Reactions of Ionic Intermediates

The encouraging results with photoredox catalysis sparked an explosion of additional investigations of visible-light-promoted oxidation of sp carbons. Many studies explored use of related polypyridyl transition metal based 

While the majority of transition metal photocatalysts are based upon ruthenium and iridium complexes, similar intermolecular cyanation reactions can also be accomplished using gold-based transition metal photocatalysts. 

The research groups of Xiao and Rueping applied this concept towards [3 -I- 2] dipolar cycloadditions using amino acetate derivatives that form azomethine ylides under photocatalysis. Xiao and co-workers performed aerobic oxidations of ethyl 2-(3,4-dihydroisoquinolin-2(lH)-yl) acetates in the 

Upon filtration of the catalysts from the reaction mixture, conversions between 84-97% were reported and were typically higher than those corresponding to the monomeric metal complex. Results were more varied among Ru-CP and Ir-CP, in which Ru-CP greatly out-performed Ir-CP for substrates that were more electron-deficient. Control experiments to support the heterogeneity of these systems employed the supernatant solution for subsequent reactions and resulted in only minimal photocatalytic activity ( 12% conversion). 

55 °C in the absence of the photocatalyst and light. However, yields and regioselectivities were generally better under photocatalytic conditions. 

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Chen, H., Eberlin, L.S., Nefliu, M., August , R., Cooks, R.G. (2008) Organic Reactions of Ionic Intermediates Promoted by Atmospheric-pressure Thermal Activation. Angew. Chem. Int. Ed. 47 3422-3425. 

R.G. Cooks Fischerindohsynthesis, Borsche-Drechselcyclization, and the pinacol rearrangement organic reactions of ionic intermediates were promoted by atmospheric pressure thermal activation. [225]... 

The kind of reaction which produces a dead polymer from a growing chain depends on the nature of the reactive intermediate. These intermediates may be free radicals, anions, or cations. We shall devote most of this chapter to a discussion of the free-radical mechanism, since it readily lends itself to a very general treatment. The discussion of ionic intermediates is not as easily generalized. 

Cycloadditions are not restricted to the reactions of combinations of neutral dienes and dienophiles. There are examples of corresponding reactions involving ionic intermediates. The addition of 2-methylallyl cation to cyclopentadiene is an example ... 

TTigh pressure mass spectrometry has recently provided much detailed kinetic data (5, 12, 13, 14, 15, 17, 22, 24, 26, 29) concerning ionic reactions heretofore unobtainable by other means. This information has led to increased understanding of primary reaction processes and the fate of ionic intermediates formed in these processes but under conditions distinctly different from those which prevail in irradiated gases near room temperature and near atmospheric pressure. Conclusive identification and measurements of the rate constants of ionic reactions under the latter conditions remain as both significant and formidable problems. 

The product selectivity arises from the reaction of this intermediate with the various nucleophiles present in the reaction medium. Therefore, the structure of this intermediate and its close environment (counter-ion, solvent molecules, etc.) determines the product selectivity. It is important to note that the relevant data concern the intermediate as it is formed during the reaction and not prepared under extreme conditions where it is stable, such as those used to obtain the nmr spectra of these ions, for example (ref. 5). Since the life-time of this ionic species in the reaction medium is very short, in the 10 9 - 10-10 s range (ref. 6), there is no way to observe it directly. Consequently, the available data, useful for understanding... 

Such radicals or ion pairs are formed transiently as reactive intermediates in a very wide variety of organic reactions, as will be shown below. Reactions involving radicals tend to occur in the gas phase and in solution in non-polar solvents, and to be catalysed by light and by the addition of other radicals (p. 300). Reactions involving ionic intermediates take place more readily in solution in polar... 

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give... 

In agreement with the involvement of ionic intermediates for electrophilic halogenation of alkenes, an important role is also exerted by the solvent. Not only the reaction rate is strongly solvent-dependent, but also the stereochemical course of the addition process may be affected by the polarity of the medium. Solvent properties determine the reaction rate the overall kinetic order the nature of the products the stereochemistry of the products... 

Lifetimes of the ionic intermediates of nucleophilic substitution are generally correlated to the pathways followed under given reaction conditions. Information on the lifetimes of ionic intermediates formed by bromine addition to olefins in methanol, as determined by the azide clock method, do not allow the different reaction pathways to be distin-... 

