Cyclic ethers to esters and monoesters to bis-esters with unconventional coupling partners under metal free conditions via sp3 C-H functionalisation

Article abstract of DOI:10.1039/c4cc05050a

An efficient metal free oxidative esterification of sp³ C-H bonds (adjacent to an oxygen atom) in simple solvents like 1,4-dioxane, tetrahydropyran, tetrahydrofuran and ethyl acetate has been achieved using terminal aryl alkenes and alkynes as

Full text of DOI:10.1039/c4cc05050a

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Cyclic ethers to esters and monoesters to bis-esters
with unconventional coupling partners under metal
3
free conditions via sp C−
Cite this: DOI: 10.1039/x0xx00000x
H functionalisation
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Ganesh Majji, Srimanta Guin, Saroj K. Rout, Ahalya Behera and Bhisma K. Patel*
www.rsc.org/
3
An efficient metal free oxidative esterification of sp C−
H bonds similar alkenylation of cyclic ethers under metal free conditions.
(
adjacent to an oxygen atom) in simple solvents like 1,4-dioxane,
tetarhydropyran, tetrahydrofuran and ethyl acetate has been
achieved using terminal aryl alkenes and alkynes as ArCOO−
sources.
Organic chemists continue to strive towards the development of
atom and step economic process for the synthesis of complex
molecular structures from simple precursors. What a better way than
1
to manipulate the ubiquitous C−H bonds to achieve this? In this
regard, the cross dehydrogenative coupling (CDC) has played a
pivotal role in bringing about transformation of C−H bonds of all
2
types to C−C and C−heteroatom bonds. In fact, the CDC has mostly
been directed towards C−C bond formation as it provides the
2a,2b,2g
Scheme 1 Differential reactivity of alkenylbenzene with cyclic
ethers.
The query that arose was whether the radically induced vinylation
requisite connectivity to build larger and complex structures.
One of the extensively studied approaches in this forum is the
1
h,3
substrate directed alkenylations using Rh, Ru and Pd catalysts.
However, scrutiny entails that most of these transformations are for
3
5
at inert α sp C−H bonds of cyclic ethers using copper salts/oxidant
can similarly be accomplished with tetrabutylammonium iodide
TBAI) and TBHP combinations. This intent encouraged a trial
2
4
sp C−H bonds facilitated by a multitude of directing groups. In
contrast, due to their intrinsic low reactivity (high pKa and bond
(
reaction between cyclic ether 1,4-dioxane (a) and styrene (1) in the
o
presence of TBAI (10 mol%) and aqueous TBHP (4 equiv.) at 80 C.
3
energy) similar alkenylation at sp C−H bonds remain virtually
unexplored. The recently developed radically induced vinylation of
The surprising outcome of this reaction was the unexpected α-
esterification of dioxane to give product (1a) in 20% yield (entry 1,
Table S1, see ESI‡). This observation clearly indicates that the
reactivity of TBAI/TBHP combination is in sharp contrast to Cu
salts/oxidant combination which gives ester (1a) (Scheme 1, path-c)
as the sole product rather than the expected vinylation (Scheme 1,
path-a). Notably, the coupling of styrene with cyclic ether in
presence of CuBr/TBHP is reported to undergo oxyalkylation i.e.
3
sp C−H bonds of cyclic ethers and cycloalkanes using copper
5
catalyst are the only two reports till date (Scheme 1, path-a). The
current trend in organic synthesis is to use and encourage metal-free
catalysts such as iodine, iodide salts and organoammonium iodides,
2e,2f,6
which are often appropriate substitutes to transition metals.
