Elimination reactions dihalides

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Reductive 1,3-elimination reaction of alkyl dihalides constitutes one of the classical methods for the preparation of cyclopropyl derivatives and is particularly useful for the synthesis of highly strained polycyclic hydrocarbons. A new preparation method of [l.l.ljpropellane, more versatile than the original Wiberg s method, has been devised3,4. Thus, treatment of l,l-dibromo-2,2-bis(chloromethyl)cyclopropane with alkyllithium or lithium powder affords [1.1. ljpropellane by two successive 1,3-eliminations of halogens by way of 1 -bromo-2-(chloromethyl)bicyclo[l. 1. Ojbutane (equation 1). This method has been... 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

Suitable candidates for a-elimination reactions are silylmethyl halides (— base-induced elimination of H-Hal), silylmethyl dihalides (— halide/metal exchange followed by elimination of a metal halide) and stable carbenoid-type compounds such as (a-halo-a-silylalkyl)mercury compounds (— thermal elimination of mercury(II) halide). Bis(phenylthio)(trimethylsilyl)methyl lithium (— elimination of LiSPh) represents a borderline case (see Section III.E.8). 

Alkynes can be prepared by elimination reactions of 1,2-dihalides, using a strong base. 

Reacts with 1,1- or 1,2-dihalides to yield alkynes by a twofold elimination reaction (Section... 

Two elimination reactions are needed to remove two moles of HX from a dihalide as substrate. 

Alkynes are prepared by elimination reactions, as discussed in Section 8.10. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne by two... 

Many of the same reagents that reduce vic-dihalides also reduce halohydrins with similar regiochemi-cal and stereochemical advantages. However, halohydrins are more useful alkene precursors because they can be prepared by a greater variety of methods. The Fischer-Zach synthesis of 3,4,6-tri-O-acetyl-D-glucal (11 Scheme 3) reported in 1913 is one of the first synthetically useful applications of reductive elimination reactions to the manipulation of natural products and the method is still widely used today. ... 

An example of this concerns the elimination reaction in which ageminal dihalide, RH2C-CHC12,... 

At this point, you know only one way to synthesize a ketone—the addition of water to an alkyne (Section 6.6). The alkyne can be prepared from two successive E2 reactions of a vicinal dihalide, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction. 

Elimination reactions can also be used to prepare alkynes. Dihalides can be converted into alkynes by treatment with base, provided that the two halogen atoms are on the same, or adjacent, carbon atoms. In general, alkyne formation requires more drastic reaction conditions than alkene formation, and a strong base, such as sodium amide, is usually employed (Reactions 5.4 and 5.5). The product of Reaction 5.5, phenylethyne, is shown in Figure 5.1 as a ball and stick model. 

Elimination reactions involving dihalides can be used to prepare alkynes. A strong base such as sodium amide is usually required. 

OTHER 1,2-ELIMINATION REACTIONS Dehalogenation of Vicinal Dihalides... 

In each of the examples of 1,2-elimination reactions discussed above, one of the groups eliminated was a proton. Another iype of 1,2-elimination reaction is the dehalogenation of vicinal dihalides. Iodide ion can be used as the dehalogenating agent in both protic and aprotic solvents (equation 10.40). 

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