Atom and radical addition to alkenes

Extensive kinetic data exist for the low temperature addition of H atoms to alkenes [65]. Activation energies vary from 17 (C2H4, high pressure) to 

However, H atom addition to alkenes is ca. IVOkJmoP exothermic, so that C—C homolysis reactions are enhanced through the chemically activated alkyl radicals formed. Relative yields of products are thus very dependent on pressure. 

Studies [23] of the separate addition of butane, butene-1 and butene-2 to H2 -I- O2 mixtures (Method I), where H atoms are important chain carrying intermediates, have confirmed that the yield of homolysis products such as C2H4 and CsHg increase significantly with the butenes even at 500 Torr total pressure. 

The addition of O atoms to alkenes has been studied extensively, particularly at low temperatures [104], although Klemm et al. [105] (244-1052 K) and Mahmud et al. [106] (290-1510 K) studied the kinetics of O -I- C2H4 over wide temperature ranges. In their 

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Radical addition to alkenes has been used in cyclizations in aqueous media. Oshima and co-worker studied triethylborane-induced atom-transfer radical cyclization of iodoacetals and iodoacetates in water.121 Radical cyclization of the iodoacetal proceeded smoothly both in aqueous methanol and in water. Atom-transfer radical cyclization of allyl iodoacetate is much more efficient in water than in benzene or hexane. For instance, treatment of allyl iodoacetate with triethylborane in benzene or hexane at room temperature did not yield the desired lactone. In contrast, the compound cyclized much more smoothly in water and yielded the corresponding y-lactone in high yield (Eq. 3.31). 

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration ... 

Selenium-containing molecules have also been used as precursors for radical seleno group transfer reactions. This is a very powerful method for radical additions to alkenes and alkynes it is especially interesting from an atom economy point of view since all atoms remain in the product molecule. The free-radical addition of selenosulfonates 146 can be initiated either photochemically or thermally using AIBN. The addition of 146 not only to alkynes 147,255-257 km also to alkenes258-261 or allenes,261 has been reported and the products such as 148 are versatile building blocks for subsequent reactions (Scheme 39). For example, vinyl selenides 148 can be easily transformed into allenes. 

Although RSEs specifically measure radical stability toward hydrogen atom abstraction, they often provide a qualitative guide to radical stability and reactivity in other types of reactions. For example, one generally finds that radicals that have high RSEs such as benzyl radicals tend to have higher barriers and lower exothermicities in radical addition to alkenes, when compared with less stabilized radicals such as... 

The finely tuned reactivity of tri-n-butyltin hydride and of the resulting tin radical allows it to mediate radical additions to alkenes and alkynes in reductive carbon-carbon bond formation and for radical ring closures . It is a highly versatile and efficient reagent, but not a very green one delivering one useful atom (hydrogen, MW = 1) for every 40 atoms (MW = 290) lost as waste. 

The addition of a radical to the C=C bond of enamines generates an a-amino radical, stabilized via spin delocalization onto the nitrogen atom. The extent of this stabilization as compared to the unsubstituted methyl radical has been determined by theoretical calculations and experimental studies to be about 9-10 kcal mol . However, the contribution of this stability effect to the reactivity in radical addition to enamines is important only if the addition process has a late transition state, which is usually not the case for radical addition to alkenes. 

In a more detailed analysis of the structural and strain-related features of radical additions to alkenes, transition states should be taken into account (261,262). According to ab initio calculations, the elongation of the carbon-carbon double bond and the pyramidalization at both centers in the transition state for the addition of a hydrogen atom to ethylene (1) are small and compatible with an early transition state. Thus, effects of strain release are expected to be small. 

Radical addition to alkenes (Section 10.10) A process by which an atom with an unshared electron, such as a bromine atom, adds to an alkene with homolytic cleavage of the tr-hond and formation of a (T-bond from the radical to the carhon the resulting carhon radical then continues the chain reaction to product the final product plus another species with an unshared electron. 

Carbon-carbon bond formation is a fundamental reaction in organic synthesis [1, 2,3,4], One way to form such a bond and, thus, extend a carbon chain is by the addition of a polyhalogenated alkane to an alkene to form a 1 1 adduct, as shown in Scheme 1. This reaction was first reported in the 1940s and today is known as the Kharasch addition or atom transfer radical addition (ATRA) [5,6], Historically, Kharasch addition reactions were conducted in the presence of radical initiators or... 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

Owing to the high electronegativity of fluorine atoms, perfluoroalkyl radicals show an electrophilic character moreover, these radicals are usually much more reactive than the nucleophilic alkyl radicals in the addition to alkenes, aromatic rings, and quinones for enthalpic reasons (Scheme 14.5b). 

