Reaction of dicyclohexylcarbodiimide with sterically hindered thiophosphoric esters 3 leads at -80 °C to observable (31P NMR) 5-phosphorylisothioureas 5. At 20 °C in methylene chloride the same reaction does not give 5 but its TV-phosphoryl isomer 4, together with S', 2, and the counterion 3'. The equilibrium quantity of the TV-phosphoryl isomer varies with the bulk of the R group (R = Ph > 2,6-Me2C6H3 > 2.5-/-Pr2C6H3). The equilibrium can also be either approached from 4 (which is isolable) or by treating the isothiouronium salt 5' with a strong amine base (such as the proton sponge), and in each case the same observed rate constant (=k, + k.f is obtained. The equilibrium can be displaced toward 5' in the presence of acid (HBF4 or Et3NH+) or toward the elimination products 2 and 3' in the presence of bases (tertiary amines). In the presence of excess acid the observed rate of reaction of 4 is independent of acid concentration and type, and the rate-determining step is assigned to the intramolecular TV--5-phosphoryl group transfer (4 -5), governed byIn the presence of excess base the rate of reaction of 4 is independent of base type and concentration, and the step which is partly rate determining in this case is consistent with the conversion of 5 to 8 (k2) which is correctly oriented for elimination. The implications of the observation of this mobile equilibrium governed by the presence of acid or base catalysts to the question of the involvement of Oor TV-phosphoryl intermediates in the mode of action of the coenzyme biotin is briefly discussed.
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