Alkenes, radical addition steric effects

No effect of this type is manifested for the addition of alkyl radicals to the same alkenes. Evidently, the steric effect involved in the addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is due to the repulsion between the carbon and silicon atom, caused by the large size of the silicon atom in the reaction center of the transition state. 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

The combined influences of polar and steric effects and of the strength of the newly formed bond was also recognized in the reaction of OE,0-unsaturated carbonyl compounds and similar electron deficient alkenes with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

Rate constants for ring openings of substituted cyclopropylcarbinyl radicals show the same types of kinetic effects as seen in radical additions to substituted alkenes with an obvious relationship to the thermodynamics of the reaction (Fig. 4.16). Minor steric effects influence the kinetics, however, and... 

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

Recent studies [382] have provided rate data for cycloaddition reactions. Accordingly, steric effects at the electrophile site, and the capacity of the added unsaturated component RCH=CH2 to stabilize radicals and cations, play a vital role, the importance of which is reflected in a rate decrease for R = Ph OR > vinyl > alkyl by a factor of 100-300. In principle, the observed trends follow those for addition of car-bocations to alkenes [292]. A study of the [2-1-1] cycloaddition of 4-methoxystyrene also emphasizes the importance of the rapid one-electron reduction of the intermediate dimer radical cation [383]. A direct view of 4-center 3-electron cyclobutane [384] and bisdiazene-oxide [385] radical cations has been obtained with polycyclic, rigid systems. 

Steric effects are used to explain the regioselective addition of radicals to alkenes. The radical preferentially attacks the less substituted (hindered) end of the double bond to give a less strained transition state with lower AH0 in an anti-Markovnikov-type reaction. If we introduce substituents on to the double bond, the rate of addition is lowered because of greater steric interaction. Thus, for example, the methyl radical will add three times more quickly to ethene (CH2CH2) than to the disubstituted alkene ( )-2-butene. 

The regiochemical outcome of a radical addition to substituted alkenes generally depends on a complex blend of bond strength, polarity and steric effects . The more common result is the attack on the less substituted carbon. Recently Shaik and Canadell used a state correlation diagram (SCO) model to derive the regiochemical trends in radical addition to substituted alkenes. Regiochemistry was discussed in terms of the relative spin density in the state of the alkene (which directs the radical... 

Cyclopentyl radicals substituted in the /1-position relative to the radical center are formed during the solvomercuration/reductive alkylation reaction of cyclopentene34. The organomer-curial produced in the first solvomercuration step is reduced by sodium borohydride and yields free cyclopentyl radicals in a radical chain mechanism. Addition of alkenes can then occur tram or cis to the / -alkoxy substituent introduced during the solvomercuration step. The adduct radical is finally trapped by hydrogen transfer from mercury hydrides to yield the tram- and ris-addition products, The transicis ratio depends markedly on the alkene employed and it appears that the addition of less reactive alkenes occurs with higher trans selectivity. In reactions of highly substituted alkenes, this reactivity control is compensated for by steric effects. Therefore, only the fnms-addition product is observed in reactions of tetraethyl ethenetetracarboxylate. The choice of alcohol employed in the solvomercuration step has, however, only a small influence on the stereoselectivity. 

Addition reactions of the 4-tm-butylcyclohexyl radical have been studied with a variety of alkenes and also in atom-abstraction reactions (see Section D.2.2.)3 58. While hydrogen or halide abstraction reactions yield preferentially the axial product, unselective addition occurs with terminally unsubstituted alkenes. In addition reactions of alkenes substituted by alkyl groups at the attacked olefinic center, the preference for axial attack decreases further and the equatorial addition product is formed. This influence of the size of the reaction partner on the selectivity is rationalized on the basis of the simultaneous presence of steric effects (hindering axial attack) and torsional effects (hindering equatorial attack), very similar to those discussed for nucleophilic addition reactions to cyclic ketones59. 

Free radicals may undergo addition to activated alkenes with rate constants on the order of 10 -10 M s but steric effects may still impose one to two orders of magnitude difference in the rates of addition at diastereotopic faces of the alkene, kR vs. ks- The relationship between transition state enthalpies and entropies and the rate constants for addition is shown in Eq. (16), in which the subscripts R and S indicate the enthalpies and entropies of activation of the transition states leading to the R and S products respectively. 

Addition to a terminal alkene shows only a P effect. Both electron withdrawing and electron donating groups increase the rate of radical addition when in this position, because the orbital resulting after radical addition is lower in energy due to delocalization of the radical by either inductive, hyperconjugative, or resonance effects. The sensitivity to the Taft steric parameter is relatively small (S = 0.28), meaning that sterics play very little role in the p-position. 

Metzger and coworkers have shown in a series of papers (269-276) that alkanes can be added to alkenes and alkynes in thermally initiated free-radical chain reactions in the neat reactants at SCF conditions. These reactions have been demonstrated with a wide variety of substrates investigating various effects, including the influence of steric and polar substituents as well as product regioselectivity. The radical chain is initiated by a bimolecular reaction of the alkane with the alkene or alkyne to give two radicals. Addition, rearrangement, and elimination reactions have also been observed. No effect on the reaction rate constant near the critical point was observed on varying the physical state of the reaction mixture from liquid to supercritical to gas-phase conditions (276). 

---