Rearrangement reaction paths

The structures in the (1,0) and (0,1) comers are not necessarily stable species, they may correspond to hypothetical structures. In the Cope rearrangement it appears that the reaction only involves a single TS, independently of the number and nature of substituents. The reaction path may change from B A C depending on the system, but there are no intermediates along the reaction coordinate. 

The tertiary alcohol m,m,/ra ,v-perhydro-9h-phcnalcnol (7) is converted stereospecifically and in high yield (92%) to /ran.v,/ran.v,/ran.v-pcrhydrophcnalcnc (10) when treated with either triethylsilane or triphenylsilane and trifluoroacetic acid in dichloromethane (Eq. 15). Studies indicate that the reaction path follows the cation rearrangement 8 9 and that the trans trifluoroacetate ester related to... 

Fig. 16 (a) Comparison of potential energy profile for the formal Cope rearrangement of 3,4-difluorohexa-l,5-diyne-3-ene with that of (Z)-hexa-l,5-diyne-3-ene, (b) Rehybridization in the C(F) bond along the reaction path. EDI = 3,4-difluoro-hex- 3-ene-l,5-diyne ED2 = 1,6-di-fluoro-hex-3-ene-l,5-diyne BZY = difluoro-l,4-didehydrobenzezne TSBC = the transition state for the Bergman cyclization TSRBC = the transition state for the retro Bergman cyclization. 

Fig. 8 shows a plot of the calculated 48) reaction path for the reaction of 1S carbon atoms with ethylene. It will be seen that the intermediate carbene (28) is formed exothermically, but that its rearrangement to allene (29) requires much activation (50 kcal/mole). At first sight this seems inconsistent with the evidence that the reaction takes place readily at -190 °C. 

The next step in complexity are systems in which alkylation competes with hydride transfer to give dimeric alkyl cations which, when hydride abstraction occurs, yield dimeric saturated hydrocarbons (equation 14). This reaction path for cyclic aliphatic alcohols and olefins is often accompanied by some rearrangement (Deno etal., 1964 Pittman, 1964). 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

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