These metal free catalysts (in combination with oxidant) either
complement or show similar reactivity to that of transition metals in
bringing about requisite transformations. In addition, their use is
highly advantageous from the enviroeconomic point of view for
sustainable future. In order to achieve the same, we attempted
7
ketone formation (Scheme 1, path-b). Thus the divergence in
selectivities achieved with various catalyst/oxidant combinations
further highlights the significance of the present finding. The in situ
_
Department of Chemistry, Indian Institute of Technology Guwahati, 781 039,
____________________________________________ generation of other coupling partner, viz. the benzoxy group
−
PhCOO ) derived from styrene is not surprising, since recently
(
under a similar condition (Cu/TBHP) is reported to generate the
8
same. Thus the TBAI/TBHP combination is also expected to give
Assam, India. Fax no. +91-3612690762; E-mail: patel@iitg.ernet.in
†
1 13
Electronic supplementary information (ESI) available: H and C NMR
spectra For ESI or other electronic format see DOI: 10.1039/xxxxxx.
This journal is © The Royal Society of Chemistry 2012
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Jou4Crnal Na0Ame
−
PhCOO group from styrene under identical conditions. Prior to this ESI‡). Thus the ideal condition arrived is the use of styrene (1
report, synthesis of α-acyloxy ethers has been achieved by other mmol), 1,4-dioxane (1 mL), Bu
strategies that can be classified into five main categories: (i) TBHP (5−6 M) (6 equiv.) and a reaction temperature of 80 C.
NI (20 mol%), decane solution of
o
4
9
esterification of hemiacetals with acids or its derivatives, (ii)
Having established the optimised parameters, the oxidative
addition of carboxylic acids to alkenyl ethers, (iii) complex routes esterifications of 1,4-dioxane (a) with various substituted styrenes
1
0
1
1
comprising of two-step synthesis, (iv) α-halo substitution of ethers were tested and the results are summarised in Scheme 2. Under the
with carboxylic acids, and (v) CDC transformations involving optimised conditions, electron neutral styrene –H (1) and styrenes
carboxylic acids and ethers in the presence of either transition metal possessing either electron-donating substituents such as p-Me (2), p-
1
2
1
3
or metal free catalysts in combination with various oxidants.
Following the CDC strategy, our group has recently developed a (5), m-CF
protocol for the synthesis of α-acyloxy ethers via a Cu mediated surrogates providing the desired α-acyloxyethers (1a−
OMe (3) or electron-withdrawing substituents such as p-Cl (4), p-Br
(6) and o-Cl (7) were found to serve as ArCOO−
3
7a) in good
14
solvent-solvent coupling between alkylbenzenes and cyclic ethers.
Apart from these results there are other oxidative C−O bond forming containing electron-donating groups provided the corresponding
yields (Scheme 2). It was observed that, substituted styrenes
reactions most of which are for acetoxylation and hydroxylation.
a,b
Scheme 2 Substrate scope for α-acyloxyether.
products in higher yields than those bearing electron-withdrawing
groups. These results imply that the electronic factor of substituents
on the phenyl ring of styrenes affects the reaction rates as well as the
product yields. This reactivity trend is consistent with our recent o-
benzoxylation of 2-phenylpyridine using styrenes as aryl carboxy
8
source. This approach was further examined with other cyclic ethers
possessing single oxygen i.e. tetrahydropyran (b) and
tetrahydrofuran (c). The oxidative esterification of tetrahydropyran
(
b) with styrenes (1, 2 and 4) were not so effective in terms of yields
as compared to 1,4-dioxane and it afforded α-acyloxy ethers (1b, 2b
and 4b) in lower yields (Scheme 2). Tetrahydrofuran (c) with
styrenes (1 and 2) provided much lower yields of α-acyloxyethers
(
1c and 2c) (Scheme 2). In the case of 1,4-dioxane (a) there are eight
3
equivalent sp C−H’s where as in tetrahydropyran (b) and
3
tetrahydrofuran (c) there are only four such sp C−H’s, thus the yield
obtained in the later two are lower compared to the former due to
statistical factor.
a,b
Scheme 3 Substrate scope for α-acyloxyether.
a
Reaction conditions: styrene (1 mmol), cyclic ethers (1 mL), TBAI (0.2
o
mmol), TBHP (6 mmol) at 80 C. Isolated yields.
