Disubstituted 1,4-dibromobenzene

This method was also used for the polymerization of disubstituted 1,4-dibromobenzene (equation 70) (272). This produced a polymer with a regioregular structure connected in a 1,4-manner. The maximum DP obtained was 13. Two types of end groups were identified (a) unreacted Br and (b) reduced Br. The proton for reduction may be from adventious water sources or from Ni-catalyzed... 

In the reaction of o-dibromobenzene, disubstitution is faster than monosubstitution, and the disubstituted product is obtained as a major product. Similarly, 1,2,4,5-tetrabromobenzene reacts with styrene to give the tetrasub-stituted product[29]. The bridged, annulated [2,2]paracyclophanedieiie 22 was... 

Terephthalic acid has been obtained from a great many /)-disubstituted derivatives of benzene or cyclohexane by oxidation with permanganate, chromic acid, or nitric acid. The following routes appear to have preparative value from />-toluic acid, />-methylacetophenone,2 or dihydro-/)-tolualdehyde by oxidation with permanganate from f>-cymene by oxidation with sodium dichromate and sulfuric acid from />-dibromobenzene or from /i-chloro- or -bromobenzoic acid by heating at 250° with potassium and cuprous cyanides and from />-dibromo-benzene, butyllithium, and carbon dioxide. ... 

In 1988, Linstrumelle and Huynh used an all-palladium route to construct PAM 4 [21]. Reaction of 1,2-dibromobenzene with 2-methyl-3-butyn-2-ol in triethylamine at 60 °C afforded the monosubstituted product in 63 % yield along with 3% of the disubstituted material (Scheme 6). Alcohol 15 was then treated with aqueous sodium hydroxide and tetrakis(triphenylphosphine)palladium-copper(I) iodide catalysts under phase-transfer conditions, generating the terminal phenylacetylene in situ, which cyclotrimerized in 36% yield. Although there was no mention of the formation of higher cyclooligomers, it is likely that this reaction did produce these larger species, as is typically seen in Stephens-Castro coupling reactions [22]. 

Yoshida et al. [63] applied microreaction technology for the sequential synthesis and the disubstitution of o-bromophenyUithiuni (Scheme 28). Lithiation of o-dibromobenzene in batch leads to the formation of the highly reactive benzyne and to the further creation of various side products thus this reaction needs to be conducted at 100°C and below to avoid this problem. This reaction was optimized under microreactor conditions for the production of o-bromophenyUithium and for its further reaction with methanol to determine the yield of bromobenzene. Microreactors were set up of stainless steel tubes with an internal diameter of 250-1,000 pm. 

The photostimulated reactions of o- and j9-dibromobenzene with the anion of pinacolone lead to the disubstitution products in 62% [78], and 64% yields [79], respectively. 

When iodide in /7-iodotoluene and /7-iodoanisole and bromide in /7-dibromobenzene serve as the leaving group, there is a fast HME reaction giving only dehalogenation products. It is noteworthy that in the case of /7-dibromobenzene, the HME reaction leads to PhBr which does not undergo an HME process, but reacts under irradiation by the S l mechanism as shown by the formation of Ph4Sn (40%) together with the disubstitution product (22%). 

In the photostimulated reaction of substrates with two leaving groups in 0-position, such as 0-dibromobenzene and acetone enolate ion, a disubstitution product (330) is formed. In the basic reaction conditions it leads, by an aldol condensation, to a final mixture of two acetylmethyl indenes 331a and 331b in 64% yield (equation 196)342. 

C NMR spectroscopy is used to determine the substitution patterns in disubstituted benzenes, because each line in a spectrum corresponds to a different kind of carbon atom. For example. 0-, m-, and p-dibromobenzene each exhibit a different number of lines in its C NMR. spectrum,... 

The first high yielding one-pot tandem Hartwig-Buchwald-Heck cyclization was reported and applied to the synthesis of 2,3-disubstituted indoles [201]. Commercially available enone 123 was coupled with 1,2-dibromobenzene to provide A -arylcncaminonc 124 which subsequently cyclized to indole derivative 125 [201]. This reaction was widely applicable to a variety of electron rich, electron poor and neutral aromatic bromides and chlorides as well as heterocyclic halides. Excellent yields were obtained regardless of the substitution pattern on the aromatic halide. 

If a disubstituted benzene ring has the same two substituents, for example, dibromobenzene, the number of unique carbon atoms in the ring can be 2, 3, or 4. The NMR spectrum depends on where the bromine atoms are located on the ring, (a) Draw the three possible dibromobenzene molecules,... 

If a disubstituted benzene ring has the same two substituents, for example, dibromobenzene, the number of unique carbon atoms in the ring can be 2, 3, or 4. The NMR spectrum depends on where the bromine atoms are located on the ring, (a) Draw the three possible dibromobenzene molecules, (b) Using the dichlorobenzene example in Section 3.6.3, draw a mirror plane in each of your dibromobenzene molecules. A mirror plane is a plane of symmetry that gives you identical halves of the molecule on each side of the plane, as was done for dichlorobenzene. Identify how many unique carbon atoms there are in each dibromobenzene molecule and predict how many peaks you will see in the NMR spectrum of each molecule. 

A more general method of assigning structures to the disubstituted benzenes was devised by Wilhelm Komer (1839-1925), who in 1874 observed that each of the three dibromobenzenes yielded a different number of tribromobenzenes on further substitution. The dibromobenzene that gave two trisubstituted products must be the 1,2-isomer, that which gave three products must be the 1,3-isomer, while the 1,4-isomer could only yield one trisubstituted product (Figure 10.9). 

Aromatic substrates with two leaving groups such as o-dibromobenzene react with CH COBu anion to give only disubstitution product in the sense of Scheme 10.6 where [29]. On... 

A Merck research group discovered that the cross-coupling reaction between aryl halides and vinylogous amides could take place under the catalysis of Pd/DavePhos. When 1,2-dibromobenzene was used, a cascade intermolecular C-N bond coupling and intramolecular Heck reaction occurred to provide 2,3-disubstituted indoles (Scheme 2) [25]. 

Fig. 9.2 Synthesis of o-disubstituted benzenes from o-dibromobenzene by space integration of reactions...

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