Dibromo-p-xylene

An important feature of the antibiotic chloramphenicol (9) is the presence of the dichloroacetamide function. Inclusion of this amide in a simpler molecule, teclozan (15), leads to a compound with antiamebic activity. Whether this is cause and effect or fortuitous is unclear. The synthesis begins with alkylation of the alkoxide derived from ethanolamine (10) with ethyl iodide to give the aminoether (11). Reaction of a,a -dibromo-p-xylene (12) with 2-nitropropane in the presence of base leads to dialdehyde (13). The reaction probably proceeds by O-alkylation on the nitropropyl anion... 

The Sandoz company used the dibromoterephthalic acid method. This acid was made from p-xylcnc by brominating it to form 2,5-dibromo-p-xylene and then oxidising this to 2,5-dibromoterephthalic acid. Reaction of one mole of this acid with two moles of an arylamine in the presence of copper(II) acetate gives 2,5-bis(arylamino)terephthalic acid, which can be ring-closed to a linear quinacridone. Unsymmetrical substitution using two different arylamines is possible. 

Another way of synthesizing B-A-B triblock copolymers is to use a coupling reaction.2 Monocarbanionic poly-B precursor is used to initiate the polymerization of A. The living two block copolymer is then reacted stoichiometrically with an efficient bifunctional coupling agent, such as dibromo-p-xylene or dimethyldichlorosilane, or even phosgene. This coupling reaction yields the triblock copolymers. 

Sodium hydride (97%), triethyl phosphite (99%), a,a dibromo-p-xylene (97%), 4-hydroxy-3-methylbenzaldehyde (97%), 3-ethoxy-4-hydroxybenzaldehyde (99%), vanillin (99%), and valeryl chloride (98%), were supplied by Aldrich Chemical Co. and were used without further purification 4-hydroxybenzaldehyde (96%, Aldrich) was resublimed prior to use. The add chlorides were supplied by Aldrich Chemical Co. and, with the exception of sebacoyl chloride (99+%), were fractionally distilled at reduced pressure through a 6-inch Vigreux column prior to use pimeloyl chloride 95-6°C at 1.1 torr, suberoyl 114-16°C at 2.2 torr, azelaoyl 104-6°C at 0.35 torr. Dibromoalkanes from Aldrich Chemical were fractionally distilled prior to use 1,7-dibromoheptane 111-114°C at 1.4 torr, 1,9-dibromononane 135-137°C at 2.7 torr, 1,11-dibromoundecane 129-131 °C at 0.85 torr. Tetrabutylammonium iodide (98%) was supplied by Aldrich. Reagent grade solvents were obtained from Fisher Scientific. 

Bisphenol Preparation. The mesogenic bisphenols were produced in a manner similar to that of Stilz and Pommer (14). First, tetraethyl-p-xylylenediphosphonate was produced in a reaction between a,al-dibromo-p-xylene and triethyl phosphite in xylene. The diphosphonate was then reacted with an excess of the appropriately-substituted p-hydroxybenzaldehyde. The preparative details and analytical results for the four monomers were given earlier (Mates, T.E. Ober, C.K. sL Polvm. Sci. Lett., to be published). 

By cathodic 1,6-dehalogenation p-xylylenes were prepared. When OLfxpifil,d-- hexachloro-p-xylene was electrolyzed at low temperatures the tetrachloro-p-xy-lene could be isolated 3231. Cpe of a, a+dibromo-p-xylene at the plateau of the first or second polarographic wave yields 5-10% [2.2] -paracyclophane and 90% poly-p-xylylene as products from the reactive unsubstituted p-xylylene (Eq. 

Amorphous, low molecular weight ( 3000) poly-p-xylylene (poly-QM) was synthesized by the Wurts-Fittig reaction of 7,8-dibromo-p-xylene with sodium or magnesium... 

Figure 2. Synthesis of the polyether of bis(4-hydroxyphenoxy)decane and a.a -dibromo-p-xylene.

Gao et al. have recently prepared a number of thermotropic liquid crystalline main-chain fer-rocenes exhibiting fluorescent properties.182 The reaction of 1,1 -bisf functionalized)ferrocenes, 95, with 1,4-dibromobutane, 97a, or a,a -dibromo-p-xylene 97b generated polyelectrolytes 97a,b with Mn ranging from 5400-14,700 (Scheme 2.29). These polymers were highly thermally stable with the first weight loss occurring between 172 and 330°C. The liquid crystalline properties showed these materials formed smectic liquid-crystal phases and displayed batonnet textures. 

An other method, described by Coessens et al. [130], is the conversion of the azide group into the phosphoranimine end groups and subsequent hydrolysis to the amino end groups (Scheme 18). This procedure was used to synthesize diamine telechelic oligomers of PSs. Styrene was initiated by a difunctional initiator (ajOf -dibromo-p-xylene) yielding a/o-dibromo PSs. The bromine atoms are then converted into amino end-groups [123]. 

N0S2 crowns (Bulkowski et al., 1977). Others used a,a -dibromo-p-xylene in acetonitrile to give the same product in a yield of 34% (Shinkai et al., 1984). These latter workers also prepared photoresponsive bis[12]NOS2 ligands as shown. 

Nearly quantitative yields were obtained when peraza-paracyclophanes were prepared from a,a -dibromo-p-xylene and pertosylated polyamines in DMF in the presence of potassium carbonate (Iwata and Kuzuhara, 1986). 

Nuclear halogen often makes oxidation more difficult it protects alkyl groups against attack. Thus oxidation of 4-chloro-m-xylene and 2,5-dibromo-p-xylene by chromic acid proceeds only as far as 4-chloro-w-toluic and 2,5-dibromo-p-toluic acid, respectively 401 the latter requires alkaline permanganate for conversion into the dibromoterephthalic acid.402... 

A special class of functional initiators are the difunctional initiators which lead to the formation of telechelic polymers, a, a -Dibromo-p-xylene, for example, was used as an initiator for the ATRP of styrene and butyl acrylate in bulk (77). The polymerizations occurred in a controlled fashion. The a, co-dibromopolystyrene was... 

Ben bromide, tetra-N-butyiammonium bromide, a a -dibromo-p-xylene, 2-bromoanisole, terephthalonitrile, and boron tribromide were> purchased from Aldrich Chemical Company and used without further purification. Tetrahydrofuran (THF) was freshly distilled under nitrogen from potassium-benzophenone. 

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