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DOI: 10.1039/C4RA08764J (Communication) RSC Adv., 2014, 4, 46494-46497

Tetraethylammonium iodide catalyzed synthesis of diaryl ketones via the merger of cleavage of C–C double bonds and recombination of aromatic groups†

Xianghua Zengab, Daqian Xua, Chengxia Miaoa, Chungu Xiaa and Wei Sun*a
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail: wsun@licp.cas.cn
bCollege of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, 314001, China

Received 16th August 2014 , Accepted 10th September 2014

First published on 10th September 2014

Abstract

An efficient method for synthesizing diaryl ketones via merging of oxidative cleavage of C–C double bonds and recombination of aromatic groups is developed with Et4NI (2.5 mol%) as the catalyst and NaIO4 as the oxidant. The control experiments provide valuable mechanistic insights into the formation of diaryl ketones, and suggest that NaIO4 serves as an epoxidation and nucleophilic deformylation reagent.

The oxidative cleavage of a carbon–carbon double bond is a fundamental transformation in organic synthesis.1 Depending on the appropriate workup, the olefin could be directly cleaved into a variety of functionalized products, such as aldehydes, ketones and carboxylic acid. Ozonolysis2 and Lemieux-Johnson protocol3 are accepted as the principal methods for this direct transformation, however, their utilities generally suffer from the safety concerns. Alternative approaches were achieved through the transition metal catalysis including Ru,4 Au,5 Pd,6 Os,7 etc., a co-oxidant was required to regenerate the active catalyst for oxidative cleavage of olefins.8 Also, the metal-free systems that employ equivalent oxidant such as aryl-λ3-iodanes, phenyliodonium diacetate, were intensively studied.9 Very recently, an environmentally benign N-hydroxyphthalimide or tert-butyl nitrite catalyzed aerobic oxidative cleavage of olefins to ketones has been developed.10 The first example of biocatalytic regioselective oxidative cleavage of dialkenes was also successfully achieved with enzyme preparation from Trametes hirsute.11 Despite great progress has been achieved in this field, almost all of these approaches involve the scission of 1,2-disubstituted alkenes to aldehydes or carboxylic acids, whereas ketone products are usually obtained from respective 1,1-disubstituted alkenes. However, the synthesis of diaryl ketone and heterocyclic ketone starting from easily available 1,2-disubstituted alkenes by R1–C[double bond, length as m-dash]C–R2 bond cleavage reaction is rarely documented (Scheme 1).12
image file: c4ra08764j-s1.tif
Scheme 1 The oxidative cleavage of alkenes.

Aryl ketones or diaryl ketones are an important class of compounds and widely used in the synthesis of various pharmaceuticals, natural products, agrochemicals and other functional materials;13 therefore, the development of efficient way toward these compounds synthesis is highly desirable.14 As a continuation of our interest in alkenes oxidation,15 we aimed to extend iodine complexes catalyzed alkenes functionalization to carbonylation using hypervalent iodine as oxidant. Herein, we report the efficient catalyst system for removing a carbon atom from C–C double bond (R1–C[double bond, length as m-dash]C–R2) via merging of oxidative cleavage and recombination of aromatic groups to synthesize symmetrical/unsymmetrical diaryl ketones, including the substrate scope, selectivity, and mechanistic studies. This method employs tetraethylammonium iodide as catalyst with NaIO4 as terminal oxidant.

An initial screening of the reaction conditions was performed with trans-stilbene (1a) as the model substrate, and the results were shown in Table 1. We chose tetraethylammonium iodide (Et4NI) as catalyst and NaIO4 as oxidant in MeCN, resulting in the desired benzophenone in trace at 105 °C for 12 h (Table 1, entry 1). The result may ascribe to the bad solubility of NaIO4 in MeCN. Gratifyingly, benzophenone was got in 53% by using the mixture solvent MeCN–H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) under the same conditions (entry 2). The choice of solvent appeared to be crucial, and water is one of the indispensable components. After rough test of the ratio of the mixed solvents and the kinds of the organic solvent, the acetonitrile-water system (4[thin space (1/6-em)]:[thin space (1/6-em)]1) gave the highest yield of benzophenone (entries 3–7). And the result indicated that only 11% yield of the benzophenone was gotten without Et4NI as catalyst (entry 8). Other catalysts, such as CuI, I2, (n-Bu)4NI and (n-Bu)4NBr were also investigated under the same reaction conditions (entries 9–12). Obviously, Et4NI was the optimal catalyst (entry 4). The yield almost remained the same by elevating the temperature to 120 °C, and lower reaction temperature resulted in reduced yield (entries 13 and 14). Other oxidants such as oxone and K2S2O8 did not perform well (entries 15 and 16). Then, the loading of Et4NI was examined (entries 17–19). To our delight, 91% yield of desired ketone was isolated when the catalyst loading was decreased from 10 mol% to 2.5 mol% (entry 18). Besides, the yield would decrease markedly by using only 2 equiv. NaIO4 (entry 20). As a result, the reaction conditions described in entry 18 were selected as the standard conditions for further investigations.

