1,2-Dihalobenzenes

Dihalobenzenes, such as p-bromoiodo-benzene, can be reduced in a stepwise manner [64]. An investigation of the electrochemistry of the entire family of chlorinated benzenes has been reported [65], and the dechlorinations of 1,2,3,5-tetrachlorobenzene [57] and of 2,4-dichlorophenoxyacetic acid (a herbicide) have been accomplished [66]. Several papers dealing with the reduction of chlorinated biphenyls have appeared [67-69]. 

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... 

Even more attractive is the prospect of preparing phenazines in a single synthetic operation by sequential inter-Zintramolecular AT-arylation. This would require the transformation of either two o-haloanilines 122 and 135 or of an o-phenylenediamine 82 with an o-dihalobenzene 136. Until recently, not even a single example could be found for this one-step synthesis of heterocycles via double Pd-catalyzed AT-arylation. Numerous experiments conducted in a variety of different combinations of various Pd catalysts, bases, and phosphine ligands in our laboratory failed to realize this novel synthetic principle. 

Boiling points of Isomeric dihalobenzenes are very nearly the same. However, the para-isomers are high melting as compared to their ortho-and meta-isomers. It is due to symmetry of para-isomers that fits in crystal lattice better as compared to ortho- and meta-isomers. 

All astatohalobenzenes were identified by GLC their thermodynamic properties were estimated by extrapolative gas chromatographic data related to the corresponding dihalobenzene isomers 17, 99, 137, 138) (see Table V). 

Since the aryldizinc species are formed in good yields (50-70%), the values reported in Table 10 are most likely affected by losses during the purification steps and depend on the vinyl dihalide, 1,4-dihalobenzene or 2,5-dihalothiophene introduced. In the case of PPVs, the original configuration of the vinyl substrates seems to be retained within the polymer structure. 

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... 

The direct synthesis of PPP by condensation polymerization represents one of the oldest preparations of a conjugated polymer. This is exemplified by the Ull-mann coupling of dihalobenzenes using copper powder [56] and by Wurtz [57, 58] chemistry where dihalobenzenes are reacted with sodium metal to give oligomeric PPPs [59]. 

An important advancement for the synthesis of well-defined PPPs was the site-controlled nickel-catalyzed coupling of Grignard reagents, derived from dihalobenzenes, to yield linear PPP as shown in Scheme 19 [63,64]. The PPPs obtained by Yamamoto chemistry range from yellow to brown infusible solids of low molecular weights typically with degrees of polymerization ranging from 10-12. 

Analytical Properties Separation of dimethylbenzene isomers, dihalobenzene isomers (Cl, Br), haloketone benzene isomers, dimethoxybenzene isomers Reference 14... 

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]. 

In presence of LDA 2,3-dibromothiophenes rearrange to 3,5-dibromothiophenes230. Aluminum halides are well known catalysts for disproportionation of aryl halides which permit the transformation of the kinetic products ortho- and / ara-dihalobenzenes to equilibrium mixtures which contain significant amounts of meta-dihalobenzenes231,232. 

In presence of very strong bases and certain phase transfer agents, raeta-dihalobenzenes are chlorinated to yield mainly 1,2,3-trihalobenzene and small amounts of 1,2,4- and 1,3,5-trihalo isomers (equation 94)697. Under photochemical conditions hexabromobenzene is a brominating agent for the benzylic bromination of substituted toluenes698. 

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. 

Carbonylations of 6>-dihalobenzenes and o-halobenzoic acids catalysed by Co2(CO)8 in aqueous NaOH and under photostimulation were also described213 to give moderate yields of the diacids. 

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. 

All the results indicate that 198 reacts with o-dihalobenzenes by the S l mechanism as a bidentate nucleophile through a stepwise process in which the monosubstitution product 351 is an intermediate, and that the oxygen functionality is able to react intramolecu-larly quite efficiently with the aromatic o radical to give the cyclized product. The proposed mechanistic steps are summarized in equations 206 and 207 for o-diiodobenzene. 

In contrast, benzenethiolate anions react at the sulfur atom (Scheme 28) [146-149] from 1,2-dihalobenzenes and the 1,2-dithiolate dianion, dithiin is obtained [148]. Analogous substitutions are observed with diarylphosphites and other phosphorus nucleophiles [150-152], as well as with arylarsenides... 

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... 

---