Alkenes chlorine atom reactions

Alkenes can be chlorinated at the allylic position by tert-huty hypochlorite, (CH l C—O—Cl, which undergoes homolytic cleavage to give the ri rt-butoxy radical and a chlorine atom. Reaction of (E)-4,4-dimethyl-2-pentene with this compound gives two CjHgCl products in the ratio 93 7. What are the structures of these two products ... 

Sulfur dichloride is difficult to handle it disproportionates readily, has an unpleasant odor, and tends to introduce extra, unwanted chlorine atoms. Several reagents have been developed, especially by Harpp and co-workers, in which these undesirable features have been modified or eliminated. Succinimide-N-sulfenyl chloride (28) and phthalimide-N-sulfenyl chloride (29) are both stable crystalline compounds which undergo many of the reactions of the sulfur dichloride itself. They can, for example, be used in a facile, high yield synthesis of thiiranes from alkenes.33... 

Table 6.14 gives the rate constants for the reactions of chlorine atoms with some simple alkenes and some biogenic hydrocarbons. As expected, the reactions are... 

TABLE 6.14 Some Reported Values of the Rate Constants for the Reactions of Chlorine Atoms with Some Alkenes at 1 atm Total Pressure and 298 K... 

However, a computational study124 shows that the Kishi model controls the stereoselectivity for (Z)-alkenes. Note also that in the Diels-Alder reactions of hexachloropentadiene with chiral alkenes, the inside alkoxy effect is attributed to electrostatic repulsion of the oxy group in the125 outside position with the chlorine atom of hexachloropentadiene in the 1-position. 

The second problem in the addition of 1,1-dichlorodifluoroethylene to 1,3-butadiene is regiospecificity. Carbon 1 of 1,3-butadiene may become attached either to the carbon holding two chlorines or to the carbon with two fluorines. The cycloaddition of fluorinated alkenes is usually not a concerted four-center reaction in which the bonds are formed simultaneously or nearly so. Instead, the reaction is a stepwise biradical process in which the first step is formation of a free-radical intermediate with a single electron at that end of the double bond that can better accomodate it. That happens at the carbon linked to two chlorine atoms. Thus, a biradical is formed that cyclizes to form 2,2-dichloro-l,l-difluoro-3-vinylcyclobutane M [118, 119, 720]. 

Both sets of starting materials look all right—the regiochemistry is correct for the first and doesn t matter for the second. However, we prefer the second because we can control the stereochemistry by using ds-butene as the alkene and we van make the reaction work better by using dichloroketene instead of ketene itself, reducing out the chlorine atoms with zinc. 

Similar conclusions are drawn by Cvetanovic et al. from their results of ozonization of alkenes in the gas phase (9) and in CC14 solution (10). The rate constants for the ozonolysis of chloroethylenes and allyl chloride, in CC14 solution, indicate (11) that the rate of ozone attack decreases rapidly as the number of chlorine atoms in the olefin molecules is increased. However, to explain the departures from simple correlations, in some cases steric effects and the dipolar character of ozone had to be invoked (10). The relevance of the dipolar character of ozone in its reactions has also been stressed by Huisgen (12), who provided evidence that the ozone—olefin reaction is usually a 1,3-dipolar cycloaddition. 

Diaiyl sulfide templates have also been used to direct chlorinations. The selectivities indicate that the chlorine atom is bound to the sulfur, but the yields are not as good as those with aryl iodide templates. The problem is that the sulfur gets oxidized under the reaction conditions. As expected, a thiophene ring is more stable to oxidation and its sulfur atom can still bind chlorine in a radical relay process." The best sulfur template so far examined is the thioxanthone system (Scheme 20). Thus with 3 equiv. PhICh compound (18) undergoes directed C-9 chlorination in 100% conversion, affording a 71% yield of the A iO-alkene sifter base treatment, along with some polar products from excessive chlorination. The thioxanthone template can be recovered unchanged. 

When metal ion complexed amino radicals are produced by the reaction of A -chloro amines with reducing metal salts in the presence of alkenes, /6-halo amines are produced12-39 41. The reaction of 1-chloropiperidine with cyclohexene, iron(II) sulfate and iron(III) chloride in methanol afforded mainly the d.s-adduct of 2. The diastereoselectivity is attributed to coordination of the unprotonated amino group with the iron(III) salt, which is mainly responsible for the chlorine atom transfer. With A-chlorodimethylarnine and 4-chloromorpholine lower yields are obtained. 

Anions of a-silyl phosphonates of type (153) also undergo additions to carbonyl compounds. The corresponding addition products, 3-silyl alkoxides, can react with ketones to yield the product of the Peterson alkenation or the Wittig reaction. In practice only the Peterson product (154) is obtained, indicating that loss of OSiMes is faster than elimination of C PPhs (Scheme 68). 72 If the a-silyl carbanion is adjacent to a chlorine atom (155), an internal displacement reaction follows the initial formation of the -silyl alkoxide, and epoxides (156) are formed (Scheme 69). 74... 

The cycloaddition of ketenes with alkenes to give cyclobutanones is a general reaction. Dimerization of the ketene is a serious side reaction with ketene itself and with monoalkyl ketenes. Dichloroketene is more reactive than simple ketenes due to the electron-withdrawing effect of the chlorine atoms and reacts readily with a wide variety of alkenes to give cyclobutanones. 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

Cyclopeutanoues, Jung and Hudspeth have devised a new cyclopentanone synthesis by three-carbon annelation of an alkene via a Diels-Alder reaction. They chose the dimethoxytetrachlorocyclopentadiene 1 as the diene component because it is remarkably reactive. The general method is outlined for methyl acrylate (2, equation I). The chlorine atoms of the adduct 3 are replaced by... 

Work on correlations of rate coefficients with ionisation potentials for the reactions of NO3 with a series of alkenes has been extended. Ionisation potentials are determined using a semi-empirical molecular orbital package (MOPAC 5.0). We are now able to estimate both room-temperature rate coefficients and Arrhenius parameters for the reaction of NO3 with a wide range of alkenes, including those containing vinylic chlorine atoms (which we earlier found difficult to handle). In an effort to improve our understanding of the physical basis of our observations, ab initio calculations have been performed on a limited number of compounds. Such calculations should overcome some of the limitations of the semi-empirical methods when applied to elements such as chlorine and bromine. 

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