You will get partial reduction of the second nitro group and the potential nitrosyl intermediates are quite reactive and capable of generating multiple side products. Try using a larger excess of triphenylphosphine at a lower concentration to minimise side products. Hopefully it will converge.

I've been tried 4 mmol of triphenylphosphine reacted with 1 mmol of the starting materials.. Same result, so many spots on TLC

Try preforming the Schiff base of the nitroaniline and benzaldehyde first before subjecting it to reduction with triphenylphosphine. You may need to change your reductant. SnCl2.H2O may be a good choice as ideally you need to go via the hydroxylamine.

Like EricBJ Harris suggested synthesize the Schiff base and then triethyl phosphite:

the substrate : P(OEt)3 = 1: 5  and 150-170 oC, under argon.  It has been already published. Find the article.

Hi,

 Two ways

Bill

The article referred to by Helena.  Your problem is the presence of two nitro groups, and hoping - probably in vain - that you can reduce just one of them.

Hi James,

Presumably, you are right. I have not noticed that there are two nitro groups. The two methods provided by William F.B. solved the problem. Best. Helena

Not quite - William's methods won't incorporate benzaldehyde.

I think the thing to do here is to start with 4-nitro-o-phenylenediamine, which you can buy.

I assumed that he wanted to reduce both nitro groups to make aminobenzimidazole. Otherwise the Cadogan reaction is an extremely poor choice of method. There is no reasonable fashion of selectively reducing only the desired nitro group.

If the nitrobenzimidazole is the desired product then it is not easily accessible through the Cadogan reaction. Condensation of 4-nitro-o-phenylenediamine with benzoic acid or benzoyl chloride would be the simplest method.

Thank you guys for your valuable comments and suggestions together with the paper. Hopefully, it will work through MW irradiation.