Olefins unsymmetrical

Hydrogen sulfide reacts with olefins under various conditions forming mercaptans and sulfides (108,109). With ethylene it can react to ultimately give diethyl sulfide (110). With unsymmetrical olefins, the direction of addition can be controlled by the choice of either a free-radical initiator, including ultraviolet light, or an acidic catalyst (110) ... 

The nature and distribution of the products of the dimerization or oligomerization of unsymmetrical olefins, such as propene, will depend, among others, on the direction of addition of the hydrido- and alkylnickel species to the olefin, i.e., on the regioselectivity of the catalyst (see Section 1 V,E). In order to define the direction of addition of hydrido- or alkylnickel species to terminal olefins, we shall adopt the convention nickel-to-C, addition [Eq. (12)] (Ni —> Cj) ... 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

In unsymmetrical olefines, a mixture of products is obtained probably due to allylic shift in the molecule... 

It has been noted that when unsymmetrical olefins are ozonized in methanol, there is often a large preference for one cleavage mode over the other. For example,... 

The photocycloaddition of carbonyl compounds to unsymmetrical olefins (electron rich) can give two products however, usually one predominates. For example, the photocycloaddition of benzophenone to isobutylene gives a mixture of the two oxetanes 30 and 31 in the ratio of 9 1.17 This ratio is consistent with the preferential formation and/or closure of the intermediate 30a relative to 31a. The diradical 30a is more stable than 3la since a tertiary radical is more stable than a primary radical by about 8 kcal.62 Many of the examples listed in Section VII are consistent with this apparent generalization there are, however, exceptions. 

Ion 24 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a (3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 5-34). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HC1 to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical olefins, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 750). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2S04, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1-methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-l-methylcyclohexene.198... 

In a senes of papers, Tedder and co-workers reported the factors determining the reactivity of perfluonnated radicals with vanous fluoroethylenes Relative Arrhenius parameters for tnfluoromethyl radicals [17] and pentafluoroethyl radicals [/ ] were determined, with higher selectivity demonstrated for the higher homologue Selectivity of addition to unsymmetrical olefins was found also to increase with greater radical branching [19]... 

In unsymmetrical olefins there is a pronounced orientating effect in addition. Both la and lb are not formed in reaction (13) but predominantly (and sometimes perhaps exclusively) the biradical in which the oxygen atom is attached to the less substituted carbon atom of the double bond. Thus in terminal olefins addition is almost exclusively terminal as evidenced by the fact that the carbonyl product consists almost entirely of the corresponding aldehyde. 

Orientation in addition to unsymmetrical olefins is determined at the addition step, which occurs so as to leave the most stable radical (Equations 9.83... 

The hydroboration of an olefin involves a cis addition of a boron-hydrogen bond to an alkene linkage, and for unsymmetric olefins occurs in a counter-Markownikoff fashion. 1-Alkenes and simple 1,2-disubstituted olefins undergo rapid conversion to the corresponding trialkylborane, whereas addition of diborane to tri- and tetrasubstituted olefins may be conveniently terminated at the respective di- and monoalkylborane stage. 1-Alkenes yield trialkylboranes in which there is a preponderant (approximately 94%) addition of the boron atom to the terminal carbon.2,3... 

While die above reactions will provide carboxylic acid products, each has problems associated with it. The cleavage of olefins to carboxylic acids [reaction (7.1)] can be carried out using potassium permanganate or by ozonolysis at low temperature followed by oxidative workup with hydrogen peroxide. Neither of diese mediods is very useful since only symmetric olefins provide a single carboxylic acid product. Unsymmetrical olefins give a mixture of two acids which must be separated. Furthermore the most useful synthetic processes are those which build up structures, whereas these reactions are degradative in nature. 

The effect of substituents on the orientation of addition of radicals to unsymmetrical olefins. 58... 

It was the observation that the presence of peroxides or ultra-violet light causes a reversal in the orientation of addition of hydrogen bromide to unsymmetric olefins as shown in (9) which led to the appreciation of the importance of... 

A similar change in orientation of attack arises with the fluorobromomethyl radicals where the bulkier the radical, the lower the relative rate of attack at the most substituted end of an unsymmetric olefin (Table 13). This increase in selectivity is believed to be primarily due to steric effects. Lone pair repulsion is unlikely to be important in the tribromomethyl radical, and bromine is less electronegative than fluorine. 

Chloroxy compounds such as C10CF2CFXS02F (X = F CF3) undergo reaction with simple olefins to give ethers (equation 10)23. With unsymmetrical olefins the reaction... 

With the exception of propynoic acid derivatives, all alkynes undergo the reaction. On the other hand, generally, only strained olefins react efficiently in the intermolecular PKR whereas electron deficient alkenes give the reaction only in limited examples. With respect to regioselectivity, the bulkier substituent of the alkyne is placed adjacent to the carbonyl in the cyclopen-tenone product. Unsymmetrical olefins usually give mixtures of regioisomers... 

It is assumed that a CT complex precedes diradical formation 140,143,146). primarily because of the regioselectivity of addition to unsymmetrical olefins. If different complexes are indeed involved, the final product ratio is a complex function of many competing rates. For example, let us assume the least complicated scheme as shown below for cyclopentenone. 

T he Criegee (1) mechanism of ozonolysis postulates that unsymmetrical olefins should give two zwitterions and two carbonyl compounds and hence postulates the possible formation of three different ozonides. This prediction has now been realized in a number of cases (2-9). It has also been shown that in many cases the ozonide stereoisomer ratio depends upon olefin stereochemistry in both normal (3, 6-12) and cross (6-9) ozonides. Since the original Criegee mechanism did not provide for these stereochemical results, a number of additional suggestions for the mechanism have been made (6,9,13, 14), all of which retain the fundamentals of the Criegee mechanism. 

The results obtained for the cis- and trans-l,2-disubstituted ethylenes are indicated, for the symmetrical and the unsymmetrical olefins, respectively, in Tables III and IV and in Table V for the monosubstituted ethylenes. 

For symmetrical 1,2-disubstituted ethylenes symmetrical olefins is, thus, twice that observed for unsymmetrical olefins, or twice that observed for unsymmetrical olefins or twice that calculated by Equation 12. 

In the ozonization of unsymmetrical olefins, cross reaction of zwitterions and carbonyl compounds can lead to six ozonides, i.e., three cis-trans pairs. This problem has recently been studied by a... 

As described in the experimental section single-step radical-molecule reactions can be studied in isolation and in a very direct way by using the rotating cryostat. The identity of the initial radical is known and the product radical can usually be identified unambiguously by e.s.r. Also the relative amounts of the primary and product radicals can be obtained by analysis of the composite e.s.r. spectrum and thus the extent of reaction can be determined directly. When there is more than one site in a molecule at which reaction can occui, the resulting e.s.r. spectrum, which consists of the spectra of the different product radicals, can often be analysed to give the relative degree of attack at the different sites in the molecule (e.g. as for H-atom addition to an unsymmetric olefin). 

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