Abstract: The acetyl, sulfonyl and phosphoryl are three amino protecting groups. The natural abundance ~(15)N-NMR of their derivatives of the phenylethylglycine and β- phenylethylamine has been studied.We found that there are great differences in the chemical shifts of the three protected compounds. δ~(15)N is shifted to upfield in turn.Among these three amino protecting groups, the diisopropoxyphosphinyl one produces the least deshielding effect, while the acetyl group gives the strongest deshielding effect, and the tosyl group is in the middle. The trend of deshielding effect of the three protecting groups occurred also in the ~1H NMR (Table 1). Furthermore, we have studied the relation of chemical shifts to the atomic charge and molecular structures by means of semiempirical molecular orbital CNDO/2 and X-ray diffraction of single crystal structure analysis.
Model moleculae has been calculated by semiempirical molecular orbital CNDO/2 method. The results were shown in Table 2. The trend of the nitrogen net charges of these moleculae is the same as the ~(15)N NMR chemical shifts. These data show that the phosphoryl group is the most effective nitrogen lone pair electron localizing group among the three kinds of protecting groups. And the acetyl one is the least one.
By using the data of the single crystal structure of N-diisopropoxyphosphinyl-N-2-(3,4-methylenedioxy) phenyl) ethylglycine. The bonding types of the P-N, S-N and C-N of the related compounds were analyzed. The localizing effect of the phosphoryl group on the lone pair electron of nitrogen has been discussed. The one bond coupling constants of the N-phosphoryl system (Table 1) showed that the nitrogen through sp~2 hybrides formed bonds with one phosphorus atom and two carbon atoms. Thus, the nitrogen is in a planar conformation. It is consistent with the results of X0ray diffraction.