Basicity of Guanidines with Heteroalkyl Side Chains in Acetonitrile

The pK(a) values of seven novel guanidine derivatives, six of them possessing heteroalkyl substituents capable of forming intramolecular hydrogen bonds, were determined in acetonitrile (MeCN) by using the UV/Vis spectrophotometric titration method. The obtained pK(a) values range from 24.7 to 27.2. The most basic among the studied guanidines was found to be by ca. 4 pK(a) units more basic than the well-known superbase N-1,N-1,N-3,N-3-tetramethylguanidine (TMG). The trends in the changes in the measured pK(a) values were compared with the experimental (determined by the extended kinetic method) and theoretical [B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d)] gas-phase proton affinities. It was shown that basicity ordering of the bases with dimethylaminopropyl substituents in acetonitrile follows the trend encountered in the gas phase. However, this is not the case for the methoxypropyl-substituted guanidines indicating that in these molecules formation of the intramolecular hydrogen bonds is to large extent hindered due to solvation by acetonitrile. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)