Carbocations bicyclic

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

The same tertiary carbocation serves as the precursor to numerous bicyclic monoterpenes A carbocation having a bicychc skeleton is formed by intramolecular attack of the rr electrons of the double bond on the positively charged carbon... 

The CIO carbocation adds to the 8,9 double bond, giving a C8 tertiary bicyclic carbocation. 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

Bicycloalkylidenes. a-Delocalization in bicyclic carbocations provides the opportunity to detect the protonation of bicycloalkylidenes. An obvious choice was the 2-norbomyl cation (66), for which symmetrical bridging (equivalence... 

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

Chapter 1 by H.-U. Siehl discusses parallel stable ion NMR spectroscopic and computational studies on various classes of silyl-substituted carbocations. Chapter 2 by K. Komatsu focuses on unusually stable n-conjugated carbocations that are formed as a result of annelation to bicyclic frameworks. 

The possibility of trapping of the carbocation by a triflate anion followed by substitution of the triflate group by sulfide has also been suggested126, at least for the addition of bicyclic dithioether dications to alkenes and alkynes. 

In contrast, in the formation of the hydroazulene ring of bulnesol (23) Marshall and Partridge [11] started from a bridged system (21) to arrive, through an intermediate carbocation 22, at the fused bicyclic system 23 (Scheme 7.10). 

Compounds containing a pyramidally arranged (hence, chiral) sulfur to which are linked three alkyl or aryl groups, resulting in a net positive charge on the sulfur. A biologically important example is S-adenosyl-L-methi-onine chloride. Sulfonium salts can also be utilized as analogs or mimics of carbocation intermediates in enzyme-catalyzed reactions. For example, methyl-(4-meth-ylpent-3-en-l-yl)vinylsulfonium perchlorate proved to be an excellent inhibitor (Ki = 2.5 tM) of the enzyme that catalyzes the formation of the bicyclic (+)- -pinene ... 

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... 

Similar isomerization occurs in the presence of silica-alumina-thoria.104 As it might be expected, this reaction is similar to the isomerization of cyclohexane to methylcyclopentane. Both processes involve the same intermediate cyclohexyl car-bocation, which is formed, however, in different reactions. It may be formed from cyclohexane by hydride ion transfer, or by protonation of cyclohexene. Bicyclic alkenes undergo complex interconversions via carbocations over acidic catalysts.105... 

This bicyclic carbocation then undergoes many reactions typical of carbocation intermediates to provide a variety of bicyclic monoterpenes, as outlined in Figure 26.7. 

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. 

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