Ions as Reaction Intermediates

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

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Thiiranium and thiirenium ions as reaction intermediates and building blocks in organic synthesis 97G177. 

Undoubtedly, such scrambling phenomena are possible only with the participation of carbenium ions as reaction intermediates or transition states. Molecular rearrangements that rationalize the scrambling of deuterium atoms are shown in Scheme 5. 

The occurrence of carbenium ions as reaction intermediates is strongly supported by the observation that the isotopic exchange can be totally suppressed in the presence of carbon monoxide.68 Furthermore, trapping of the intermediate carbenium ions by CO and water has been observed by insitu NMR spectroscopy when isobutane, water, and CO reacted on HZSM-5 zeolite to form pivalic acid.69,70 Regarding the small conversion, only a limited number of acid sites are suggested to be strong enough for the initial protolytic activation to take place. 

The first infrared evidence of the possible occurrence of formate ions as reaction intermediates on the surface of metals resulted from the research of Hirota and his colleagues (46, 56), who showed formate ions to be present on powders of silver, copper, nickel, palladium, rhodium, platinum, and zinc, after adsorption of formic acid at room temperature. Moreover, in the far-infrared region they observed bands at 410 and 130 cm-1, which is an indication of bonding between metal atoms and formate ions via oxygen atoms [Hirota and Nakai (57)]. 

As discussed before there are, however, indications of the importance of formate ions as reaction intermediate. The work of Scholten revealed that the rate of decomposition of formate ions situated on the surface is well within the range of the rate of the decomposition of formic acid on magnesium oxide. 

The idea of carbonium ions is quite old in organic chemistry. Olah has traced the early history (15). In 1902 Von Baeyer wrote of carbonium salts in explaining the deep color formed when triphenylmethyl chloride was dissolved in sulfuric acid. Carbonium ions as reaction intermediates were proposed by Meerwein in 1922, and much used by Ingold, Hughes, and others in England soon thereafter. F. C. Whitmore in the USA from 1932 on showed how carbonium ions as reaction intermediated could explain the acid-catalyzed reactions of alkylation, polymerization, and isomerization. His studies were summarized in a review article in 1948 ( ). More recently, of course, there have been many spectroscopic studies of stable carbonium ions formed in highly acidic solutions at low or moderate temperatures, as, for example, in the works of N. C. Deno and G. A. Olah. 

Apart from generating the cupric ion, the acidic oxidation reaction (loss of electrons) produces cuprous ion as an intermediate from any cuprous oxide that may be present in the deposit. It is therefore neces-... 

These ideas are speculative only and cannot be considered conclusive until other cases are tested. Unfortunately, information on states of polyatomic ions is meager. Two ions which have been studied are C02 + and CS2 +, both of which have 2w ground states and 22 excited states. It would be interesting to look for ion-neutral reactions having these ions as possible intermediates. 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

The addition of organodichalcogenides to alkynes does not necessarily require the aid of transition metal catalysts. Indeed, the reaction proceeds via different mechanisms under various conditions (radical, In, CsOH,183 SnCl4,184 and phase-transfer catalysts185). Treatment of a mixture of (PhS)2 and terminal alkynes with GaCl3 affords (E)-products (E Z=>20 1).186 The reaction is assumed to involve a thiirenium ion as the intermediate (Scheme 38). 

Meyerson, S. Cationated Cyclopropanes As Reaction Intermediates in Mass Spectra an Earlier Incarnation of Ion-Neutral Complexes. Org. Mass Spectrom. 1989, 24, 267-270. 

For the sake of simplicity, carbenium ions, carbonium ions or protonated cyclopropane rings were used as reaction intermediates, omitting the anionic zeolite framework in the illustrahon of the reaction mechanisms for the reactions discussed here. Furthermore, it is conceivable that many such reachon paths involve alkoxide intermediates, instead of carbenium and carbonium ions. 

The reaction mechanism of and active sites for the reaction have been studied using various techniques combined with an isotopic tracer method. Infrared spectra were measured for detection of surface adsorbates on the Cu-ZSM5 zeolites ESR, XPS, phosphorescence, diffuse reflectance UV, and Cu-MASNMR have been used to reveal the states of the copper ions in the catalysts. CO adsorption and TPD experiments have been employed to measure quantitativdy the amounts of Cu ions, NO adsorbed, and O2 remaining on the surface. Based on these investigations, we can propose a reasonable reaction mechanism which includes Cu ions as active sites and NO" species as reaction intermediates. The reaction cycle is suggested to be as foDows elevated temp. + 2NO... 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Adducts 119 are of relevance as reaction intermediates in the chemistry of several uracil and cytosine derivatives, which show a strong tendency to undergo covalent nucleophilic addition across the 5,6 double bond with such reagents as water, alcohols, hydroxylamine, and bisulfite ion. 

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