Cyclohexene, bromine addition

McMillen, D. W., Grutzner, J. B. Radical Bromination of Cyclohexene in CCI4 by Bromine Addition versus Substitution. J. Org. Chem. 1994, 59, 4516 528. 

The mechanism in Figure 9.5 also provides a rationalization for some observations of solvent effects in bromine addition reactions. The major product of the reaction of bromine with cyclohexene in methanol is trans-2-bromo-l-methoxycyclohexane. traMS-l,2-Dibromocyclohexane is observed if Br is added to the solution, but the yield of the dibromo adduct approaches 0% as [Br ] approaches 0 M. This result stands in contrast to the 27% of 1,2-dibromo adduct obtained from the corresponding reaction in acetic acid. It appears that the greater polarity of methanol accelerates the dissociation of IIP to DI, and the greater nucleophiliciW of methanol enhances the reaction of solvent with IIP, SSIP, DI, and SSIP. ... 

There are significant differences between the bromine addition and chlorine addition reactions, however. The addition reaction of ethene and chlorine is exothermic by 44 kcal/mol, which is 15 kcal/mol more exothermic than the addition of ethene and bromine. Poutsma noted that addition of chlorine to alkenes in nonpolar solvents can occur by either radical or ionic pathways, but that oxygen inhibits the radical reaction. For example, addition of chlorine to neat (i.e., not diluted by solvent) cyclohexene (19) gave frans-l,2-dichlorocyclohexane (20), 3-chlorocyclohexene (21), and 4-chloro-cyclohexene (22) in a 1.95 1.00 0.60 ratio when the reaction was carried out imder a nitrogen atmosphere. In the presence of oxygen, the ratio was 3-4 1.00 0. ... 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that then undergoes a ring flip to the diequatorial conformation of rn r-l,2-dibromo-cyclohexane. However, when the unsaturated bicyclic compound I is the alkene, instead of cyclohexene, the addition product is exclusively in a stable diaxial conformation. Account for this. (You may find it helpful to build handheld molecular models.)... 

A study of the influence of allylic hydroxy- or methoxy-substituents on the steric course of bromine addition to cyclohexenes has been carried out with substrates (245)—(250). The hitherto unknown compounds (249) and (250) were obtained (in a ratio of 1 9) by LiAlH reduction of the corresponding ketones HQO -catalysed equilibration changed this ratio to 36 64, and pure materials were obtained by chromatographic separation. The equilibrium is similar to that for (248) and (247),... 

The propellane (239) contains both cyclobutene and cyclohexene groups but only the latter undergoes direct electrophilic addition. Bromine addition to 1-cyano- and l-alkoxycarbonyl-3,3-dimethylcyclobutene has been... 

The radical addition of halogen to an alkene has been referred to briefly in Section 9.3.2. We saw an example of bromination of the double bond in cyclohexene as an unwanted side-reaction in some allylic substitution reactions. The mechanism is quite straightforward, and follows a sequence we should now be able to predict. 

The silver(I) fluoride/bromine and silver(I) fluoride/iodine systems can be exploited for generation in situ and electrophilic addition of "bromine monofluoride and iodine monofluoride to cyclohexene, mcthylenecyclohcxane, styrene, indene, and acenaphthylene.55... 

Novel results were reported for allylic bromination. In radical bromination of cyclohexene in CCI4 under light the selectivity of substitution over addition was shown to be controlled by bromine concentration.304 Substitution via the corresponding allyl radical, while relatively slow, is irreversible and fast enough to maintain the concentration of bromine at a sufficiently low level to prevent significant addition. The reaction of two strained alkenes, fZ)-1,2-dimethyl-1,2-di-ferf-butylethylene and the -isomer (14), leads to the corresponding bromosubstituted product, instead of addition 305... 

In a 3-I. three-neck round-bottom flask fitted with a 500-cc. separatory funnel, a mechanical stirrer, and a thermometer, is placed a solution of 123 g. (1.5 moles) of cyclohexene (Note 1) (Org. Syn. 5, 33) in 300 cc. of carbon tetrachloride. The flask is surrounded by an ice-salt bath. The entire apparatus is assembled in direct sunlight (Note 2). The stirrer is started, and when the temperature has reached o° a solution of 210 g. (1.31 moles) of bromine in 145 cc. of carbon tetrachloride is added from the separatory funnel at such a rate that the temperature of the reaction mixture does not exceed 30 (Note 3). The addition requires about two hours. 

Figure 17-8 Apparatus for coulometric titration of cyclohexene with Br2. The solution contains cyclohexene, 0.15 M KBr, and 3 mM mercuric acetate in a mixed solvent of acetic acid, methanol, and water. Mercuric acetate catalyzes the addition of Br2 to the olefin. [Adapted from D. H. Evans, Coulometric Titration of Cyclohexene with Bromine," J. Chem. Ed. 1968,45,88.]... 

Oxidation of thiochromones does not lead to the sulfoxide or sulfone derivatives these are better prepared from the corresponding thiochromanones by oxidation followed by bromination and dehydrobromination. The 1-oxide is an extremely reactive species and gives a Diels-Alder adduct with butadiene. The 1,1-dioxide is also very reactive. Photolysis in benzene affords a 2 1 adduct in 30% yield (116) (79TL1097) via an initial [2 + 2] addition followed by a Diels-Alder reaction. Simple [2 + 2] addition is found with cyclohexene. 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

LiBr and in the presence of cyclopentene as a scavenger olefin. The kinetics, determined by monitoring the formation of strong acids (TfOH or HBr), show that the rate of solvolysis of 65 is dependent on [Br-] (at a constant ionic strength). In the presence of Br-, the products are trans- 1,2-dibromides and bromo-solvates of both cyclohexene and cyclopentene. The cyclopentenyl products have been shown to arise from the electrophilic addition of Br2/Br3 to cyclopentene, while trans-l, 2-dibromocyciohexane 67 is formed by Br- capture of the bromonium ion 66 on carbon. The Br2 required for bromination of cyclopentene results from attack by Br- on the bromonium ion 66 on Br+. On the basis of the ratio of the cyclopentyl products to 67, Br- capture of the solvolytically produced bromonium ion 66 (by attack on Br+) is 4-5 times more prevalent than attack on carbon in AcOH, and ca 25 times more preferred in MeOH123. 

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