Carbocations Wagner-Meerwein rearrangement

The "classical" - "non-classical" carbocation controversy concerned the Wagner-Meerwein rearrangement of norbomyl systems ... 

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

At this point, H+ transfer could occur directly to give the product [Cu2(R—XYL—0—)(0H)]2+ (12), or Y could undergo a Wagner-Meerwein rearrangement [170] (i.e., formal migration of Y in a N.I.H. shift) to produce another resonance stabilized carbocation intermediate. 

In the conversion of a-pinene (2.8) into bornyl chloride (2.9) endo isomer), the rearrangement to a 2° carbocation is favoured by relief of small-ring strain (Scheme 2.9). In a similar manner the conversion of camphene hydrochloride (2.10) into isobornyl chloride (2.11) involves rearrangement known as the Wagner-Meerwein rearrangement (Scheme 2.10). 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

Thirdly, the carbocation may initiate a series of rearrangement reactions with either the eventual loss of a proton or attack by a nucleophile elsewhere in the molecule (Scheme 3.8). The classical examples of the Wagner-Meerwein rearrangements of terpenoids follow this pattern. 

Clearly the course of these cyclization reactions is dependent upon the silicon group. In this regard, cyclization of (82) affords the steroid nucleus (83 Scheme 40). The formation of (83) was attributed, in part, to a transition state preference for the formation of the linear vinyl carbocation (84b) rather than the bent vinyl cation (84a), which would be produced in an endocyclic cyclization. The formation of (81) was controlled by the generation of the -silyl carbocation (85a), which may be a precursor to an a-silyl ketone, which undergoes protodesilylation. It is not known whether the formation of (81) as the major cyclization product occurs through a kinetic pathway or by Wagner-Meerwein rearrangement of the kinetically prefened linear carbocation (85b). 

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