Carbocations, continued

Fluorosubstituted carbocations continue to be a subject of interest. Diflu-oro(phenylthio)methylation has been achieved through the silver-promoted generation... 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Carbocations are highly reactive species that can be used for C-C bond formation. One driver for using continuous micro chemical processing is to employ also unstable cations, which are not amenable to batch synthesis because they decompose before they can actually be used [66, 67]. 

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

In the extreme carbocation limit of (3.163) and (3.164), the stereoelectronic secondary-hyperconjugation effects therefore blend seamlessly into ordinary pi-type conjugation phenomena (Section 3.3), the two extremes always being linked by electronic continuity. 

Tripheny(methyl cation (10 ) is effectively stabilized by electron donating substituents. B. W. Laursen et al. reported the highly stable carbocation 11+ with a pKr+ value of 19.7 (10). Recently, they reported a similar carbocation with a much higher p R+ value (23.7) (11). In our continuing efforts to prepare extremely stable carbocations, we have investigated the effect of introduction of electron donating substituents into each azulenyl group. 

In continuation of our investigations on asymmetric nucleophilic acylations with lithiated a-aminonitriles [40], we envisaged the asymmetric synthesis of 3-substituted 5-amino-4-oxo esters A, bearing both a-amino ketone and 5-amino ester functionalities (Scheme 1.1.14) [41]. Since a-amino ketones are precursors of chiral p-amino alcohols [42, 43] and chiral amines [43], their asymmetric synthesis has the potential to provide valuable intermediates for the synthesis of biologically active compounds, including peptidomimetics [44]. The retrosynthetic analysis of A leads to the a-aminoacyl carbanion B and p-ester carbocation... 

Formed carbocations can undergo p scission [Eq. (2.8)] to yield propylene (or the corresponding alkene, but not ethylene) and a new primary cation. The primary ion rapidly rearranges to a secondary ion involving a hydride shift [Eq. (2.9)], which, in turn, can continue the process. Isomeric cations may also be formed through intermediate alkenes [Eq. (2.10)] ... 

Carbenes (Continued) substituents on, 115 Carbocations, 105-108 acids and electrophiles, 97 dialkoxy, 105 ethyl, 84 fluoro, 105 norbornyl, 84 reactions, 107 substituents on, 106... 

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