Copper-catalyzed oxidative esterification of unactivated C(sp3)-H bonds with carboxylic acids via cross dehydrogenative coupling

Article abstract of DOI:10.1039/c4ra14586k

An effective copper-catalyzed esterification of unactivated (non-benzylic and allylic) C(sp³)-H bonds of hydrocarbons with Selectfluor as an oxidant has been developed. This reaction could provide a direct, new and useful strategy for the synthesis of esters and alkyl alcohols by ester hydrolysis.

Full text of DOI:10.1039/c4ra14586k

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Copper-Catalyzed Oxidative Esterification of
Unactivated C(sp3)−H Bonds with Carboxylic
Acids via Cross Dehydrogenative Coupling †
Cite this: DOI: 10.1039/x0xx00000x
Jiadi Zhou1,2, Can Jin1,2, Xiaohan Li1,2 and Weike Su1*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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An effective copper-catalyzed esterification of unactivated C(sp3)−F bonds with Selectfluor has been developed.11 The Lectka
(non-benzylic and allylic) C(sp3)−H bonds of hydrocarbons group11b unveiled C(sp3)−H fluorination employing a combination of
with Selectfluor as an oxidant has been developed. This Selectfluor, a copper(I) bisimine complex, an anionic phase-transfer
reaction could provide a direct, new and useful strategy for catalyst and N-hydroxyphthalimide (NHPI). The Chen group11e
the synthesis of esters and alkyl alcohols by esters hydrolysis.
developed a photolytic method for direct benzylic fluorination in the
presence of Selectfluor. Besides its fluorinating ability, the
combination of Selectfluor with copper has shown a considerable
oxidation ability that could facilitate C−H abstraction.12 The Zhang
group12a,b reported a Selectfluor-mediated copper-catalyzed highly
selective benzylic C−O cyclization. The Baran group12c reported an
intermolecular Ritter-type C−H amination of unactivated sp3 carbons
with F-TEDA-PF6 as an oxidant. Herein, we report an effective copper-
catalyzed intermolecular C(sp3)−H esterification of unactivated (non-
benzylic and allylic) hydrocarbons with a broad spectrum of carboxylic
acids (Scheme 1).
Esterification is one of the most fundamental and important reactions
widely employed in organic synthesis as well as in the synthesis of
medicinal and natural products.1 Traditional approaches for the
formation of esters rely on prefunctionalized starting materials, such
as alcohols, carboxylic acids and acyl chlorides. In recent years, the
direct and catalytic activation of C(sp3)−H bonds adjacent to an
oxygen atom, phenyl group or double bond, to form esters via
coupling has attracted much attention(Scheme 1). Several transition-
metal catalysts, such as Ru,2 Rh,3 Pt,4 Pd,5 Cu,6 and Fe7 have been
developed for this new cross dehydrogenative coupling reaction. The
Wan group,8 Wang group,9a and Patel group9b,c have reported the use
of Bu4NI as a nonmetal catalyst for the activation of the C(sp3)−H
bonds to form C−O bonds via coupling. However, direct esterification
of the unactivated C(sp3)−H bonds has been rarely explored. Recently,
the Han group6e and Patel group6h reported
a Cu-catalyzed
dehydrogenation−olefination and esterification of C(sp3)−H bonds of
cycloalkanes with TBHP as an oxidant.
Selectfluor, a commercially available electrophilic fluorinating
reagent, is widely employed for fluoro functionalization of organic
compounds.10 Recently, a direct conversion of C(sp3) −H bonds to
1. Collaborative Innovation Center of Yangtze River Delta Region Green
Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014,
China.
E-mail: pharmlab@zjut.edu.cn; Tel: +86 57188320899
2. College of Pharmaceutical Sciences, Zhejiang University of
Technology, Hangzhou 310014, China.
