azoles have found wide applications as structure motifs
in natural products, pharmaceutically active compounds, agro-
chemicals, and organic functional materials such as liquid crystals
and fluorescent dyes.1 Over the many decades, enormous efforts
have been devoted to the development of efficient methods for
synthesizing this type of aromatic heterocycles, especially those
with appropriate substitutions.2 In this context, transition-metal-
catalyzed C-H functionalizations of heterocycles have wit-
nessed explosive developments in the past few years due to their
advantages over the traditional coupling approaches, in which
not easily available heteroaromatic organometallic reagents have
to be employed.3 Up to now, most of the studies on direct C-H
functionalization are focused on the direct arylation of 1,3-azoles,
namely the formation of sp2-sp2 C-C bonds.4-9 These direct
arylations are supposed to proceed through similar reaction
mechanisms, which include initial deprotonation of a relatively
acidic sp2 C-H bond of heterocycles by an alkali metal base,
oxidative addition, transmetalation and reductive elimination.10
A wide range of metal catalysts, including palladium,4,7d,8a,b,d
copper,5,7f rhodium,6d,e ruthenium,6h cobalt6g and nickel,6a,b,7e,8e
have been exploited for these catalytic processes. Among these
transition metal catalysts, inexpensive copper salts are particularly
attractive due to their potential applicability in industry.
alkyne. Subsequent transmetalation and dediazotization of the
in situ-generated diazo substrate lead to the formation of a Cu
carbene species, which undergoes a migratory insertion process.14
We conceived that the heteroaromatics bearing similar acidic
C-H bonds as those of terminal alkynes, may also be suitable
cross-coupling partners to couple with N-tosylhydrazones under
similar Cu-catalyzed conditions (Scheme 1). We found that this
was indeed the case, and we report herein a highly efficient
Cu(I)-catalyzed direct benzylation or allylation of heteroaro-
matic compounds with N-tosylhydrazones. Mechanistically, this
direct C-H bond functionalization process is fundamentally
different from those recently reported in the literature. More-
over, a transition-metal-catalyzed direct C-H functionalization
of heteroaromatics with secondary benzyl groups is achieved.
At the outset of this investigation, we explored the Cu-
catalyzed reaction of benzo[d]oxazole 1a and N-tosylhydrazone
2a (eq 1). After extensive screening of the reaction conditions
(the copper salts, ligands, solvent, base, and temperature),15 it
was concluded that under the following reaction conditions the
expected product 3a could be obtained in good yield: CuI as
catalyst and tBuOLi as base in toluene at 110 °C.
Despite tremendous developments in direct C-H arylation, the
transition-metal-catalyzed cross-coupling of heteroarene C-H
bonds with sp3 carbon remains largely unexploited.9 Direct benzyla-
tion of heteroarenes and directing-group-containing arenes
have been recently developed by Hoarau,11a Fagnou9c and
Ackermann.11b Very recently, Miura reported a Pd-catalyzed direct
benzylation of 1,3-azoles with benzyl carbonates,9b and Nakao and
Hiyama reported
a Ni-catalyzed hydroheteroarylation of
vinylarenes.12 We have noted that in most of the previously reported
direct benzylation reactions, only primary benzylation is achieved
except for the Ni-catalyzed reaction reported by Nakao and Hiyama.
This inflicts significant limitations on these methodologies. It can be
anticipated that β-hydride elimination may become a problem when
secondary benzyl halides are employed as coupling partners.
Recently, we developed a copper-catalyzed cross-coupling
reaction of N-tosylhydrazones with terminal alkynes, which
afforded allene products.13 It has been supposed that the reaction
is initialized with deprotonation of the relatively acidic terminal
With the optimized reaction conditions established, the scope
of this transformation was subsequently investigated. The ex-
amples of the reactions between oxazoles and N-tosylhydrazones
Received: December 14, 2010
Published: February 22, 2011
r
2011 American Chemical Society
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are presented in Figure 1. The reaction could be carried out with
a series of substituted aryl N-tosylhydrazones 2a-u and oxazoles
1a-e, affording the corresponding products 3a-s and 4a-h in
moderate to good yields.
