Riveros reaction

We can, however, form alkoxide ions that are monosolvated by a single alcohol group, via the Riveros reaction [Equation (7)]. When the monosolvated methoxide is reacted with acrylonitrile, the addition process reaction (8a), is the major pathway, because there is a molecule of solvent available to carry off the excess energy. The proton transfer pathway, reaction (8b), becomes endothermic, because the methoxide-methanol hydrogen bond, at about 29 kcal/mol, must be broken in order to yield the products. Thus, one can observe either the unique gas phase mechanism in the gas phase, reaction (6b), or the solution phase mechanism in the gas phase, reaction (8a), and the only difference is in the presence of the first molecule of solvent. 

Reaction (58) is an efficient way of generating a solvated ion in the gas phase at low pressures. The general requirements for this type of reaction, usually referred to as the Riveros reaction, have been advanced for a number of nucleophiles (Isolani and Riveros, 1975). A further example is shown in (64c). 

A variation of the halide affinity approach was used by Riveros et al. in the investigation of the enthalpy of formation of o-benzyne. Reaction of bromo- or iodobenzene with base in an ICR leads predominantly to the formation the expected M-1 anion, but also leads to the formation of solvated halide ions (Eq. 5.15). By using substrates with known halide affinities, it was possible to assign limits to the enthalpy of formation of the benzyne product. Ultimately, the experiment is comparable to that outlined in Eq. 5.14, although the acidity and halide affinity measurements are made in a single step. 

A. Mantilla-Ramirez, G. Ferrat-Torres, J.M. Dominguez, C. Aldana-Rivero, and M. Bernal. Influence of reaction parameters and comparison of fluorinated alumina and silica supports in the heterogeneous alkylation of isobutane with olefins. Appl. Cat. A, 143 203-214, 1996. 

Despite this information, the question remains as to the exact structure of the associated ions and their possible electronic resemblance to an SN2 transition state. An icr experiment (Riveros et al., 1973) generated a CH3C12 species of mixed isotopic composition from the ion-molecule reaction (33). 

Faigle et al., 1976 Takashima and Riveros, 1978). Kinetic studies reveal this to be a fast reaction (kicr = 5 x 101 M s"1 at room temperature for R = H). Mechanistic studies, based on the use of18OH, are consistent with a reaction that proceeds substantially both by attack at the carbonyl centre (Bac2), and at the methyl group (SN2). The reactions of hydroxide ions and esters in the gas phase are also known to give rise to further reactions ... 

Jose and Riveros, 1977). This reaction becomes overshadowed in higher alkyl acetates because attack at the P-hydrogen to induce elimination as in... 

A reversal of this trend has been observed in the reaction of RO with alkyl pivalates (Takashima and Riveros, 1978) as shown in (65). 

The trends in the gas-phase reaction (67) follow closely the correlations encountered for the similar reaction in protic solvents (Jencks and Carriuolo, 1960). This observation, plus the fact that in the case of esters SN2 reactions become competitive with attack at the carbonyl, has been rationalized by several authors (Asubiojo and Brauman, 1979 Comisarow, 1977 Takashima and Riveros, 1978) on the basis of the poor solvation expected for the Sn2 transition state due to charge delocalization. Thus, SN2 reactions are expected to display much larger differences in the gas phase than in solution, when compared with nucleophilic displacement at carbonyl centres. This is reflected in a larger sensitivity of the activation parameters. 

Tiedemann and Riveros (1974) first discussed the gas-phase reactions which are equivalent to an esterification reaction. An icr study of alcohols and acetic acid revealed that the formation of protonated acetic acid by ion-molecule reactions of fragment ions with acetic acid is followed by the rapid reaction (72). 

There is some doubt as to whether the formic acid-2-propanol system reacts according to reaction (72). Tiedemann and Riveros (1974) observed reaction (75) in their icr study claiming that this process occurs by alkyl-oxygen cleavage in the alcohol. This is a slow reaction, and due to the low abundance of the product ion, the lsO experiment (Pau et al., 1978) was inconclusive. 

An analogous mechanism is capable of explaining similar reactions, for example (90), which occur in systems involving protonated esters and t-butyl halides (Riveros et al., 1979). 

An early report (Briscese and Riveros, 1975) revealed that in the gas phase, alkoxide ions can displace fluoride from fluorobenzene (91). Hydroxide ion fails to react because C6H5F is more acidic than H20 and thus proton transfer becomes the most important channel. Similar reactions with other monohalobenzenes are complicated because these substrates usually generate halide ions directly by dissociative electron attachment. 

