Halogenation cyclopentene

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

In 2000 the Miller group provided a proof-of-principle study of Pd pi-allyl chemistry for library selection in the presence of a biomolecule [44]. In this approach, Pd(0) chemistry was employed to generate a library of cyclopentene-1,4-diesters in halogenated solvent (Fig. 1.10). This was allowed to equilibrate across a dialysis membrane with an enzyme target (pepsin) in buffered aqueous solution. LC-MS analysis of the library allowed identification of compound 24 as a library member amplified in the presence... 

Stereochemistry of Halohydrin Formation Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation. For example, the addition of bromine water to cyclopentene gives fran.v-2-bromocyclopentanol, the product of anti addition across the double bond. 

Halohydrins are easily generated by treating alkenes with aqueous solutions of halogens. Bromine water and chlorine water add across double bonds with Markovnikov orientation (Section 8-11). The following reaction shows cyclopentene reacting with chlorine water to give the chlorohydrin. Treatment of the chlorohydrin with aqueous sodium hydroxide gives the epoxide. 

How does the proposed mechanism invoking a bridged halonium ion intermediate explain the observed trans products of halogenation For example, chlorination of cyclopentene affords both enantiomers of rram-l,2-dichlorocyclopentane, with no cis products. 

An alternative approach to alkylidene carbenes uses the deprotonation or halogen-lithium exchange of vinyl haUdes. Hence, treatment of the vinyl chloride 103 with potassium hexamethyldisilazide (KHMDS) resulted in the formation of the cyclopentene 104 via the intermediate alkylidene carhene (4.83). The carbene... 

Scheme 7.20 Halogenated cyclohexenyl oxyallylic cations as cyclopentene equivalents via a r4+3i-cycloaddition/quasi-Favorskii reaction sequence.

The ketens used in cycloadditions with cyclopentenes are frequently generated in situ from an acid chloride and a tertiary amine, and the mechanism of these dehydro-halogenations has been investigated. The results suggested that the keten is formed via an acylammonium salt rather than via an enolate. Either species may be trapped under suitable conditions, indicating a complex series of equilibria. 

From the mechanism of catalytic hydrogenation we notice that hydrogen adds to the alkene molecule in such a way that both H-atoms are added to the same side of the molecular plane. The stereochemistry of this reaction is called syn-addition. Addition of halogen follows qnite different stereochemistry two halogen atoms approach the alkene molecular plane from the opposite sides. Such stereochemistry is called anrt-addition. The mechanism is confirmed by the analysis of configurations of products of addition of chlorine to cyclopentene. 

The intramolecular cyclization was also in DMF under photoirradiation (halogen lamp, applied to a sequential radical reaction. The elec- 150 W) produced a bicyclic compound Eq. 12 trolysis of 3-(2 -bromo-r-ethoxy)cyclopenten [24]. This product is the precursor of methyl with 1-cyanovinyl-acetate in the presence of jasmonate or epituberolide. vitamin Bi2a (0.75 mol %) at -1.1 V versus SCE... 

Halogen addition is stereospecifically anti the groups add to opposite sides of the alkene. For example, addition to cyclopentene gives only fraMr-l,2-dibromo-cyclopentane (Fig. 10.8). 

The alcohol adducts from Q -(trimethylsilyl)vinyllithium and aldehydes have found many uses in organic synthesis. The ketones obtained by oxidation (eq 4) are especially good Michael acceptors and have been used in a modified Robinson annulation reaction for the construction of cyclohexenones. Cyclopropana-tion leads to cyclopropylsilane adducts, which can be converted into a variety of cyclopentenes (eq 5). Halogenation followed by stannylation gives synthons (eq 6) useful for making substituted vinylsilanes through radical reactions. ... 

High conversions of cyclopentene were obtained by Nuetzel et al. [59] with hydroperoxides, inorganic peroxides, or aromatic nitroderivatives. To increase the stability of some tungsten-based catalytic systems, Witte et al. [63] employed a-halogenated alcohols such as chloroethanol, 2-chlorocyclohexanol, bro-moethanol, l,3-dichloro-2-isopropanol, o-chlorophe-nol, and 2-iodocyclohexanol. It was observed that methyl and ethyl acetals of formaldehyde, acetaldehyde, chloroform, and benzaldehyde impart good staiblity to the binary systems of tungsten, whereas epoxides such as ethylene oxide and butylene oxide lead at the same time to an increase in activity and stability. 

---