Structure-basicity relations among phosphate and phosphite esters. CNDO/2 and protonation studies

Protonation in HFSO; at -50 O C o f the phosphite triesters 1"'-5"' revealed a steady increase in ' J ~(H826 -928 Hz) upon increased constraint of the alkoxy groups. CNDO/2 calculations showed a concomitant rise in the positive charge on phosphorus and the phosphorus-bound hydrogen in the protonated species, but no trend was observed in the P hybridization. A plot of the cube of the sum of these charges vs. 'JpH (correlation coefficient 0.92) suggests that the charge contribution is probably dominant in the Fermi contact term. These results are consistent with stereochemically dependent orbital effects proposed earlier. A similar rise in the calculated negative charges at the ring oxygens of the phosphate analogues 1"-4" along with smaller differences in the protonation energies of the phosphoryl oxygen on constraint parallels the decreased hydrogen bonding tendency of the phosphoryl oxygen as shown by phenol shift experiments. A minor reversal in basicity of the first two members of the phosphite series is indicated by the BH stretching frequency trend of the BH; adducts 1*-5* and is rationalized in terms of the greater polarizing power of the proton than BHj