The reaction was found not significantly affected by the
substituents on the aromatic ring of N-tosylhydrazones. Both
electron-donating and electron-withdrawing groups were toler-
ated under the reaction conditions (Figure 1, 3a-e). The reaction
isalso not noticeably affected by the position of the substituents on
the aromatic ringof aryl N-tosylhydrazones. However, the reaction
of benzo[d]oxazole 1a with 2j only afforded the product 3j in 46%
yield under the standard conditions. This diminished yield is
attributed to the formation of (E)-1,2-diphenylethene, which is
derived from 1,2 hydrogen shift of carbene intermediate. This side
reaction seems not avoidable under the standard conditions, but
the yield of the expected product could be improved by simply
changing the substrate ratio of 1a to 2j from 1.0: 1.2 to 1.0: 2.0.
This reaction proceeded smoothly with other substituted
benzo[d]oxazoles (Figure 1, 3k-q and 4h). The reaction was
not affected by the substituents on the phenyl ring of benzo-
[d]oxazoles. However, slightly diminished yields were observed
for the reaction with 5-phenyloxazole (Figure 1, 3r, s). The
reaction also proceeded smoothly with N-tosylhydrazones de-
rived from diaryl ketones, affording the corresponding benzyla-
tion products 4a-d in moderately high yields. Moreover, the
N-tosylhydrazone derived from benzaldehyde was also suitable
substrate for the reaction, affording 4e in 53% yield.
We also applied this reaction system to the direct allylation.
Thus, the N-tosylhydrazones derived from 3-methylcyclohex-2-
enone and 4-methylpent-3-en-2-one were subjected to the slightly
modified reaction conditions with increased loading of catalyst
CuI (20 mol %). Allylation products 4f-h could be obtained in
moderate yields.
Encouragedby theresults obtained with oxazoles, weproceeded
to apply this catalytic system to other aromatic heterocycles with
similar acidic C-H bond. However, direct benzylation of aromatic
heterocycles such as benzofuran, benzo[b]thiophene, and benzo-
[d]imidazole under similar conditions with N-tosylhydrazone did
not proceed well.15 Toourdelight, it was foundthatthiazoles could
be employed in this benzylation reaction, with increased catalyst
loading and with dioxane as solvent. The requirement for the
higher catalyst loading as compared to the reaction for oxazoles is
assumed to be due to the relatively lower activity of the C-H bond
of thiazoles. A series of N-tosylhydrazones 2a, 2h, 2i, 2m, 2o, 2v-y
and thiazoles1f, g weretested, giving product5a-k in moderateto
high yields (Figure 2). It is worthy to note that the reaction is
substantially affected by the loadingof CuIcatalyst. For example, as
shown by the synthesis of 5d, the yield could be improved by using
30 mol % of CuI catalyst. Finally, we were delighted to find that
benzylation of 1,2,4,5-tetrafluorobenzene 1g also worked, albeit
with low efficiency (eq 2).16
Figure 1. Cu-Catalyzed benzylation of oxazoles with N-tosylhydra-
zones. Reaction conditions if not otherwise noted: a1a-e (0.5 mmol),
t
2a-u (0.6 mmol), CuI (10 mol %), BuOLi (1.25 mmol), toluene
(2.0 mL), 110 °C, 1.5 h. b All the yields refer to isolated product after
chromatography with silica gel if not otherwise noted. c The reaction was
carried out under these conditions: 1a (0.5 mmol), 2j (1.0 mmol), CuI
(10 mol %), LiOtBu (1.75 mmol), toluene (2.0 mL), 110 °C, 1.5 h.
dYield of isolated product after chromatography with neutral Al2O3.
eReaction conditions: 1a (0.5 mmol), 2s (1.0 mmol), CuI (20 mol %),
A possible mechanism for this novel direct C-H bond
benzylation has been depicted in Scheme 1. The key step in
the reaction is the migratory insertion of Cu carbene species. We
have proposed a Cu carbene migratory insertion in our previous
publication on Cu-catalyzed coupling of terminal alkyne and N-
f
tBuOLi (1.75 mmol), dioxane (2.0 mL), 120 °C, 1.5 h. Reaction
conditions: 1a, 1c (0.5 mmol), 2t, 2u (0.6 mmol), CuI (20 mol %),
tBuOLi (1.25 mmol), toluene (2.0 mL), 110 °C, 1.5 h.
