Radical reactions alkenes

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

The anti-Markownikoff addition of hydrogen bromide to alkenes was one of the earliest free-radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as a peroxide, a radical-chain mechanism becomes competitive with the ionic mechanism for addition of hydrogen bromide ... 

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

We had a brief introduction to radical reactions in Section 5.3 and said at that time that radicals can add to alkene double bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let s now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. 

The same high reactivity of radicals that makes possible the alkene polymerization we saw in the previous section also makes it difficult to carry out controlled radical reactions on complex molecules. As a result, there are severe limitations on the usefulness of radical addition reactions in the laboratory. Tn contrast to an electrophilic addition, where reaction occurs once and the reactive cation intermediate is rapidly quenched in the presence of a nucleophile, the reactive intermediate in a radical reaction is not usually quenched, so it reacts again and again in a largely uncontrollable wav. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

The (TMS)3Si radical addition to terminal alkenes or alkynes, followed by radical cyclization to oxime ethers, were also studied (Reaction 50). The radical reactions proceeded effectively by the use of triethylborane as a radical initiator to provide the functionalized pyrrolidines via a carbon-carbon bond-forming process. Yields of 79 and 63% are obtained for oxime ethers connected with an olefin or propargyl group, respectively. 

Analogous to terminal alkenes, the reaction of 123 with valeraldehyde and cyclohexanone under radical-based conditions allowed for the preparation of the corresponding functional polysilanes 126 (Reaction 90). The efficiency of Si-H bond replacement was 80-85%... 

Sml2 has also been used to form cyclooctanols by cyclization of 7,8-enones.262 These alkene addition reactions presumably proceed by addition of the ketyl radical to the double bond, followed by a second electron transfer. 

The most general method for formation of new carbon-carbon bonds via radical intermediates involves addition of the radical to an alkene. The reaction generates a new radical that can propagate a chain sequence. The preferred alkenes for trapping alkyl... 

There are also reactions in which electrophilic radicals react with relatively nucleophilic alkenes. These reactions are exemplified by a group of procedures in which a radical intermediate is formed by oxidation of readily enolizable compounds. This reaction was initially developed for /3-ketoacids,311 and the method has been extended to jS-diketones, malonic acids, and cyanoacetic acid.312 The radicals formed by the addition step are rapidly oxidized to cations, which give rise to the final product by intramolecular capture of a carboxylate group. 

The sequence of S l reactions (see the section 7.1.1 discussing the radical reaction) and elimination of HN02 provide a new synthetic method for various kinds of alkenes. For example,... 

All reactions of benzotriazole derivatives of the type Bt-CR RbS discussed above are based on electrophilic or nucleophilic substitutions at the ot-carbon, but radical reactions are also possible. Thus, the first report on unsubstituted carbon-centered (benzotriazol-l-yl)methyl radical 841 involves derivatives of (benzotriazol-l-yl)methyl mercaptan. 3 -(Benzotriazol-l-yl)methyl-0-ethyl xanthate 840 is readily prepared in a reaction of l-(chloromethyl)-benzotriazole with commercially available potassium 0-ethyl xanthate. Upon treatment with radical initiators (lauroyl peroxide), the C-S bond is cleaved to generate radical 841 that can be trapped by alkenes to generate new radicals 842. By taking the xanthate moiety from the starting material, radicals 842 are converted to final products 843 with regeneration of radicals 841 allowing repetition of the process (Scheme 134). Maleinimides are also satisfactorily used as radical traps in these reactions <2001H(54)301>. 

Table 4.39. Non-radical reaction types for interactions of transition metals (M) with alkenes ( ), showing principal donor-acceptor combinations in each case (the symbol denotes a vacant valence orbital [formal hypovalency] on the...

Addition of halocarbons to alkenes in the presence of transition metals is a well-known radical reaction. Weinreb etal. have now reported an intramolecular version leading to cyclic esters or bicyclic lactones. Typical substrates are the a,a-dichloro ester 1 or the a,a-dichloro acid 2, readily available by reaction of ethyl lithiodi-chloroacetate with 5-bromo-l-pentene. When 1 is heated in benzene at 160° with a metal catalyst, mixtures of epimeric ot,w-dichloro esters 3 and 4 are obtained. The ratio and yields of 3 and 4 are dependent on the catalyst and concentration of 1, but 3 and 4 are the major products formed in the presence of Ru(II) and Fe(II) catalysts. In contrast cyclization of 2 under the same conditions gives the bicyclic y-lactone 5 in high yield. 

Curran2 has reviewed recent applications of the tin hydride method for initiation of radical chain reactions in organic synthesis (191 references). The review covers intermolecular additions of radicals to alkenes (Giese reaction) as well as intramolecular radical cyclizations, including use of vinyl radical cyclization. 

Keywords Alkene radical cations Ion pairs Kinetics Stereochemical memory effects Tandem reactions... 

At one time considered as two distinct reactions occurring by different mechanisms [51], the fragmentations of Scheme 2 and the rearrangments of Scheme 5 are now seen as different facets of the same fundamental heterolysis of -substituted alkyl radicals into alkene radical cations, with the eventual outcome determined by the reaction conditions [52],... 

Relatively few kinetic data are available for the carbon-carbon bond forming reactions of alkene radical cations. Nevertheless, rate constants for the cyclization illustrated in Scheme 9, with generation of the alkene radical cation by the fragmentation method, have been measured. These cyclization rate constants are significantly faster than those of the corresponding neutral radicals [89]. 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

The regiochemistry of nucleophilic addition to alkene radical cations is a function of the nucleophile and of the reaction conditions. Thus, water adds to the methoxyethene radical cation predominantly at the unsubstituted carbon (Scheme 3) to give the ff-hydroxy-a-methoxyethyl radical. This kinetic adduct is rearranged to the thermodynamic regioisomer under conditions of reversible addition [33]. The addition of alcohols, like that of water, is complicated by the reversible nature of the addition, unless the product dis-tonic radical cation is rapidly deprotonated. This feature of the addition of protic nucleophiles has been studied and discussed by Arnold [5] and Newcomb [84,86] and their coworkers. 

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