Alkene dihalogenated

Geht man von 1,2-Dihalogen-Derivaten Oder 1,2-Halogenhydrinen aus, so werden un-ter Eliminierung Alkene erhalten. 

Numerous reagents may react with alkenes to yield mixed 1,2-dihalogenated compounds. Interhalogens prepared on mixing two different halides or their stoichiometric equivalents are used in mixed halogenations. 

In accordance with an earlier suggestion by Olah and Hockswender,240 the initial 1 1 adducts have been shown to be n complexes572 and a second species, a 2 1 bromine-alkene complex was directly observed.573 In the course of the bromination of crowded alkenes, such as ( )-2,2,3,4,5,5-hexamethylhex-3-ene (46), the bro-monium ion intermediate was shown to be reversibly formed.574 However, steric hindrance prevents the formation of the usual dihalogenated product. Instead, it loses a P proton to give an allylic bromo derivative, which is then dehydrobromi-nated to diene 47 ... 

We ve already seen several methods for preparing alkyl halides, including the reactions of HX and X2 with alkenes in electrophilic addition reactions (Sections 6.8 and 7.2). The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine and chlorine yield trans 3,2 dihalogenated addition products. 

As expected, halogeno compounds also undergo [l,2,3]-eliminations (cf. footnote 68)). Thus, in the gas phase the dehydrochlorination of neopentyl chloride (59) inter alia yields the alkene 13142). From gem-dihalogen alkylcyclopropanes such as 132,... 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

Zur Eliminierung von Halogen aus vicinalen Dihalogen-alkanen unter Alken-Bildung kann ebenfalls Eisen eingesetzt werden (s. Ullmann, 3. Aufl., Bd. 5, S. 428 s.a. ds. Handb. Bd. V/lb, S. 182). Am gebrauchlichsten ist jedoch Zink. 

Durch Chlor- oder Brom-Addition an 1-Alkoxy-l-halogen-1-alkene bei — 15 bis0° erhalt man u.a. folgende 1-Alkoxy-l,l-dihalogen-alkane,06 U2 ... 

Of two possible diastereomeric dibromides, only one is formed. As noted before, bromination of alkenes is said to be a diastereospecific reaction and dihalogenation is generally diastereospecific. Once the dibromide product is formed, there is free rotation around the carbon-carbon bonds, and 44A is drawn to show other rotamers in Figure 10.2. What is the point of redrawing this molecule It is essential to recognize the diastereomer as any of its possible rotamers. In all representations of 44A, the absolute configuration remains (2S,3S), which is the only reliable way to identify the product. 

It is clear from the preceding section that alkenes react as Lewis bases with polarizable dihalogens and with a classical Lewis acid such as borane. Can similar reactions occur with other Lewis acids The answer is yes, and an important class of Lewis acid is composed of mercury(II) compounds. 

The analogy with alkene chemistry can be continued in that alkynes react with bromine, chlorine, or iodine, but only one of the two Ji-bonds is used. The reaction is known as dihalogenation of alkynes. When 2-hexyne reacts with chlorine (CI2), the alkyne reacts as a Lewis base and the isolated product is the vinyl dichloride 110 (J5-l,2-dichloro-l-pentene). Formation of this product is explained by an intermediate vinyl-chloronium ion, 109, which is analogous to the halonium ions formed from alkenes in Section 10.4.1. As with alkenes, the chloronium ion reacts with the nucleophihc chloride ion (CL) via anti attack... 

Dihalogenation of alkynes gives a dihalogenated alkene, which is also susceptible to reaction with bromine, chlorine, or iodine. Tetrahalo derivatives are available from dihalogenated alkenes (vinyl dihalides). When 1-pentyne reacts with one molar equivalent of diatomic bromine. 111 is the product. Because alkenes are also subject to reaction with halogens. 111 can react with a second molar equivalent of bromine to give 1,1,2,2-tetrabromopentane, 112. 

The potential of chiral sulfonium reagents to affect asymmetric halo-nium ion addition to alkenes was studied by Snyder et al. (13S1886). Although racemic dihalogenation and halohydroxylation of alkenes are well known, the corresponding enantioselective variants have remained elusive. Snyder first prepared a series of chiral, crystalline, sulfur-derived halonium reagents and indeed was able to carry out enantioselective dichlorination and iodohydroxylation of 1,2-dihydronaphthalene. Other chiral sulfonium reagents and the scope of the reaction and reaction conditions were also extensively studied. 

A diversity of purines could be accessed with good functional group tolerance for alkene, bromo, and ester substituents. Different halogenated alkanes could also be employed including alkyl, alkenyl, acrylate, and dihalogenated species, albeit a decrease in yield was observed for longer alkyl chains. Benzoimidazoles were also viable substrates, providing oxidized C8 imidazoles (72-94% yield). 

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