The Markedly Enhanced Basicity of Selenenamides vs Sulfenamides and the Mechanism of the Methanolysis of & Nitro - and 2 , 4 , 6 - Tri - tert

Measurement in acetonitrilemethanol of the basicity of selenenamides shows that they are approximately 3 pK units more basic than the corresponding sulfenamides. Since other explanations do not seem tenable, this apparently results from the fact that selenium is less electronegative than sulfur, although it is somewhat surprising that the small difference in the electronegativity of the two elements should lead to such a large difference in basicity. In the same solvent the kinetics of the acid-catalyzed methanolyses (eq 2) of o-nitroand 2,4,6-tritert-butylbenzeneselenenamides are similar (large dependence of rate on [MeOHl, unusual dependence of rate on [H+l) to those for the methanolysis of the corresponding o-nitrobenzenesulfenamides. In the latter case the kinetics have been shown6 to be indicative of a mechanism where a sulfuranide (3, eq 5) is the key intermediate on the reaction coordinate; as a consequence, a similar kind of mechanism (eq 6) involving a hypervalent selenium intermediate (4) is proposed for eq 2. The unusual dependence on [H+l is because acidcatalyzed reversion of 4 to protonated selenenamide (2-H+) and methanol (step k-6) is faster under certain conditions than cleavage of 4 to give the final products (step kd. Comparison of the methanolysis kinetics of o-nitro (2a) and 2,4,6-tri-tert-butylbenzeneselenenamides (2c) shows that the coordination of an o-nitro group to selenium that stabilizes 2a and 2a-H+ does not appear to change the mechanism for methanolysis although it does cause the partitioning of 4 (k*[H+l/k7) to be much less favorable to the formation of products (step k7).