Relative olefin addition rates

In a series of papers in the early 1980s, Sokolov s group reported relative rate studies which were similar in nature to those of the early Szwarc studies. Sokolov generated various perfluoroalkyl radicals via thermal decomposition of the respective perfluoro diacyl peroxides in heptane containing various olefins [89] or arenes [90]. Determination of the ratio of olefin addition products to hydrogen abstraction products provided the relative rate data given in Table 4 [89]. 

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. 

There is little rate data available for addition and abstraction reactions involving higher alpha-olefins. Where such data are available, they have usually been obtained from lower temperature studies (12,13,14), However, the available information indicates that addition should be competitive with abstraction in the temperature range of this study. For example, Steacie s data for ethyl radical reactions with 1-hexene and 1-heptene indicate that at 525°C, the addition rate would be about seven-tenths of the abstraction rate. For dodecene, one would expect this ratio to decrease because of the increase in abstractable hydrogen, but addition should still be a significant pathway. For methyl radicals and H atoms, available data (13,14) indicate that addition is somewhat faster relative to abstraction. 

It has also been shown in radical substitution at the 2-position of a series of 4-substituted (CN, MeO, Me) protonated pyridines, that the cyclopropyl radical is the least nucleophilic of the cycloalkyl radicals This low nucleophilicity is consistent with the observed difficulty in oxidizing the cyclopropyl radical by Cu ". The lack of reactivity of the 2-phenylcyclopropyl radical, generated by the thermal decomposition of the 2-phenyl-cyclopropanepercarboxylic acid, towards the 0-0 peracid bond to yield 2-phenylcyclo-propanol is also in line with the radical s weak nucleophilicity However from a study of relative rates of hydrogen abstraction to olefin addition of the cyclopropyl radical to a variety of olefins (Table 7) Stefani and coworkers concluded that the cyclopropyl radical was decidedly nucleophilic. 

The much slower addition of acetic acid to the [4.2.1]propellane and -ene as compared to that to [3.2.1]propellane is, of course, attributable to their relative strain. The rate differences between the olefinic (ti/2 = 19.2h at 50°C with AcOH) and saturated (ti/2 = 1.6 h) [4.2.1]substrates are perhaps not so large to worry about excessively. 

In some situations, the slower rate of phenyl selenide transfer can lead to advantages over other faster atom transfer additions. This is particularly useful when a relatively slow rearrangement step is desired prior to atom transfer. In a synthesis of a precursor to the all-cw Corey lactone, Renaud has shown that radical addition is followed by rearrangement prior to PhSe-transfer (Scheme 19) [56]. Products arising from unrearranged olefin addition were observed when the reaction was attempted using BrCCl3. 

Determination of Relative Reaction Ratios of Olefins Comparison of relative reaction rates was done by gas chromatographic analysis of reaction mixtures with addition of a suitable internal standard such as biphenyl. A mixture of the olefin to be compared and norbornene (both olefins in excess) were readed with a defidency of ethyl mercaptoacetate at various temperatures. Relative retention times were compared with those of independently prepared and characterized mercaptoacetate-olefin adducts. Comparison of the relative amounts of each product in the reaction mixture gave the relative, competitive reaction rate. The independently synthesized adducts were also examined to determine that... 

Product mixtures from radical chain addition of hydrogen chloride to olefins are much more complicated than is the case for hydrogen bromide. The problem is that the rate of abstraction of hydrogen from HCl is not large relative to addition of the alkyl radical to the olefin, and this results in the formation of short polymers called telomers ... 

This reaction is an efficient method of synthesis of cyclopropanes. Singlet car-benes add at the double bond stereospecifically unlike triplet carbenes. The structure of olefin, naturally, affects the addition rate. Below we present the relative rates of CCl2 addition in dimethoxy hane at 353 K with respect to cyclohexene... 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

Radical additions lo double bonds are, in general, highly exothermic processes and rates increase with increasing temperature. The rcgiospccificity of addition to double bonds and the relative reactivity of various olefins towards radicals are also temperature dependent. Typically, specificity decreases with increasing temperature (the Reactivity-Selectivity Principle applies). However, a number of exceptions to this general rule have been reported. 8 63... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Already, at an early stage of the studies on the captodative effect, Viehe s group (Lahousse et ai, 1984) measured relative rates for the addition of t-butoxyl radicals to 4,4 -disubstituted 1,1-diphenylethylenes and to substituted styrenes. This study did not reveal a special character of captodative-substituted olefins in such reactions. It might be that the stability of the radical to be formed does not influence the early transition state of the addition step. The rationalization of the kinetic studies mentioned above in terms of the FMO model indicates, indeed, an early transition state for these reactions, with the consequence that product properties should not influence the reactivity noticeably. 

Table 11. Relative rates of addition of singlet and triplet bis(methoxy carbonyl) carbene to olefins...

If the addition of the second hydrogen atom is the rate-controlling surface reaction, then the preceding steps would tend to be reversed, the degree of reversibility being a function of the relative rates of the several reactions. Two effects are expected (1) the isomerization of the initial olefin is pronounced and (2) the proportion of saturated products should tend towards the equilibrium distribution. Indeed, such effects are commonly observed when palladium catalysts are employed (5, 65, 66) (Fig. 9). 

---