Dihalobenzenes, reactions

Since the reactivities of the species R1B.2CO, RiR2C(OH)2 and RiR2C(0H)0 will normally be very different, the variation of reaction velocities with pH can sometimes give information about the equilibria involved. Thus Bunnett ef ai. (1961) showed that the alkaline cleavage of 2,6-dihalobenzaIdehydes to m-dihalobenzenes and formate ion took place with an apparent first-order velocity constant given by k — klKi[OK ] l (1 -f- 4[0H ]), where (defined above) was measured independently by... 

Benzene derivatives with two nucleofuges have been used in the preparation of polymeric materials with varying degrees of success. Poly(l,4-phenylene sulfide) has been prepared by condensation of p-di-chlorobenzene with sodium sulfide,99,100 and in a related process, diazonium ions have been shown to initiate the polymerization of p-halobenzenethiolate ions.101 In a preliminary study, poorly characterized polymers were obtained from reaction of equimolar amounts of p-dihalobenzenes and the enolate ions from ketones in the presence of excess base. When an excess of the ketone enolates was used, the normal p-disubstituted derivatives were formed.102... 

Two further quite different cyclization processes each have an initial intermolecular Srn 1 reaction followed by a second in which the ring closure is made. The reactions of o-dihalobenzenes (24) and (25) with the dithiolate (26) to give (27) have already been mentioned (see equation 28), but the yield of analogous product formed between (26) and 1 -brotno-2-iodonaphthalene is only 24% and use of 1,2-eth-anedithiolate with o-diiodobenzene gives only 13% of benzo-l,4-dithiane.98 A rather convenient one-step synthesis of [m.m]-mera-cyclophanediones (m = 3-8) in most acceptable yields is obtained on tandem SrnI reactions of the ketones (45 equation 50).165... 

Disubstitution products are obtained when dihalobenzenes (Cl, Br, I) react with aliphatic ketone enolate anions. Conversely, the reactions of o-iodohalobenzenes (X = I, Br, Cl) with the enolate anions of aromatic ketones, such as acetophenone, propiophenone and 2-naphthyl methyl ketone in DMSO yield mainly monosubstitution with the retention of one halogen (Scheme 10.7). The extent of dehalogenation is explained in terms of the energetics of the intramolecular ET from the ArCO-7t-system to the C—X bond in the monosubstituted radical anions proposed as intermediates [19]. 

The fragmentation of the radical anion (RNu)- along the chain propagation cycle of the process has been taken as evidence of the proposed mechanism. For example, in the case of dihalobenzenes YArX, the radical anion formed upon the first substitution YArNu- may transfer the extra electron to the C—Y bond (intramolecular ET) or to YArX (inter-molecular ET). The ratio between monosubstituted and disubstituted products formed will depend on the relative rate constants for both types of competing ET reactions. 

Many dihalobenzenes afford disubstitution products when they react with PhS ions and little, if any, monosubstitution products are formed. For instance, ra-chloroiodo, />-chloroiodo, m- and / -dibromo benzenes gave the disubstitution products2823, whereas m-fluoroiodobenzene gave 96% of the monosubstitution product270. However, with other sulphanions, the ratio of mono- to disubstitution varies, depending on the nature of the nucleophile and the haloarenes279. Thus, the sulphanions 221-225 were studied in the photostimulated reaction with chloroiodobenzenes. 

Aromatic rings can also trap aryl radicals in the propagation cycle of the S l mechanism to give ring closure product. The reaction of o-dihalobenzenes 341 with 2-naphthalenethiolate ion (342) in liquid ammonia under photostimulation gives the ring closure product 343 as well as the substitution product 344 (equation 201 )344. 

In the photostimulated reaction of 2-naphthoxide ion (198) with an o-dihalobenzene, an aromatic a radical may be formed very close to the oxygen functionality along the chain propagation cycle of the S l mechanism. This spatial proximity and the fact that the intramolecular coupling between the two moieties will form a relatively stable radical anion will favour the reaction between both reactive centres. Thus in the photostimulated reaction of o-dihalobenzenes with 198 in liquid ammonia, the formation of the monosubstitution 351 and of the cyclization product 352 were reported in yields that depend on the substrate and on the reaction conditions (equation 205)346. 

Such dilithium reagents should make available many of the triptycenes suggested in Fig. 1, but the recent withdrawal of (the very expensive) 1,2-diiodotetrafluorobenzene from commercial catalogs is regrettable. Unfortunately, 1,2-dilithiobenzene cannot be prepared from the reaction of 1,2-dihalobenzenes with either lithium or alkyl-lithium reagents due to the exceedingly rapid formation of benzyne ... 

