Copper-catalyzed esterification of alkylbenzenes with cyclic ethers and cycloalkanes via C(sp3)-H activation following cross-dehydrogenative coupling

Article abstract of DOI:10.1021/ol5011906

A copper-catalyzed cross-dehydrogenative coupling strategy has been developed for the synthesis of two classes of esters from simple solvents. The reaction of methylarenes with cyclic ethers resulted in α-acyloxy ethers involving four sp³ C-H cleavages, while treatment of methylarenes with cycloalkanes led to the formation of allyl esters at the expense of six consecutive sp³ C-H bonds.

Full text of DOI:10.1021/ol5011906

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Esterification of Alkylbenzenes with Cyclic Ethers
and Cycloalkanes via C(sp3)−H Activation Following Cross-
Dehydrogenative Coupling
Saroj Kumar Rout,† Srimanta Guin,† Wajid Ali, Anupal Gogoi, and Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati 781039, India
S
* Supporting Information
ABSTRACT: A copper-catalyzed cross-dehydrogenative cou-
pling strategy has been developed for the synthesis of two
classes of esters from simple solvents. The reaction of
methylarenes with cyclic ethers resulted in α-acyloxy ethers
involving four sp3 C−H cleavages, while treatment of
methylarenes with cycloalkanes led to the formation of allyl
esters at the expense of six consecutive sp3 C−H bonds.
Scheme 1. Representative Examples of Ester Synthesis
he direct C−H activation path has streamlined the
synthesis of functionalized molecules by minimizing the
T
number of synthetic steps, and the processes are more atom
economic. One such strategy, cross-dehydrogenative coupling
(CDC), has played a vital role by providing synthetic values to
such methodologies.1 The CDC protocols have been employed
to access a diverse array of C−C and C−heteroatom bonds, by
functionalizing C−H bonds of all types (sp, sp2, sp3).2 The
extreme reluctance of sp3 C−H bonds to enter into the periphery
of chemical reactions makes their selective functionalization a
formidable challenge. The solutions to these problems have
resulted in some appealing results on sp3 C−H functionaliza-
tions.1a,b In a limited hemisphere of sp3 C−H functionalizations
via CDC, the alkylbenzenes are found to be useful precursors and
have been used as ArCOO−, ArCO−, ArCH2O−, and ArCH2−
surrogates;3 most of which have ultimately led to the synthesis of
esters.3a−d
Esters are important building blocks in organic synthesis, so
the most revisited strategies are the ones where the installation of
ester C−O bonds are via C−H activation.3a−d,4 Hence, syntheses
of different classes of esters by unconventional approaches are
always appreciable, particularly through functionalizations of
inert C−H bonds. In this regard, our group has developed a
protocol for the synthesis of benzylic esters involving only
alkylbenzene(s) as the self- or cross-coupling partners under
metal-free conditions (path a, Scheme 1).3a The most remarkable
outcome that has emerged out of this ester synthesis is the
involvement of solvents (methylarenes) as substrates for sp3 C−
H functionalizations via CDC. In pursuit of utilizing this “solvent
chemistry” in a CDC reaction, cyclic ether (1,4-dioxane) was
attempted as the potential coupling partner with alkylbenzenes.
Cyclic ethers are widely used as solvents and are also amenable to
functionalizations at sp3 C−H α to the ethereal oxygen.5 A
relatively weak bond dissociation energy of α sp3 C−H helps
generate a radical center under oxidative conditions, thereby
allowing a radical/nucleophile to attack at that position.1a,6 This
leads to the question of how these two (alkylbenzene and cyclic
ether) solvents cum reagents can behave as mutual cross-
coupling partners under favorable conditions.
To give a practical shape to the above-mentioned concept,
toluene (1) and 1,4-dioxane (a) were allowed to react under
conditions similar to the coupling between alkylbenzene(s).3a
However, an attempt to achieve the desired cross coupling was
not so fruitful as the reaction gave predominantly benzyl
benzoate obtained by the self-coupling of toluene along with a
trace (<10%) of the desired α-acyloxy ether (1a) (entry 1, Table
S1, Supporting Information, SI). Hence, for the exclusive
formation of α-acyloxy ether it was necessary to avoid the
competing self-coupling of toluene by changing the reaction
conditions. During the self-coupling of alkylbenzene, the use of
Cu(II)/TBHP combination instead of metal-free conditions
(Bu4NI/TBHP) was unproductive. Interestingly, in the present
case the use of Cu(II)/TBHP changed the course of reaction,
giving the desired cross-coupled ester (1a) exclusively without
any trace of benzyl benzoate (entry 2, Table S1, SI).
