Carbanions dimerization

The second electronic transfer to the oxygen produces the diradical (C) which evolves into monomer formation. The latter possibility (IV) is a homolytlc cleavage giving another anion radical. If the process follows scheme III or IV, we must obtain monomer formation after the oxidation reaction in all cases. We have carried out the oxidation of carbanionic dimers derived from isoprene, crmethylstyrene, styrene, 1,1-diphenylethylene. 

The last observation concerns the oxidation of the lsoprene carbanionic dimers.— We do not observe the monomer formation, but the presence of monoalcohols with a high yield. In "Inverse oxidation we obtain 52% of monoalcohol and 80% In "direct oxidation . These monoalcohols have been Isolated and characterized (Fig. 1). They come from the transfer reaction from the primary radical to the solvent. The high yield of monoalcohol shows that the hydrogen transfer from the solvent Is a competitive reaction towards functionalization. This fact Is confirmed by the similar values of the two rate constants ( 108 M-i.S-1) . . ... 

Citrate synthase in mammals is a dimer of 49-kD subunits (Table 20.1). On each subunit, oxaloacetate and acetyl-CoA bind to the active site, which lies in a cleft between two domains and is surrounded mainly by a-helical segments (Figure 20.6). Binding of oxaloacetate induces a conformational change that facilitates the binding of acetyl-CoA and closes the active site, so that the reactive carbanion of acetyl-CoA is protected from protonation by water. 

The dimer behaves simultaneously as a radical and as a carban-ion, and thus the radical end might grow by a radical mechanism, anionic polymerization proceeding from the carbanion end. This behavior is particularly interesting when two monomers are present in the system, one polymerizable by a radical but not by an anionic mechanism, the other behaving in the opposite sense. In such a hypothetical case the resulting product would be a block polymer, -A—A. . . A—B—B. . . B-. 

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

The oxidative dimerization of the anion of methyl phenyl sulfone (from a Grignard reagent) in ethereal solution in the presence of cupric chloride in 5% yield has been reported47. Despite the reported48 poor stability of the a-sulfonyl C-centered radicals, Julia and coworkers49 provoked the dimerization (in 13 to 56% yields) of the lithiated carbanion of alkyl phenyl sulfones using cupric salts as oxidants. The best results are obtained with cupric triflates in THF-isobutyronitrile medium (56% yield for R = H). For allyl phenyl sulfones the coupling in the 3-3 mode is predominant. 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

Mixed carbanion/alkoxide complexes were formed from combining n,sBu2Mg, TuOM (M = Na or K), and TMEDA to form the dimeric CIPs [(Bu)2(tBuO)MMg(TMEDA)]2 (M = Na and K) 437, 438.442 While the two structures are identical in their atom connectivity, they are not precisely isostructural in that K shows a bias toward C-coordination while Na is inclined toward N. The alkali metal cations are both formally five-coordinate bonding with two a-C(Bu) atoms, two N(TMEDA) atoms, and a single 0(cBu) atom. The dominant feature in both structures though is the [(Bu)2Mg(/x-tBuO)2MgBu2]2 dianion. 

Reviews covering the chemistry of group 2 metal complexes with phosphorus-stabilized carbanions,279 and of molecular clusters of magnesium dimetallated primary phosphanes, are available.2 u Magnesium phosphanes remain rare compounds.281 Lithiation of bromide 98 with BuLi in the presence of tmeda in pentane produces a lithium phosphine dimer subsequent treatment with MgCl2 in EtzO gives the phosphane 99 in 69% overall yield (Equation (19)). The centrosymmetric 99 has Mg-C = 2.217 A Mg-P = 2.77 A (av.).282... 

In a direct comparison of the reactivity of 1-alkyl- and 2-alkylbenzotriazoles, compound 393 was lithiated in the presence of benzophenone with 1 equiv of LDA to give a mixture of alcohol 394 and dimer 395 (Equation 12) <1996LA745>. No reaction was detected at the carbon adjacent to the benzotriazol-l-yl moiety. When benzaldehyde was used instead of benzophenone, only dimer 395 was obtained. This suggests that a-benzotriazol-2-yl carbon radical reactions are much faster than those of a-benzotriazol-l-yl) carbanions. 

The reaction (a Schlenk dimerization) between the phosphine-borane-substituted alkene nPr2P(BH3) (Me3Si)C=CH2 and elemental lithium in THF yields the complex [(pmdeta)Li Prn2P(BH3) (Me3Si)CCH2]2 123 which in the solid state has a lithium bound to the BH3 hydrogens of the ligand, and no Li-C(carbanion) contact (pmdeta = N,N,N, N",N"-pentamethyldiethylenetriamine).85... 

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

Reactive donors can be generated ca-thodically by reductive cleavage of hahdes to carbanions or by reduction of double bonds to radical anions. Using these methods, two acceptors can be dimerized in one step by reductive Umpolung , for example, two molecules of acrylonitrile to adipodini-trile - a reaction, which normally needs two or more steps. 

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