Alkanes radical addition

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

The compounds CCI4, BrCCla, ICF3, and similar simple polyhalo alkanes add to alkenes in good yield. These are free-radical additions and require initiation, for... 

Alkane free radical addition to aklenes and alkynes generates products with more carbon atoms than the reactants—that is, the result is alkylation. Generally,... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

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... 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

Once formed, the radical intermediate (R ) can couple to afford a dimer (R2), can disproportionate to give an alkane (RH) and an olefin (R(—H)), or can accept a hydrogen atom from a donor (such as the solvent, SH) to give an alkane. A carbanion (R ) can be protonated by the solvent (or a deliberately added acid, HB) to yield an alkane. In addition, RX can undergo E2 and Sn2 reactions with B , and R can attack RX to form a dimer. 

For substituted tricyclo[4.1.0.02,7]heptanes, similar addition of benzenethiol in diethyl ether gave an isomeric mixture of bicyclo[3.1.1]heptanes.35 As shown in the mechanistic scheme, the 1,3-disubstituted patterns of the bicyclo[3.1, l]heptanes are governed by the regiospecific attack of the thiol radical on the sterieally less hindered bridgehead carbon. The results of these radical additions arc summarized for bicyclo[n.l.l]alkanes (Table 8)35 and bicyclo[1.1.0]butanes (Table 9). 

TableS. Bicyclo[n.l.l]alkanes by Radical Addition Using Benzenethiol...

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

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. 

From C-H bond-dissociation energies of alkanes (see Table 4-6), the ease of formation and stabilities of the carbon radicals is seen to follow the sequence tertiary > secondary > primary. By analogy, the secondary l-bromo-2-propyl radical, 5, is expected to be more stable and more easily formed than the primary 2-bromo-1-propyl radical, 6. The product of radical addition should be, and indeed is, 1-bromopropane ... 

Indoles can be used as radical acceptors instead of 63 [120, 121]. Simple and twofold reactions giving either 3-alkylindoles [120] or l,l-bis(3-indolyl)alkanes [121] were observed in 16-72% and 54-90% yield, respectively. In both methods the indole is subject to radical addition in 3-position. The resulting a-amino radical undergoes a further oxidation and deprotonation to the 3-substituted indole. In the case of twofold additions, the second indole unit is introduced by a subsequent polar Friedel-Crafts-type alkylation. 

Traditionally, this radical addition reaction has been used for the preparation of new poly halo organic compounds by reaction of simple polyhalo alkanes with alkenes689 698. The products from these reactions have often been used as key intermediates for the synthesis of heterocyclic compounds690. In some cases it has been found that metal complexes significantly speed up reactions and/or increase yields. 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

This type of reaction is important industrially since it is one of the few that allows compounds containing functional groups to be made from alkanes. As you might guess, since it needs light for initiation, the process is another example of a radical chain reaction. As with the radical addition of HBr to alkenes, we can identify initiation, propagation, and termination steps in the mechanism. 

This access to a , y-bis(tricyclohexyltin)alkanes proves to be very convenient when the corresponding organic dihalides are available. When this is not the case, and when a , y-dienes are more easily accessible, a double hydrostannation of these unsaturated compounds can be used. As indicated above, this radical addition has to be conducted in less mild conditions than for the addition of tri-n-butyltin hydride, for instance, because of the bulkiness of the organotin center and also of the more nucleophilic character of the tricyclohexyltin radical. Nevertheless, the treatment of 4,4 -di(butenyl)biphenyl or 4,4 -bis(but-3-enyloxymethyl)biphenyl with tricyclohexyltin hydride at 130 °C over seven days affords the corresponding adducts 72 and 75 in good yields (Scheme 3.7.11). Due to its rapid decomposition at this temperature, the AIBN initiator has to be added in small portions for the duration of the reaction. 

Three major types of cationic species that can be derived from saturated hydrocarbons are alkyl carbenium ions (R+), alkane radical cations (RH +) and alkyl carbo-nium ions (RH2+). The term carbocations is usually reserved to denote alkyl carbenium and carbonium ions only. Pentacoordinated alkyl carbonium ions (proton-ated alkanes) are the species that result from protonation of alkane molecules they are of paramount importance as reactive intermediates/transition states in the initiation of (Br0nsted) acid-catalyzed conversions of saturated hydrocarbons. Upon dissociation of alkyl carbonium ions, trivalent alkyl carbenium ions are formed and these are responsible for the further progression of acid-catalyzed conversions of alkanes. Alkyl carbenium ions may also be formed by ionization of neutral alkyl radicals and by proton addition to olefins. In both carbenium and carbonium ions, the positive charge is very much located on a particular part of the cation. 

In addition, proton-transfer reactions take place. In irradiated mixed alkanes the higher-alkane radical cations are trapped next to matrix molecules and part of them react by proton transfer. 

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