Dibromine formation

Brick et al. have studied this bromination in more detail and showed that the extent of the bromination can be controlled by changing the ratio of the reagents. The first substitution was found to be in the para position but subsequent intramolecular rearrangements allowed the formation of 2-5-dibrominated species. Brick et al. also reported the functionalization of such species using Pd-catalyzed reactions such as Heck and Suzuki couplings to give fully substituted p-stilbenes, p-biphenyls, diarylamines, and methylcinnamates. Hydrogenation of... 

In spite of the numerous spectral observations of complex formation between aromatic and olefinic donors with the dihalogens, the preparations of the corresponding crystalline complexes have been hindered by their enhanced reactivity (as well as the relatively weak bonding). As such, only few examples of the X-ray structural characterization of the corresponding intermolecular associates are reported, the most notable exception being the dibromine complex with benzene. 

Similar oxidations also occurred with enamine substrates. In the reaction outlined in Scheme 8 [18], the formation of a dibrominated product arose because of the initial oxidation of an enamine... 

Substituents at the 3-position, if capable of stabilizing an adjacent positive charge by resonance and/or inductive effects, direct strongly to the adjacent (i.e. 2) position (equation 3). Stabilization of the allylic carbonium ion intermediate is obviously involved. This effect can be quite pronounced for R = Ph in equation (3) the ratio of 2- to 5-bromo derivatives for electrophilic bromination is ca. 660 (68JOC2902). Even more striking is the exclusive formation of the 2,2 -dibrominated product (19) from 3,3 -dithienyl (equation 4) (69JOC343). As expected, substituents not capable of stabilizing the cationic intermediate direct substitution to the 5-position. 

Conceptually, a much different approach was used to prepare 8,10-dideazaAP 1418) 3 years later (Scheme 3.80) [ 190a,b]. Here, (418) was assembled by a convergent pathway involving formation of the C-9,C-10 bond as the critical step. Until Broom s synthesis of (160) from (159) using PBr3-THF [86], this labile material was accessible only with great difficulty from either (416) or (141) due to competitive dibromination in the former case [42] or the formation of complex, inseparable mixtures in the latter [98]. Once in hand, (160) was converted to the corresponding ylide, which was condensed with (252) [140] to yield the 9,10-dehydro derivative (417). Catalytic reduction of (417) followed by saponification produced (418). 

Epoxides do not, normally, react with dibromine or diiodine unless a reducing agent is present. This has been demonstrated in formation of bromo- and iodohydrine from oxiranes in presence of sodium thiosulfate1103. 

Dimethylbutane reacts with bromine in the presence of light to give a monobrominated product. Further reaction gives a good yield of a dibrominated product. Predict the structures of these products, and propose a mechanism for the formation of the monobrominated product. 

The intramolecular Wurtz-type coupling of dihaloorganic compounds with use of metallic zinc is a classical synthetic route to cyclic compounds. For example, cyclopropane derivatives can be prepared from 1,3-dihalo-propanes (29, 189a, 248, 451), and cyclobutane derivatives from 1,4-dihalobutanes (71). These reactions presumably proceed via the intermediate formation of organozinc compounds. The reaction of diethylzinc with esters of a,a -dibrominated aliphatic dicarboxylic acids leads to the... 

Cumulene modification of carbyne (P-c arb5me) has been received on specially developed by Sladkov two-stage method [3], At the first stage, spent polycondensation of suboxide of earbon (CjO ) with dimagnesium dibromine acetylene as Grignard reaction with formation polymeric glycol ... 

Preferential formation of a-bromoketones from ketones via cnamines, and dibromination... 

Various other mixed oligomers and modified [3,2-b]TTs have been prepared, and a selection is described in what follows. The reaction of 2,5-dibromothieno [3,2-b]thiophene (56) (easily prepared from TT (1) using two equivalents of NBS) with tributyl(thien-2-yl)stannane (57) in a Stille cross-coupling protocol resulted in the formation of 2,5-di(thien-2-yl)thieno[3,2-ft]thiophene (58). The successive treatment of 58 with -BuLi and BusSnCl gave (59), which was Stille cross-coupled to obtain 2,5-bis(5-phenylthien-2-yl)thieno[3,2-ft]thiophene (61). Dibromination of 88 furnished 2,5-bis(5-bromothien-2-yl)thieno[3,2-f>]thiophene (60) and... 

Dibromine aqueous solutions are very unstable, because of bromine s volatility. They cannot be more concentrated than 0.015 mol/L for solubility reasons. Bromine is more soluble in the presence of chloride ions by formation of the complex ion [Br2Cl ] and in the presence of bromide ions by formation of complex ions [Brs"] and [Br5 ]. Its solubility is 0.4 mol/L in a 0.4 mol/L NaCl solution and 1.4 mol/L in a 1.0 mol/L KBr solution. 

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