Alkenes with hydrogen halides, free-radical

Reactions of lithium alkyls are generally considered to be carbanionic in nature, but in reactions with alkyl halides free radicals have been detected by electron spin resonance.32 Lithium alkyls are widely employed as stereospecific catalysts for the polymerization of alkenes, notably isoprene, which gives up to 90% of 1,4-cA-polyisoprene numerous other reactions with alkenes have been studied.33 The TMED complexes again are especially active not only will they polymerize ethylene but they will even metallate benzene and aromatic compounds, as well as reacting with hydrogen at 1 atm to give LiH and alkane. 

Hydrogen halides also add to alkynes. The addition of HBr to alkjmes can be difficult to interpret because (as with alkenes) both ionic and free radical mechanisms may occur, and the free radical process can be difficult to suppress. Reaction of HBr with propjme (63) in the liquid phase at —78°C led to the formation of (Z)-l-bromopropene (64, equation 9.63), indicating stereoselective anti addition. When the reaction was carried out at room temperature, however, a mixture of Z (64) and (65) isomers was obtained (equation 9.64). The results suggested that addition of a bromine atom to propyne produces the vinyl radical 66, which abstracts a hydrogen from HBr to produce 64 at -78°C but which can isomerize (with Eg > 17kcal/mol) to the radical 67 at room temperature. 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

It is significant that no other hydrogen halides add to alkenes, contrary to the Markovnikov rule, even in the presence of free radicals. This appears to be due to the difficulty with which other halogen atoms, particularly fluorine and chlorine, are produced by reaction of the hydrogen halides with radicals ... 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

The catalyzed additions of —OH and —SH bonds to alkenes is largely in the domain of organic chemistry, and standard textbooks in that field supply such information. The same is true for the hydrogen halides. The ease with which —SH compounds undergo free radical catalyzed additions to olefins is not emulated by —SeH and —TeH systems. Such reactions have, however, been used for the synthesis of mixed arylalkyl selenides. ... 

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