Acylation of aromatic compounds with isocyanates catalyzed by ytterbium(III) triflate : environmentally friendly

Aromatic compounds can be acylatcd by isocyanates in the presence of catalytic amount of ytterbium(III) trinate [Yb(OT0 3] to obtain the corresponding N-substitutcd benzamides in good yields under neutral conditions. The catalyst can easily be recovered and reused without any loss of activity.

Since the first utilization of Yb(0Tf) 3 by Forsberg et al. 1 , Yb(0Tt) 3 as an all-purpose Lewis acid, it has been found widely used in organic syntheses 2 . For instance, Yb(0Tf) 3 was found to catalyze the Aldol reaction 3 , Mannich-type reaction 4 , Diets-Alder reaction 5 , Friedel-Crafts acylation and alkylation 6 . However, to the best of our knowledge Yb(0Tf) 3 catalyzed acylation of aromatic compounds by isocyanates has not been reported up to date. Herein, we wish to report the reaction using Yb(0Tf) 3 as the catalyst (Scheme 1). The acylation reaction of aromatic compounds with isocyanates in the presence of aluminum chloride was first reported by Leuckart 7 . Billeter found that benzenesulfonyl isocyanate was mixed with benzene in the presence of AlCl~, similarly N-(benzenesulfonyl)benzamide was obtained . This reaction has been evaluated elaborately more recently by Effenberger and Gleiter 9 using Friedel-Crafts-type catalysts. Recently, Mcfarland reported the optimal ratio (substrate : catalyst) and the mechanism of the reaction catalyzed by aluminum chloride 10 . However, a common problem, particularly in industrial process, is that these reactions require excess amount of the conventional Lewis acid which cannot be reused because of its instability in the usual aqueous media. Unlike conventional Lewis acids Yb(0Tf) 3 can be easily recovered and be reused after the reaction is completed 11 . These properties of Yb(OTf) 3 made them attractive alternative to Lewis acids such as aluminum JICS-10 chloride. Actually, this reaction catalyzed by Yb(OTf) 3 was possible and run well in RT.

Results and discussion
From Table 1, it can be found that p-(toluenesulfonyl) isocyanate or phenyl isocyanate can react with aromatic compounds to afford the desired benzamides in satisfactory yields. Unfortunately, no desired products were detected when the substrate was nitrobenzene even for a long time under reflux conditions (i, j).
The experimental evidence showed that catalytic amount of Yb(0Tf) 3 (0.1 equiv.) could promote this reaction smoothly at RT. Smaller amount of catalyst resulted in the . In addition, it was found in our experiment that the reaction did not take place in the absence of Yb(0Tf) 3 or in the presence ofYb(OTf) 3 . (H 2 0)x.Various reports on the use of lanthanide catalysts have focused on their recyclability 2 a,b_ We decided to demonstrate that this was possible in our system and the catalyst remaining in the aqueous phase was recovered by removing the water through heating and then drying under vacuum at 100° for 2 h in nearly quantitative yield. According to the typical procedure (3a), we repeated three times by recycle catalyst with the following yields: 87, 86 and 86% (Table 2). In conclusion, we have demonstrated that the Yb(0Tf) 3 can catalytically promote the reaction smoothly under suitable conditions with simple operation. As the catalyst, Yb(0Tf) 3 was superior to previous Bn)>nsted and Lewis acids, and overcame the problems encountered with classical Lewis acids. The mechanism of the reaction is not known, but the advantages were obvious for synthesis of this type of benzamides.

Experimental
Isocyanates were prepared from their corresponding amines 11 . Benzene and toluene were dried over sodium prior to use. Anisole, chlorobenzene and CH 2 CI 2 were dried over anhydrous calcium chloride. M.ps. were obtained with a capillary melting point apparatus and were uncorrected. IR spectra were recorded using KBr pellets on Vector-22 IR spectrophotometer. 1 H NMR was determined on Bruker AC-400 spectrometer with TMS as an internal standard. Organic solvents were obtained from commercial sources. Preparative TLC separations were carried out with silica gel GF-254 coated glass plates.
Typical procedure for synthesis of N-substituted benzamides: A mixture of Yb(0Tf) 3 (62 mg, 0.1 mmol), ptoluenesulfonyl isocyanate (0.20 g, I mmol), benzene (0.94 g, 1.2 mmol) in anhydrous CH 2 CI 2 (15 ml) was stirred at RT for 2 h. Subsequently, the solution was washed with 886 water(5 x 2 ml). The aqueous layer was extracted with diethyl ether (15 x 3 ml). Yb(0Tf) 3 can be recovered from aqueous mixture by removing the water. The combined organic layer was evaporated under reduced pressure. The crude product was purified by preparative TLC (cyclohexane : ethyl acetate= 2 : I) to afford 3a (0.249 g, 90% yield) as a white solid. The characterization and spectral data of the products are as follows : 3a m.p. 145-146° (1it. 13