2- Butene, allylic chlorination

Evidently there is not much difference in the overall conversions of styrene in polymerizations using rcrt-butyl chloride and 3-chloro-1 -butene coinitiators under similar conditions (Figs. 5 and 6). This is expected because of the similar reactivities of the tertiary and substituted allylic chlorines in these coinitiator molecules. In addition, since rates of the various individual competing reactions in nonpolar solvents (i.e. methylcyclohexane and n-heptane) are expected to be higher at higher temperatures (20° C) than at lower temperatures (—50° C), and if the propagation or initiation rate overrides that for termination, it would not be unexpected to find that yields increase with increasing temperature. Also, it is... 

The vapor-phase chlorination reaction occurs at approximately 200-300°C. The dichlorobutene mixture is then treated with NaCN or HCN in presence of copper cyanide. The product 1,4-dicyano-2-butene is obtained in high yield because allylic rearrangement to the more thermodynamically stable isomer occurs during the cyanation reaction ... 

All this was later put on a sound basis as a result of more precise measurements of rate constants and of activation energies. However, it did not require precise measurements to predict which chlorinated hydrocarbons would decompose by a radical chain mechanism and which by the unimolecular mechanism. Clearly, if the chlorinated hydrocarbon, or the product from the pyrolysis of the chlorinated hydrocarbon reacted with chlorine atoms to break the chain then the chain mechanism would not exist. Such chlorinated hydrocarbons would decompose by the unimolecular mechanism. Mono-chlorinated derivatives of propane, butane, cyclohexane, etc. would afford propylene, butenes, cyclohexene, etc. All these olefins are inhibitors of chlorine radical chain reactions because of the attack of chlorine atoms at their allylic positions to give the corresponding stabilized allylic radicals which do not carry the chain. 

More recently, the dehydrohalc enation of chlorinated butyl rubber, i. e. iso-butene-isoprene copolymer containing 1.4—1.6 wt % of chlorine, essentially in allylic positions, was performed successfully thus poljdsobutraie chains carrying randcxn groups of two conjugated double bonds were obtained. 

Another example is the vapor-phase chlorination of butadiene, which gives a mixture of dichlorobutenes of which 3,4-dichloro-l-butene (12) is the only desired isomer for the chloroprene synthesis [23, 24] (cf. eq. (14)). It is easily boiled off from the cis/trans 1,4-dichloro-2-butenes (123 vs. 155 °C). The migratory isomerization of residual 1,4-isomers 11 is effected by Cu complexes and seems to operate through -olefin/ -allyl intermediates. The chloride probably migrates via the copper center, but no mechanistic details are available. 

The use of organic halogen compounds as the starting products for the synthesis of other organic chemicals is too immense a field to do more than indicate some of the commercial applications. In his book I4S) on the chemistry of petroleum derivatives, Ellis includes a chapter on the production of alcohols and esters from alkyl halides, and also one on miscellaneous reactions of halo-paraffins and cycloparaffins. The manufacture of amyl alcohols and related products from the chlorides has been well covered 14 ) 1 )-A two-step process for the synthesis of cyclopropane by chlorinating propane from natural gas and dechlorinating with zinc dust was devised in 1936 152). A critical review of syntheses from l,3-dichloro-2-butene was published in Russia in 1950 (1-54). The products obtainable from the allylic chlorides are covered in a number of articles 14If 14 157). 

Poutsma, M. L., Chlorination Studies of Unsaturated Materials in Nonpolar Media. 3. Competition between Ionic and Free radical Reactions During Chlorination of Isomeric Butenes and Allyl Chloride, J. Am. Chem. Soc. 1965, 87, 2172 2183. 

Addition of chlorine to neat f-butylethylene gave not only the expected dichloride but also 10% of 4-chloro-2,3-dimethyl-l-butene, indicating that a rearrangement consistent with a methyl shift in a carbocation had occurred. Poutsma, M. L. /. Am. Chem. Soc. 1965,87,4285. Small amoimts of substitution products accompanied the addition. Addition of chlorine to linear alkenes gave mostly addition, but addition of chlorine to branched alkenes gave mostly allylic substitution products. Addition is favored at lower temperatures, as is the case with the competition between SnI and El reactions. 

In l,4-dichloro-2-butene, two equivalent sites are available for hydrobora-tion. Both are (3 to one chlorine and y to others and exhibit cumulative effect of (3- and y-chlorine substitution. Employing the data for allyl chloride and 1-chloro-3-methyl-2-butene (Table 4.14) [1], the rate change is calculated as (J,25) 0,1.77) = 44, which is in good agreement with experimentally observed values ofi48. 

In the case of the system natural rubber-chlorine the reaction is complex. Fully chlorinated natural rubber contains about 65% chlorine compared with the 51% that would be obtained if the only reaction were one of addition. Hence at some stage some substitution is necessary. This is not altogether surprising since although linear alkenes give predominantly addition products (A), alkenes branched at the double bond give predominantly allylic substitution, for example as with 2-methyl-2-butene (B). 

Synthesis of dienyl chloride 2.349, the dithiane coupling partner for dithiane aldehyde 2.338, was accomplished in five steps from commercially available cis-2-butene-l,4-diol (Scheme 2.72). To this end, monosilylation of 2.344 as the TBS ether and this allyl alcohol was then oxidized with accompanying Zto E isomerization by PCC [122] to afford the enal 2.346 in 73 % yield. a,]S-unsaturated ester 2.347 was accessed by the Still—Gennari modification [219] of the Homer-Wads worth-Emmons olefination in good yield and selectivity (92 %, Z/E — 16 1). Reduction of the resulting ester 2.347 to produce allyl alcohol 2.348, in turn was chlorinated by treatment with LiCl and methanesulfonyl chloride to furnish the requisite dienyl chloride 2.349. 

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