Critical appraisal of the method, 23,26 using attempts to synthesize nonsymmetrically substituted lanthionines, resulted in rearrangement of the products, presumably due to phosphine-catalyzed disproportionation of the unsymmetrical disulfides. This reaction should proceed more rapidly than the desulfurization process and is thought to occur because sulfur extrusion takes place via a reversible reaction by recombination of ionic intermediates (Scheme 3). 21-22-24 Thus, the reaction of nonsymmetrical cystine derivatives results in the formation of a mixture of three different lanthionines. 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

Propadiene also appears to have the potential for much easier formation of a biradical than does ethene, as you will see if you work Exercise 21-27. Not all [2 + 2] cycloadditions proceed by biradical mechanisms, some clearly occur by stepwise reactions involving ionic intermediates (see Exercise 21-43). 

Despite our increasing knowledge of the structures and reactions of organometallic intermediates, it is virtually impossible to plan complex ionic reactions by using known rate constants from simple model systems. Rate constants are potentially more useful when planning pericyclic reactions because they are easier to measure and because pericyclic reactions are much less susceptible to medium effects than ionic reactions. However, the need to evaluate the rates of competing pericyclic reactions is relatively rare (often there is only one reasonable possibility). 

Qualitative evidence that ionic species were significant intermediates was obtained from a study of the radiation induced polymerisation of isobutene28,29. Since this monomer was known to be readily polymerised by ionic initiators, polymerisation by 2 MeV electrons at —80 °C seemed to indicate the existence of ionic intermediates. However, the polymerisation was inhibited by oxygen and benzoquinone which are known to be inhibitors for free radical polymerisations. It was subsequently suggested30 that polymerisation was caused by the positive ion (CH3)3C+ produced by the reactions... 

Intermolecular hydroamination or hydroarylation reactions of norbornene and cyclo-hexadiene carried out with catalytic amounts of Brpnsted or Lewis acid in ionic liquids have been found to provide higher selectivity and yields than those performed in classical organic solvents. This effect was attributed to the increases of the acidity of the medium and stabilization of ionic intermediates through the formation of supramolec-ular aggregates with the ionic liquid.38... 

H, Me, r-Bu, or Ph or R = H and R = Me, r-Bu, or Ph), was performed. Two possible reactions were investigated (a) the reactions suitable for the gas-phase interactions, which start from a 1 1 Br2-alkyne r-complex and do not enter into a 2 1 Br2-alkyne jt-complex and (b) the processes passing through a 2 1 Br2-alkyne 7r-complex, which appear more realistic for the reactions in solutions. The structures of the reactants and (g) the final products and also the possible stable intermediates have been optimized and the transition states for the predicted process have been found. Both trans- and cw-dibromoalkenes may ensue without the formation of ionic intermediates from a n-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond formed in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The cluster model was used for the prediction of the solvent influence on the value of the activation barrier for the bromination of the but-2-yne.35... 

Moreover, the rate constants for these substitutions appear to decrease as the reaction progresses. This is because the initial dissociation is significantly reversible. As the concentration of Cl resulting from the reaction builds up, recombination of Cl with the 5-covalent intermediate, Co(en)2Cl f, begins to compete with the reaction of the intermediate with NOj, Br, or SCN, and a smaller and smaller percentage of the acts of dissociation result in net reaction. The same effect may be observed by carrying out the reaction in the presence of an excess of ionic chloride here, the observed rate of substitution is greatly decreased. This, the so-called mass-law effect, constitutes one of the surest tests for the dissociation mechanism. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

The much more common case of hydrolysis reactions, which seem to correspond to the intermediate case of Eq. (XVI.1.4) with feCX ) = kz(H20)j is difficult to classify with certainty because, with the increase in salt content of the system, there is an increased tendency toward ionization (i.e., increase in Xion = ki/k2 due to increased ionic strength), which tends to compensate for the mass-law retardation. Because of these ambiguities, other approaches have been employed to throw light on the mechanism. One of these is to study stereochemical changes of RX during reaction, while another is to study competitive reaction of the intermediate R+. Thus f-butyl chloride in formic acid solution exchanges with radio-... 

Carbocations are electrophiles and carbanions are nucleophiles. Reactions of these intermediates involving, at some stage, the bonding of a nucleophile to an electrophile are sometimes called ionic reactions. 

If the homogeneously catalyzed reaction is performed in the phase-transfer catalyzed mode (e. g., [27, 39]), mass transfer rates of ionic intermediates between the organic and aqueous phases, their phase and partition equilibria as well as... 

In 1902 Semmler observed the formation of tricyclanes in the reaction products from the Wagner rearrangements of terpenes5. A tricyclane mechanism was later proposed by Ruzicka to account for the Wagner rearrangement. However, Ruzicka failed to recognize that the process involes the formation of ionic intermediates. 

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