Carbanion mechanism

Bunnett (1991) expresses doubts that the aryldiazene can be formed at all under these strongly basic conditions, after Huang and Kosower (1968) showed that phenyldiazene is destroyed in water at pH 13.8 (25 °C) with a half-life of <10s, doubtless via C6H5-N2. In addition, Broxton s proposal (Scheme 8-53) does not provide a satisfying explanation of why the ortho halogen has such a strong effect in favor of the carbanion mechanism. 

If the mechanism for nucleophilic addition is the simple carbanion mechanism outlined on page 975, the addition should be nonstereospecific, though it might well be stereoselective (see p. 166 for the distinction). For example, the E) and (Z) forms of an alkene ABC=CDE would give 6 and 7 ... 

In the El mechanism, X leaves first, and then H. In the E2 mechanism, the two groups leave at the same time. There is a third possibility the H leaves first, and then the X. This is a two-step process, called the ElcB mechanism, or the carbanion mechanism, since the intermediate is a carbanion ... 

The relative amounts of these produced at various temperatures are shown in Table VII. The formation of these products may be explained using carbanionic mechanisms. The cyclic material may form by addition of an allylic carbanion to a molecule of the styrene, followed by a cyclization to yield a benzylic carbanion [Reaction (33a, b, c)]. 

Correct At least three mechanisms should be taken into consideration carbanion mechanism, hydride mechanism, and addition-elimination mechanism. 

Carbamazepine, interaction with lithium, 36 66 Carbanion mechanism, 36 277-282 Carbenes... 

Flavin mononucleotide, 3absorption coefficients, 36 270 active site, 36 265-267 catalysis and electron transfer, 36 275-287 carbanion mechanism, 36 277-282 electron acceptors, 36 285-287 electron transfer pathway, 36 275-276, 282-285... 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. 

The reaction can also be base-catalyzed, in which case there is nucleophilic addition and a carbanion mechanism.418 Carbanions most often used are those stabilized by one or more u-aryl groups. For example, toluene adds to styrene in the presence of sodium to give 1,3-diphenylpropane 419... 

The experimental observations presented can be explained by a carbanionic mechanism with prototropic rearrangement. On heterogeneous catalysts this involves the formation of an ally lie carbanion by proton abstraction112,125 [Eq. (4.24)]. The carbanion then participates in transmetalation to yield a new anion and the isomerized product [Eq. (4.25)] ... 

The mechanism Favorskii envisioned involved the initial attack of the ethoxy anion on the triple bond to form a vinyl ether. The now accepted carbanionic mechanism assumes the formation of resonance-stabilized anions, allowing the stepwise interconversion of 1- and 2-alkynes, and allenes143 147 (Scheme 4.9). 

Biochemical reactions often involve addition to C = C bonds that are not conjugated with a true carbonyl group but with die poorer electron acceptor - COO. While held on an enzyme a carboxylate group may be protonated, making it a better electron acceptor. Nevertheless, there has been some doubt as to whether the carbanion mechanism of Eq. 13-6 holds for these enzymes. Some experimental data suggested a quite different mechanism, one that has been established for the nonenzymatic hydration of alkenes. An example is the hydration of ethylene by hot water with dilute sulfuric acid as a catalyst (Eq. 13-11), an industrial method of preparation of ethanol. The electrons of the double bond form the point of attack by a proton, and the resulting carbocation readily abstracts a hydroxyl... 

Molecular oxygen usually reacts rapidly with only those organic substrates, such as dihydroflavins, that are able to form stable free radicals. However, the endiolate anion of Eq. 13-50 may be able to donate a single electron to 02 to form a superoxide-organic radical pair prior to formation of the peroxide (see also Eq. 15-30). Similar oxygenase side reactions have been observed for a variety of other enzymes that utilize carbanion mechanisms.283 The reaction of rubisco with 02 is of both theoretical and practical interest, the latter because of its significance in lowering the yield in photosynthesis (Chapter 23). 

