Boron heterocycles derived from 2-guanidinobenzimidazole

The syntheses and structure determinations of a series of boron heterocycles derived from 2-guanidinobenzimidazole 1 are reported. Structures of new compounds, 2-guanidino-1-methyl-benzimidazole 2, diphenyl-(2-guanidinobenzimidazole-N,N?)-borate 3, diphenyl-(2-guanidino-1-methyl-benzimidazole-N,N?)borate 4, hydroxy-phenyl-(2-guanidino-benzimidazole-N,N?)borate 5, hydroxy-phenyl-(2-guanidino-1-methyl-benzimidazole-N,N?)borate 6,methoxy-phenyl-(2-guanidinobenzimidazole-N,N?)borate 7, isopropoxy-phenyl-(2-guanidinobenzimidazole-N,N?)borate 8, acetoxy-phenyl-(2-guanidinobenzimidazole-N,N?)borate 9, methoxy-phenyl-(2-guanidino-1-methyl-benzimidazole-N,N?)borate 10, dihy-droxy-(2-guanidino-1-methyl-benzimidazole-N,N?)borate 16, difluoro-(2-guanidinobenzimidazole-N,N?)borate, 17, dihydroxy-(2-guanidino-1-benzimidazole-N,N?)borate potassium salt 19, diphenyl-(2-guanidinium-10H-benzimidazole-N,N?)borate hydro-chloride 20, methoxy-phenyl-(2-guanidinobenzimidazole-N,N?)borate hydrochloride 21, and N10-borane-(diphenyl-2-guanidinobenzimidazole-N,N?)borate 22, were determined based on 1H, 13C, 15N, and 11B spectroscopy. The X-ray diffraction structures of 37, 19, and 20 were obtained. The formation of N3-borane adducts 11 and 12 derived from compounds 1 and 2, respectively, and the dihydride-(2-guanidinobenzimidazole-N,N?)borate 13 and dihydride-(2-guanidino-1-methyl-benzimidazole-N,N?)borate 14 were observed by 11B NMR. The results show that 2-guanidinobenzimidazole gives stable borate heterocycles with a delocalized ? electronic system. A dynamic exchange of NH protons was observed with preferred protonation at N-12. The new heterocycles are protonated at N-10 by acidic substances to give pyridinium-type heterocycles or can lose a proton to give iminium salts