Rearrangements, carbocations selectivity

Similar to linear and branched alkanes, cycloalkanes also give rise to radical cations in zeolites, spontaneously or upon y-radiolysis. This brief discussion of selected examples is intended only to give a flavor of the work being done. Thus, a 13-line radical cation spectrum (a = 0.17 mT, g = 2.003) obtained upon incorporation of 1-methylcyclohexane, 43, into zeolites [71] was identified as 1,2-dimethylcyclopentene radical cation, 44 + (two sets of protons with hyperfine couphng constants in the ratio of ca 2 1 a = 1.67 mT, 2 CH3 a = 3.42 mT, 4H) [72]. The formation of 44 + was rationalized by protonation of the 3° carbon of 43, followed by loss of H2. Loss of a proton from a rearranged carbocation may generate 44, which is oxidized to 44 + by a Lewis site. 

Carbocation intermediates are involved and the structure and stereochemistry of the product are determined by the factors that govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. Boron trifluoride is frequently used... 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Specific acid-catalysed solvolysis of l-methoxy-l,4-dihydronaphthalene or 2-methoxy-l,2-dihydronaphthalene in 25% acetonitrile in water has been found to yield mainly the elimination product, naphthalene, along with a small amount of 2-hydroxy-1,2-dihydronaphthalene, there being no trace of either the 1-hydroxy-1,4-dihydronaphthalene or the rearranged ether. The nucleophilic selectivity, ns/ hoh = 2.1 X 10", between added azide ion and solvent water has been estimated for the relatively stable = 1 x 10 s ) intermediate benzallylic carbocation for which the barrier to dehydronation is unusually low k = 1.6 x 10 ° s ), as evidenced by the large elimination-to-substitution ratio with solvent water as base/nucleophile. The kinetics of acid-catalysed solvolysis of 1-hydroxy-1,4-dihydronaphthalene and 2-hydroxy-1,2-dihydronaphthalene have also been studied. 

Fluoride ion-assisted desilylation has been extensively used to create an ylid from a /V-silyl methyl-quaternary ammonium salt. Its evolution to final produces) is variable and Sommelet-Hauser and Stevens rearrangement products were obtained (often as major products) in a ratio that can be shifted from one structure to another very close one, as in examples 1 and 2 dealing with //-benzyl salts.246,366 Differences in the solvents used are not significant because in the first example, HMPA does not reverse the ratio, yields and selectivity being just a bit lower, /so-toluene was proposed as an intermediate in example 1 it might also be the intermediate in example 2. Thus product partition reflects the relative ability of the C-H or the C-C bonds to be cleaved to produce aromatization with proton or a-amine carbocation migration. 

In view of these results, it is surprising that no rearrangement occurs during the aluminium chloride-catalyzed reaction of terf-butyl chloride with ethylene [97]. The intermediacy of a primary carbocation can, therefore, be ruled out and the selective formation of 18 may be rationalized by assuming the intermediacy of a bridged cation (tert-butyl-bridged ethylene). Alternatively, the attack of the /er/-butyl cation at ethylene may be nucleophilically assisted by the AIC14 ion because of the low stability of the 3,3-dimethyl-l-butyl cation (Scheme 22) [98],... 

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