Several phosphorus acids (methyl-and phenylphosphonic, diphenylphosphinic, and ethyl phosphoric acids) were found to give esters on reaction with feri-butyldimethylsilanol. The reactions occurred simply on heating the reactants in an inert solvent (hexane or toluene). The reaction is reversible, and yields of esters were improved by continuous water removal. The dibasic acids gave either mono-or disilyl esters; the latter were more easily purified and characterized. The OH groups on the surface of silica gel were also found to react with various phosphorus acids under similar conditions to give surface-bonded esters. Products were characterized by CP/MAS 3IP and 29Si NMR. In the case of phenylphosphonic acid and ethyl phosphoric acids, the products were spectrally identical with those obtained by phosphorylation of the surface OH groups with the highly reactive 3-coordinate anhydrides of these acids when generated in the presence of silica gel.In recent publications,1•2 we have described the action of alkyl metaphosphates, highly reactive and known only as transient species, on the surface OH groups of silica gel. This produces a phosphate group directly bonded to the silica surface: Three different precursors of metaphosphate have been used in these studies, illustrated in Scheme 1 for the ethyl derivative. The same phosphorylated product is formed in each reaction, as revealed by cross polarization-magic angle spinning (CP/MAS) 3IP NMR spectroscopy. The proposal that phosphorus is indeed bonded covalently to the surface is supported by the position of the31P NMR signal at -10; simple phosphates have shifts around ±1, but upfield shifts of about 10 ppm occur routinely when siloxy groups replace alkoxy or hydroxy groups (cf. (EtO)2P-(O)OH, +1, to (EtO)2P(0)OSiMe3, -91), and a similar shift can be expected for the silica phosphate. The possibility was also considered that the metaphosphate was simply hydrolyzed to ethyl phosphate by water remaining on the surface, and adsorbed

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