In carbocation-forming

Another way in which the electron-donating ability of cyclopropyl is manifested is in the influence of this group on the sensitivity of reaction centers to the effect of other substituents. This is conveniently examined in carbocation forming reactions by the values which measure the dependence of the reaction to the electron-donating ability of aryl substituents as expressed by their Brown-Hammett values by the relation log k/kii = p (7 as shown in Table 9 ... 

To be able to prepare and study these elusive species in stable form, acids billions of times stronger than concentrated sulfuric acid were needed (so called superacids). Some substituted carbocations, however, are remarkably stable and are even present in nature. You may be surprised to learn that the fine red wine we drank tonight contained carbocations which are responsible for the red color of this natural 12% or so alcoholic solution. I hope you enjoyed it as much as I did. 

Treating 5.5 g of 2-amino-4,5-dimethylthiazole HCl with 0.66 g of solid sodium hydroxide 15 min at 220°C yields 53% of 4.4. 5.5 -tetramethyT 2,2 -dithiazolylamine, whose structure w as proved by identification with the produa obtained from the reaction between dithiobiuret and 3-bromo-2-butanone (467). This result is comparable to the reaction between 2-aminopyridine and its hydrochloride to yield bis(pyridyl-2)amine (468). Gronowitz applied this reaction to 2-aminothiazole, refluxing it with its hydrochloride 4 hr in benzene and obtained the dimeric 2-aminothiazole (236). He proposed a mechanism (Scheme 143) that involves the addition of a proton to the 5-position of the ring to give 234. The carbocation formed then reacts on the 5-position of a second... 

The first step is rate determining In it a carbocation forms when the pair of tt electrons of the alkene is used to form a bond with the electrophile Following its formation the carbocation undergoes rapid capture by some Lewis base present m the medium... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

The first step, protonation of the double bond of the enol, is analogous to the protonation of the double bond of an alkene. It takes place more readily, however, because the carbocation formed in this step is stabilized by resonance involving delocalization of a lone pah of oxygen. 

The carbocation formed in this step is a cyclohexadienyl cation. Other commonly used terms include arenium ion and a-complex. It is an allylic car bocation and is stabilized by electron delocalization which can be represented by resonance. 

The rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

How can we describe the carbocation formed in the first step of die leaction of ethvJene with HBr The carbocation is clearly different from the reactants, yet it isn t a transition state andit isn t a final product. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Figure 1. Isomerization selectivity on Pt/WOx-Zr02 (0.3% wt. Pt, 12.7% wt. W, H-atoms formed from H2 or adamantane on Pt, 073 calcination) and Pt/SOx-ZrOj sites are involved in carbocation hydrogen (0.4% wt. Pt, 4.5% wt. S, 723 K...

The activation energies for the fragmentation of the carbene in CH2C12 were calculated by the B3LYP/6-31G method to be 14.6, 2.2, and —0.95 for the bicyclo[2.2.1]heptyl, bicyclo[2.2.2]octyl, and adamantyl systems, respectively. Are the product trends consistent with these computational results, which presumably reflect the relative stability of the carbocation formed by the fragmentation ... 

A molecule of water removes one of the hydrogens from the P carbon of the carbocation. An electron pair moves in to form a double bond between the a and P carbon atoms. 

As the understanding of the ionic intermediates has progressed, advantage has been taken of the fact that bromination, like SN1 heterolysis, is a carbocation-forming reaction. Kinetic data on this addition have therefore been used to examine in detail how the basic concepts of physical organic chemistry work as regards transition-state shifts with reactivity (Ruasse et al, 1984). Bromination lends itself particularly well to the quantitative application of the BEMA HAPOTHLE (acronym for Bell, Marcus, Hammond, Polanyi, Thornton and Leffler Jencks, 1985). In particular, it has been possible to evaluate the transition-state dependence on the solvent and substituents. The major disadvantage that bromination shares with many... 

For a long time, it was considered that the formation of a bromonium ion from olefin and bromine is irreversible, i.e. the product-forming step, a cation-anion reaction, is very fast compared with the preceding ionization step. There was no means of checking this assumption since the usual methods—kinetic effects of salts with common and non-common ions—used in reversible carbocation-forming heterolysis (Raber et al., 1974) could not be applied in bromination, where the presence of bromide ions leads to a reacting species, the electrophilic tribromide ion. Unusual bromide ion effects in the bromination of tri-t-butylethylene (Dubois and Loizos, 1972) and a-acetoxycholestene (Calvet et al, 1983) have been interpreted in terms of return, but cannot be considered as conclusive. 

The proanthocyanidin assay is carried out in a solution of butanol-concentrated hydrochloric acid, where proanthocyanidins (condensed tannins) are converted to antho-cyanidins (products of autoxidation of carbocations formed by cleavage of interfla-vanoid bonds) (Matus-Cadiz and others 2008). 

Not only is the leaving group important, but if the reaction has SnI character, the stability of the positive charge left behind is also important. In the examples above, a primary carbocation is difficult to form and therefore the reaction of busulfan with glutathione would likely be a SN2-type reaction, whereas the nitrenium ion and carbocation formed from N -acetylaminofluorene and safrole, respectively, are relatively stable and likely to be Sn 1 -type reactions. 

Rearrangements of long-lived carbocations formed via ewfo-9-hydroxy-1,8,9,10,10,11,12-heptamethyltricyclo[6.2.2.02,7]dodeca-2(7),3,5,11 -tetraene (5) in FSO3H-SO2CIF-CD2CI2 at low temperature (-95 °C) were studied by H and... 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

Finally, as is the case for the secondary a-deuterium KIEs, the /3-deuterium KIE is assumed to vary in magnitude from near unity for a reactant-like transition state to a maximal value for a transition state resembling the carbocation formed in an SN1 reaction. The experimentally determined KIE may, therefore, be used as a measure of transition state structure provided that the maximum value of the KIE, i.e. the EIE for the formation of the carbocation, is known. 

Oxidative S—C bond cleavage, followed by the attack of a nucleophile on the carbocation formed, is a classical anodic substitution reaction. In this way, OH [60], AcNH [61], AcCH2 [62], and CH2=CHCH2 [63] groups were introduced to replace the RS fragment. 

One of the first notions of EGA-catalyzed reactions was the rationalization [8, 14] of the unexpected outcome of anodic oxidation of methyl arenes, (1), in MeGN containing various amounts of water. Preferentially A-benzyl acetamides, (3), rather than the benzyl alcohols, (2), were formed [15, 16] (with increasing amounts of water, increasing amounts of aldehyde was formed as a side product [16]). Since water is a more powerful nucleophile than MeCN, it is reasonable to believe that the carbocation formed by overall two-electron oxidation and deprotonation is initially trapped by water. However, the process is reversible in the presence of a strong EGA (protons liberated from the oxidized substrate), and the carbocation is eventually trapped by the excess MeCN, Scheme 1. 

As the carbocation formed in the slow, rate-limiting step of the reaction is planar, it might be expected that subsequent attack by a nucleophile such as OH, or the solvent (H2O ), would take place with equal readiness from either side of this planar carbocation leading to a 50/50 mixture of species having the same, and the opposite, configuration as the starting material, i.e. that racemisation would take place yielding an optically inactive ( ) product. 

The H-Br molecule is the electrophile and is already polarised. Its atom attacks the double bond in propene, forming an intermediate carbocation. At the same time, the bond in the H-Br molecule breaks heterolytically and a Br" ion is generated. 

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