Scheme 1. Direct Esterification of C(sp3)−H
†Electronic Supplementary Information (ESI) available. Experimental
details, mechanism investigation and characterization data. See DOI:
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DOI: 10.1039/C4RA14586K
as the additive. Selectfluor (1.0 equiv) was used. gSelectfluor (2.5 equiv) was
f
Initially, a mixture of p-nitrobenzoic acid 1a (1.0 mmol, 1.0 equiv),
h
i
used. NFSI (2.0 equiv) was used as the oxidant. F-TEDA-PF6 (2.0 equiv)
was used as the oxidant. jPyridine (2.0 equiv) or Et3N (2.0 equiv) was used as
base. kK2CO3 (2.0 equiv) was used as base.
adamantane 2a (3.0 equiv), Selectfluor (2.0 equiv) and CuBr2 (20
mol %) was stirred in acetonitrile at 60 °C for 4 h, and the reaction
afforded only 20% yield of 1-adamantanol 4-nitrobenzoate 3a (Table
1,entry 1). According to the literature,12c the intermolecular Ritter- when additional base such as pyridine and Et3N was added (Table 1,
type C−H amination of adamantane 2a with CH3CN was inevitable. entry 20). The yield has no significant increase when K2CO3 was
Other solvents such as DCE, acetone, THF, and CH3NO2 were added (Table 1, entry 21).
screened in order to avoid the C−H amination. However, no reaction
With the optimized conditions established (Table 1, entry 10), we
occurred in the above mentioned solvents. Further optimization of the next screened different acids aiming to improve the esterification
C(sp3)−H esterification was carried out by screening on a range of yield (Table 2). Using 4-nitrobenzoic acid (1.0 mmol) and CF3COOH
additives. Compound 3a was obtained in 15% yield by adding 20 (1.0 mmol) as acids, gave esterification product in 30% and 24% yield,
mol % of 1,10-phenanthroline in CH3NO2 at 60 °C (Table 1, entry 2). respectively (Table 2, entries 1 and 2). No desired product was
Other additives such as benzonitrile (1.0 equiv), CH2ClCN (1.0 equiv), obtained when CH3COOH (1.0 mmol) was used (Table 2, entry 3). The
NH(Tf)2 (1.0 equiv), and 4-fluorobenzonitrile (1.0 equiv), gave 3a in reason might be that CH3COOH was failed to deprotonated by F− due
55%, 25%, 32%, and 53% yield, respectively (Table 1, entries 3
the presence of pentanenitrile (1.0 equiv), the yield of 3a was mmol) resulted in a significant increase in the yield of esterification
improved to 60% (Table 1, entry 7). When the amount of (55%), along with 9% yield of
(Table 2, entry 4). Increasing the
pentanenitrile or Selectfluor was reduced by half, the yields dropped CCl3COOH to 2.0 equiv gave
in 60% yield, along with only 6% yield
accordingly to 41% or 32% (Table 1, entries 8 and 9). To our delight, of
(Table 2, entry 5). Unexpectedly, replacing CCl3COOH with
when CuBr2 (30 mol %) and Selectfluor (2.5 equiv) was used, the yield CF3SO3H provided no esterification product
, while the amination
of 3a improved to 72% (Table 1, entry 10). Other copper salts such as product
was obtained in 45% yield (Table 2, entry 6). When the
CuBr, Cu(OTf)2, CuCl, and CuI were less active (Table 1, entries 11−14). amount of pentanenitrile was doubled, the yield of
improved to 76%
−6). In to it’s weak acidity. Gratifyingly, the application of CCl3COOH (1.0
4
3
4
3
4
4
No reaction was observed when the copper catalyst or additive was (Table 2, entry 7). While the role of CF3SO3H remains unclear, it might
−
absent (Table 1, entries 15−17). Switching the N-F reagent from be that the nucleophilicity of CF3SO3 was not strong enough to
Selectfluor to NFSI or F-TEDA-PF6 gave 3a in 0% and 54% yield, undertake the esterification. On the other hand, the strong acidity of
