Cu( Ⅱ )-promoted oxidative C-N bond cleavage of N-benzoylamino acids to primary aryl amides

A novel protocol for CuCl 2 -promoted oxidative C-N bond cleavage of N-benzoyl amino acids was developed. It is the first example of using accessible amino acid as an ammonia synthetic equivalent for the synthesis of primary aryl amides via CuCl 2 -promoted oxidative C-N bond cleavage reaction. The present protocol shows excellent functional group tolerance and provides an alternative method for the synthetic of primary aryl amides in 84-96% yields.


Introduction
Amide bond forming reactions are among the most important and widely studied transformations in organic chemistry, 1 because they are widely present in detergents, lubricants, biologically active molecules, agrochemicals, pharmaceuticals and polymers. 2Amide is the most important functional group necessary to maintain biological systems, 3 as well as the importance intermediates in organic syntheses. 4Therefore, various approaches to the construction of the amide have been developed.The traditional methods for the synthesis of primary amides rely on the treatment of acyl halides, acid anhydrides, esters or acids with ammonia. 5However, the use of ammonia can often be hampered by handling issues, importantly, since ammonia can react with carboxylic acids, esters and other functional groups, these methods are sometimes not suitable for selective amide generation.Other methods for amide synthesis include the Beckmann rearrangement 6,7 and the Schmidt reaction. 8However, both of them involve hazardous reagents and produce stoichiometric quantities of noxious by-products.Recently, metal complexes have been used to access amides through the dehydrogenative reaction between amines and alcohols or aldehydes. 9,10Furthermore, the hydration of nitriles 11 and the rearrangement of aldoximes 12,13 can also provide alternative synthetic routes to amides.Very recently, Sekar and coworkers developed an efficient process for the direct synthesis of amides from methylarenes and amines using an iron catalyst. 14In this new catalytic reaction, the methyl group of the methylarene is oxidized to the corresponding aldehyde through non-directed C(sp3)-H oxidation followed by its oxidative amidation with N-chloroamine, yielding the carboxylic amide.However, these above-mentioned methods suffer several drawbacks, such as low functional group tolerance, undesired hydrolysis of the amides into carboxylic acids.Therefore, development of a general, practical and efficient method for the construction of primary aryl amides is still a challenge, and such a method is highly desirable.
The C−N bond is one of the most abundant chemical bonds and widely exists in many organic molecules and biomacromolecules.The formation and transformation of C−N bond are among the central topics in organic chemistry, organometallic chemistry, and biochemistry. 15The C-N bond is strong in organic molecules, thus, the transformations via C-N bond cleavage are usually difficult. 160][21][22] Based on the previous work, we reasoned that the C-N bond cleavage of N-benzoyl amino acids 1 can also be realized via oxidative decarbonylation/dehydration reaction, followed by hydrolysis to afford the corresponding primary amides 2 (Scheme 2).Herein, we describe the first example of the use of amino acids as an ammonia synthetic equivalent for the synthesis of primary aryl amides via Cu(Ⅱ)-promoted oxidative C-N bond cleavage of N-benzoyl amino acids.

Results and Discussion
In our initial study, 1a containing a 2-aminoisobutyric acid was chosen as a model substrate and selected results from our screening experiments are summarized (Table 1).Only trace amount of product 2a was detected by using 1 equivalent of CuO as oxidant in DMSO at 120 °C for 11 h (Table 1, entry 1).Then, other copper precursors were tested, only 12% and 17% yields of 2a were obtained with CuSO4 and Cu(OAc)2 as oxidants, respectively (Table 1, entries 2,3).Surprisingly, the yield was dramatically improved to 96% by using CuCl2 as oxidant, (Table 1, entry 4).In addition, we found that 34 % yield of 2a was obtained by using AgOAc as oxidant (Table 1, entry 5).To our delight, we found that MnO2 can afford the desired product 2a in good yield (Table 1, entry 6).To further optimize the reaction, an extensive screening of various solvents was conducted by using CuCl2 as the oxidant; we found that DMF as solvent gave a 56% yield (Table 1, entry 7).Notably, DMA showed similar reactivity compared to that of DMSO and 86% yield was obtained (Table 1, entry 4 vs 8).However, we found that nonpolar solvents and protic solvents were ineffective in the present protocol (Table 1, entries 9-15).Then, the effect of variation of reaction temperature was studied; when the reaction temperature was decreased to 100 o C, only 56% yield of 2a was obtained (Table 1, entry 16).Finally, the substrates employing other amino acids were tested, for example, 1b containing 2-amino-propionic acid showed almost the same reaction efficiency compared to that of 1a (Table 1, entry 17).Interestingly, substrate 1c containing an aminoacetic acid, only less than 5% yield of 2a was detected (Table 1, entry 18).Therefore, by a systematic variation of the reaction parameters, the optimtimal reaction conditions were found to be: CuCl2 (1 equiv) as oxidant in DMSO at 120 °C for 11 h (Table 1, entry 4).

