Alkenes, radical halogenation reactions

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

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. 

In these reactions (Scheme 3.1), the first electron addition is to the alkene giving a radical-anion. This interacts with the alkyl halide to transfer an electron, in a process driven by simultaneous cleavage of the carbon-halogen bond. The alkyl radical formed in this manner adds an alkene radical-anion [25]. Aluminium ions generated at the anode are essential to the overall process. They coordinate with the intermediate carbanion, which then interacts with the second halogen substituent in an Sn2 process to form the carbocycle. 

Cyclohexene was shown to react partly by a radical mechanism when chlorinated in the absence of oxygen even in the dark.247 The reaction is slightly less stereoselective than the ionic process (96% trans-1,2-dichlorocyclohexane vs. 99%). Isomeric butenes under similar conditions give products nonstereoselec-tively.248 Branched alkenes are less prone to undergo free-radical halogenation. 

A significant observation concerning bromine addition is that it and many of the other reactions listed on page 360 proceed in the dark and are not influenced by radical inhibitors. This is evidence against a radical-chain mechanism of the type involved in the halogenation of alkanes (Section 4-4D). However, it does not preclude the operation of radical-addition reactions under other conditions, and, as we shall see later in this chapter, bromine, chlorine, and many other reagents that commonly add to alkenes by ionic mechanisms also can add by radical mechanisms. 

Photoinduced free-radical halogenation is very useful for the functionalization of electron-deficient alkenes such as vinyl halides. Among halides, the addition of bromine is by far the most useful reaction, as shown in Scheme 3.25. In the first example, a polyfluoroalkene was brominated in a high yield by using a 300 W light bulb as the light source. Of note, the synthesis of l,2,4-tribromo-l,l,2-trifluorobutane (38) was carried out under solvent-free conditions in a near-quantitative yield and on a multigram scale (Scheme 3.25a) [67]. l,2-Dibromo-l,l,2-trichloroethane (39) was likewise obtained in about 250 g amounts by the solar lamp irradiation of neat trichloroethylene to which bromine was continuously added (Scheme 3.25b) [68],... 

The halogenation reaction proceeds in darkness and is reasonably considered as ionic. It has been shown that if chlorine gives primarily free radical addition on mono- and di-substituted alkenes, it gives ionic substitution products with tri- and tetra-substituted alkenes [73], A model compound study, together with NMR analysis of commercial chloro and bromobutyl samples, confirmed that the reaction on isoprenyl unit leads predominantly to the exomethylene-substituted structure A, and this is explained by steric hindrance due to the tetra-substituted carbon in f3-position which favors proton elimination rather than the nucleophilic attack of halide counter ion in the second phase of addition (Fig. 11, Table 1) [74,75]. 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

An example of 1,3-asymmetric induction has been illustrated in the copper-mediated addition of electron-deficient radicals to alkenes [48]. The reaction is shown as in Eq. (13.36). The mechanism involves a single-electron transfer from copper, which forms the copper(I) halide as a by-product. This reaction also uses atom-transfer methodology to obtain halogen transfer at the y position (116), which then readily lactonizes with the ester to form the product 117. 

Addition Reactions.- The photoelectron transfer process of the iminium salt (38) with the 3-butenoate anion results in the formation of the allylated product (39). The reaction involves decarboxylation of the 3-butenoate followed by a radical coupling reaction. The photoaddition of halogenated alkenes to the tetraraza phenanthrene (40) yields products (41) of (2+2)-addition. The Eu(III)/Eu(II) photoredox system has been studied with regards to its reactivity toweu ds a-methylstyrene. Irradiation of the system at > 280 nm in methanol yielded the products (42) and(43). ... 

The second important type of propagation reaction is addition to multiple bonds addition to C=C is particularly important. In reaction (6.34), R can be an atom or a group centred on carbon or any element which forms a bond stronger than the n bond which is broken in the reaction (about 250 kJ mol-1). If the alkene is unsymmetrical, addition can in principle take place at either end of the double bond. Addition normally takes place at the end of the double bond which will generate the more stable free radical. Thus for addition of a halogen atom to propene, attack at the CH2 position will give the secondary radical 47 (reaction 6.35) rather than attack at the central carbon atom which would give the less stable primary radical 48 (reaction 6.36). 

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