Existing methods to α-acyloxy ethers can be broadly classified
into five main categories: (i) addition of a carboxylic acid to an
alkenyl ether,7 (ii) α-halo substitution of an ether with a
carboxylic acid,8 (iii) treatment of a hemiacetal with an acid or its
derivatives,9 (iv) CDC reaction between a carboxylic acid and an
Received: April 25, 2014
© XXXX American Chemical Society
A
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Letter
ether,10 and (v) miscellaneous methods comprising of two-step
synthesis.11 The present method on synthesis of α-acyloxy ether
with a higher degree of C−H activation (four sp3 C−H
cleavages) from solvents is unprecedented (path b, Scheme 1).
Inspired by this unique α-esterification of ether originating
from solvent−solvent coupling, we examined other Cu catalysts
in a quest to improve the yield (Table S1, SI). Among all other
Cu(I) [CuBr, CuCl, and CuI] (Table S1, entries 3−5, SI) and
Cu(II) [CuBr2, CuCl2, and Cu(OTf)2] salts (Table S1, entries
6−8, SI) tested, Cu(OAc)2 was found to be the best (Table S1,
entry 2, SI). By increasing the Cu(OAc)2 quantity from 10 to 20
mol %, the yield improved from 63% to 71% (Table S1, entry 9,
SI). No significant improvement in the product yield was
observed with 30 mol % of the catalyst (Table S1, entry 10, SI).
Even when the oxidant quantity was increased from 6 to 7 equiv,
the yield could not be enhanced further (Table S1, entry 11, SI).
Unexpectedly the yield decreased when the reaction was
performed at 100 °C instead of 80 °C (Table S1, entry 12, SI).
The use of a decane solution of TBHP (5−6 M) (Table S1, entry
13, SI) was found to be slightly ineffective compared to its
aqueous solution (70%). Control experiments suggest that a
catalyst-oxidant combination is indeed essential to bring about
the desired transformation (Table S1, entries 14 and 15, SI).
Having established the standard reaction conditions, the
present oxidative esterification of cyclic ether were then
implemented on cross couplings between 1,4-dioxane (a) and
a series of substituted methylarenes. As can be seen in Scheme 2,
the developed methodology was applicable to a diverse
methylarenes irrespective of the nature of their substituents
present. In particular, methylarenes possessing electron-donating
groups such as o-Me (2), m-Me (3), p-Me (4), 3,5-diMe (5), o-
OMe (6), p-OMe (7), and 2,6-diOMe (8) showed higher
reactivities compared to methylarene (1), giving their respective
α-acyloxy ethers (2a−8a) in moderate to good yields. Lower
yields obtained for o-subtituted methylarenes (2, 6, 8) compared
to their p- or m- analogues (3, 4, 5, 7) could be attributed to steric
hindrance imparted by the ortho-substitutents. The structure of
α-acyloxy ether (8a) has been confirmed by single-crystal X-ray
crystallography as shown in Figure S1 (SI). Note that for
polymethylated benzenes (2−5) one of the −Me groups is
functionalized, while the other −Me group(s) remains intact,
consistent with the previous esterification processes with
alkylbenzenes.3a,c,d The presence of electron-withdrawing groups
in methylbenzenes such as p-Br (9) and p-Cl (10) lowered the
yields of their corresponding α-acyloxy ethers (9a−10a). The
yield dropped further when the electron-withdrawing substituent
−Br (11) was present in the ortho-position, which is due to steric
as well as electronic factors. The fused bicyclic methylarene viz. 1-
methylnaphthalene (12) when coupled with a provided the
desired α-acyloxy ether 12a in a slightly lower yield.
The successful esterification of 1,4-dioxane (a) with various
methylarenes led us to examine the feasibility of this approach
with other cyclic ethers possessing a single oxygen, such as
tetrahydropyran (b) and tetrahydrofuran (c). Unlike in dioxane
(a), the oxidative esterifications of tetrahydropyran (b) with
methylbenzenes (1, 4, and 10) were not so effective, giving α-
acyloxy ethers 1b, 4b, and 10b in modest yields. Five-membered
cyclic ether tetrahydrofuran (c), when reacted with methyl-
benzene (4), gave a negligible amount of α-acyloxy ether 4c.
Lower yields were obtained when tetrahydropyran (b) was used
instead of 1,4-dioxane (a) because in the former there are four
equivalent sp3 C−H’s, whereas in the latter the chances are
doubled due to the presence of eight equivalent C−H’s. A
substantial drop in the yield was observed when tetrahydrofuran
(c) was used, which could be due to the above-mentioned
statistical factor as well as the instability of the desired radical
intermediate. Acyclic ethers such as dimethoxyethane (DME)
was found to be completely inert under the reaction conditions
when treated with 4. In spite of the favorable statistical
possibilities, the 18-crown-6 failed to undergo esterification,
which may be due to steric crowding or its fluxional nature.
