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Experimental and Quantum Chemical NMR Studies of Carbocations - Degenerate (1, 5) and Nondegenerate (1, 2) Hydride Shifts

Carbocations are important reactive intermediates in chemical reactions in polar media and in the course of biochemical transformations. 1, 2- and higher order hydride shifts in carbocations are involved in the biogenetic synthesis of terpenes and steroids. As a model, two different rearrangements in tertiary carbocations were studied by a combination of NMR spectroscopy and quatum chemical ab initio methods: I) 1, 5 hydride shift in 2, 6-dimethyl-heptyl cation and II) 1, 2 hydride shift in isopropylcyclohexyl cation. The 2, 6-dimethyl-2-heptyl cation (Scheme I) undergoes a rapid degenerate rearrangement (Ia Ib), which is a 1, 5-hydride shift between two equivalent forms Ia and Ib and, thus, does not show temperature-dependent chemical shifts. In the case of isopropylcyclohexyl cation (Scheme II) the time-averaged NMR spectrum is created by a nondegenerate equilibrium (IIa IIb). While the equlibrium constant for the 1, 2 hydride shift (IIa IIb) differs substantially from unity (K = IIb/IIa = 17 at -120 oC), for the 1, 5 hydride shift (Ia Ib) the equlibrium constant is unity (K = Ib/Ia = 1) by a definition. For the rearrangement (Ia Ib) the time averaged NMR spectrum and quantum chemical ab initio calculations suggest a low energy barrier ( 5 kcal/mol) reflecting the low energy of a symmetrical six-membered ring hydrido-bridged transition state Ic. Quantum chemical ab initio calculations of NMR chemical shifts support the interpretation of the NMR studies. The combination of experimental and quantum chemical investigations allow a detailed description of the structure and the reactions of carbocations

quantum Chemical NMR; carbocations; degenerate and nondegenerate hydride shifts

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