Carbanions rate-limiting step

The reaction is promoted by a variety of bases, usually in catalytic quantities only, which generate an equilibrium concentration of carbanion (92) it is reversible, and the rate-limiting step is believed to be carbon-carbon bond formation, i.e. the reaction of the carbanion (92) with the substituted alkene (91). Its general synthetic utility stems from the wide variety both of substituted alkenes and of carbanions that may be employed the most common carbanions are probably those from CHjfCOjEtlj—see below, MeCOCHjCOjEt, NCCH -COjEt, RCH2NO2, etc. Many Michael reactions involve C=C—C=0 as the substituted alkene. 

Active methylene compounds have more than one activating group such as carbonyl, cyano, sulfonyl, or aryl bound to a methylene carbon. Bases such as hydroxide ion easily remove a proton to form a reactive carbanion. The most widely studied example is the alkylation of phenylacetonitrile (Scheme 1). The abstraction of the proton is generally the rate limiting step. 

On the other hand, at high concentrations of acetaldehyde, when the intermediate enolate carbanion is rapidly captured by another molecule of aldehyde, reverse of the initial parallel proton-abstraction steps is prevented (k3[CH3CHO] k-1 and k 2[BH 1 ]), and the rate of the overall reaction is effectively limited by the initial proton abstractions these then constitute (parallel) rate-limiting steps. The overall process is now first order in acetaldehyde and shows general-base catalysis [5], i.e. the rate law is given by Equation 3.13 ... 

Investigations of the mechanism of the a/3 reaction by steady-state and transient kinetic methods have determined the rate constants for intermediate steps in the reaction.I04 The transient kinetic results show that diffusion of indole and condensation of indole with the aminoacrylate intermediate (ES III in Fig. 7.6) are rapid steps that occur without a lag reprotonation of the nascent tryptophan carbanion (ES IV in Fig. 7.6) is the rate-limiting step. The data rule out diffusion of free indole through the bulk solvent and support the channeling mechanism. 

The rate-limiting step for the reaction of CH30 with CF2=CHC6H5 generates a carbanion intermediate, CH30CF2CHC6H5, and would result in bimolecular kinetics. This reaction will be discussed in Section 18.2. 

Qualitatively the transition state structure for the rate-limiting step will bear little relationship to the product aci-compound to which the equilibrium is measured. If the equilibrium were measured to the pyramidal carbanion then normality should exist. 

Rates of potassium / r/.-butoxide-catalyzed isomerization of l-alkenes and methylenecycloalkanes in dimethylsulfoxide at 55°C are summarized in Table 2. For the acyclic as well as exocyclic alkenes, isomerization rates parallel rates of base-catalyzed enolization of structurally analogous ketones. This is further evidence that the rate-limiting step in the alkene isomerizations is proton abstraction to form the allylic carbanion. 

A central question about this reaction concerns the H—C bond shown in bold in Figure 6.48. Does the rate-limiting step occur before this bond is broken (perhaps during formation of the chromate ester), is it the breaking of this bond, or does it occur after ttiis bond is broken (e.g., through the decomposition of a carbanionic intermediate in which the oxygen—chromium bond is still intact) ... 

When the BC salt of 1 is used in the reaction with N=C-NR2 (R = Me, Et) a long-lived intermediate can be detected at -40 C and characterized IR-spectroscopically. It very likely has the structure of a BCl4" adduct of A or that of a rotational isomer. This adduct can be trapped with carbanions. When adding LiMe to the solution at -40 C (R = Me), Cp(CO)2Mn=C(Ph)[N=C(Me)NMe2] is isolated from the reaction mixture. Cp(CO)2Mn=C(NMe2)[N=C(Me)Ph], the expected reaction product from addition of Me" to B, cannot be detected. Therefore we believe that the 1,3-migration of Cp(CO)2Mn (A B) is the rate-limiting step in the formation of 2 from 1 and N=C-NR2. 

In contrast to the El reaction, which involves a carbocation intermediate, the ElcB reaction takes place through a carbanion intermediate. Base-induced abstraction of a proton in a slow, rate-limiting step gives an anion, which expels... 

When the reaction is carried out in MeOH neither step (2), the formation of the carbanion (127), nor step (3), addition of this carbanion to the carbonyl carbon of the acceptor molecule (128), is completely rate-limiting in itself. These steps are followed by rapid proton transfer, (129)— (130), and, finally, by rapid loss of eCN—a good leaving group—i.e. reversal of cyanohydrin formation (cf. p. 212) on the product... 

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