Wagner-Meerwein rearrangement, solvent

Acid-induced wagner-meerwein rearrangements in chiral alcohols. In view of the considerable interest on ion-molecule complexes involved in gas-phase analogues of solvolytic reactions," ° " a sustained research effort has been directed to the study of Wagner-Meerwein rearrangements in cationized )8-arylaIkyl systems, under conditions excluding nucleophilic assistance by the solvent which in these systems normally interferes with anchimeric assistance of groups adjacent to the... 

Rearrangement reactions, where the first step corresponds to the ionization of a tertiary haloalkane, also proceed faster with increasing solvent polarity, as shown by the Wagner-Meerwein rearrangement of 3-chloro-2,2,3-trimethylnorbornane to 2-exo-ehlorobornane [49]. 

Plieninger, H., Kraemer, H. P. Enantioselective Wagner-Meerwein rearrangement in chiral solvents under high pressure. Angew. Chem. 1976, 88, 230-231. 

The case history of the Wagner-Meerwein rearrangement may serve as a guideline to an extensive discussion of mechanism and stereochemistry of cationic 1,2 shifts. First principles may be derived from the behavior of free carbocations as they occur in the gas phase (Section 2) and in solvents of extremely low nucleophilicity ( superacids ) (Section 3). While such conditions provide valuable information on energetics and some insight into mechanism, they virtually exclude stereochemical studies. 

Scheme 8.43. Enantioselective Wagner-Meerwein rearrangement in chiral solvents.

Rearrangement of the primary halide product (Wagner-Meerwein rearrangement) occurs when HC1 or HBr is added to bicyclic terpenes, especially in solvents of high dielectric constant and when an excess of hydrogen halide is used. For the mechanism see Gould.18d... 

Rearrangement of quadricyclyl-7-carbinyl triflate (2). The triflate ester (2), prepared as formulated, on solvolysis in the nonacidic, nonnucleophiUc solvent trifluoroethanol with 1 eq. of triethylamine (scavenger for TfOH) is converted into (3) as the major product. This solvolysis involves a Wagner-Meerwein rearrangement, but does not result in an olefinic product. However, the present... 

For example (Scheme 7.48), it might be supposed that in an ionizing solvent, 3-iodo-2,2-dimethylpentane, following an SnI pathway, undergoes solvolysis with loss of iodide anion and formation of secondary carbocation. Then, in a subsequent step, a 1,2-methyl shift (a Wagner-Meerwein rearrangement) from the initially... 

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

Kneen showed that a Wagner-Meerwein type rearrangement occurred on reaction of AuBrs(vp) with methanol and ethanol, the substituted product having a 6-membered ring system. Ionisation, rapid rearrangement of the primary carbonium ion to a more stable secondary, benzylic carbonium ion and subsequent attack by solvent can account for this reaction. 

---