The mechanism of dehydrohalogenation

No extensive investigation of mechanism has been undertaken for any of the methods of dehydrohalogenation described. 17-Bromo-20-ketones appear to undergo preferential /ran -elimination. 2-Halo-3-ketones suffer predominant loss of the la (axial) hydrogen, but the geometry of bromine loss is not known. 7 fl -diaxial elimination has sometimes been assumed in configurational assignments, but this is not necessarily correct (see ref. 6). 

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The mechanism of dehydrohalogenation of haloalkanes varies from a concerted E2 mechanism to a carbanionic ElcB mechanism, where the /j-hydrogen has been made sufficiently acidic to be removed, leaving a carbanion.4,5 The E2 mechanism of dehydrohalogenation takes place in one step. The ElcB or carbanionic mechanism of dehydrohalogenation has two steps the first step being deprotonation, and the second one elimination of the halide ion. 

The mechanism of dehydrohalogenation under basic conditions of trons-fused bicyclo[4,n,0]alkane halohydrins (563)—(565) has been studied. Three reaction types are noted (i) epoxide formation, (ii) ketone formation, and (iii) ring contraction. trans-Diaxial chlorohydrins corresponding to (563)—(565) gave epoxides (566)—(568) with relative rates (derived from bimolecular rate constants) of 1 3 17. This rate sequence was rationalized in terms of deformation of the cyclohexane ring brought about by the nature of the fused ring. In particular, deformation is probably towards the half-chair conformation favoured by the cyclohexane epoxide which is formed in the slow step. trans-Diequatorial chlorohydrins represented by (563)—(565)... 

A complex of homoadamantyne (273) has been prepared by dehalogena-tion of 272 in the presence of tris(triphenylphosphine)platinum(0) [Eq. (43)]." Trapping free homoadamantyne was presumed to be the mechanism of this reaction. However, reduction of neither an initially formed 77-com-plex2,82 nor a C-Br insertion product93 was excluded. Interestingly, the free alkyne, generated by dehydrohalogenation of the vinyl bromide, was successfully trapped with diphenyl-wo-benzofuran. 

The key step in the first reaction is, once again, a 1,4-dehydrohalogenation followed by a stereocontrolled prototropic shift and electrocyclic closure of the derived triene to a bicy-cloheptadiene (shown below) the dichlorocyclopropane then undergoes a typical sequence of 1,2-dehydrochlorination followed by prototropic shifts to lead to the [10]annulene. The chlorodiene 45 may be produced by a similar sequence of 1,4-dehalo-genation, prototropic shift and cyclization 00b. The mechanisms of these and related reactions have been the subject of labelling studies. 

The mechanism of the above elimination reaction is similar to the E2 mechanism of dehydrohalogenation. The reaction proceeds by nucleophilic attack of a base on silicon, simultaneous elimination of halide ion and formation of double bond (Scheme 4.4). 

There are four main general methods [40 2] for the preparation of perfluorinated alkenes, namely dehydrohalogenation, dehalogenation, pyrolysis and halogen exchange reactions of appropriate fluorinated precursors. The overall features of the mechanisms of each of these processes have already been discussed (Chapters 6 and 7, Sections 1 and 11). Representative examples of each of these types of synthesis are collated in Table 7.5 clearly the method of choice for the synthesis of a particular fluoroalkene will depend... 

The kinetics of dehydrohalogenation from the configurational isomers of vinyl halides have been determined for numerous reactions. Thus cw-/7-nitro-P-bromo-styrene in the presence of ethanolic NaOH is converted quantitatively by trans elimination to /7-nitrophenylacetylene within a few minutes (equation 21), whereas the trans isomer hardly reacts at all in that short time. However, the latter affords l,l-diethoxy-2-p-nitrophenylethane in high yield when kept under the above conditions for 20 days (equation 22). The mechanism of the latter reaction could not... 

The stereochemistry of dehydrohalogenation with DBU has been studied by several authors. Wolkoff (82JOC1944) studied in detail the stereochemical consequences of dehydrohalogenation of secondary and tertiary alkyl and cycloalkyl halides with DBU. A comparison of the product distributions obtained in the elimination reactions of alkyl halides with DBU, with weak bases, and with anionic bases indicated that the elimination reactions with DBU very probably follow an E2C-like mechanism. 

In dehydrohalogenation of alkyl halides (Sec. 5.13), we have already encountered a reaction in which hydrogen ion and halide ion are eliminated from a molecule by the action of base there —H and —X were lost from adjacent carbons, and so the process is called p-eHmination. In the generation of the methylene shown here, both —H and -X are eliminated from the same carbon, afid the process is called a-eliminaiion. (Later on, in Sec. 24.12, we shall see some of the evidence for the mechanism of -elimination shown above.)... 

Problem 11.12 Dehydrobromination by C2HsO Na of ordinary isopropyl bromide and of labeled isopropyl bromide, (CD3)2CHBr, at 25 has been studied, and the rates found to be in the ratio 1.76 0.26. (a) What is the value of the isotope effect (b) Is this isotope effect consistent with the mechanism for dehydrohalogenation given in Sec. 5.13 (c) With the following two-step mechanism involving a carbonium ion ... 

The general mechanism of dehydrohalogenation may be illustrated by considering the reactions of 4-halo-3-methoxymethyl-6-methylpyridazine with amide ion in ammonia, Scheme 2. Amide ion removes a proton at a position adjacent to a halogen ... 

Though the above heats of reaction values calculated are not the absolute values they can be taken as relative values to predict the probability of formation of various components. Compared to hydrogenation products (steps A, B and C ), the formation of dehydrohalogenated products (Steps C D) are difficult. However, since the formation of each hydrogenated product is followed by the formation of dehydrohalogenated product (as per the following reaction sequence, which is the most probable mechanism)... 

The mechanism of formation of 1 and 2 has yet to be determined however, the simple scheme shown in Figure 4 can be used to rationalize the formation of both species in the reaction. Thus, the initially formed 4-CBgHi3 can react with PhPCl2 to form a bridge-substituted species which can then undergo dehydrohalogenation with loss of PSH+Cl- to yield the RP inserted species. 

The mechanisms of base-induced beta-eliminations leading to alkynes are highly dependent on the type of base employed, and for this reason the dehydrohalogenations induced by metal alkoxides are considered separately from those initiated by metal amides or metal alkyls. 

The mechanism of the dehydrohalogenation process has not been directly studied. A reasonable explanation is shown in Scheme 69 it is based on observations made on the course of events of the reverse process, the reaction of phospholes with anhydrous HCl <86JCS(P2)629> (see Section 2.15.4.3). A key feature is the formation of a transient 2//-phosphole derivative which undergoes a [1,5]-sigmatropic shift to the phosphole. 

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