Radical hydrobromination

Synthesize the compound shown below from methylcyclopentane and 2-methylpropane using those compounds as the source of the carbon atoms and any other reagents necessary. Synthetic tools you might need include Markovnikov or anti-Markovnikov hydration, Markovnikov or anti-Markovnikov hydrobromination, radical halogenation, elimination, and nucleophilic substitution reactions. 

This change caused considerable confusion in the early days of alkene chemistry, because one researcher would obtain only one hydrobromination product, whereas another would obtain a different product or mixtures from a seemingly identical reaction. The mystery was solved by Kharasch in the 1930s, when it was discovered that the culprits responsible for anti-Markovnikov additions were radicals formed from peroxides, ROOR, in alkene samples that had been stored in the presence of air. In practice, to effect anti-Markovnikov hydrobromination, radical initiators, such as peroxides, are added deliberately to the reaction mixture. 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

In gas-phase hydrobromination, where a radical mechanism is operative, the bromine atom always adds to the central carbon atom of the allenic system. As a result, vinylic bromides are formed through the stable allylic radical. In the solution phase under ionic addition conditions, either the vinylic or the allylic cation may be the intermediate, resulting in nonselective hydrobromination. Allylic rearrangement or free-radical processes may also affect product distributions. 

Figure 4. Examples of low-temperature limit of rate constant of solid-state chamical reactions obtained in different laboratories of the USSR, United States, Canada, and Japan (1) formaldehyde polymerization chain growth (USSR, 1973 [56]) (2) reduction of coordination Fe-CO bond in heme group of mioglobin broken by laser pulse (United States, 1975 [65]) (3) H-atom transfer between neighboring radical pairs in y-irradiated dimethylglyoxime crystal (Japan, 1977, [72], (4, 5) H-atom abstraction by methyl radicals from neighboring molecules of glassy methanol matrix (4) and ethanol matrix (5) (Canada, United States, 1977 [11, 78]) (6) H-atom transfer under sterically hampered isomerization of aryl radicals (United States, 1978 [73]) (7) C-C bond formation in cyclopentadienyl biradicals (United States, 1979 [111]) (8) chain hydrobromination of ethylene (USSR, 1978 [119]) (9) chain chlorination of ethylene (USSR, 1986 [122]) (10) organic radical chlorination by molecular chlorine (USSR, 1980 [124,125]) (11) photochemical transfer of H atoms in doped monocrystals of fluorene (B. Prass, Y. P. Colpa, and D. Stehlik, J. Chem. Phys., in press.).

The mechanism of the addition reaction under these conditions is not an ionic sequence rather, it is a much faster radical chain sequence. The reason is that the activation energies of the component steps of radical reactions are very small, as we observed earlier during the discussion of the radical halogenation of alkanes (Section 3-4). Consequently, in the presence of radicals, anti-Markovnikov hydrobromination simply outpaces the regular addition pathway. The initiation steps are... 

In radical chain adflitions to alkenes, the chain carrier adds to the rr bond to create the more highly substituted radical. This method allows for the anti-Markovnikov hydrobromination of alkenes, as well as the addition of thiols and some halomethanes. 

The extreme readiness with which tetrafluoroallene homopolymerises has prevented a study of the orientation of free-radical attack on the allenic system. Perfluoropenta-1,2-diene, however, does not polymerise readily, and so its photochemical chlorination and hydrobromination have been studied. ... 

Though inert towards chlorine at room temperature in the dark, perfluoropenta-1,2-diene is rapidly converted into the saturated tetrachloride, 1,2,2,3-tetrachloro-octafluom-n-pentane (95 %), and a small amount of mixed dimers, (CjFjIj (4%), when photolysed in the presence of an excess of chlorine. Photochemical hydrobromination yields mainly 3/f-2-bromo-octafluoropent-1-ene (34) and 1 W-2-bromo-octafluoropent-2-ene (35), a result which indicates that bromine atom preferentially attacks the central carbon of the allenic system to give an intermediate radical (36) which exists long enough to permit 90° rotation of the orbital containing the unpaired electron, so that it is transformed into an allylic radical (37) (see Scheme 14). 

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