respectively (Table 1, entries 18 and 19). No reaction was observed
CF3SO3H would favor the amination, which might be similar to the
role of Zn(OTf)2 as was reported by Baran group.12c
Table 1. Optimization of the Reaction Conditionsa
Table 2. Optimization of the Acidsa
entry
catalyst
(mol %)
additive
yield
(%)b
20
15
55
25
32
53
60
41
32
72
34
56
18
0
0
0
0
0
54
0
52
entry
acid
1
yield (%)b
1c
2d
3
4
5
6
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (30)
CuBr (20)
Cu(OTf)2 (20)
CuCl (20)
CuI (20)
AgNO3 (20)
Bu4NBr (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
CuBr2 (20)
-
3
30
24
0
55
60
0
4
10
30
0
9
6
1
2
3
4-nitrobenzoic acid
CF3COOH
CH3COOH
CCl3COOH
CCl3COOH
CF3SO3H
1,10-phenanthroline
benzonitrile
CH2ClCN
4
NH(Tf)2
5c
6
4-fluorobenzonitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
-
45
76
7
7d
CF3SO3H
0
8e
9f
aReaction conditions: acid 1 (1.0 mmol, 1.0 equiv), cyclohexane 2b (10.0
equiv), CuBr2 (30 mol %), Selectfluor (2.5 equiv), pentanenitrile (1.0 equiv),
10g
11
12
13
14
15
16
17
18h
19i
20j
21k
b
CH3NO2 (10.0 mL), N2, at 60 °C for 4 h, unless otherwise noted. Yield was
determined by 1H NMR analysis of the crude mixture, based on 1.0 mmol of
c
acid 1. CCl3COOH (2.0 equiv) was used. dpentanenitrile (2.0 equiv) was
used.
With the optimized conditions in hand, we subsequently explored
pentanenitrile
pentanenitrile
pentanenitrile
pentanenitrile
the reaction scope by using a variety of carboxylic acids
1 and
adamantane 2a (Table 3). Benzoic acids with electron-withdrawing
and electron-donating substituents could be converted to the desired
products (3a and 3i). Remarkably, the ortho-substituted benzoic acids
aReaction conditions: 4-nitrobenzoic acid 1a (1.0 mmol, 1.0 equiv),
adamantane 2a (3.0 equiv), catalyst, Selectfluor (2.0 equiv), additive (1.0
(
1b and 1c) with steric hindrance exhibited the similar activity as para-
equiv), CH3NO2 (10.0 mL), N2, at 60 °C for 4 h, unless otherwise noted. substituted benzoic acids (1a and 1f). Adamantane 2a reacted with
bIsolated yields based on 1a. cUsing CH3CN as solvent. dUsing 1,10-
CCl3COOH and gave 3l in 80% yield. Compound 3m was obtained in
e
phenanthroline (20 mol %) as the additive. Using pentanenitrile (50 mol %)
40% yield, along with some unidentified by-products. Surprisingly, the
2 | J. Name., 2012, 00, 1-3
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Table 3. Substrate Scope for the C(sp3)−H Esterificationa
reaction showed a high regioselectivity which indicated that the
esterification of the tertiary C−H was more accessible than secondary
C−H, and the regioisomer was not found according to the 1H NMR
analysis of the crude mixture. The method tolerates various types of
functional groups including nitro, tert-butyl, fluoro, chloro, bromo,
iodo, and aldehyde group, and 3a−3k were obtained in 40%−78%
yield. Notably, the aldehyde group was sensitive toward other
di
converted to the desired products even if K2CO3, NaOH, or Cs2CO3
was added, due to acidity of -methoxybenzoic acid was too weak.
ff p-methoxybenzoic acid was failed to
erent oxidants.6c,e,8a However,
p
aReaction conditions: acid (1.0 mmol, 1.0 equiv), hydrocarbon (3.0 equiv),
CuBr2 (30 mol %), Selectfluor (2.5 equiv), pentanenitrile (1.0 equiv),
CH3NO2 (10.0 mL), N2, at 60 °C for 4 h, isolated yield based on acid.
bhydrocarbon (10.0 equiv) was used. cIsolated yield of the recovered acid. d2-
methyl-6-nitrobenzoic acid (1.0 mmol) was used as substrate.
