Implications of protonation and substituent effects for C-O and O-P bond cleavage in phosphate monoesters.

A recent study of phosphate monoesters that broke down exclusively through C-O bond cleavage and whose reactivity was unaffected by protonation of the nonbridging oxygens (Byczynski et al. J. Am. Chem. Soc. 2003, 125, 12541) raised several questions about the reactivity of phosphate monoesters, R-O-P(i). Potential catalytic strategies, particularly with regard to selectively promoting C-O or O-P bond cleavage, were investigated computationally through simple alkyl and aryl phosphate monoesters. Both C-O and O-P bonds lengthened upon protonating the bridging oxygen, R-O(H(+))-P(i), and heterolytic bond dissociation energies, DeltaH(C)(-)(O) and DeltaH(O)(-)(P), decreased. Which bond will break depends on the protonation state of the phosphoryl moiety, P(i), and the identity of the organosubstituent, R. Protonating the bridging oxygen when the nonbridging oxygens were already protonated favored C-O cleavage, while protonating the bridging oxygen of the dianion form, R-O-PO(3)(2)(-), favored O-P cleavage. Alkyl R groups capable of forming stable cations were more prone to C-O bond cleavage, with tBu > iPr > F(2)iPr > Me. The lack of effect on the C-O cleavage rate from protonating nonbridging oxygens could arise from two precisely offsetting effects: Protonating nonbridging oxygens lengthens the C-O bond, making it more reactive, but also decreases the bridging oxygen proton affinity, making it less likely to be protonated and, therefore, less reactive. The lack of effect could also arise without bridging oxygen protonation if the ratio of rate constants with different protonation states precisely matched the ratio of acidity constants, K(a). Calculations used hybrid density functional theory (B3PW91/6-31++G) methods with a conductor-like polarizable continuum model (CPCM) of solvation. Calculations on Me-phosphate using MP2/aug-cc-pVDZ and PBE0/aug-cc-pVDZ levels of theory, and variations on the solvation model, confirmed the reproducibility with different computational models.