Experimental support for the mechanism of Eq. 15-26 has been obtained using D-chloroalanine as a substrate for D-amino acid oxidase.252-254 Chloro-pyruvate is the expected product, but under anaerobic conditions pyruvate was formed. Kinetic data obtained with a-2H and a-3H substrates suggested a common intermediate for formation of both pyruvate and chloro-pyruvate. This intermediate could be an anion formed by loss of H+ either from alanine or from a C-4a adduct. The anion could eliminate chloride ion as indicated by the dashed arrows in the following structure. This would lead to formation of pyruvate without reduction of the flavin. Alternatively, the electrons from the carbanion could flow into the flavin (green arrows), reducing it as in Eq. 15-26. A similar mechanism has been suggested for other flavoenzymes 249/255 Objections to the carbanion mechanism are the expected... 

The mechanism of dehydrohalogenation of haloalkanes varies from a concerted E2 mechanism to a carbanionic ElcB mechanism, where the /j-hydrogen has been made sufficiently acidic to be removed, leaving a carbanion.4,5 The E2 mechanism of dehydrohalogenation takes place in one step. The ElcB or carbanionic mechanism of dehydrohalogenation has two steps the first step being deprotonation, and the second one elimination of the halide ion. 

In the other three variations of the carbanion mechanism, an equilibrium concentration of carbanion is formed, which then either returns to starting material or decomposes to products. 

Carbanion mechanisms may give either syn or anti elimination. For example, Hunter and Shearing studied the butoxide-catalyzed elimination of methanol from 35 and 36. Since deuterium exchange with solvent is in close competition with elimination, the mechanism is probably (EjcB)b. The ratio ofsyn/anti... 

In the carbanionic mechanisms for elimination, if the substrate has two proton-bearing ft carbons, the more acidic protons will be removed. Thus in alkylated substrates the double bond will be oriented toward the less substituted carbon and Hofmann elimination is obtained. 

The alkaline cleavage of picrocrocin has been discussed by Isbell,2 who rationalized the apparent fact of D-glucose elimination on the basis of a carbanion mechanism in which the unshared electron pair (Villa), result-... 

Rates of exchange with NaOMe/MeOH at 50°C of the 2-, 4-, and 6-positions of 3-chloropyridine N-oxide and of the 2-position of 3,5-dichlor-opyridine N-oxide, relative to the 2-position of pyridine N-oxide, were 1840, 0.37, 12.2, and 11,800, respectively. These showed that the normal carbanion mechanism applied and emphasized further the importance of inductive effects in base-catalyzed exchange. The activating effects of substituents (relative to a position in benzene, at 50°C) were calculated as 2-C1, 1800 4-C1, 9 2-N + 0, 3.8 x 109 4-N+0", 7.6 x 10s (69JOC1405). 

A similar difference in reactivity is evident in the N-oxides pyrazine N-oxide is even more reactive than pyrimidine or pyridine N-oxide. Substituents in the 3-position appear to act mainly through their inductive effects (Table 10.3). Log k2 varies linearly with p/Ca, and the carbanion mechanism probably operates (there is, however, doubt about the mode of substitution at C-6 in pyrazine N-oxides) (70JOC3467). All four hydrogens of pyrazine N,AT-dioxide are exchanged in MeOD at 65°C (68MI2). 

In NaOD-D20 (or with added DMSO) 1,2,4-triazine and its 5- or 6-methyl and 5-phenyl derivatives exchange H-3 for deuterium with an opposite reactivity order to that observed under acid catalysis (vide supra) (73T2495). The observation that the 5-phenyl derivative reacts 15 times faster than the unsubstituted parent (Table 10.4 comparison was made in D,0-DMS0 because of solubility problems) clearly indicates that the carbanion mechanism operates in basic media rather than a process involving covalent hydration. 

In trans-1 -bromo-2-fluorocyclohexane, E2 elimination occurs in the treatment of the compound with sodium methoxide or potassium tert-butoxide. There is only one hydrogen antiperiplanar to bromine, and its elimination leads to 3-fluorocyclohexene W. On the other hand, when, sodamide is used as a base, hydrogen fluoride is eliminated, not by the E2 mechanism but by a cA-elimination leading to X, 1-bromocyclohex-ene, probably by Elcb (carbanion) mechanism [132. ... 

Fig. 13. Model of propylene molecule adsorption on clusters containing a bridged hydroxyl group. The arrows indicate the atom motions corresponding to different mechanisms of the double-bond migration (black arrows, carbocation mechanism white arrows, carbanion mechanism in the case of the synchronous mechanism these motions are simultaneous).

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