Alkyl dibromides, formation

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

The reaction of phosphines and alkyl halides presents an alternative way to generate phosphonium electrophiles (Scheme 3.8). In particular, the combination of a phosphine and carbon tetrabromide (the Appel reaction) allows for in situ formation of a phosphonium dibromide salt (48, X = Br). Treatment of a hemiacetal donor 1 with the phosphonium halide 48 initially provides the oxophosphonium intermediate 38 (X = Br). However, the oxophosphonium intermediate 38 can react with bromide ion to form the anomeric bromide intermediate 49 (X = Br) with concomitant generation of phosphine oxide. With the aid of bromide ion catalysis (i.e. reversible, catalytic formation of the more reactive P-anomeric bromide 50) [98], the nucleophile displaces the anomeric bromide to form the desired glycoside product 3. The hydrobromic add by-product is typically buffered by the presence of tetramethyl urea (TMU). 

The methylene hydrogens between the two carbonyls are the most acidic, so this is where enolate anion formation occurs. Now follows an Sn2 reaction with the dibromide reagent. It is soon apparent that this sequence of enolate anion formation and Sn2 displacement can be repeated, since the substrate still contains an acidic hydrogen. We soon end up with an alkylated ketoester. 

A study of the mono- vs di-alkylation reactions of dibromide (9) with carbanions (lOc-g), covering a range of >15 pK units in DMSO, has revealed that the carbanions (lOd-g) derived from the less acidic carbon acids give exclusively the bis(monoalkylated) product (ii) however, carbanions (lOa-c) give the cyclic product (12) of dialkylation.21 This dichotomy is apparently a consequence of the relative rates of formation (by proton transfer, /id) and cyclization (kc) of die conjugate base of the monoalkylated intermediate. 

The reaction of alkenyl alkyl ethers and ketene acetals with PX3 (X = Cl or Br) has been reported to occur readily in the presence of an organic base and to result in the electrophilic substitution of a vinyl hydrogen atom with the PX2 group. Thus, (2-alkoxyalkenyl)-, (l-bromo-2-alkoxyalkenyl)- and (2,2-dialkoxyalkenyl)-phosphorus dichlorides and dibromides were obtained in 70-98% yield. The reaction proceeds regio- and stereo-selectively and is believed to involve formation of a cyclic phosphirenium ion.74... 

Alkyl and cycloalkyl halides react with benzenetellurolate to yield (cyclo)alkyl phenyl telluriums, which are converted to (cyclo)alkyl phenyl tellurium dibromides upon treatment with bromine. Stirring of a mixture of these tellurium dibromides and 0.5 M aqueous sodium hydroxide at 20° resulted in the formation of (cyclo)alkenes5. Alkyl phenyl tellurium oxides were postulated as intermediates that eliminated phenyl tellurium hydroxide. 

The cleavage of tetrahydrofuran and its alkylated derivatives with halogen acids is an excellent method for the preparation of, A-dihalo-alkanes.The reaction of tetrahydrofuran with the less-reactive hydrogen chloride stops at the chlorohydrin stage, whereas the reaction in the presence of zinc chloride catalyst leads to the formation of the dichloride. The crude reaction mixture containing the intermediate chlorohydrin may be treated directly with phosphorus tribromide, yielding tetramethylene chlorobromide. The preparation of dibromides can be accomplished easily with hydrogen bromide or phosphorus and bromine and diiodides, by the action of potassium iodide and orthophosphoric acid. ... 

Alkynes (acetylenes, RCsCR) may be prepared by the elimination of a hydrogen halide from alkenyl halides under vigorous conditions. This is exemplified by the preparation of phenylacetylene from cinnamic acid via the dibromide and (o-bromostyrene (Scheme 3.26). The contrast between the conditions required for the bromodecarboxylation and for the second elimination to form the alkyne reveals the difference in reactivity between an alkyl and an alkenyl halide. Alternative modes of elimination, such as allene formation or rearrangement reactions, restrict the use of this procedure. 

While no higlier alkylated product could be detected in the CCI4 reaction the formation of both, the phosphonous and phosphinous derivative was observed in the reaction of white phosphorus with bromoform at 190 °C for 1 h But also here the amount of the phosphonous dibromide was 4 to 6 times that... 

Deprotonation to the enamine anion, selective coupling with the allylic terminus of dibromide 114, followed by an intramolecular enamine alkylation, afforded reduced isoquinoline 119. A rather elegant conversion to aminoaldehyde 122 ensued. Immonium ion formation in 119 via protonation with perchloric acid at first yielded the kinetic trans isomer, which underwent equilibration upon reflux in methanol to give the corresponding crystalline cis product 120. Diazomethane treatment led to aziridinium salt 121, which upon exposure to DMSO, ring opened with concomitant oxidation in a Komblum fashion to the aldehyde 122.63 Treatment with Lewis acid effected B-ring closure, thus... 

The formation of self-filled phanes in coupling reactions between diphenolic cuppedophanes and capping units (Scheme 31) could also be attributed to the nature and reactivity of the capping unit. For example, with p-xylylene dibromide as the capping unit, the rate of the second alkylation of the phenol was apparently faster than the rate of conformational flip, thus leading to the predominance of 208v. However that was not the case when the capping unit... 

A drawback of the Tebbe and related reagents is that they are generally suitable only for methylenation and do not permit the formation of higher alkyl analogues. However, the alkenylation of esters (or amides) has been found possible using the Oshima-Lombardo conditions in the presence of TMEDA (tetramethylethylene-diamine) (2.106). This chemistry requires the prior formation of the alkyl gem-dibromide and a more-convenient method, using a dithioacetal, has been reported (2.107). ... 

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