Benzoxazole arylation

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

The direct arylation of heteroaryls is particularly attractive due to the fact that these moieties are present in many biologically active compounds [58], Recently, etinkaya and co-workers reported the direct arylation of benzoxazoles and ben-zothiazoles with aryl bromides catalysed by a bis-NHC-palladium complex [59], Also, Sames and co-workers have described the C-H arylation of different SEM-protected heteroarenes, catalysed by NHC-Pd complex 28 (Scheme 7.12, pathway a) [60],... 

In general, symmetrical oxo-squaraines having the same end-groups are synthesized by reacting squaric acid with two equivalents of quatemized indolenine, 2-methyl-substituted benzothiazole, benzoselenazole, pyridine, quinoline [39, 45, 46] (Fig. 4) in a mixture of 1-butanol - toluene or 1-butanol - benzene with azeotropic removal of water in presence [39, 45] or absence [47] of quinoline as a catalyst. Other reported solvent systems include 1-butanol - pyridine [48], 1-propanol - chlorobenzene, or a mixture of acetic acid with pyridine and acetic anhydride [49]. Low CH-acidic, heterocyclic compounds such as quatemized aryl-azoles and benzoxazole do not react, and the corresponding oxo-squaraines cannot be obtained using this method [23, 50]. 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

Table 2 contains the characteristics of the amic ester-aryl ether copolymers including coblock type, composition, and intrinsic viscosity. Three series of copolymers were prepared in which the aryl ether phenylquinoxaline [44], aryl ether benzoxazole [47], or aryl ether ether ketone oligomers [57-59] were co-re-acted with various compositions of ODA and PMDA diethyl ester diacyl chloride samples (2a-k). The aryl ether compositions varied from approximately 20 to 50 wt% (denoted 2a-d) so as to vary the structure of the microphase-separated morphology of the copolymer. The composition of aryl ether coblock in the copolymers, as determined by NMR, was similar to that calculated from the charge of the aryl ether coblock (Table 2). The viscosity measurements, also shown in Table 2, were high and comparable to that of a high molecular weight poly(amic ethyl ester) homopolymer. In some cases, a chloroform solvent rinse was required to remove aryl ether homopolymer contamination. It should also be pointed out that both the powder and solution forms of the poly(amic ethyl ester) copolymers are stable and do not undergo transamidization reactions or viscosity loss with time, unlike their poly(amic acid) analogs.

Whereas UL 94 delivers only a classification based on a pass-and-fail system, LOI can be used to rank and compare the flammability behavior of different materials. In Figure 15.2 the increasing LOI values are presented for different polymers as an example POM = poly(oxymethylene), PEO = poly(ethyl oxide), PMMA = poly(methyl methacrylate), PE = polyethylene), PP, ABS, PS, PET = polyethylene terephthalate), PVA = poly(vinyl alcohol), PBT, PA = poly(amide), PC, PPO = poly(phenylene oxide), PSU, PEEK = poly(ether ether ketone), PAEK = poly(aryl ether ketone), PES, PBI = poly(benzimidazole), PEI = poly(ether imide), PVC = poly(vinyl chloride), PBO = poly(aryl ether benzoxazole), PTFE. The higher the LOI, the better is the intrinsic flame retardancy. Apart from rigid PVC, nearly all commodity and technical polymers are flammable. Only a few high-performance polymers are self-extinguishing. Table 15.1 shows an example of how the LOI is used in the development of flame-retarded materials. The flame retardant red phosphorus (Pred) increases... 

Photocycloaddition reactions of alkyl and aryl 2-thioxo-3//-benzoxazole-3-carboxylates 142 to alkenes afforded stable isolable spirocyclic aminothietanes 143 <02HCA2383> similar reactions with both electron-poor and electron-rich alkenes were also performed on 2-methyloxazolo[5,4-h]pyridine <02EJO4211>. 

It is interesting to note that 5-nitrobenzoxazole has two absorption maxima 224 (4.39) and 270 nm (3.84), while 6-nitrobenzoxazole shows only one maximum in area Amix 282 nm (4.01) [1200], A similar pattern is observed for 5-nitro- and 6-nitro-2-alkyl(aryl)benzoxazole too. The available experimental data are not enough to explain spectral differences of this kind. Apparently, it would be well to carry out quantum-chemical calculations or to involve other physical-chemical methods. An investigation of chromophoric 2-(4 -diphenyl)-5-nitrobenzoxazole with the help of UV (A 305 nm), fluorescence and laser spectroscopy has been reported by Chinese chemists [1201], Chromophores 2-(2 -hydroxy-4 -aminophenyl)-6-nitrobenzoxazole [ 1202], 2- [4-[4-(V,V-dihydroxyethyl-amino)-phenylazol]-phenyl -6-nitrobenzoxazole, 2-[4-(/V-mcthyl,V-hydroxyethyl-amino)... 

N-Arylation of azonane with 2-chloro-5-nitrobenzoic acid was reported <1998JME5219>. Arylation of anion formed from 1,6-diazonane (PhLi, diethyl ether) with 4-chloropyridine resulted in mixture of mono- (38%) and disubstituted (13%) products <1998CC1625>. A novel 1,4,7-triazonanes bearing thiazol-2-yl and benzoxazol-2-yl substituents were synthesized by high-pressure S r reactions <1995H(41)237>. Arylation of 1,4,7-triazonane with 5equiv of 4,7-dichloroquinoline in dimethylformamide (DMF) at reflux in the presence of potassium carbonate afforded a mixture of mono- and disubstituted products, while formation of the trisubstituted derivative was not indicated <2001JME1658>. 

Benzoxazol 5-Chlor-3-phenyl- -2-oxid E14b, 916 [aus 2-(2-OH -aryl)-oxim ... 

An interesting result has been obtained in a study of the dechlorination of polychlorobenzenes by direct irradiation in acetonitrile in addition to dechlorination, products resulting from rearrangement of the chlorine atoms to positions mefa to their original point of attachment are observed. Thus orfho-dichlorobenzene was found to yield mainly chlorobenzene along with small amounts of para-dichlorobenzene the authors speculate that the rearrangement products are formed by recombination of an aryl-chlorine radical pair. In another novel result photolysis of pentachlorophenol in acetonitrile is found to give small amounts of the benzoxazole (160). ... 

Anodic addition of an oxygen function to a C = N or C = C double bond may be exemplified by the oxidative cyclization of aryl semicarbazones to 1,3,4-oxadiazoles in acetonitrile-acetic acid containing H2SO4 or, under strictly anhydrous conditions (in the presence of acetic anhydride), to triazolinones [40]. Other examples are the anodic oxidation of benzaldehyde-2-hydroxyanil to 2-phenyl-1,3-benzoxazole derivatives [41] and the indirect oxidation of 2-hydroxychalcones to flavonoids using tris(4-bromophenyl)amine in MeOH-CH2Cl2 as mediator [42] ... 

Aryl hydroxy groups have been removed by first converting them into an ether by reaction with a heterocycle such as l-phenyl-5-chlorotetrazole (61) and then hydrogenolyzing the ether with platinum or palladium (Eqn. 20.42).97-99 other aryl ether groups that are readily hydrogenolyzed to give the benzene are the benzoxazole, 62, > sulfonates, 63, and phosphates, 64. ... 

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