Carbanions resonance-stabilized

FIGURE 24.20 The methylmalonyl-CoA epimerase mechanism involves a resonance-stabilized carbanion at the oj-position. 

Mechanism of nucleophilic aro-malic substitution. The reaction occurs in two steps and involves a resonance-stabilized carbanion intermediate. 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Arnett and colleagues [219,220] measured the enthalpies of a considerable number of processes where a resonance-stabilized carbenium ion R was reacted with a resonance stabilized carbanion, oxanion, thioanion, or nitroanion, R(T, in mixtures of sulfolane (95%) and 3-methylsulfolane (5%), at 298.15 K ... 

I > II > III. All three compounds afford resonance-stabilized carbanions. However, COOEt has an electron-releasing O bonded to carbonyl C, which decreases resonance stabilization. There are two COOEt groups in III and one in II, while I has only ketonic carbonyl groups. 

Acetoacetic ester is acidic (pX = 10.2) and forms a resonance-stabilized carbanion whose negative charge is delocalized over one C and two O s. 

Many carbon acids, upon losing the proton, form carbanions that are stabilized by resonance. Structural reorganization (movement of atoms to different positions within the molecule) may accompany this. Chloroform, HCN, and 1-alkynes do not form resonance-stabilized carbanions, and these78 behave kinetically as normal acids.79... 

The oxidation occurs first, creating a /3-keto-carboxylate, which readily loses C02 via a resonance-stabilized carbanion. 

A typical cyclic voltammetric trace for the anodic oxidation of the fluorenyl anion 2 at platinum is shown in Figure 1. The oxidation potential for this and several other resonance stabilized carbanions lies conveniently within the band gap of n-type Ti02 in the non-aqueous solvents, and hence in a range susceptible to photoinduced charge transfer. Furthermore, dimeric products (e. g., bifluorenyl) can be isolated in good yield (55-80%) after a one Faraday/mole controlled potential (+1.0 eV vs Ag quasireference) oxidation at platinum. 

Benzocyclobutene-l,2-dione (74) undergoes base-catalysed ring fission between the carbonyls to give 2-formy I benzoate (75). Rate constants, activation parameters, isotope effects, and substituent effects have been measured in water.107 Rapid reversible addition of hydroxide to one carbonyl is followed by intramolecular nucleophilic attack on die otiier, giving a resonance-stabilized carbanionic intermediate (76a)-o-(76b). 

The 1,4-addition (or conjugate addition) of resonance-stabilized carbanions. The Michael Addition is thermodynamically controlled the reaction donors are active methylenes such as malonates and nitroalkanes, and the acceptors are activated olefins such as a,P-unsaturated carbonyl compounds. 

Treatment of 2,6-bis(trialkylsilyl)-4/7-thiopyrans with strong bases generates a deep red color, indicating formation of the resonance stabilized carbanion subsequent reaction with iodomethane results in exclusive alkylation at C-4. However, with other alkylhalides varying amounts of the 2-alkylated 2/7-thiopyran are also formed and the overall yield decreases. Two products also result from reaction of the organolithium derived from 300 with benzaldehyde. The benzyl alcohol is the major product and the minor 2-benzylidenethiopyran results from a Petersen reaction (Scheme 46) <2005PS(180)1303>. 

The alanine racemization catalyzed by alanine racemase is considered to be initiated by the transaldimination (Fig. 8.5).26) In this step, PLP bound to the active-site lysine residue forms the external Schiff base with a substrate alanine (Fig. 8.5, 1). The following a-proton abstraction produces the resonance-stabilized carbanion intermediates (Fig. 8.5, 2). If the reprotonation occurs on the opposite face of the substrate-PLP complex on which the proton-abstraction proceeds, the antipodal aldimine is formed (Fig. 8.5,3). The subsequent hydrolysis of the aldimine complex gives the isomerized alanine and PLP-form racemase. The random return of hydrogen to the carbanion intermediate is the distinguishing feature that differentiates racemization from reactions catalyzed by other pyridoxal enzymes such as transaminases. Transaminases catalyze the transfer of amino group between amino acid and keto acid, and the reaction is initiated by the transaldimination, followed by the a-proton abstraction from the substrate-PLP aldimine to form a resonance-stabilized carbanion. This step is common to racemases and transaminases. However, in the transamination the abstracted proton is then tranferred to C4 carbon of PLP in a highly stereospecific manner The re-protonation occurs on the same face of the PLP-substrate aldimine on which the a-proton is abstracted. With only a few exceptions,27,28) each step of pyridoxal enzymes-catalyzed reaction proceeds on only one side of the planar PLP-substrate complex. However, in the amino acid racemase... 

