The energy barriers in our recently discovered Walden-type inversion of chlorophosphonium salts are similar to those for Cope rearrangements of caged cyclic hydrocarbons. Therefore, we have designed a molecular system that integrates the two processes, thereby producing the first embodiment of a chemical species that can undergo two entirely different and independent stereomutation mechanisms at the same nominal asymmetric center. Thus, the energy barrier to the rearrangement of 9-phenyl-9-phosphabarbaralane oxide, which is easily prepared by a new high-yielding synthesis, was found to be roughly 11 kcal mol-1 . This value is in contrast to the parent barbaralane (7.3 kcal mol-1 ) but in good agreement with our computational results for the rearrangement barriers. Crucially, in the corresponding chlorophosphonium derivative, two stereomutations occur simultaneously: a fast Cope rearrangement (barrier ≈12 kcal mol-1 ) and a slow Walden-type inversion of the phosphorus center (barrier ≈21 kcal mol-1 ). The computational model also revealed a relationship between the Cope rearrangement barrier and the bridgehead distance. The phenomenon of two independent and geometrically orthogonal stereomutations at a single asymmetric center provided important general insights into reaction pathway bifurcation, microscopic reversibility, and dynamic stereochemistry. This first example of coexisting alternative mechanisms that involve covalent bonds may encourage the design of new types of dynamic molecular structures.
Keywords: cycloaddition; electrocyclic reactions; microscopic reversibility; nucleophilic substitution; stereomutation.
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