Quantum chemical calculations 1,5-hydride shift

According to the authors of Ref. in passing from the gas phase to the solution the rates of degenerate 1,2-hydride shifts in ethyl and cyclohexadienyl ions should decrease due to more effective solvation of the carbocation in the ground state, see also Ref. cf., however. Ref. This conclusion agrees with the results of quantum chemical calculations. The experimental data available, however, are either insufficient for verifying this conclusion or contradict it. For example, for the 1,2-hydride shifts in the ethyl cation only the upper limits of the activation barrier are known g 1.9kcal/mole for the solution in SbF,—SO (cf. Ref. >)... 

A cytochrome P450-catalyzed Friedel-Crafts reaction is proposed in the biosynthesis of viridicatumtoxin (225, Scheme 45). ° The transformation involves a spirocyclization of the geranyl-substituted substrate (226). This biosynthetic chemistry is notable as the first terpene cyclization catalyzed by a P450 enzyme (VrtK). It suggests a mechanism with oxidation to the allyl cation (227), with subsequent ring formation, hydride shift, and Friedel-Crafts cyclization steps. The proposed mechanism was further studied by quantum chemical calculations. 

At -115 °C the first set of signals disappears within 10 minutes and only the peaks for cation 9 remain (Fig. 8 upper spectrum). Structural assignment for cation 9 was confirmed by HC-COSY-and COSY45-NMR spectra shown in Figs. 9 and 10, respectively. Experiments with (3-CD2-labeled progenitors and quantum chemical model calculations of transition states for 1,3-hydride shifts indicate that the rearrangement of the 1-silyl-substituted bicyclobutonium ion 8 to the 3-silyl-sub-stituted bicyclobutonium ion 9 occurs most probably by a 1,3-hydride shift from Cy to C across the bridging bond. 

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