Table 1. Examination of reaction conditions
With the optimized conditions in hand, we investigated the scope of substrates (Table 2).We found that substrates 1 containing either electron-donating or electron-withdrawing group smoothly reacted to afford the corresponding products 2 in 84-96% yields.For example, the substrates 1d-h containing a substituent group at the para position such as p-Me, p-MeO, p-Cl, p-F, and p-CF3 gave the corresponding aryl primary amides in excellent yield (Table 2, entries 2-6).
As for different substitution patterns in the arene ring, meta-and ortho-substituted substrates1i-j all worked well in this reaction to afford the desired products 2g-2h.For example, substrates 1i bearing a meta-substituent such as m-Me gave an 86% isolated yield (

Conclusions
In conclusion, a novel Cu(Ⅱ)-promoted oxidative C-N bond cleavage of N-benzoylamino acids has been developed.Readily available benzoylamino acids with broad substrate scope were fully compatible by employing CuCl2 as the oxidant.The present protocol shows excellent functional group tolerance and provides an efficient method for the synthesis of aryl amides in excellent yields.Further applications to the synthesis of biologically important molecules are in progress.

Experimental Section
General.All reagents were obtained from commercial sources and used without further purification.The reactions were carried out in air; all products were purified by column chromatography on silica gel (200-300 mesh) using petroleum ether (60-90 o C) and ethyl acetate as eluate.Compounds described in the literature were characterized by HRMS and NMR.The 1 H and 13 C NMR spectra were recorded on a Bruker AC-400 (400 MHz) spectrometer with TMS as an internal standard.High-resolution mass spectral (HRMS) data were recorded on Bruker APEX IV Fourier transform ion cyclotron resonance mass spectrometer using electrospray ionization (ESI).Melting points were determined on an XT-4 electrothermal micro-melting-point apparatus.
Synthesis of N-benzoylamino acids 1: typical procedure. 23Amino acid (100 mmol) was dissolved in 10% sodium hydroxide solution (100 mL), and then benzoyl chloride or aryl sulfonly chloride (100 mmol) was added in portions to this solution, stirred vigorously after each addition.Crushed ice (100 g) was added to the solution and then concentrated HCl was added dropwise until the mixture was acidified (pH 2-3).The resulting compounds 1a-1m were obtained as a white or yellow crystalline solid.General procedure for CuCl2-promoted oxidative C-N bond cleavage of N-benzoyl amino acids for primary aryl amides 2. A mixture of benzoyl amino acid 1 (0.5 mmol), CuCl2 (85 mg, 0.5 mmol), and DMSO (0.5 mL) was placed in a 25 mL flask under air.The tube was heated at 120 ℃ for 11 h.The reaction mixture was cooled, diluted with ethyl acetate (10 mL), filtered through Celite, and concentrated in vacuo.The residue was purified by silica gel column chromatography with ethyl acetate/ petroleum ether (ethyl acetate/ petroleum ether = 1:3) to afford the desired product 2.

Table 2
, entry 7); substrate 1j bearing ortho-substituent such as o-Cl afforded the corresponding products in 88% isolated yield (Table2, entry 8).Substrate derived from 2-Naphthalene such as 1k was suitable, giving the corresponding product 2i in 84% yield (Table2, entry 9).Notably, a substrate 1l, bearing a thiophenyl ring underwent reaction smoothly to afford the desired product 1j in 89% yield (Table2, entry 10).Unfortunately, the present protocol does not appear to be applicable to the similar arylsulfonyl amino acid 1m, only a trace amount of the primary sulfonamides product 2k was obtained (Table2, entry 11).On the basis of these results and previous reports, a plausible reaction mechanism is proposed (Scheme 4).Initially, interaction of CuCl2 and benzoylamino acid 1a afford organocopper species A, The oxidative decarboxylation of A gives the radical intermediate B and simultaneously generates CO2.Subsequently, B is further oxidized to afford the iminium intermediate C, which affords the imine D after deprotonation.Finally, the imine D is hydrolyzed to afford the primary arylamide 2a and acetone.