b
After the preliminary success on unusual esterification further
attempts were made to improve the yield of (1a) by varying reaction
parameters. When the above coupling reaction was performed with a (0.2 mmol), TBHP (6 mmol) at 80 C. Isolated yields.
decane solution of TBHP (5-6 M) (4 equiv.) instead of an aqueous
Beside styrenes, phenylacetylenes are efficient arylcarboxy source
a
Reaction conditions: phenylacetylene (1 mmol), cyclic ethers (1 mL), TBAI
b
o
8
TBHP (70% in water) the yield improved up to 45% (entry 2, Table under Cu(II)/TBHP conditions. Thus we were curious to see
S1, see ESI‡). By increasing the TBHP quantity from 4 to 6 equiv. whether under the present metal free conditions phenylacetylenes
and Bu NI from 10 to 20 mol% the yield improved from 45% to alike styrene can similarly generate arylcarboxy group and couple
4
7
0% (entries 3−5, Table S1, see ESI‡). Further, no enhancement in with cyclic ethers. With this objective phenylacetylene (1′) was
the yield was observed with increase in the catalyst Bu NI (30 treated with 1,4-dioxane (a) under the previous optimised conditions.
4
o
mol%), oxidant TBHP (7 equiv.) and reaction temperature (100 C) It was heartening to observe that the product α-acyloxyethers (1a)
(
butyl peroxide (DTBP), aq. H O , oxone, K S O and (PhCOO ) can be generated from phenylacetylene under a metal free
entries 6−8, Table S1, see ESI‡). Other oxidants such as di-tert was obtained in 48% yield. Thus alike styrene the benzoxy group
−
2
2
2
2
8
benzoylperoxide were completely ineffective for this transformation condition which is unprecedented. In order to generalise this
entries 9−13, Table S1, see ESI‡). Instead of Bu NI other halogen methodology, 1,4-dioxane (a) was treated with various
(
4
species such as Bu NBr, KI and I (entries 14−16, Table S1, see phenylacetylenes (2′−8′) and the results are summarised in Scheme
4
2
ESI‡) were completely unproductive. The control experiments 3. Phenylacetylenes having either electron-donating groups such as
carried out with either TBHP or catalyst (Bu NI) alone were p-Me (2′), p-OMe (3′), p-tBu (8′) or electron-withdrawing groups
4
ineffective towards this CDC coupling (entries 17−18, Table S1, see such as p-Cl (4′), p-Br (5′), m-CF (6′) and o-Cl (7′) served as
3
2
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ArCOO− surrogates under the present conditions providing their towards double esterification of ethyl acetate. Various substituted
desired esters (2a−
8a) in moderate yields (Scheme 3). Additionally, phenylacetylenes with electron neutral –H (1′), electron-donating
the oxidative esterification of tetrahydropyran (b) with various groups such as p-Me (2′), p-OMe (3′) and electron-withdrawing
phenylacetylenes (1′, 2′ and 4′) gave α-acyloxy THP ethers (1b, 2b groups such as p-Cl (4′), p-Br (5′), m-CF (6′), o-Cl (7′), m-Cl (9′)
and 4b) in lower yields (Scheme 3). Tetrahydrofuran (c) with and o-Br (10′) enacted as corresponding arylcarboxy surrogates for
3
phenylacetylenes (1′ and 2′) provided much lower yields of α- the formation of unsymmetrical gem-diacylates (1d−
acyloxy THF ethers (1c and 2c) (Scheme 3). The lesser yields of but in relatively lower yields (Scheme 5).
7d, 9d and 10d)
a,b
products obtained with phenylacetylenes compared to styrenes is due Scheme 5 Substrate scope for esterification of ethyl acetate.
to competitive homocoupling of terminal alkynes to diynes under the
reaction conditions.
