MEMORY EFFECTS IN MULTIPLE CARBONIUM ION REARRANGEMENTS .2. BEHAVIOR OF POTENTIALLY SYMMETRICAL CATION IN RING-EXPANSION REACTIONS OF 7-NORBORNYLCARBINYL SYSTEMS

Carbonium ion ring expansions of stereospecifically labeled syn- and anti-7-norbornylcarbinyl-2,3-d2 derivatives lead to bicyclo[2.2.2]oct-2-yl and bicyclo[3.2.1]oct-exo-2-yl products. The ratio of these two products is barely higher in the ring expansions than in “conventional” entry into this system from bicyclo[2.2.2]oct-2-yl or bicyclo[3.2.1]oct-exo-2-yl substrates. External nucleophilic capture of an “extra” precursor of bicyclo[2.2.2]oct-2-yl product in the ring expansions thus is rather inefficient. The distribution of the deuterium label in the bicyclo-[3.2.1]oct-exo-2-yl product demonstrates the existence of a memory effect in the second rearrangement step. The first-formed potentially symmetrical ring-expanded cation behaves unsymmetrically; the bond that migrates preferentially in the second rearrangement step is the one remote from the initial site of reaction in the starting material. The magnitude of this preference in solvolytic ring expansion is ather insensitive to environmental factors, and the small changes produced by large changes in solvent ionizing power are opposite in direction to those expected for ion-pair return phenomena. Deaminative ring expansions at room temperature give higher memory effect selectivities than solvolytic ones at elevated temperature. Independent synthesis and solvolysis of the hypothetical ion-pair return product, stereospecifically labeled bicyclo[2.2.2]oct-2-yl-5,6-d2 p-bromobenzenesulfonate, show that this cannot be the sole intermediate in the ring expansion of the labeled 7-norbornylcarbinyl p-bromobenzenesulfonate. © 1969, American Chemical Society. All rights reserved.