Cationic reactions dimerization

R = Me) is reported. (323) is unreactive towards CO and protic acids, but reacts with triflic acid at the 3-position of the ring to give an indolenine cation. Reaction of trans-Ir(PR3)2(CO)Cl (R = Me, Et) and LiNHPh in THF gives /rart.v-Ir(PR3)2(CO)(NHPh), (324).520 Benzene solutions of tra i-Ir(PEt3)2(CO)(NHBut) readily convert to the dimer species (325) when heated. 

Radical cations can dimerize in a radical-radical coupling reaction (Scheme la) to afford dimer dications. An alternative pathway to form the dimer dication is a radical-substrate coupling in an electrophilic addition of the radical cation to the nucleophilic substrate. The dimer dication can lose two protons to form a bis-dehydro dimer or react with two nucleophiles to yield a disubstituted dimer. 

Oxidation of enaminone 1 is initiated by electron loss from the dimethylamino moiety leading to radical cation, RH". The following chemical reaction would be an intramolecular cyclization through addition of a hydroxy group on the radical cation site yielding a cyclic radical cation, cRH ". This step is most likely the rate-determining step. The cyclic radical cation then dimerizes... 

The calculations of Pabon and Bauld on the hole-catalyzed ethylene dimerization were among the first ab initio studies on radical cation reactions [46]. 

A kinetic study of nitrous acid-catalyzed nitration of naphthalene with an excess of nitric acid in aqueous mixture of sulfuric and acetic acids (Leis et al. 1988) shows a transition from first-order to second-order kinetics with respect to naphthalene. (At this acidity, the rate of reaction through the nitronium ion is too slow to be significant the amount of nitrous acid is sufficient to make one-electron oxidation of naphthalene as the main reaction path.) The reaction that initially had the first-order in respect to naphthalene becomes the second-order reaction. The electron transfer from naphthalene to NO+ has an equilibrium (reversible) character. In excess of the substrate, the equilibrium shifts to the right. A cause of the shift is the stabilization of cation-radical by uncharged naphthalene. The stabilized cation-radical dimer (NaphH)2 is just involved in nitration ... 

Reaction conditions were as follows Bz/02/He = 10 5 85 (mol.%) W/F = 37 gcat h (g molBz) 1. The conversion of benzene was taken at 20-30 min on stream. Using this catalyst [146], the phenol yield at 20-30 min on stream was approximately twice that reported previously [144, 145]. UV/Vis and Raman data indicated that the production of phenol was maximized in the presence of copper-polymeric (size-limited) species, though isolated copper species such as cations and dimers also catalyzed the benzene-to-phenol transformation. 

This linear polymerization represents one unusual case of diazoacetophenone oxidation. For instance, upon the action of copper oxide, diazoacetophenone gives the ketocar-bene, which is involved in typical carbene reactions—dimerization, addition to olefins, insertion in the O—H bonds of alcohols, etc. If the amine cation radical is used as an oxidant instead of copper oxide, only the polymer is formed. The ketocarbene was not observed, despite careful search (Jones 1981). 

Cationic species are also formed when sulfides (Meissner et al. 1967 Adams 1970 Bonifacic et al. 1975a Janata et al. 1980 Hiller et al. 1981 Davies et al. 1984 Ramakrishna Rao et al. 1984) or thioureas (Wang et al. 1999 Schuchmann et al. 2000) react with OH. Especially stable are the dimeric radical cations [reactions (48) - (50)]. In the case of thiourea, the high stability of the dimeric radical cation may contribute to the driving force which leads, in acid solution, to its forma-... 

An ion-pair derived from the substrate and solid NaOH forms a cation-assisted dimeric hydrophobic complex with catalyst 39c, and the deprotonated substrate occupies the apical coordination site of one of the Cu(II) ions of the complexes. Alkylation proceeds preferentially on the re-face of the enolate to produce amino acid derivatives with high enantioselectivity. However, amino ester enolates derived from amino acids other than glycine and alanine with R1 side chains are likely to hinder the re-face of enolate, resulting in a diminishing reaction rate and enantioselectivity (Table 7.5). The salen-Cu(II) complex helps to transfer the ion-pair in organic solvents, and at the same time fixes the orientation of the coordinated carbanion in the transition state which, on alkylation, releases the catalyst to continue the cycle. 

Treatment of arenes or heteroarenes with oxidants can lead to the formation of radical cations by SET. These radical cations can dimerize, oligomerize, or react with other radicals present in the reaction mixture deprotonation of the resulting intermediates yields the final products (Scheme 3.18). 

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

A mechanism in which the chain-growth step occurs by the radical coupling of two of these cation radicals to form a dication dimer is illustrated in Fig. 60. The reaction begins with oxidation of the monomer to form cation radical 411. The radical coupling of two cation radicals produces the dication dimer 412. Two protons are eliminated to form aromatic dimer 413. This dimer is readily oxidized to cation radical 414 due to its close proximity to the electrode and its ease of oxidation. Cation-radical dimer 414 then couples with a cation radical monomer 411 to form a dicat-... 

