1.2- Dibromides from olefins

Vicinal dibromides from olefins, acetylenes and a,p-unsaturated ketones. 

Synthetic Methods.— Direct routes to phosphonic acid derivatives from olefins and phosphorus halides continue to be explored. In the presence of oxygen, phosphorus tribromide reacts with olefins by a radical mechanism in a manner analogous to the corresponding chloride, giving 2-bromoalkyl-l-phosphonyl dibromides. The reaction of phosphorus trichloride and per-chloryl fluoride with hex-1-ene gives moderate yields of (55), the direction of... 

When dibromides from alkyl vinyl ethers were condensed with Grignard reagents and the resulting )0-bromoethers were treated with zinc, 53-84% of olefins could be yielded, which is 10-30% higher than the traditional procedure from aldehydes. ... 

Further examples of the conversion of dibromides into acetylenes are included in section 14 (Acetylenes from Olefins)... 

The formation of diequatorial and diaxial dibromides from 3- and 4-sub-stituted cyclohexenes has prompted examination of the factors which may influence the steric course of the reactions. All additions of bromine are anti and all adducts are formed under kinetic control. The diequatorial diastereo-isomer (207) content of the reaction product decreases when a basic solvent such as diethyl ether is used the same result pertains in the presence of tertiary amines or when the brominating agent is either pyridine perbromide or pyri-dinium hydrobromide perbromide. Of the olefins examined, (205b) yields the highest amount of diequatorial adduct. 

Since 1,2- to 1,6-difunctional opengeneral procedures (see chapter 1), it is useful to consider them as possible starting materials for syntheses of three- to seven-membered heterocycies 1,2-heterocycles can be made from 1,2-difunctional compounds, e.g. olefins or dibromides 1,3-difunctional compounds, e.g. 1,3-dibromides or 1,3-dioxo compounds, can be converted into 1,3-heterocycles etc. 

If return occurs during the bromination of cw-stilbenes and rotation around the C-C bond is faster than collapse of the intermediates to dibromides, this process will lead to fra j-stilbene (Scheme 9). We used this test to check the possibility of return in the bromination of unsubstimted, 4-methyl, 4-trifluoromethyl-, and 4,4 -bis(trifluoromethyl)-stilbenes in DCE (ref. 24). All these olefins gave clean third-order rate constants spanning 7 powers of 10. For each cis-trans couple the cis olefin was brominated 3.5 to 5.5 times faster than the trans isomer. Reactions for products analysis were performed at initial molar ratios of Br2 to olefin of 1 to 2, so that products arose only from the cis olefin, the trans isomer being accumulated in the reaction medium. 

To test this point, McKeon and Koch examined the preparative scale electrochemical reduction of the diaxial bromides 43 and 46 in aqueous DMF 49>. They found only the corresponding olefins as products, under analytical conditions which could have detected as little as 3% of the alkanes, had they been formed. It is unfortunate that a dibromide for which = 90° was not investigated by these workers. Reduction of such a compound ought to be stepwise, and alkane could be formed, especially in a proton-donating medium. Finally, Nelson and coworkers have suggested from their study of a series of rigid vi-... 

It is noteworthy that the nature of the ionic intermediate formed in bromine addition to olefins and the solvent properties also govern the competition between nucleophilic trapping and elimination. Thus 1,1-diphenylethylene, 11, gives the corresponding dibromide 13 (or solvent incorporated products, 14) and vinyl bromide, 12, in a ratio changing from 99 1 to 5 95 depending on solvent and on bromine concentration.(20) (see Table III results)... 

Examination of the peak potential locations and transfer coefficient values in a series of 16 cyclic and acyclic dibromides according to the procedures detailed in Chapter 3 points to a first dissociative electron transfer rate-determining step (Scheme 4.1). It is followed by another dissociative electron transfer step, leading directly to the olefin. Intrinsic barriers for the first, rate-determining step range from 0.6 to 0.8 eV, consisting mostly of the bond dissociation contribution (one-fourth of the bond dissociation energy). 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

Several methods achieving the debromination of v/c-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inermess of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threo-and eryf/iro-dibromides, respectively. 