Now the question arises whether cycloalkane possessing no
oxygen atom would serve as the coupling partner with
alkylbenzenes? The inert cycloalkanes have previously been
used as precursors for C−C1a,b and C−N12 bond formations.
Pertaining to C−O bond formation at cyclohexane, the Giff
process13 is well-known, although a direct construction of ester
C−O bond is unexplored. To reveal the answer to these queries,
toluene (1) and cyclohexane (a′) were reacted under the above
optimized conditions. Unfortunately, other than the detection of
benzaldehyde originating from 1 no desired coupling product
was observed. However, when the reaction was performed at 120
°C formation of an allyl ester (1a′) was observed in a mere yield
of 10%.14 We envisaged the formation of an ester C−O bond at
cyclohexane (a′) with PhCOO− derived from toluene, but the
generation of an olefinic system and further formation of C−O
bond at its allylic position leading to an allyl ester (1a′) is
unprecedented (path c, Scheme 1). This process is taking place at
the expense of six sp3 C−H bond cleavages, three each from
either of the coupling partners. The dehydrogenative olefination
of cyclohexane to cyclohexene has been reported with various Pt,
Ir, or Re catalysts.15 Very recently, Perez et al. have observed
dehydrogenative olefination of cyclohexane using hydrogen
peroxide and a Cu complex.16 Prior to this report, such allyl esters
a c
Scheme 2. Substrate Scope of α-Acyloxy Ethers −
a
Reaction conditions: 1−12 (5 mmol), a−c (2 mL), Cu(OAc)2 (0.2
b
c
mmol), aq TBHP (6 mmol), 80 °C. Isolated yield. Catalyst and
oxidant were for 1 mmol of substrates.
B
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Letter
have been synthesized following the Kharasch−Sosnovsky
reaction.17 In the initial attempt, the yield of the allyl ester
(1a′) obtained was quite low (10%); hence, rigorous
optimizations were carried out to arrive at the best conditions
leading to maximum possible yield. It was found that the use of
Cu(OAc)2 (20 mol %) and TBHP (5−6 M in decane) (8 equiv)
at 120 °C gave an improved yield of 33%. Even with this level of
optimizations the yield could not be improved beyond this. This
is possibly because of the existence of these two solvents in their
vapor phase, which limits their opportunity to react at the
interface containing the catalyst and oxidant. To prevent the
escape of vapors from the flask, a reaction was performed in a
Teflon-lined stainless steel autoclave under otherwise similar
conditions. Surprisingly, the strategy did not work, and
benzaldehyde was the major product detected along with a
trace (<5%) of 1a′. Thus, it seems the poor yield obtained in this
esterification is due to the intrinsic low reactivity of sp3 C−H
bonds in cyclohexane.
Several control experiments were carried out to elucidate the
mechanism of these solvent−solvent couplings. Analysis of the
reaction mixture between 1 and a divulged the formation of
benzaldehyde and benzoic acid, both of which could possibly
couple with dioxane to give 1a. To find the optimal coupling
partner among aldehyde and acid, a control experiment was
carried out where an equimolar mixture of p-methylbenzalde-
hyde and benzoic acid were reacted with dioxane a under
identical reaction conditions (Scheme S2, SI). Exclusive
formation of 4a derived from aldehyde and no trace of 1a
(expected from benzoic acid) imply an aldehyde−dioxane
coupling. To ascertain the radical nature of the reaction, 4 and
a were reacted in the presence of radical scavenger 2,2,6,6-
tetramethylpyridine N-oxide (TEMPO) under otherwise iden-
tical conditions. Along with the formation of traces (<10%) of
(4a), a TEMPO ether (H) was isolated, confirming the
formation of benzyl radical intermediate (Scheme S3, SI). The
trapped benzyl radical (H) inhibits the formation of key coupling
partner p-methylbenzaldehyde, thereby considerably lowering
the yield. To determine the possible rate-limiting step in this
reaction, an intermolecular competing kinetic isotope effect
(KIE) was performed by reacting dioxane a with an equimolar
mixture of toluene and toluene-d8. A kinetic isotope effect (KH/
KD ∼ 1) ruled out the involvement of a benzylic C−H cleavage as
the rate-determining step, thereby indicating dioxane C−H
cleavage as the possible rate-limiting step as reported by the Pan
group.10e The results of the above experiments indicate that
esterification of cyclic ethers comprises the following steps.
