4-bromo-2,5-diphenyloxazole

Bromination of substituted oxazoles can yield normal aromatic substitution products or 4,5- and 2,5-addition products, depending on the reaction conditions. For example, Hassner and Fischer " brominated 2,5-diphenyloxazole 111 with bromine in acetic acid and sodium acetate to prepare 4-bromo-2,5-diphenyloxazole 595 (Scheme 1.163). Similarly, Belen kii and co-workers isolated a mixture of 5-bromo-2-phenyloxazole 596 and 4,5-dibromo-2-phenyloxazole 597 from treatment of 2-phenyloxazole 5 with bromine in refluxing benzene. Lawson and VanSant " isolated 2-amino-5-bromo-4-(trifluoromethyl)oxazole 599a and 5-bromo-2-(methylamino)-4-(trifluoromethyl)oxazole 599b from bromination of 2-amino-4-(trifluoromethyl)oxazole 598a and 2-(methylamino)-4-(trifluoromethyl) oxazole 598b, respectively, with bromine in acetic acid and sodium acetate. 

Clapham and Sutherland developed a general synthesis of 2,5-diphenyl-4-substituted oxazoles 1659 in the context of preparing monomers for scintillating polymers. Stille coupling of 4-bromo-2,5-diphenyloxazole 1658 with a variety of stannanes produced the target oxazoles in modest to excellent yield (Scheme 1.426). Alternatively, the authors reversed the StiUe-couphng approach... 

A recent approach to the synthesis of 5-bromooxazoles reacted 2-phenyl-4,5-dihydrooxazole with NBS in the presence of a.a-azobisisobu-tyronitrile (AIBN). The 2-aryloxazole product (83) resulted from a bromi-nation-dehydrobromination process. Further bromination then gave 84. With two equivalents of NBS the yields of 83 and 84 were 45 and 10%, respectively, whereas three equivalents raised the yield of dibromo product to 56% (89S873) (Scheme 29). When treated with bromine in neutral conditions, 2,5-diphenyloxazole formed an N-bromo adduct [59LA(626)83]. 

Bromo- and 4-bromoalkyl-2,5-diphenyloxazole 38 were subjected to the Stille coupling with a range of commercially available tributyltin reagents. Tri-2-furylphosphine/ Pd2(dba)3 was used as an effective catalyst. Copper(n) oxide enhanced the Stille coupling reactions of 2,5-diphenyl-4-tributylstannanyloxazole with various electrophiles [41]. Such method offered an efficient synthetic route to prepare resins from oxazole-containing monomers such as 2,5-diphenyl-4-vinyloxazole. 

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