1- Bromo-2-iodobenzene

A flask was charged with 4-bromo-iodobenzene (0.079 mol), 4-methoxy-2-methyl-phenyl boronic acid (0.087 mol), palladium acetate (0.004 mol), and triphenyl phosphine (0.008 mol) and then treated with 200 ml acetone and 250 ml 2M NaHCO i. The mixture was refluxed at 65°C for 18 hours and was then treated with water and diethyl ether and the organic layer isolated. This layer was washed with 40 ml saturated sodium chloride solution and water, dried over MgSC>4, filtered, and concentrated. The residue was purified by column chromatography using silica gel with CH2C12/ hexane, 1 1, and then recrystallized in / , 7 3, respectively, and 16.4 g of product isolated. 

ROM has been used to prepare phosphine-containing polymer supports (Scheme 20). Norbornyl-substituted monomer 22 was prepared in two steps from d-bromo-iodobenzene. This was then polymerized with diene 23. It was initially envisioned that it would be necessary to convert the phosphine to the borane adduct in order not to poison the metathesis catalyst. Although protection was needed when using the Grubb s type 1 complex as a catalyst, when employing the more active second-generation complex 24, the free phosphine monomer could be used. This has been attributed to the lower affinity of the active form of the catalyst toward coordination of phosphines due to the presence of the electron-rich heterocyclic carbene ligand. 

Some cinnamic acids have been efficiently prepared by coupling halogeno-benzenes with acrylic acid using catalytic quantities of Pd(OAc)2. 2-Bromo-iodobenzene reacts specifically to give 2-bromocinnamic acid addition of a triarylphosphine is necessary to effect displacement of bromide. 

Metalation. Benzene reacts with alkaH metal derivatives such as methyl or ethyUithium ia hydrocarbon solvents to produce phenyUithium [591 -51 -5], CgH Li, and methane or ethane. Chloro-, bromo-, or iodobenzene will react with magnesium metal ia ethereal solvents to produce phenyHnagnesium chloride [100-59-4], C H MgCl, bromide, oriodide (Grignard reagents) (32). 

The coupling of bromo- or iodobenzene to styrene yields regioselectively a mixture of E- and Z-stilbenes 12 and 13. An electron-withdrawing substituent at the olefinic double bond often improves the regioselectivity, while an electron-donor-substituted alkene gives rise to the formation of regioisomers. 

A variation of the halide affinity approach was used by Riveros et al. in the investigation of the enthalpy of formation of o-benzyne. Reaction of bromo- or iodobenzene with base in an ICR leads predominantly to the formation the expected M-1 anion, but also leads to the formation of solvated halide ions (Eq. 5.15). By using substrates with known halide affinities, it was possible to assign limits to the enthalpy of formation of the benzyne product. Ultimately, the experiment is comparable to that outlined in Eq. 5.14, although the acidity and halide affinity measurements are made in a single step. 

Hydrodehalogenations of chloro-, bromo-, and iodobenzene were carried out individually as well as in competitive reactions. When the reactions were carried out separately, the reduction of chlorobenzene closely paralleled that of bromobenzene, whereas the reduction of iodobenzene was slower. When they were allowed to react competitively, the reduction was highly selective, and the reaction was delayed, but iodobenzene reacted first followed by bromobenzene and then chlorobenzene. 

Conditions a. Trimethylsilylacetylene, Pd(PPh3)2Cl2, Cul, PPh3, Et3N, THF. b. 1-Bromo-4-iodobenzene, Pd(dba)2, Cul, PPh3, K2C03, MeOH, THF. 

Changing to a trisubstituted aromatic precursor such as 1,3,5-tribromobenzene (153) yields the trisallene 155 when l-bromo-2-butyne (154) is used as the coupling partner. When leaving groups of different reactivity are present in the aromatic substrate, such as in l-bromo-4-iodobenzene, different propargyl halides can be connected to the aromatic core, resulting in the formation of arylallenes with different allene substituents. 