Cyclic ethers such as 1,4-dioxane, tetrahydrofuran and
tetrahydropyran are the only common solvents utilised for α C−H
3
Ethyl acetate (bearing sp C−H α to oxygen)
13,14
functionalisations.
is predominantly used as a solvent and a diluent. The oxidative α
C−H functionalisation of ethyl acetate (d) is a very exigent task
because it is prone to hydrolysis and trans-esterification. Thus
selective functionalisations of ethyl acetate would not only be a
challenging task but also exciting if achieved. To explore this, the
above optimised conditions were implemented on ethyl acetate (d)
by treating it with styrene (1). Interestingly, the product obtained
was found to be a double ester (1d) formed via functionalisation at
3
the α sp C−H of ethyl acetate without affecting the existing ester
functionality. The double ester is nothing but an unsymmetrical gem-
16
diacylates. This double esterification of ethyl acetate was then
tested with other styrenes having electron-donating groups such as p-
Me (2), p-OMe (3) and electron-withdrawing groups such as p-Cl
a
Reaction conditions: phenylacetylene (1 mmol), ethylacetate (2 mL),
o
TBAI (0.2 mmol), TBHP (6 mmol) at 80 C. Isolated yields.
b
(
4), p-Br (5), m-CF3 (6), o-Cl (7), m-Cl (9) and o-Br (10) all
Several control experiments were carried out to depict a plausible
mechanism for these coupling reactions. Analysis of the reaction
mixture obtained by reacting 1,4-dioxane (a) with styrene (1)
revealed the presence of intermediate benzaldehyde and benzoic acid
in the medium (Scheme 6). Formation of benzaldehyde from styrene
is expected to go via decarbonylation of in situ generated
provided unsymmetrical gem-diacylates (2d−7d, 9d and 10d) in
moderate yields as shown in Scheme 4. In these cases also the
8
benzoxy groups originated from respective styrenes. It may be
3
noted here that active methylene compounds possessing sp C−H α
to oxygen upon reaction with carboxylic acids under similar
conditions give double esters via oxidative esterification at the active
8
phenylglyoxal intermediate (see ESI). Under oxidative reaction
conditions, benzaldehyde gets converted to benzoic acid which could
6a
be the possible source of carboxy group in these reactions. When
17
methylene C−H.
Scheme 4 Substrate scope for esterification of ethyl acetate.
a,b
benzoic acid was treated with 1,4-dioxane (a) under the present
reaction condition exclusive formation of (1a) was observed (90%)
1
3a
implying the coupling partners are carboxylic acid and dioxane.
Apart from benzoic acid when phenylglyoxal and benzaldehyde
were reacted independently with 1,4-dioxane (a), the desired ester
(1a) was obtained in 84 and 87% yields respectively; thus
confirming their intermediacy in this transformation.
a
Reaction conditions: styrene (1 mmol), ethylacetate (2 mL), TBAI (0.2
o
mmol), TBHP (6 mmol) at 80 C. Isolated yields.
b
Since phenylacetylenes too are potential benzoxy surrogates as
observed during esterification of cyclic ethers, the same was utilised
Scheme 6 Proposed mechanism for oxidative esterification
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8,13a
experiments and related literature reports
convey the operation of
5
6
following paths for the oxidative esterification. Styrene (1) in the
presence of TBAI/TBHP is converted to benzoic acid via the
intermediacy of phenylglyoxal and benzaldehyde (Scheme S1, see
ESI‡). To rule out the source of oxygen from molecular oxygen,
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an argon atmosphere under anhydrous conditions. The reaction
proceeded smoothly giving product (1a) in identical yield suggesting
that the oxygen atom in ester/carboxylic acid is indeed originating
7
8
18
from TBHP only. The benzoic acid so generated is deprotonated to
3
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9
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2
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2
1
0
7
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1
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−
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4
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B. K. P acknowledges the support of this research by the
Department of Science and Technology (DST) (SB/S1/OC-53/2013),
New Delhi, and the Council of Scientific and Industrial Research
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