Though the reduction of Flox by NADH is a facile process in solution, the reactions of Equations 5 and 6 are characterized by large kinetic barriers. The second grouping of carbon acid substrates are those that dissociate to resonance-stabilized carbanions and that, due to internal electron release, do not yield a carbonium ion on two-electron oxidations, for example ... 

The third category of carbon acid substrates is nitroalkanes that readily dissociate to resonance-stabilized carbanions. The carbanions cannot be oxidized by loss of two electrons because of the instability of the resultant carbonium ion. Oxidation of nitroalkanes occurs through loss of nitrite ion. 

Alkyl substituents in aromatic azoloazines are reactive towards electrophilic reagents in basic media. Basic reagents readily abstract protons from such alkyl groups yielding resonance stabilized carbanions. Thus, treatment of the methyl derivatives (243) with aldehydes gives the alkenes (245) (Scheme 21) <84H(22)174i). Ready formation of the resonance stabilized anions (244) is behind the activity of the methyl group. 

This resonance-stabilized carbanion must be sp2 hybridized and planar for effective delocalization of the negative charge onto oxygen (Section 2-6). Resonance-stabilized carbanions are the most common type of carbanions we will encounter in organic reactions. 

A 1,4-addition (conjugate addition) of a resonance-stabilized carbanion (the Michael donor) to a conjugated double bond such as an a,/3-unsaturated ketone or ester (the Michael acceptor), (p. 1086)... 

The synthetic drawbacks of amino-substituted reagents are not observed when using resonance stabilized carbanions such as a-deprotonated nitriles 77) and carbonyl compounds 25), or allyl-21 and benzylmagnesium bromide 77), the corresponding very reactive tris-aminotitanium reagents undergoing rapid (—78 ° to —20 °C, 0.5 h) Grignard-type addition (80-95 % yields). 

Dihydrothiophene 1,1-dioxide in the presence of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) reacts with CO2 to give the carboxylic acid (Equation 69), which is a stable precursor to l,3-butadiene-2-carboxylic acid <2003SC3643>. The reaction proceeds through initial deprotonation at the 2a-position the resonance-stabilized carbanion thus generated reacts with CO2 to form the carboxylate. Abstraction of a proton from the 3-position by another molecule of the base generates a dianion, which isomerizes to the stable dianion as shown in Scheme 39. Final protonation produces 3-sulfolene-3-carboxylic acid. 

When dissolved in nonpolar solvents such as benzene or diethyl ether, the colourless (2a) forms an equally colourless solution. However, in more polar solvents [e.g. acetone, acetonitrile), the deep-red colour of the resonance-stabilized carbanion of (3a) appears (1 = 475... 490 nm), and its intensity increases with increasing solvent polarity. The carbon-carbon bond in (2a) can be broken merely by changing from a less polar to a more polar solvent. Cation and anion solvation provides the driving force for this heterolysis reaction, whereas solvent displacement is required for the reverse coordination reaction. The Gibbs energy for the heterolysis of (2a) correlates well with the reciprocal solvent relative permittivity in accordance with the Born electrostatic equation [285], except for EPD solvents such as dimethyl sulfoxide, which give larger values than would be expected for a purely electrostatic solvation [284]. 

Aromatic aldehydes generally do not produce cyanohydrins on reaction with hydrogen cyanide, but undergo the benzoin condensation (Scheme 6.12). The initial product from nucleophilic attack by cyanide ion is depro-tonated to form a resonance-stabilized carbanion, which attacks a second molecule of the aldehyde. Elimination of HCN leads to an a-hydroxy ketone, benzoin (2-hydroxy-1,2-diphenylethanone). The benzoin condensation is catalysed specifically by cyanide ion, which assists in both the formation and stabilization of the carbanion. The reaction is limited to aromatic aldehydes, since the aryl ring also stabilizes the anion. 

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