Aromatic dibromides

Polycarbosilanes with the structure [SiR2—C=C—Z—C=C] (R2SiI 2,3,4,5-tetraphenyl-l-sila-2,4-cyclopentadiene Z 1,4-benzene, 4,4 -biphenyl, 9,10-anthracene, 2,7-fluorene and 2,6-pyridine, 6,6 -bipyridine, 2,5-thiophene, 2,6-p-dimethylaminonitrobenzene, 2,6-p-nitroaniline, 2,6-p-nitrophenol and 2,7-fluoren-9-one) were prepared by the reaction of l,l-diethynyl-2,3,4,5-tetraphenyl-l-sila-2,4-cyclopentadiene with the appropriate (hetero)aromatic dibromide or diiodide in the presence of [(PPh3)2PdCl2] and Cul182 (equation 69). 

Polycondensation of aromatic primary diamines and aromatic dibromides gave high molecular weight polyamine (Mw > 15 000) irrespective of the m-and p-substituted positions of both monomers (Scheme 12). In this polycondensation, BINAP was effective as a ligand of the Pd(0) catalyst use of P(f-butyl)3, which was effective for the above polymerization using secondary diamines, afforded polymer in a low yield [65,66]. When BINAP was used... 

Various aromatic dibromides have been cross-coupled with difunctional tributyltin aromatic compounds in the presence of palladium-based catalysts to yield poly(arylene)s [scheme (12)] [149-151]. The mechanistic pathway of this coupling, known as Stille coupling [152], follows an oxidative addition-trans-metallation-reductive elimination sequence. 

Aromatic dibromides (1,2-dibromobenzene, 1,4-dibromobenzene, 1,2-dibromo-6-methoxybenzene, 2,7-dibromo-3,6-dimethoxynaphthalene, 5,10-dibromoanthracene) can be converted to lithioaryl bromides and subsequently to lithium bromoarenetellur-olates (Table 1 p. 155). An example of such a reaction sequence is given for 1,4-dibromobenzene3-4. With four moles of tert.-butyl lithium per mole of dibromoarene, the dilithio compounds that were formed were converted to the ditellurolates4 (Table 1 p. 155). 

Fig. 1-30. Polyester synthesis from the carbonylative polycondensation of aromatic dibromides and diols (adapted from [237]).

Some industrially important polymeric materials can be prepared using the basic strategies discussed earlier. A representative example can be found in the synthesis of polyesters using the carbonylative polycondensation of aromatic dibromides and diols (Fig. 1-30) [237]. The underlying principle is no different from the fundamentals of carbonylative coupling presented earlier in Section 1.5.1.3. Replacement of the diols with hydrazides 86 similarly yields poly(acylhydrazide)s 87 [238]. The catalytic... 

The two disadvantages of the Grignard method, i.e, synthesis of the aromatic dibromide and the difficulty of initiating the reaction of an aryl chloride with magnesium, have led to a search for other routes to... 

Syntheses of 5j8-cholest-l-ene and 5/3-cholest-2-ene have been reported from 2/3-bromo-5/S-cholestan-3-one, which was prepared from the 2/3,4/3-dibromo-ketone by reaction with chromous acetate. A series of 5/3-substituted-5a-hydroxy-5a-cholest-2-enes was prepared from 5a,6a-epoxy-cholestanes. Acid-catalysed dehydrobromination of the 7,11,22,23-tetra-bromide (254) gave the aromatic dibromide (255). ... 

An aromatic dibromide C HGBr2 reacted with aqueous sodium hydroxide. The product of this reaction had lost only one bromo group to give the product CTHrBrO. When the dibromide was converted to a Grignard reagent and then hydrolyzed, the product was toluene. Determine the structure of the dibromide. 

The fact that the aromatic dibromide reacted indicates that an alkyl halide group must be attached to the aromatic (or benzene) ring. Since only 1 bromine was lost, the other bromine must be directly bound to the aromatic ring, where NaOH is ineffective in hydrolysis. 

Recently, the Heck reaction was extended to carbonvlation of aromatic dibromides with aromatic diamines in the presence of carbon monoxide. High molecular weight aromatic polyamides form with the help of palladium catalysis ... 

The polymerization reaction takes place in a homogeneous dimethylacetamide solution, with catalytic amounts of PdCl2(PPh3)2 and an HBr scavenger. The carbonylation polycondensation proceeds rapidly at 115 C and is almost complete in 1.5 hours. This reaction was also used to prepare many aromatic-aliphatic polyamides from corresponding aliphatic diamines with aromatic dibromides. 

Palladium is a relatively high-priced catalyst and it would be preferable if a lower-priced nickel catalyst could be used instead. All attempts, however, to form polymers by nickel-catalyzed carbonylation poly condensations of aromatic diamines with aromatic dibromides failed to yield high molecular weight materials. 

The amount of reaction solvent should be kept to a minimum to maximize reaction rate and concentration of reactive end groups as a means of obtaining PAEs of sufficiently high molecular weight. These reactions are stirred for 48-72 h at temperatures ranging from 20°C to 50°C if diiodides are utilized. For dibromides, reaction around 100°C is necessary, due to their significantly decreased reactivity. There are now more active catalysts available [24]. It will be interesting to see if these catalysts work well in the formation of PPEs and PAEs fi om aromatic dibromides or even dichlorides. 

FIGURE 5.15 Synthetic scheme for the synthesis of polyfluorenes. Note that the dibromofluorene can be ftdly or partially replaced with other aromatic dibromides, like the ones drawn here, to synthesize copolymers containing fluorenes. 

Heck Reaction. The Heck reaction [870] allows the double bond to be directly introduced into the polymer chain. A variety of aromatic dibromides have been polymerized with ethylene to produce both PPV and soluble derivatives [871] (Fig. 66). Other dihalogen derivatives can also be used in this reaction [872]. Besides many possible side reactions, another disadvantage of this method is that one of the reactants is gaseous and must be added in precise amounts. Reaction conditions are mild, however, and are tolerant of many functional groups [873-877]. 

It is possible to introduce the double bond directly into the polymer chain. Griener and coworkers polymerized ethylene with a variety of aromatic dibromides via a Heck reaction to produce both PPV and soluble deriva-... 

The most widely employed synthetic route to aramids is based on the polycondensation of dicarboxylic acids with diamines in the presence of condensing agents. Good reviews on the synthesis of aramids have recently appeared (1-3). Recently, promising alternative synthetic routes to aramids have been reported and are described herein. These include the polycondensation of N-silylated diamines with diacid chlorides, the addition-elimination reaction of dicarboxylic acids with diisocyanates, and the palladium-catalyzed carbonylation polymerization of aromatic dibromides, aromatic diamines and carbon monoxide. 

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