Another way to obtain suitably substituted diphenylamines is the condensation of o-aminophenols with activated 1,2-dihalobenzenes.24 The reaction was performed in nonalkaline medium. It is interesting... 

To aiq>ly (478) as a dienophile in Diels-Alder reactions it is, therefore, prepared in situ in the presence of a suitable diene, while maintaining its stationary concentration as low as possible. The most popular methods of preparation (Scheme 112) include dehydrohalogenations of halobenzenes, e.g. (479), me-talladon of o-dihalobenzenes, e.g. (4M)," as well as thermal decompositions of benzenediazonium-2-... 

Silphenylenes, aromatic-containing silicon monomers, can be produced in 50 88% yields by the reaction of halo- and dihalobenzenes with chloro- and alkoxysilanes under Barbier conditions. Iodine, bromine, and cuprous chloride are used as initiators with THE, TMEDA, toluene, or xylene as solvents. Shavings or powdered magnesium can be used [Eqs. (66) and (67) 134,135]. 

Reactions of Miscellaneous Haloketones and Acid Chlorides. - The reactivity of anions such as (91) with 1,2-dihalobenzene (a. 1,2-diiodo, b.l-chloro-2-iodo and c. l-bromo-2-iodo) derivatives has been studied. The irradiations are brought about in DMSO as solvent. The reaction affords (92) as the principal product in variable yields (71 % from b, 86% from c and 50% from a), depending on which of the arene derivatives is used. The other product (93,17%) is formed only from reaction with o-diiodobenzene. The irradiation of the acid fluoride CF3CF2C(0)F has been described as an efficient route for the production of the radical, CF3CF2. This radical can recombine with CFO to afford the starting... 

We report the first syntheses of the selenium and tellurium analogs of the thermoplastic poly-p-phenylene sulfide (PPS), i.e., PPSe and PPTe, by reaction of p-dihalobenzenes with new alkali chalcogenide reagents under conditions significantly milder than those reported for PPS. The chemical, thermal, structural, and electrical properties of PPSe and PPTe are compared to those of PPS. While PPSe exhibits good thermal stability,... 

Edmonds and Hill of Phillips Petroleum Co. reported that poly(phenylene sulfides) may be prepared by the reaction of p-dihalobenzene with sodium sulfide in N-methylpyrrolidone or dimethylformamide [118,119]. Copper metal or cuprous chloride was also shown by Edmonds and Hill to give improved yields [118]. Table XIII lists examples of various poly(phenylene sulfides) and the conditions used for their preparation. Philllips Petroleum Co. is now producing poly(phenylene sulfides) under the name R)don (trademark of Phillips Petroleum Co.) [98]. 

Although the vast majority of stepwise polar additions to ortho-benzyne involve nucleophilic attack on the aryne, electrophilic attack is also possible provided that the aryne is generated by a method that does not involve strongly basic conditions. Few such additions are synthetically useful, with the exception of the formation of 1,2-dihalobenzenes by reactions of ortho-benzynes with halogens, although alternative mechanisms initiated by nucleophilic attack of halide may be envisaged. Radical reactions of ortHo-benzyne, on the other hand, are extremely rare. 

A standard condition has been optimized for this reaction, in which the aryl amine is diazotized in 10 times its amount of acetic acid, followed by the addition of one equivalent of cuprous halide in hydrohalic acid. Under these conditions, the acetate salt of aryl amine is relatively soluble, and less froth and tarry material are formed during diazo transformation. In addition, chlorination, bromination, and iodonation of p-haloaniline to dihalobenzenes under such standard conditions give almost comparable average yields. Other modifications of this reaction include the formation of phenyl selenocyanate by the reaction with potassium selenocyanate, and aryl nitrile by the reaction with nickel cyanide. Moreover, this reaction has been extended to the preparation of phenyl thiocyanate, phenyl isothiocyanate and aromatic sulfonyl chloride. ... 

An alternative approach to the synthesis of triarylamine polymers is the reaction of a dihalobenzene with a primary arylamine. In this case, the catalyst must be able to form both diarylamines from anilines and triarylamines from the diarylamine product in nearly quantitative yields. The discussion of catalysts above shows that complexes of P(t-Bu)3 effectively catalyze both these transformations. Kanbara and co-workers have used this... 

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