SrnI processes have been shown to be relevant to the synthesis of simple thianthrenes which is done by irradiating the disodium salt of 4-methylbenzene-1,2-dithiol in the presence of 1,2-bromochlorobenzene (55%) or 1,2-di-iodobenzene (64%). More complex, fused thianthrenes result from l-bromo-2-iodonaphthalene (24%, 2 isomeric products) and 2,3-dichloroquinoxaline (100%). These clearly hold considerable promise for the controlled construction of unsymmetrical thianthrenes (87JOC1089). 

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

It is noticeable from the data in Table 1, that the number of examples of reactions involving the enolates of aryl alkyl ketones is limited. Bromo- and iodo-benzene were reported not to react with the enolate of acetophenone112 but the reaction with iodobenzene under more intense and longer irradiation was reported to give a 67% yield of phenacylbenzene.45 Sufficient examples of the ot-arylation of the enolates of aryl (and heteroaryl) alkyl ketones have been cited to indicate that these reactions are worth further investigation. 

Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p-dibromobenzene in yields of 70-80% (isolated after oxidation, as the phosphine oxides).143 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the 5rn1 reaction, had the mechanism been recognized at the time. Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett,48 but no extension to the scope of the reaction was made. Rossi and coworkers have developed a procedure for one-pot preparation of triarylphosphines starting from elemental phosphorus (Scheme 6).146 As an example of the synthesis of a symmetrical tri-arylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i-iii only). Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (AiOC) were used. 

The role of molecular dimensions is well demonstrated by complex formation with halogen-ated benzenes. 1 1 complexes may be prepared from chloro-, bromo-, and iodobenzenes but from chlorobenzene only witto-CD, from bromobenzene witb- andp-CDs, and from iodobenzene with P- andy-CDs. 

Dienes (allenes) are also used for heteroannulation with 68 and 69. The eight-membered nitrogen heterocycle 78 is constructed by the reaction of 1,2-undecadiene (77) with o-(3-aminopropyl)iodobenzene (76) [34]. The lactones are prepared by trapping the 7i-allyl intermediates with carboxylic acids as an oxygen nucleophile. The unsaturted lactone 81 is prepared by the reaction of /1-bromo-v,/ -unsaturated carboxylic acid 79 with the allene 80 [35]. In the carboannulation of 82 with 1,4-cyclohexadiene (83), the 1,3-diene 85 is generated by / -elimination of 84, and the addition of H-PdX forms the 7i-allylpalladium 86, which attacks the malonate to give 87 [36],... 

Common Name 4-Bromoiodobenzene Synonym 4-bromo-l-iodobenzene Chemical Name ... 

At about the same time, Seddon reported the Heck reactions of bromo-and iodoarenes in a series of /V-hexylpyridinium [C6py] + and N,N dialkylimidazolium based liquids with [PF6] and [BF4] anions.20 The effects of changing the ionic liquid and reaction conditions upon the Heck reaction of iodobenzene with ethyl acrylate were reported using 2 mol% Pd(OAc)2. 

As shown in other sections of this chapter, overall attention has shifted from diazonium salts as aryl radical sources to bromo- or iodobenzenes. One of the few recent attempts to improve the classical Pschorr cyclization using diazonium ions as starting materials led to the discovery of new catalysts [119]. Results from a first samarium-mediated Pschorr type show the variety of products that can be expected from intramolecular biaryl syntheses under reductive conditions (Scheme 22). Depending on the substitution pattern of the target aromatic core and the reaction conditions, either the spirocycle 60, the biphenyl 61, or the dearomatized biphenyl 62 were formed as major product from 63 [120]. 

The corresponding chloro and bromo compounds as well as the 3-iodo and 4-iodophenyl compounds are photostable. A mechanism is proposed in which the primary excited S, state crosses into an ncr state in the iodobenzene manifold. This is followed by homolysis of the C—I bond. The aryl radical may undergo ring closure or abstract a hydrogen atom from the solvent. The primary ring-closed product is photo-oxidized under the reaction conditions. 

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