Alkenes, addition reactions rate-determining step

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

The main steps in the currently accepted catalytic cycle of the Heck reaction are oxidative addition, carbopalla-dation (G=G insertion), and / -hydride elimination. It is well established that both, the insertion as well as the elimination step, are m-stereospecific. Only in some cases has formal /r/ / i--elimination been observed. For example, exposure of the l,3-dibromo-4-(dihydronaphthyloxy)benzene derivative 16 and an alkene 1-R to a palladium source in the presence of a base led to a sequential intra-intermolecular twofold Heck reaction furnishing the alkenylated tetracyclic products 17 in good to excellent yields (Scheme 9). " In the rate-determining step, the base removes a proton in an antiperiplanar orientation from the benzylic palladium intermediate. The best amine base was found to be l,4-diazabicyclo[2.2.2]octane, which apparently has an optimal shape for this proton abstraction. 

Although these additions to CO double bonds have some superficial similarities to the electrophilic additions to CC double bonds that were presented in Chapter 11, there are many differences. The acidic conditions mechanism here resembles the mechanism for addition to carbon-carbon double bonds in that the electrophile (the proton) adds first, followed by addition of the nucleophile. However, in this case the first step is fast because it is a proton transfer involving oxygen, a simple acid-base reaction. The second step, the attack of the nucleophile, is the rate-determining step. (Recall that it is the first step, the addition of the electrophile, that is slow in the additions to CC double bonds.) Furthermore, in the case of additions to simple alkenes there is no mechanism comparable to the one that operates here under basic conditions, in which the nucleophile adds first. Because the nucleophile adds in the slow step, the reactions presented in this chapter are termed nucleophilic additions, even if the protonation occurs first. In... 

The pattern you saw for epoxidation with peroxy-acids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is the bromination reaction itself. The chart shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1.1- S... 

Migration of the hydride locks the configuration of the enantiomeric alkene-carbon centre in general this step is also reversible but probably not in this instance. In the examples studied neither the dihydride intermediates nor the alkyl intermediates have been observed and so it seems reasonable to assume that addition of H2 is also the rate-determining step. Since the latter is a bimolecular reaction and the other ones are monomolecular rearrangement reactions, one cannot in absolute terms say that oxidative addition is rate-determining. 

The time is ripe for truly exciting developments in the reactivity of dinuclear transition metal compounds. The potential for cyclic sequences of reactions, as is required for catalytic reactions, has already been realized. (1) It has been shown, by Muetterties, et al. (53), that alkynes can be selectively hydrogenated to alkenes (cU 2H-addition) by Cp2Mo2(C0) the rate determining step involves CO dissociation from the acetylene adducts Cp2MO2(C0) (R2C2). (2) We have found that... 

The coordinatively unsaturated species that adds to the alkene is believed to be the product of CO dissociation from HCo(CO)4 (3), since the overall reaction is retarded by high CO pressure, and dissociation of CO from (3) or a phosphine substitution product (equation 24) is probably the rate-determining step. After alkene addition to HCo(CO)3, CO adds to the metal center only to form a corresponding acyl via migratory insertion see Migratory Insertion). Addition of H2, followed by heterolytic cleavage, results in the product aldehyde and regeneration of the catalyst (Scheme 2). ... 

The rate-determining step in the ionic hydrogenation reaction of carbon-carbon double bonds involves protonation of the C==C to form a carbocation intermediate, followed by the rapid abstraction of hydride from the hydride source (equation 45). ° There is a very sensitive balance between several factors in order for this reaction to be successful. The proton source must be sufficiently acidic to protonate the C—C to form the intermediate carbocation, yet not so acidic or electrophilic as to react with the hydride source to produce hydrogen. In addition, the carbocation must be sufficiently electrophilic to abstract the hydride from the hydride source, yet not react with any other nucleophile source present, i.e. the conjugate anion of the proton source. This balance is accomplished by the use of trifluoroacetic acid as the proton source, and an alkylsilane as the hydride source. The alkene must be capable of undergoing protonation by trifluoroacetic acid, which effectively limits the reaction to those alkenes capable of forming a tertiary or aryl-substituted carbocation. This essentially limits the application of this reaction to the reduction of tri- and tetra-substituted alkenes, and aryl-substituted alkenes. 

There are four steps in these reactions as exemplified in the oxidative addition of acetic acid (1) to an alkene and the oxidative cyclization of 7b (Scheme 2). The first, rate-determining step is the slow reaction of acetic acid 1 or y9-keto ester 7b with Mn(OAc)3 to give Mn(III) enolates 2 and 8b, respectively [12, 13]. The rate of proton loss is proportional to the acidity of the hydrogen. Acetic acid, pK = 25,... 

The stereochemistry of bromination is usually anti for alkyl-substituted alkynes. A series of substituted arylalkynes has been examined in dichloroethane. As with alkenes, a TT-complex intermediate was observable. The A// for formation of the complex with 1-phenylpropyne is about —3.0kcal/mol. The overall kinetics are third order, as for an Ad S mechanism. The rate-determining step is the reaction of Br2 with the TT complex to form a vinyl cation, and both syn and anti addition products are formed. 

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