After testing the coupling reaction of adamantane 2a with
carboxylic acids, other target hydrocarbons were investigated under
the similar reaction conditions (Table 3). Compounds 3o and 3p were
obtained in 26% and 20% yield, respectively. Methylcyclohexane
reacted with p-nitrobenzoic acid 1a, provided a mixture of unassigned
regio- and diastereoisomers 3q in a total yield of 28%. Isooctane gave
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA14586K
the corresponding product 3r in 17% yield. Attempts to perform the
In summary, we have successfully developed a novel, effective, and
esterification of cyclododecane with CCl3COOH using this method direct method of copper-catalyzed esterification of unactivated (non-
failed, due to the product 3s would further react with CCl3COOH. benzylic and allylic) C(sp3)−H bonds of hydrocarbons via a cross
Other cycloalkanes such as cyclopentane, cyclohexane, cycloheptane, dehydrogenative coupling reaction. A variety of hydrocarbons could
and cyclooctane, reacted with CCl3COOH to give 3t, 3u, 3v and 3w in react smoothly with various carboxylic acids to give esters in 17−80%
35%, 55%, 50%, and 75% yield, respectively. Notably, this method was yields. Additionally, this reaction could also provide a direct, new and
further expanded to the benzylic and allylic C(sp3)−H bonds, and useful strategy for the synthesis of alkyl alcohols by esters hydrolysis.
providing the corresponding products 3x and 3y in 70% and 38% yield, Based on the literatures and our experiments, a radical process was
respectively. Unfortunately, the activation of the C(sp3)−H bonds proposed in the catalytic cycle. Further studies to refine the
adjacent to an oxygen atom was failed.
mechanism and to expand the application scope of this reaction are
The present esterification could be inhibited by the radical currently underway in our laboratory.
scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), which is
consistent with a radical based mechanism. Xu,12d Zhang,12a,b and
Baran12c have demonstrated that the copper(I) salt or copper(II) salt
Acknowledgements
We are grateful to Collaborative Innovation Center of Yangtze River
Delta Region Green Pharmaceuticals and National Natural Science
Foundation of China (No. 21176222 and No. 21406200) for financial
help.
can be oxidized to generate
a fluorinated copper(III) species by
Selectfluor. Based on our experiments, no reaction was observed
when pentanenitrile was absent. We inferred that pentanenitrile could
improve the solubility of CuBr2 in CH3NO2 by coordination. In addition,
the coordination of pentanenitrile with copper(III) species might
disperse copper(III) species’ charge that would help to stabilize the
copper(III) intermediate. The structure of a complex obtained by
mixing copper(II) trifluoroacetylacetonate and F-TEDA-BF4 has been
previously characterized by X-ray crystallography.13 Crystalline H-
TEDA-BF4 has been isolated by Baran,12c and the salt was
characterized by X-ray crystallography. At the completion of the
present esterification reaction, a white precipitate of H-TEDA-BF4
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CopperꢀCatalyzed Oxidative Esterification of Unactivated C(sp3)−H
Bonds with Carboxylic Acids via Cross Dehydrogenative Coupling
A table of contents entry
RSC Author Templates ꢀ ChemDraw (CDX) ꢀ Graphical Abstracts
Selectfluor (2.5 equiv)
CuBr2 (0.3 equiv)
O
H
O
R
O
R
OH
pentanenitrile (1.0 equiv)
N2, CH3NO2, 60 o
C
Unactivated C(sp3)-H
esterification
R = aryl, alkyl