Issue in Honor of Prof. P.T.Narasimhan ARKIVOC 2004 (viii) 46-51
In general, benzimidazoles with p-substituted aryl groups are formed in higher yields than the o-substituted ones under thermal conditions although in all cases, good yields are obtained under microwave irradiation. This is in accordance with earlier observation that the microwave effect becomes important when steric effects impede a reaction.(14)
The reaction may tentatively be visualized to occur via a tandem sequence of reactions depicted in Scheme 2 involving (i) formation of dibenzylidene-o-phenylenediamine, (ii) protonation of the dibenzylidene-o-phenylenediamine by clay and ring closure leading to a five membered ring in either a sequential or a concerted manner, (iii) 1,3-hydride transfer and (iv) deprotonation. While the aryl groups/nitrogen atom could stabilize the positively charged intermediates involved in the intermediate steps, the aromatic stabilization of the resulting benzimidazoles could provide the impetus for the transformation.
NH2 N NNH2 N N+ 2 ArCHO Ar Ar K-10 N ArN Ar ArNNArH Hclay H+
Ar
CH2Ar +
Scheme 2
In the previously reported syntheses of benzimidazoles by (i) the reaction of orthophenylenediamine with aromatic aldehydes at 120-160 °C(18) and (ii) heating the dry diamine hydrochloride with the aldehyde until evolution of hydrogen chloride ceased, either the yields are not good for all substituted cases or the protocols are not eco-friendly.(18)
Conclusions
The present synthetic protocol for the preparation of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles is advantageous over the previous methods as (i) the reaction could be performed with an environmentally benign clay catalyst, (ii) it provides a good yield of product and (iii) reaction occurs more rapidly.
ISSN 1424-6376 Page 48 ©ARKAT USA, Inc