Table 1 Optimization of conditions for cascade oxidative cleavage of trans-stilbenea

image file: c4ra08764j-u1.tif
Entry Solvent Catalyst (mol%) Oxidant Yieldb [%]
a Reaction conditions: 1a (0.2 mmol), catalyst, oxidant (3.0 equiv.) and solvent (1.0 mL) at 105 °C (oil bath temperature) for 12 h in a sealed tube.b Isolated yield.c The reaction was performed at 120 °C for 12 h.d The reaction was performed at 95 °C for 12 h.e NaIO4 (2.0 equiv.).f 20% epoxide and 5% 2,2-diphenylacetaldehyde were obtained determined by GC.
1 MeCN Et4NI (10) NaIO4 Trace
2 MeCN–H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 53
3 MeCN–H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]4) Et4NI (10) NaIO4 10
4 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 83
5 THF–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 43
6 DMF–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 Trace
7 Acetone–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 11
8 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) — NaIO4 11
9 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) CuI (10) NaIO4 30
10 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) I2 (10) NaIO4 17
11 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) (n-Bu)4NI (10) NaIO4 78
12 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) (n-Bu)4NBr (10) NaIO4 41
13 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 81c
14 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) NaIO4 65d
15 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) Oxone 47
16 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (10) K2S2O8 17
17 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (5) NaIO4 87
18 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (2.5) NaIO4 91
19 MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (1) NaIO4 80
20e MeCN–H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) Et4NI (2.5) NaIO4 41f

To explore the generality and scope of the methodology, a variety of stilbene derivatives and styrenes were examined under the standard reaction conditions. As shown in Scheme 2, trans-stilbenes with different substituted groups at one of the aromatic rings (ortho-, meta- and para-position), such as CH3, OCH3, Cl, Br, F, CF3, could be converted to the corresponding ketones 2a–n in excellent yields. Furthermore, unsymmetrical heteroaromatic ketones containing thienyl group 2o, 2p were obtained in good yields at 130 °C for 15 h. The reaction of trans-stilbenes with both substituted aromatic rings also proceeded well with excellent yields (Scheme 2, 2q–u). Unfortunately, trans-stilbene with strong electron-withdrawing group was inert in the catalytic system (Scheme 2, 2v). Besides, the cis-stilbenes could be smoothly transformed into the desired ketone products in excellent yields under the standard reaction conditions (Scheme 2, 2nc). It should be noted that terminal alkenes such as styrene or 1,1-disubstituted alkenes could also be converted into the corresponding benzaldehyde or ketones in Et4NI/NaIO4 system as previously reported (see ESI, Table S1† benzaldehyde 2x, benzophenone 2a, acetophenone 2z).5,10a


image file: c4ra08764j-s2.tif
Scheme 2 Cascade oxidative cleavage of various alkenes. a Reaction conditions: unless otherwise noted, 1 (0.2 mmol), Et4NI (0.005 mmol, 2.5 mol%), NaIO4 (0.6 mmol, 3 equiv.), MeCN (0.8 mL) and H2O (0.2 mL) were stirred at 105 °C (oil bath temperature) for 12 h in a sealed tube. Isolated yield. b The reaction was performed at 130 °C for 15 h. c Cis-alkene.

A series of control experiments were carried out to gain insights into the reaction mechanism (Scheme 3). Recently, we developed a chemoselective protocol for the synthesis of benzil derivatives via oxidation of stilbenes in an I2–H2O system under air.15b Thus, the reaction was firstly carried out with benzil 3, however, no further reaction took place [Scheme 3(I)]. Taking into account that NaIO4 could be used as an oxidant for the epoxidation of olefins,16 epoxide may be an intermediate of the present catalytic system. We were delighted to find that trans-stilbene oxide was converted to desired enzophenone in good yield (Scheme 3(II)). 2-Iodo-1,2-diphenylethanol was thought to be a further intermediate generated from the ring-opening of trans-stilbene oxide, but it could not be synthesized because of its instability. Accordingly, 2-bromo-1,2-diphenylethanol 5 instead of 2-iodo-1,2-diphenylethanol was examined in the presence of NaIO4 to give the benzophenone in 95% yield in the absence of Et4NI [Scheme 3(III)].


image file: c4ra08764j-s3.tif
Scheme 3 Control experiments for proving the mechanism.

On the basis of these studies, the following tentative mechanism for this transformation is proposed (Scheme 4). Firstly, epoxide A was formed in the presence of Et4NI/NaIO4 (ref. 17) and then it could be transferred to B via ring-opening reaction. The loss of iodide from B gives a carbocation intermediate C, which undergoes rearrangement to provide a more stable aldehyde D.18 For the reaction of trans-stilbene, the corresponding trans-stilbene oxide and diphenylacetaldehyde could be observed during the reaction by GC-MS and 1H NMR (see ESI, Fig. S1–S7†). Subsequently, aldehyde deformylation by a nucleophilic reaction with NaIO4 takes place, providing the desired diarylketone 2, along with the release of formic acid.19


image file: c4ra08764j-s4.tif
Scheme 4 Tentative mechanism for cascade oxidative cleavage of various alkenes.

Conclusions

In summary, we have developed an Et4NI-catalyzed method for the synthesis of symmetrical/unsymmetrical diaryl ketones from stilbenes, through merging of oxidative cleavage and recombination of aromatic groups with NaIO4. Through the mechanistic studies in this transformation, NaIO4 may play dual roles, serving as epoxidation and nucleophilic deformylation regents. The further application of this methodology to other reactions is now in progress in our laboratory.

Acknowledgements

We thank the Chinese Academy of Sciences and the National Natural Science Foundation of China (21133011 and 21103207).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, NMR data and spectra of the products, GC-MS of mechanism study. See DOI: 10.1039/c4ra08764j
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