Toluene (1) gets converted to benzaldehyde (A) via radical
oxidation in the presence of Cu/TBHP. The tert-butylperoxy
radical generated from TBHP adds to benzaldehyde (A),
providing tert-butyl benzoperoxate (B). Homolytic cleavage of
peroxy species (B) affords benzoxy radical (C) along with tert-
butoxyl radical. On the other hand, abstraction of α ethereal
hydrogen from dioxane by tert-butylperoxy radical gives the
radical intermediate (D). The radical coupling of C and D leads
to the formation of ester 1a (path I, Scheme 4). During the
Proceeding further toward the substrate exploration, cyclo-
hexane (a′) was reacted with various methylarenes possessing
electron-donating and electron-withdrawing groups (Scheme 3).
a,b c
,
Scheme 3. Substrate Scope of Allyl Esters
Scheme 4. Proposed Mechanism for Esterification
a
Reaction conditions: 1−12 (2.5 mmol), a′−c′ (4 mL), Cu(OAc)2
b
(0.2 mmol), TBHP (5−6 M) (8 mmol), 120 °C. Isolated yield.
c
Catalyst and oxidant were for 1 mmol of substrates.
For methylbenzenes possessing electron-donating substituents
such as o-Me (2), m-Me (3), p-Me (4), and p-OMe (7), the
yields of the corresponding allyl esters 2a′−7a′ ranged from 26%
to 41%. The allyl ester (9a′) derived from methylbenzene, having
a moderately electron-withdrawing substituent p-Br (9), was
obtained in 21% yield. The electronic and steric factors in
alkylbenzenes have a similar effect on the yields of allyl esters as
was observed during synthesis of α-acyloxy ethers. Bicyclic
methylarene such as 1-methylnaphthalene (12) upon coupling
with a′ afforded its allyl ester 12a′ in a poor yield of 19%. Other
cycloalkanes viz. cyclopentane (b′) and cyclooctane (c′) when
reacted with 4 gave their corresponding allyl esters 4b′ and 4c′,
however, in poor yields (Scheme 3). The lower yields obtained in
five- and eight-membered cycloalkanes are in part due to
instability of their radical and unfavorable strain of their
corresponding cycloalkenes. To see if a regioselective
esterification could be achieved, methylcyclohexane (d′) was
reacted with 4. 1H and 13C NMR analysis revealed the formation
of at least three regioisomeric esters obtained in a combined yield
of 30% (Scheme S1, SI).
generation of tert-butylperoxy radical from TBHP, Cu(II) gets
reduced to Cu(I). Reoxidation of Cu(I) in the medium
regenerates Cu(II) for further reaction (Scheme 4). The
intermediacy of tert-butyl benzoperoxate (B) in this trans-
formation has been supported by an independent experiment
where the treatment of presynthesized B with dioxane a under
similar reaction conditions afforded the α-acyloxy ether 1a.
To gain insight into the mechanistic path for the allyl ester
formation, various control experiments were again performed.
The first query that arose was whether the dehydrogenative
olefination of cyclohexane occurs first followed by the
C
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Letter
10821. (l) Liu, L.; Yun, L.; Wang, Z.; Fu, X.; Yan, C.-h. Tetrahedron Lett.
2013, 54, 5383.
construction of an allylic ester C−O bond or a reverse sequence
operates. To reveal the exact sequence, two independent
reactions were performed. In the first reaction, toluene and
cyclohexene (E) were reacted (Scheme S4, SI), while in the
second a presynthesized cyclohexyl benzoate (G) was treated
under the standard reaction conditions (Scheme S5, SI). The
formation of allyl ester (1a′) in the former and failure in the latter
suggests that dehydrogenative olefination precedes ester C−O
bond formation. In another experiment, when B was reacted with
cyclohexane, formation of 1a′ was observed, suggesting a
Kharasch−Sosnovsky-type path. Details of this reaction
sequence are depicted in path II, Scheme 4, having common
intermediates A−C as that of path I.
In conclusion, the present protocol demonstrated a copper-
catalyzed synthesis of two classes of esters from simple solvents.
The combination of methylarenes and cyclic ethers resulted in
the formation of α-esterification of ethers via four sp3 C−H
cleavages, while the combination of methylarenes and cyclo-
alkanes led to the synthesis of allyl esters involving six
consecutive sp3 C−H bond cleavages.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: patel@iitg.ernet.in.
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.K.P. acknowledges the support of this research by the Science
and Engineering Research Board (SERB) (SB/S1/OC-53/
2013), New Delhi, and the Council of Scientific and Industrial
Research (CSIR) (02(0096)/12/EMR-II). S.G. and S.K.R. thank
CSIR.
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