Dihalo-elimination reaction

Compounds with halogens on neighboring carbons can produce an unsaturated derivative through a dihalo-elimination reaction ... 

Analogous sulfur compounds are also prepared. 1,2-Dihalo compounds lead to the formation of alkenes (see above), as do l-halo-2-X-substituted alkanes (X = NRjOR, SR, etc.). In the aromatic series 2-heterosubstituted reagents can be stable , for elimination would lead to highly strained arynes. Similarly, the elimination reaction may be difficult when allenes ... 

One of the best known a elimination reactions occurs when chloroform is treated with base. This Is the most Important way of making dichlorocarbene, CCl2, and other dihalo-carbenes too, although it must be said that the widespread use of dichlorocarbene in chemistry is due mainly to the ease with which it can be made using this method ... 

With a,p-dihalo- or a-halo-a,p-unsaturated carbonyl derivatives A significant number of examples exist in which a,P-dihalogenated carbonyl derivatives undergo reactions with thioglycolates in the presence of base to produce thiophenes. " The reactions have been shown to occur through intermediate a-halo-a,P-unsaturated carbonyl derivatives produced by the elimination of HX. Thus the use of a-halo-a,P unsaturated carbonyl systems in place of the a,P-dihalocarbonyl compounds was found to efficiently provide thiophenes upon reaction with thioglycolates. In a modification of the work of Fiesselmann, readily accessible methyl-2-chloroacrylate 23 and 2-chloroacrylonitrile 24 have been used in this sense to provide 25 and 26, respectively. 

Halo-l-benzothiepins 4 can be synthesized by the treatment of 7a-halobenzo[fe]cyclopropa-[e]thiopyran-7-ols 2 with hydrogen bromide and subsequent hydrogen bromide elimination from the 2,4-dihalo-2,3-dihydro-l-benzothiepins 3 by l,5-diazabicyclo[4.3.0]non-5-ene (DBN).9 The alcohols 2 are prepared by Grignard reaction of the corresponding 7a-halobenzo[ft]cyclopropa[c]thiopyran-7-ones l18 and are used for the synthesis without purification. The intermediate dihalodihydro-l-benzothiepins 3 are not isolated due to their thermal lability related and more stable compounds are described in Section 2.1.2.1. 

Although alcohols are oxidized by tetra-n-butylammonium persulphate when the reaction is conducted in dichloromethane, tetrahydropyranyl ethers have been produced (>90%) when attempts to oxidize the alcohol are conducted in tetrahydro-pyran (see Chapter 10) [ 19], Tetrahydrofuranyl ethers have been prepared by an analogous method [20,21 ]. Base-mediated elimination of halo acids from P-halo alcohols under phase-transfer catalysed conditions produce oxiranes in high yield (70-85%). The reaction has particular use in the synthesis of epihalohydrins from p,y-dihalo alcohols [22],... 

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

A second important reaction of L-AuR species is the oxidative addition (see Oxidative Addition) of halogen or alkyl halides, which leads, at least in a first step, to organo(dihalo)gold(IIl) or diorgano(halo)gold(III) products. The structures are transformed from linear to square planar, and therefore cis and trans isomers are possible. The products may undergo secondary reactions and/or become subject to a reductive elimination (see Reductive Elimination) of other substituent combinations (equations 41 and 42). ... 

One of the most useful routes to 2-oxyallyl cations (7) is the reaction of a,a -dihalo ketones (9) with reducing agents (Scheme 3). Initially, halogenated metd enolates of type (8) are formed and oxyallyl cations (7) are then generated by subsequent elimination of the halide ion. Copper-sodium iodide in ace-... 

A copper-containing oxocarbenoid is assumed to be involved in the formation of both the cyclopropanes 9 and the 4,5-dihydrofurans 10. It is therefore not surprising that the formal cyclotrimers of oxocarbenes, triacylcyclopropanes 12, were obtained in the absence of suitable reagents that could trap the oxocarbenoid. Cyclopropanes 12 are likely to be formed via the formal oxocarbene dimers 11, which in some cases are also found among the reaction products. In addition to the copper-mediated debromination of a,a-dibromo ketones, triacylcyclopropanes can also be obtained from a,a-dihalo ketones by metal-induced a-elimination in other cases, e.g. from a,a-dibromo ketones with Ni(0), Fe(0), or Co(0) complexes,", and from a,a-dichloro ketones with a zinc-copper couple. 

In combination with protic acid, NIS has been used for the preparation of a-iodo enones and j8, -dihalo enones (eqs 4 and 5 HTIB = [Hydroxy(tosyloxy)iodojbenzene). More reactive alkenes such as enol acetates react with NIS to afford a-iodo ketones. The reaction of enol ethers with NIS in the presence of an alcohol, affording iodoacetals, allows the synthesis of acetals by dehalogenation or mixed ketene acetals by elimination. ... 

A route to a halogen-substituted polymer is through a reaction of dihalo-p-xylylenes with caustic. It results in 1,6 elimination of HCl and formation of a chlorine-substituted polyCp-xylylene)" ... 

Baciocchi E, Ruzziconi R, Sebastiani GV (1983) Anti and syn eliminations from 2,3-dihalo-2,3-dihydrobenzofurans. The role of the substrate structure and the base-solvent system on the reaction mechanism. J Am Chem Soc 105 6114-6120... 

A new one-pot procedure has been described for the dialkylation of ew-dihalo-cyclopropanes. The method allows for the unsymmetrical double alkylation to give mixtures of the products (59) and (60). The reaction proceeds via initial halogen-copper exchange to give (61), then consecutive alkyl migration under X elimination conditions produces (62) which undergoes a second alkylation by R X (Scheme 7). 

The pseudopara dichloro- and dibromo- [2.2]paracyclophanes are also available in modest yields ( 20 %), and free from other regioisomers, from Winberg 1,6-elimination-dimerization [80, 81], of (2-halo-4-methylbenzyl)lrimethylammonium hydroxides (Fig. 14) [82]. Similarly, reaction of (2,5-dihalo-4-methylbenzyl)trimethyl-ammonium hydroxides affords the cross tetrahalo[2.2]paracyclophane substitution pattern previously reported by de Meijere [79]. Additional value of this route might also lie in the ability to prepare new highly halogenated [2.2]paracyclophanes, including 4,8,12,16-telrabromo[2.2]paracyclophane and 4,5,7,8,12,13,15,16-octabromo[2.2] paracyclophane. 

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