Carbanion mechanism, and

Basran, J., Sutcliffe, M. J., Hille, R., and Scrutton, N. S., 1999a, Reductive half-reaction of the H172Q mutant of trimethylamine dehydrogenase evidence against a carbanion mechanism and assignment of kinetically influential ionizations in the enzyme-substrate complex, Biochem. J. 341 3079314. 

When the carbanion decomposes more readily than it reprotonates, kinetic behaviour intermediate between that of the carbanion and bimolecular mechanism is predicted. For only a small extent of substrate ionisation in low conjugate acid concentration (k-i s>, [6h]), general base catalysis is observed. At constant buffer ratio, an increase in base concentration causes a linear increase in observed rate coefficient until / [6h] approaches/ .2. Under this condition the rate coefficient attains a maximum with further increase in base concentration, the kinetics parallel the carbanion mechanism and specific base catalysis is observed . ... 

There is ample scope for further investigations of the mechanism of beta-eliminations yielding imines and nitriles. Some of the base-catalysed reactions may be examples of the carbanion mechanism and hydrogen isotope studies and the application of the Bronsted catalysis law to rates of elimination under various reaction conditions could prove informative. 

MAO is a flavin-dependent enz5mie in which the cofactor is cowilendy bound to the enzyme via a cysteine residue Four mechanisms have been proposed for this complicated oxidation reaction. A hydride mechanism, an addition-elimination mechanism, a carbanion mechanism, and a radical mechanism have all been... 

As with the reduction of carbonyl compounds discussed in the previous section, we ll defer a detailed treatment of the mechanism of Grignard reactions until Chapter 19. For the moment, it s sufficient to note that Grignard reagents act as nucleophilic carbon anions, or carbanions ( R ), and that the addition of a Grignard reagent to a carbonyl compound is analogous to the addition of hydride ion. The intermediate is an alkoxide ion, which is protonated by addition of F O"1 in a second step. 

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 ... 

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

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

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. 

The carbanion can be destroyed in two ways, k2 or k, and two limiting types of behavior are expected for ElcB mechanisms. If k2 3> fe-i[BH], then the carbanion always decomposes to the alkene product and the rate law simplifies to feobs = fei. In other words, the rate is only dependent on carbanion formation and the rate law has a form that is identical to what would be obtained in a concerted E2 reaction. This has been referred to as an EIcBirr mechanism, where IRR indicates that deprotonation is irreversible. On the other hand, if k2 fc i[BH], then carbanion formation rarely leads to the product and the rate law simplifies to the following ... 

The mechanism suggested243 includes successive, intermediate formation of the 4"-ulose ester 114, the carbanion 115, and the 4"-ulose 116, all of which are firmly bound to the enzyme-NADH complexes. The sequence for the 5"-labeled substrate is given in Scheme 13. 

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). 

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