Polyfluorobenzenes were the first compounds to be shown to undergo a rather unusual gas-phase reaction. Typical examples (93, 94) involve alkoxide ions (Briscese and Riveros, 1975). In (93) the relative yields of C6H4FO for different isomers of difluorobenzene are influenced by the alkyl group (R) since the different isomers display acidities comparable to the aliphatic alcohols. Reaction (93) is particularly important for p-difluorobenzene, the least... 

Dyatkin, A. B. Rivero, R. A. The Solid Phase Synthesis of Complex Propargylamines Using the Combination of Sonogashira and Mannich Reactions, Tetrahedron Lett. 1998,39, 3647-3650. 

J. R. Pliego, Jr., and J. M. Riveros, Free energy profile of the reaction between the hydroxide ion and ethylacetate in aqueous and dimethylsulfoxide solutions A theoretical analysis of the changes induced by the solvent on the different reaction pathways, J. Phys. Chem. A, 108 (2004) 2520-2526. 

Studies of proton transfers involving small ions with localized charge have shown that these reactions may proceed indeed with rate constants close to or even slightly larger than the collision rate constants predicted by the ADO theory (Mackay et al., 1976). However, rate-constant measurements of proton-transfer reactions between delocalized anions (Farneth and Brau-man, 1976) and sterically hindered pyridine bases (Jasinski and Brauman, 1980) and of SN2 displacement reactions (Olmstead and Brauman, 1977 Pellerite and Brauman, 1980 Pellerite and Brauman, 1983 Caldwell et al., 1984 for a review see Riveros et al., 1985) have shown that the rate constants can span the range from almost collision controlled values down to ones too slow to be observed. For these reactions the wide variation in rate constants has been explained on the basis of a double potential-well model which for a hypothetical SN2 substitution is schematically shown in Fig. 4. 

Another example, which shows an effective competition channel in the nucleophilic attack upon a carbonyl centre concerns the reactions of OH-and Ov with methyl formate. Using lsO-labelled reactant ions, a significant contribution of the SN2 mechanism (— 20% for OH- and 40% for Ot) in addition to the pathway via a tetrahedral structure has been found for formation of the formate anion as summarized in (20) and (21) (Dawson and Nibbering, 1978a Johlman and Wilkins, 1985 van der Wei and Nibbering, 1986). Similar observations have been made for other methyl esters, such as those of trifluoroacetic and benzoic acid (Takashima and Riveros, 1978). 

Although the attack of nucleophiles upon aromatic rings in the gas phase was reported more than a decade ago, not many publications on this topic have appeared since then. Thus, it was shown in earlier work that alkoxide ions react with fluorobenzene to give F and with hexafluoro-benzene to give pentafluorophenoxide anions (Briscese and Riveros, 1975), that (M + N02) adducts can be formed from reaction of N02 with < -, m-and /7-dinitrobenzenes (Bowie and Stapleton, 1977) and that (M — H + 0) ions are generated in reactions of Ot ions with benzene, naphthalene, pyridine, alkylbenzenes, methylpyridines and fluorotoluenes by displacement of a ring hydrogen atom (Bruins et al., 1978). 

Riveros, J. M. Ingemann, S. Nibbering, N. M. M. Formation of gas phase solvated bromine and iodine anions in ion/molecule reactions of halobenzenes. Revised heat of... 

Rivero, M. R., Adrio, J., Carretero, J. C. Pauson-Khand reactions of electron-deficient alkenes. Eur. J. Org. Chem. 2002, 2881-2889. 

Molander, G. A., Rivero, M. R. Suzuki Cross-Coupling Reactions of Potassium Alkenyltrifluoroborates. Org. Lett. 2002,4,107-109. 

Mclver, Scott, and Riveros [37] have reported equilibrium constants at 298 K, for example, of each of reaction types 7, and 9 ... 

GntoLDO, T. Riveros, J. M. Keto nol isomerization of gas-phase 2 -methylacetophenone molecular ions probed by high-temperature near-blackbody-induced dissociation, ion-molecule reactions, and ah initio calculations. J. Phys. Chem A 2002,106, 9930 9938. 

Adrio J, Rivero MR, Carretero JC. The phenylsulfonyl group as an endo stereochemical controller in intramolecular Pau-son-Khand reactions of 3-oxygenated 1,6-enynes. Angew. Chem. Int. Ed. 2000 39(16) 2906-2909. 

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