The free-radical bromination of 3, 5-di-tert-butyl-2-hydroxyl, 2-oxaphosphol-3-ene 2-oxide (lb) with N-bromosuccinimide (NBS) gave the corresponding 5-bromo-3, 5-di-tert-butyl-2-hydroxyl, 2-oxaphosphol-3-ene 2-oxide (3) in good yield. Bromide 3 was extremely labile and could not be purified rigorously, but it readily underwent methanolysis to give 3, 5-di-tert-butyl-2-hydroxy-5-methoxy-l, 2-oxaphosphol-3-ene 2-oxide (7) or hydrolysis to 3, 5-di-tert-butyl-2, 5-dihydroxyl, 2-oxaphosphol-3-ene 2-oxide (8), both crystalline compounds. Compounds 7 and 8, though somewhat less reactive than 3, were readily interconverted. Treatment of 8 with diazomethane led to dimethyl (Z)-2, 2, 6, 6-tetramethyl-3-oxohept-4-en-5-ylphosphonate (10), indicating that 8 is in equilibrium with its open phosphonic acid isomer. Ketal 7 underwent methoxy exchange at 35.5 °C with a first-order rate constant of 0.075 m-1, and the rate was only slightly increased by a large excess of trifluoroacetic acid. The conjugate base of 7 did not undergo exchange. By contrast, 3, 5-di-tert-butyl-2, 5-dimethoxy-l, 2-oxaphosphol-3-ene 2-oxide (11), the methyl ester of 7, was totally inert toward methoxy exchange except in the presence of excess HBr at high temperature for extended periods. The contrasting solvolytic behavior of 7 and 11 under acidic conditions has been interpreted as evidence for an intramolecular proton transfer from an oxygen on phosphorus to the nucleofuge. Ketal-ester 11 underwent slow ring opening in base, which was immediately reversed upon neutralization. The methyl ester (lc) of lb, the 4-bromo derivative (1d) of lb, and its methyl ester (le) all undergo similar reactions with NBS. However, 4, 5-dibromide 20 was anomalously unreactive toward nucleophilic substitution. © 1990, American Chemical Society. All rights reserved.
