Relative rates of addition to alkenes

CHAPTER 8 REACTIONS INVOLVING ELECTRON-DEFICIENT INTERMEDIATES 

Dimethoxycarbene, for example, exhibits no electrophilicity toward alkenes This has been attributed to efficient electron release from oxygen. 

5 R2CH-X halides strong base or organometallic compounds R2C + BH + X- 

Decomposition of diazo compounds to carbenes is a quite general reaction. Examples include the simplest diazo compound, diazomethane, as well as diaryl diazomethanes and diazo compounds in which one or both of the substituents is an acyl group. The inability to synthesize the required diazo compound is sometimes a limitation on the method. The low-molecular-weight diazoalkanes are toxic and unstable, and are usually prepared and used in situ rather than isolated. The simple aliphatic diazo compounds are synthesized from derivatives of the corresponding amine. All the common precursors of diazomethane, for example, are derivatives of methylamine. The details of the base-catalyzed decompositions vary somewhat from 

Alkene CCI3 CBr2 Br2 Epoxidation SECTION 9.1. CARBENES 

When an a-diazoketone is needed, the usual synthesis starts with an acid halide. Reaction with a diazoalkane gives the diazoketone as a result of nucleophilic attack, with displacement of the chloride ion  

Under the usual conditions one equivalent of the diazoalkane is consumed by the liberated acid but this can be avoided by use of an added base such as triethylamine/ Cyclic a-diazoketones, which are not available from acid chloride, can be prepared by reaction of a nucleophilic derivative such as the a-hydroxymethylene enolate with arylsulfonyl azides. Several combinations of carbon nucleophile and sulfonyl 

Addition of certain copper salts to solutions of diazo compounds also leads to evolution of nitrogen and formation of products of the same general types as those formed in thermal and photochemical decompositions of diazoalkanes. The weight of the evidence, however, indicates that free carbene intermediates are not involved in such reactions. Instead, complexes of the carbene unit with the metallic catalyst 

---

Problem 6.31 Compare and explain the relative rates of addition to alkenes (reactivities) of HCl, HBr and HI. 

Olah and coworkers, ° and Mayr and Striepe discussed the scope and limitations of aliphatic Friedel-Crafts alkylations. In particular, they considered factors that would favor reactions of the type shown in equation (121), where an alkene is alkylated by an alkyl halide. ° They reasoned that formation of the 1 1 addition products (42) can be expected, if (41) reacts faster with the alkene than (42), otherwise higher addition products will be formed. Mayr ° suggested that the relative dissociation rates of (41) and (42) induced by the Lewis acid should reflect their relative rates of addition to a common alkene. Furthermore, it was assumed that the solvolysis rates in 80% ethanol were proportional to the Lewis acid induced dissociation constants. A few examples where good yields of alkylated (addition) products were obtained are shown in equations (122) and (123). 

Similarly the introduction of alkyl substituents into the radical has only a small effect. Table 2 illustrates the effect that alkyl substituents attached to the trivalent carbon atom have on the rate of addition to ethylene. The variation in rate is very small and in fact the apparently regular decrease in rate of the more branched radicals is always within possible experimental error (the data for CH3CH2CH2- gives a relative rate of 0.25). In general both Tables 1 and 2 show that the introduction of methyl substituents into either the alkene or the alkyl radical reduces the rate of addition. This could be accounted for either by polar forces or by steric hindrance. 

Table 7.9 Relative rates of addition of perfluoroalkyl radicals to alkenes versus their rates of hydrogen-atom abstraction from heptane at 50° C [186]...

Relative rates of addition of carbena-cyclopentadiene with olefins show no electrophilic order. In contrast to 2 a, for phenyl- and phenyl-bromocarbene a clear increase of addition rate with more nucleophilic alkenes is observed. For cyclohexadienyUdene 3e) an electrophilic character was demonstrated. 3e is not a cycloalkenecarbene stabilized by resonance but a simple divinylcarbene. 3e should have the same steric requirements as 2 a. Thus, if there is a difference between 2 a and 3e it cannot be due to steric but rather to electronic effects. This means that there must be a special effect operating in 2 a. 

The isomerization of an alkene often accompanies the addition of hydrogen but goes unnoticed unless the isomer is appreciably less reactive, results in the loss of chirality in the alkene, or differs in stereoselectivity. - The relative rate of hydrogenation to isomerization depends on the structure of the alkene, the catalyst and the conditions used. - - ... 

Factors that influence the ground state conformation of 1 are also important in the transition state for addition of a radical at the carbon bearing the carboxamide. For I, the proximal methyl of the pyrrolidine protects the back face of the alkene, and addition occurs predominantly from the front, or Si, face. Figure 2 shows the relative rates of addition of cyclohexyl radical to 1 and 2 at 22 C [9]. The proximal methyl group on 1 reduces the rate of cydohexyl radical addition to the Re face of the alkene by a factor of 20 at room temperature, relative to addition to the model alkene 2. Addition to the 57 face of 1 occurs some 30% faster than addition to the... 

The relative rates of addition of PhCHa to various allyl bromides were also determined via competition experiments i.e, a mixture of two allyl bromides were allowed to react widi toluene). The relative rates of addition (for CH2=C(2 H2Br, Z = CN (180) > C02Et (110) > Ph (65) > H (1.0) at 80 C) (27) nicely parallel the relative rates of addition of PhCH2 to similarly substituted alkenes (e.g., CH2=C(Z)CH3) reported by Fischer) (28). 

Relative rates of addition of a given radical to series of alkenes can be determined by means of the rates of consumption of the alkenes, but in practice most studies have utilised... 

Alternatively, the rates of addition to a series of alkenes can be measured relative to the rate of addition to one specific alkene (usually ethylene) as a reference reaction. The relative rate constants are then given by ... 

Product mixtures from radical-chain addition of hydrogen chloride to alkenes are much more complicated than is the case for addition of hydrogen bromide. The problem is that the rate of abstraction of hydrogen from hydrogen chloride is not fast relative to the rate of addition of the alkyl radical to the alkene. This results in the formation of low-... 

The efficiency of the halomethane addition process depends on the relative rate of halogen-atom abstraction versus that of addition to the alkene ... 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

At that time, however, it was not possible to measure the absolute rates of carbene additions to alkenes, which were too rapid. Accordingly, relative reactivities... 

---