Another mechanistic possibility is the attack of the thiophene cation radical (420) upon a neutral thiophene monomer (419) to form a cation-radical dimer (421) [247]. The oxidation and loss of two protons leads to formation of the neutral dimer (422). Once again, rapid oxidation of the dimer occurs upon its formation due to its close proximity to the electrode surface and its lower oxidation potential. The cation-radical dimer (423) which is formed then reacts with another monomer molecule in a similar series of steps to produce the trimer 425. A kinetic study of the electrochemical polymerization of thiophene and 3-alkylthiophenes led to the proposal of this mechanism (Fig. 61) [247]. The rate-determining step in this series of reactions is the oxidation of the thiophene monomer. The reaction is first order in monomer concentration. The addition of small amounts of 2,2 -bithiophene or 2,2 5, 2"-terthiophene to the reaction resulted in a significant increase in the rate of polymerization and in a lowering of the applied potential necessary for the polymerization reaction. In this case the reaction was 0.5 order in the concentration of the additive. 

When cadmiiun atoms are sorbed into Cd2+-containing zeolites, redox reaction occurs to give cadmium ions in the 1+ oxidation state. These are found both as discrete Cdl" " cations and dimers. In addition, neutral atoms may be sorbed without oxidation, in one... 

For mesitylene and durene, the kinetics have been followed by specular reflectance spectroscopy [17]. The results indicated that mesitylene produces a fairly stable radical cation that dimerizes. That of durene, however, is less stable and loses a proton to form a benzyl radical, which subsequently leads to a diphenylmethane. The stability of the radical cation increases with increasing charge delocalization, blocking of reactive sites, and stabilization by specific functional groups (phenyl, alkoxy, and amino) [18]. The complex reaction mechanisms of radical cations and methods of their investigation have been reviewed in detail [19a]. Fast-scan cyclovoltammetry gave kinetic evidence for the reversible dimerization of the radical cations of thianthrene and the tetramethoxy derivative of it. Rate constants and enthalpy values are reported for this dimerization [19b]. 

Several years ago dimerization was essentially achieved (and is still currently performed) by means of acidic catalysts, sometimes as liquids but mainly as solids. However, in spite of its economic interest owing to its low price and low sensitivity to impurities, cationic oligomerization is limited in scope, the main drawbacks being its poor selectivity and low activity toward linear olefins. Organometallics of highly electropositive metals (aluminum, potassium) afford better selectivities but their specificity and their poor activity restrict their use to some specialized syntheses, e. g., dimerization of propene into 2-methyl-1-pen-tene (Al(/-Pr)3) or 4-methyl-1-pentene (K). Coordination catalysts offer a broader spectrum of activity (which is often the opposite of that observed in cationic reactions) and more diversified selectivities their practical use can be expected to grow. 

Similarly, direct reactions between anthocyanins and a flavanol dimer (namely epicatechin-epicatechin 3-gallate) were investigated in model solutions at pH 2 and pH 3.8 (31). Compounds with mass values corresponding to epicatechin-anthocyanin adducts and to anthocyanin-epicatechin-epicatechin 3-gallate, were detected at pH 2 and pH 3.8, respectively. Given the structure of the flavanol precursor, the former can be interpreted as F-A+ arising from cleavage of the flavanol dimer followed by addition of the anthocyanin AOH form onto the epicatechin carbocation whereas the latter is presumably an A+-F species formed by nucleophilic addition of the flavanol dimer onto the flavylium cation. Anthocyanin dimers (A+-AOH) were detected in model solutions at pH... 

Autoxidation of 1,3,4,7-tetramethylisoindole with excess oxygen gives the hydroperoxide 101. When oxygen supply is limited, the hydroxy derivative 102 is obtained. In an inert atmosphere the latter compound is formed by reaction of 101 with the isoindole.102 In the presence of only traces of oxygen, the postulated radical-cation intermediate dimerizes to give 100 (Section V,F,4).123... 

Dimeric alkenes from stabilized Wittig reagents. In the presence of traces of water, the reaction proceeds by a single electron transfer mechanism. The cation radical dimerizes, and the dimer loses two phosphine molecules. 

The radiolysis/EPR study of isobutene was supported by experiments on Cj-Cg olefins on both HZSM5 and NaZSMS (Table 1). This eillowed screening for possible radical cation reactions (minimal for these compounds), and the survey of related compounds aided spectroscopic assignment and tested the catalytic reaction steps from different starting points. For example, 2 was also formed from C and Cg feed molecules, corroborating the conclusion that 2 can be formed from isobutene by cracking the dimer as opposed to addition of C4 and Cj units. 

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