Method a + c. The tellurium dibromide (1.0 mmol) in THF (20 mL) is treated with 0.5 N NaOH (10 mL) at room temperature with stirring, and the resnlting solntion is stirred for 1 h, the colour of the solution turning to orange. The mixtnre is dilnted with brine and extracted with ether. The extracts are dried (MgS04), evaporated nnder vacnnm and the residue purified from diphenyl ditelluride by Si02 chromatography (elntion with hexane). The pure olefin is isolated by distillation of the residne. 

Allylic and vinylic ethers via methoxytelluration of olefins (general procedure) To a solntion of phenyltellnrinm tribromide (2.22 g, 5 mmol), prepared from diphenyl diteUnride and bromine in methanol (5 mL), is added the olefin (10 mmol) and the mixture refluxed for 1 h. On cooling, the ()3-methoxy)alkyl or cycloalkylphenyltellnrinm dibromide precipitates and is separated by filtration. 

Bromine atoms may be expected to show far reaching similarity to chlorine atoms in their reactions with olefins. This seems to be largely true (97), although complications arise from the existence of equilibria between the dibromide formed and the original reagents (Br2 and olefin). 

LiBr and in the presence of cyclopentene as a scavenger olefin. The kinetics, determined by monitoring the formation of strong acids (TfOH or HBr), show that the rate of solvolysis of 65 is dependent on [Br-] (at a constant ionic strength). In the presence of Br-, the products are trans- 1,2-dibromides and bromo-solvates of both cyclohexene and cyclopentene. The cyclopentenyl products have been shown to arise from the electrophilic addition of Br2/Br3 to cyclopentene, while trans-l, 2-dibromocyciohexane 67 is formed by Br- capture of the bromonium ion 66 on carbon. The Br2 required for bromination of cyclopentene results from attack by Br- on the bromonium ion 66 on Br+. On the basis of the ratio of the cyclopentyl products to 67, Br- capture of the solvolytically produced bromonium ion 66 (by attack on Br+) is 4-5 times more prevalent than attack on carbon in AcOH, and ca 25 times more preferred in MeOH123. 

The authors assumed that dibromide 10 formed as major product may arise from the dehydrofluorination of monoadduct 9 followed by a radical addition of HBr to the intermediate olefin, according to the following scheme ... 

The dibromides are often used for the purification of olefins since the double bond is easily regenerated by zinc and alcohol treatment. Compounds of the type RR CHCHBrCHBrR", where R" is an alkyl group or a hydrogen atom are prepared directly from the corresponding tertiary alcohols. Under the conditions of bromination, simultaneous dehydration and addition occurs e.g., Namyl alcohol to ttimethylethylene dibromide (70%). ... 

A modification of this reaction is the hydrolysis and rearrangement of olefin dibromides. The most successful of these conversions is the preparation of methylisopropyl ketone (59%) from trimethylethylene dibromide. ... 

Vinylidenes have been transferred from a variety of precursors to olefins to produce methylenecyclopropanes . Because of ready intramolecular hydrogen shifts to give terminal acetylenes, the addition of vinylidene to olefins is rather limited to 2,2-disubstituted species. The methodologies so far developed include (1) gem-dibromides, 7, with MeLi, (2) vinyl halides " or vinyl triflates, 8 " with r-BuOK, (3) the fluoride ion promoted decomposition of vinylsilanes, 92 2,243 4 thermolysis of mercuric derivatives, 10, at 250 (5) decomposition of vinylazo compounds, 11, at 25 (6) the alkaline... 

Casanova and Rogers [59] as well as Fry [69] postulate that the reduction of vicinal dihalides is a concerted process in which both carbon-halogen bonds are partially cleaved as a carbon-carbon double bond starts to form. Nonelectrochemical evidence [70] suggests that a vicinal dihalide undergoes one-electron reduction to a radical anion, which loses the first halide ion to form a neutral radical, after which the neutral radical accepts an electron to become a carbanion that eliminates the second halide ion to yield an olefin. From a study of the behavior of meso- and c/,/-l,2-dibromo-l,2-diphenylethane, Fawell and coworkers [71,72] concluded that the reduction of vicinal dihalides is a stepwise process. Andrieux and coworkers [73] have examined the reductive elimination of vicinal dibromides at carbon in MeCN. 

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