Hydride shifts unsymmetrical

The reduction of ketone 559 by LiAlH results mainly in the endo alcohol 556. The thermodynamic equilibrium of exo and endo alcohols leads mainly to exo alcohol 560. Hence, the formation of endo alcohol 556 upon solvolysis of tosylate 558 cannot be due to steric factors — the endo-side attack is sterically less favourable than the exo-side one the resulting alcohol 556 is not a thermodynamically controllable product. Consequently, the data obtained cannot be used to assume a classical structure of the intermediate cation 561. At the same time the participation of the C —C bond and the intermediate non-classical ion are quite compatible with these facts. The acetolysis of the optically active tosylate 558 is accompanied by complete racemization, the rate of the latter being 3 times as high as that of acid elimination (internal return). The completeness of racemization shows the reaction to proceed via a symmetrical trishomocyclopropenyl ion or rapid equilibration of unsymmetrical cations 561 or to be accompanied by a 1,3-hydride shift from to C . 

The close proximity of the acridine and acridan moieties in 45 provides the opportunity to generate 46 possessing a bridging hydrogen atom (C-H C ) between the methylacridan and methylacridinium groups (Fig. 12) [46]. The X-ray structure of 46 shows an unsymmetrical bridge with a distance of 1.08 A for the localized C-H bond and a distance of 2.51 A for HC. The distance between the two carbon atoms is 3.14 A. The C4-C4a-C4b-C5 torsional angle is 29.7°. The activation barrier for the hydride shift, determined by variable-temperature NMR studies, is 12.4 kcal/mol at 5°C. 

As depicted in Scheme 10.2, the mechanism of the reaction presumably involves protonation of 1-hexene to afford the secondary carbocation 43. Attack of bromide ion on this ion then leads to 2-bromohexane (41). The carbocation may also rearrange by way of a hydride shift (Sec. 10.3) to provide a different secondary carbocation, 44, which would provide 3-bromohexane (45) upon reaction with bromide ion. Alternatively, it may deprotonate to form 2-hexene (46), addition of H-Br to which could afford both 41 and 45. In this experiment, you will determine the regiochem-istry of the addition of H-Br to the unsymmetrical alkene 1-hexene (40) and thereby assess whether this ionic reaction proceeds according to Markovnikov s rule. 

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