Carbocations bicyclic, rearrangement

For example, acetolysis of r/n /-6-tosyloxytricyclo[5.2.0.02 5]nona-3.8-diene proceeded smoothly at 35 C with stereospeeific rearrangement to c.Y0..mj-9-acetoxylricyclo[4.2.1,02 5]nona-3,7-diene (l).30 Interestingly, the rate of acetolysis of the substrate was considerably enhanced kre[25 C = 6.8 x 104) as compared with that of its bicyclic counterpart 2.30 An important conclusion from these studies is that the anchinteric assistances by cyclobutene in the form of homoallylic participation is effective in the stabilization of the carbocation intermediate. It was found in another study that cyclobutene is better than cyclobutane in terms of anchimeric assistance.31... 

Exercise 30-7 Camphor can be made on an industrial scale from a-pinene (turpentine) by the following reactions, some of which involve carbocation rearrangements of a type particularly prevalent in the bicyclic terpenes and the scourge of the earlier workers in the field trying to determine terpene structures. 

The theory of nonclassical ions offers an explanation of many unique chemical, stereochemical and kinetic peculiarities of bicyclic compounds. It has expanded our knowledge on chemical bonds in carbocations by introducing electron-deficient bonds (as in boron hydrides). It has accounted for many rearrangements of stable cations. As a side result our knowledge has been extended about ionization process in a solution, as well as about stereochemical methods. 

Sdieme 6,59. Representations of the 1,2-hydride migration and Wagner-Meerwein rearrangements on a bicyclic carbocation. 

Thus, in Scheme 6.59, l,7,7-trimethylbicyclo[2.2.1]hept-2-ene ( bornene ) is protonated to yield a secondary carbocation, capable of undergoing 1,2-enrfo-hydride migration and/or the migration of a bond in a Wagner-Meerwein rearrangement to a new bicyclic (but now tertiary) carbocation and thence, by proton loss, to 3,3-dimethyl-2-methylenebicyclo[2.2.1]heptane ( camphene ). 

Figure 11.10 shows the products from the solvolysis of exo-2-chloronorbornane (one carbon is labeled with an asterisk in order to follow the carbon skeleton rearrangement). Only exo products are obtained, which means that the endo face of the bicyclic system is protected by the bonding geometry of the intermediate carbocation (see the definition of "endo" and "exo" in the margin and in Chapter 6). In addition, the product is racemic, suggesting an intermediate with a plane of symmetry. 

The reaction of pinene to form or-terpineol has been known since the late nineteenth century. Pinene is relatively abundant in nature, with the major source being pine trees, a- and pinene (1) will hydrolyze in aqueous add to form a tertiary carbocation, as shown in Fig. 9.30, followed by a series of carbonium ion rearrangements, which results in a multitude of products. The major product of the reaction is or-terpineol (2) however, as a consequence of carbonium ion rearrangements, terpinen-4-ol (3) and y-terpineol (4,5) are also produced. These monocyclic alcohols are collectively known as terpineol. Smaller amounts of the bicyclic alcohols fenchol (6) and borneol (7) are also formed. Undesirable side reactions... 

The major component of pine oil is a-terpineol (26). Other components in the product include (3-terpineol (154), y-terpineol (155), a-fenchol (156), bomeol (157), terpinen-l-ol (158), and terpinen-4-ol (159), /i-menthadienes (mainly limonene and terpinolene), 1,4-cineole (102), and 1,8-cineole (103). The mechanisms of some of these reactions are shown in Fig. 8.31. The ethers, 1,4- and 1,8-cineole, are also formed by cyclization of the / -menthane-l,4- and 1,8-diols. The bicyclic alcohols, a-fenchol [512-13-0] and borneol (c Wagner-Meerwein rearrangement of the pinanyl carbocation and subsequent hydration. 

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