L- dibromide

Let us now turn to the experimental results to see if these predictions are borne out in fact. It has long been known that E2H reactions normally give preferentially anti elimination. For example, reaction of ffz fo-stilbene dibromide with potassium ethoxide gives a j-bromostilbene (Reaction 7.39), whereas reaction of the D,L-dibromide gives the trans product (Reaction 7.40).102 A multitude of other examples exist—see, for example, note 64 (p. 355) and note 82 (p. 362). 

Reduction of l l-dibromides. Chlorobis(cyclopentadienyl)titanium, generated in situ by zinc reduction of (C5H5)2TiCl2 in THF, effects rapid reductive de-bromination of vic-dibromides to alkenes by an anft-elimination. ... 

The rate-determining step was identified as an inner-sphere electron transfer between Bi2Co(I) and the all l dibromide, Eq. (15b). This electron transfer may alternatively be due to a concerted E2 mechanism. Ultramicroelectrode voltammetry was used to determine the kinetics in AOT-isooctane-water microemulsions. The polar vitamin B12 resides entirely in the water pools, whereas the substrates are present primarily in the continuous oil phase. The rate constant k was determined from the increase of the limiting current caused by the addition of the substrates [ethylene dibromide (EDB), 1,2-dibromobutane (DBB), and /rarw-l,2-dibromocyclohexane (/-DBCH)] to the AOT microemulsion. As shown in Table 5 [36], the apparent rate constants were three orders of magnitude lower in the microemulsion than in the acetonitrile-water system. The rate decrease was attributed to the spatial separation of the catalyst and the substrate [36]. 

PEROXIDES AND PEROXIDE COMPOUNDS - ORGANIC PEROXIDES] (Vol 18) l,ly-Diheptyl-4,4y-bipyridimum dibromide... 

A solution of 16jS-methyl-l la,17a,21-trihydroxy-5j5-pregnane-3,20-dione 21-acetate (52), 45 g, in dioxane (297 ml) is cooled to 15° and treated over a 5 min period with a solution of bromine (34.2 g) in dioxane (594 ml) precooled to 18°. After 2 min a solution of sodium acetate (60 g) in water (600 ml) is added and the mixture poured into ice water (8 liters). The precipitate is filtered off, washed to neutrality with water, and dried to give the crude dibromide (53), 55.7 g mp 125-126° (dec.) [aJu 58°. A mixture of dibromide (53), 55.5 g, lithium bromide (27.8 g), lithium carbonate (27.8 g) and DMF (1.11 liters) is refluxed under rapid stirring for 6 hr. The mixture is concentrated under vacuum to about 250 ml, poured into ice water (8 liters) containing hydrochloric acid (250 ml), and extracted with methylene dichloride. The extracts are washed to neutrality with water and evaporated to dryness. The residue is dissolved in acetone, evaporated to dryness under reduced pressure, redissolved in acetone and crystallized by the additon of hexane. This gives the dienone (54) 24.4 g, mp 236-239°. 

Adopting Auwers original method, Milton and Stephen prepared 2-chloroflavonol 16 from 4-chlorocoumaran-2-one 13 in 3 steps in 70% overall yield. 4-Chloroflavonol was synthesized via the same sequence. The same group also carried out the bromination of 2-benzylidenedihydro-b-naphthafurano-l-one (17) and subsequently treated the dibromide with aqueous potassium hydroxide to give 5,6-benzflavonol 18. However,... 

Dibenzo[d,g][l,5]telluroazocines 65 are the only representatives of Te, N-containing eight-membered heterocycles known thus far. Treatment of bis(2-bromomethylphenyl)tellurium dibromide 66 with methylamine followed by reduction of the reaction mixture with sodium sulfide gives, instead of the expected... 

A -methyl-5//,7//-dibenzo[d,.g][l,5]telluroazocine 65, its thiooxide 67a (X=S, R= Me) (95JA6388). The same type of reaction with benzylamine and sodium selenide gives A-benzyl-5//,7//-dibenzo[d,g][l,5]telluroazocine selenoxide 67b (X = Se, R = CH2Ph). Reaction of the tellurium dibromide 66 with sodium sulfide carried out in the absence of the primary amines affords the eight-member Te,S-containing heterocycle 5//,7//-dibenzo[d,g][l,5]telluroazocine (92CL151). 

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

Scheme 6.1-3 The formation of bis(l-butyl-3-methylimidazolylidene)palladium(ll) dibromide in [BMIM]Br...

Whereas l,2-0-isopropylidene-5,6-di-0-methyl-D-glucofuranose was found to be unreactive towards triphenylphosphite dibromide, triphenylphosphite methiodide or phosphorus pentachloride, the related methyl 2,5,6-tri-0-methyl-/ -D-glucofuranoside (59), in which the hindrance caused by the ketal group is absent, reacted with triphenylphosphite methiodide to give the 3-deoxy-3-iodo derivative 60 in 31% yield. 

For the preparation of 3-benzothiepin 3,3-dioxide (11) a multiple-step synthesis has been described.82 Oxidation of 4,5-dihydro-3-benzothiepin-l(2//)-one to its S,S-dioxide with hydrogen peroxide in glacial acetic acid, is followed by the multiple-step preparation of dibromide 10, which is dehydrohalogenated by triethylamine to provide sulfone 11. 

For example, while reaction of 2-butoxynaphtalene with copper(II) bromide in benzene at 50°C for 2 h. produced only 6 % yield of l-bromo-2-butoxynaphtalene, similar reaction with Kieselguhr-supported copper(II) bromide gave 86 % yield of the monobromide. On the other hand, reaction using alumina as a support in benzene proceeded completely even at 10°C in 1 h. to give in addition to the monobromide (77 %), the dibromide (21 %) and the binaphtyl (2 %). 

CN N,N,V,A/, W,/V-hexamethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-l, 16-diaminium dibromide... 

Two-electron reduction of 4,4 -(l,3-propanediyl)bis-(iV-methylpyridinium) dibromide with sodium amalgam affords bis-l,4-dihydropyridine 46 <96CL151>. 

---