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Scheme 3. Effect of the Amount of Base on the Reaction
Scheme 4. Benzylation of 1a with Diphenyldiazomethane 6
Next, diphenyldiazomethane 6 was synthesized and was
subjected to the benzylation reaction (Scheme 4). In the absence
of base, less than 20% of the benzylation product 4a was identi-
fied under the otherwise standard reaction conditions, whereas
the yield could be increased to 70% when the same reaction was
carried out in the presence of 1.2 equiv of base.
Moreover, we found that the reaction did not show kinetic
isotope effect (eq 3). If the reaction followsa Cu carbene insertion
mechanism as depicted in Scheme 2, kinetic isotope effect may be
expected since the C-H insertion process is considered to follow
a concerted reaction mechanism.18 Conversely, if the Cu carbene
migratory insertion mechanism as shown in Scheme 1 operates,
this result can be reasonably explained by considering the
stepwise reaction mechanism, in which the deprotonation is a
fast equilibrium and should not be the rate-limiting step.
Figure 2. Cu-Catalyzed benzylation of thiazoles with N-tosylhydra-
a
zones. Reaction conditions if not otherwise noted: 1e (0.5 mmol),
2 (0.6 mmol), CuI (20 mol %), tBuOLi (1.25 mmol), dioxane (2.0 mL),
120 °C, 1 h. b Yield of isolated product after chromatography with silica
t
gel. c1e (0.5 mmol), 2x (0.6 mmol), CuI (30 mol %), BuOLi
(1.25 mmol), dioxane (2.0 mL), 120 °C, 1 h.dReaction conditions: 1g
(0.75 mmol), 2a, h, i, m, v (0.5 mmol), CuI (20 mol %), tBuOLi (1.25
mmol), dioxane (2.0 mL), 110 °C, 1.5 h.
Scheme 2. Proposed C-H Bond Insertion Mechanism
Finally, it is noteworthy that Cu carbene such as B in Scheme 2 is
electrophilic in character. Similar to the corresponding Rh(II)
carbene, Cu carbene C-H insertion is sensitive to the electronic
density of the targeted C-H bond.17 From the acidity of the
C-H bond of the aromatic heterocycles concerned in this inves-
tigation, the direct carbene insertion is expected to be difficult to
occur due to low electronic density of the C-H bond. As far as we
know, metal carbene C-H insertion into the 2-C of oxzoles or
thiazoles is not known in the literature. On the contrary, the
heterocycle-copper species has been generally suggested as the
intermediate in the Cu-catalyzed C-H functionalization reaction of
heteroaromatic compounds under basic conditions.5 Summarizing
all the information, we consider the reaction mechanism shown in
Scheme 1 is more probable, while the Cu carbene direct C-H bond
insertion as shown in Scheme 2 seems less likely. However, further
study is needed to unambiguously establish the reaction mechanism.
In conclusion, we have developed an efficient Cu-catalyzed
cross-coupling of azoles with N-tosylhydrazones through direct
C-H bond functionalization, affording the corresponding ben-
zylated aromatic heterocycles in moderate to good yields. This
reaction provides an efficient access toward C-H bond functio-
nalization by secondary benzyl group, which is difficult to achieve
tosylhydrazone.13 However, for the direct C-H bond benzyla-
tion described in this paper, an alternative mechanism which
involves a direct Cu carbene insertion into C-H bond of
heterocycles cannot be strictly eliminated (Scheme 2).17
To gain insight into the reaction, several control experiments
were carried out. First, we studied the effect of the amount of base
on the reaction (Scheme 3). If the reaction follows the Cu
carbene insertion mechanism, theoretically only 1 equiv of the
base, required for converting the N-tosylhydrazone into diazo
substrate, is sufficient for completing the benzylation reaction.
It was observed that the yield of benzylation product was
significantly diminished when the amount of base was reduced
to less than 2.0 equiv. The benzylation product was only formed
in trace amount when the base was reduced to 1.4 equiv.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
wangjb@pku.edu.cn
’ ACKNOWLEDGMENT
The project was supported by the NSFC (Grant Nos.
20902005, 20832002, 21072009, 20821062), 973 Program
(No. 2009CB825300).
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