Although scheme (138) is the standard mechanism for the radical-catalyzed isomerization of isomeric alkenes, kinetic data for both substitution and isomerization are sparse. Using cis- or frcms-diiodo-ethene and labeled iodine atoms, Noyes et al. (1945) demonstrated that iodine atoms exchanged with predominant retention isomerization was the slower process, the barrier being <4 kcal/mole. Corresponding studies with dibromoethene and bromine atoms indicate a barrier of ca. 3 kcal/mole (Steinmetz and Noyes, 1952) in which bromine-atom departure from and isomerization of the intermediate were competitive. Qualitative selective or stereospecific radical-initiated additions to alkenes have since indicated that radical intermediates probably have stereostability, but the studies cited are definitive. The kinetic analysis provided the essential model for SS in mechanistic schemes such as (138), whether for SE, SH or SN processes. 

Radical iodine atom transfer [3 + 2]-cycloaddition with alkene (118) using dimethyl 2-(iodomethyl)cyclopropane-l,l-dicarboxylate (117) forms cyclopentane derivative (119), through the formation of an electron-deficient homoallyl radical, followed by the addition to alkene, and cyclization via 5-exo-trig manner as shown in eq. 4.41. 

In the first propagation step of the Wohl-Ziegler bromination, the bromine atom abstracts a hydrogen atom from the allylic position of the alkene and thereby initiates a substitution. This is not the only reaction mode conceivable under these conditions. As an alternative, the bromine atom could react with the C=C double bond and thereby start a radical addition to it (Figure 1.27). Such an addition is indeed observed when cyclohexene is reacted with a Br2/AIBN mixture. 

Fiirstner reported the first McMurry-type reactions working with 5-10 mol% of titanium trichloride and stoichiometric amounts of zinc powder in the presence of chlorotrimethylsilane. The amount of TiCl3 could be reduced to 2 mol% when (ClMe2SiCH2)2 was used as a reagent [125, 131]. At the same time, Burton and coworkers reported atom transfer radical additions of perfluoroalkyl iodides 39 to alkenes 40 catalyzed by 20 mol% of a low-valent titanium compound generated from TiCLt and zinc powder affording 41 in 10-85% yield (Fig. 13). A tandem radical addition/5-exo cyclization/iodine transfer reaction with diallyl ether proceeded in 66% yield [132]. 

Nedelec and coworkers reported a manganese(III)-initiated cyanoacetate-catalyzed atom-transfer radical addition of polyhalomethanes or dibromomalonate 172 to alkenes 126 (Fig. 48) [272]. Since neither Mn(II) nor Mn(III) is useful to initiate Kharasch-type additions, an organocatalyst served this purpose. Thus, a short electrolysis of a mixture of 126,172,10 mol% of Mn(OAc)2, and 10 mol% of methyl cyanoacetate 171 led to initial oxidation of Mn(II) to Mn(III), which served to form the cyanoacetate radical 171A oxidatively. The latter is able to abstract a halogen atom from 172. The generated radical 172A adds to 126. The secondary... 

Electron transfer from copper or copper salts to alkyl halides has been used to initiate atom transfer radical additions. One modification of this process involves catalytic amounts of copper powder and fluorinated alkyl iodides the radicals so generated may react in either inter- or intramolecular fashion with alkenes (equation 13)19. Alternatively, a-chloroesters with remote alkene functions undergo cyclization in the presence of cat-... 

Alkyl radicals generated from the reduction of halides or sulfones with Sml2 have been successfully exploited in intramolecular additions to alkenes that result in the generation of a variety of functionalised small carbocyclic and heterocyclic ring systems. Substrates containing an oxygen atom within the... 

Radical stabilization energies for a wide variety of carbon-centered radicals have been calculated at G3(MP2)-RAD or better level. While the interpretation of these values as the result of substituent effects on radical stability is not without problems, the use of these values in rationalizing radical reactions is straight forward. This is not only true for reactions involving hydrogen atom transfer steps but also for other reactions involving typical elementary reactions such as the addition to alkene double bonds and thiocarbonyl compounds. 

In all telomerisations, the distribution of molecular weights increases, i.e. n increases with (a) an increasing rate of free-radical addition to the alkene, (b) a decreasing rate of the atom-transfer step, i.e. the A—B bond is stronger, (c) a higher concentration of alkene relative to the telogen and (d) temperature. 

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