Novel Route to Azobenzenes via Pd-Catalyzed Coupling Reactions of Aryl Hydrazides with Aryl Halides, Followed by Direct Oxidations

Abstract

[reaction: see text] N-Boc aryl hydrazines undergo Pd-catalyzed coupling reactions with aryl halides to provide N-Boc diaryl hydrazines in excellent yields. The resulting N-Boc diaryl hydrazines were directly oxidized with NBS/pyridine in CH(2)Cl(2) at room temperature to the azobenzenes.

Shape‐Switchable Azo‐Macrocycles

“…[35] The reaction sequence of this coupling protocol is shown in Scheme 4. In a palladium catalyzed Hartwig Buchwald type reaction [36] [37] an aryl halide A17 is coupled with Boc protected hydrazine A18 to provide a N-Boc aryl hydrazine A19 as one of the coupling partner.…”

“…[38] Scheme 4 Reaction mechanism of palladium catalyzed azo formation via the introduction of a Boc protected hydrazine followed by oxidation to an asymmetric azo compound A21. [35] …”

“…Subsequent Suzuki cross coupling reaction turned out to be challenging under conventional conditions. [149] [192] With [Pd(PPh 3 ) 4 ] as catalyst and potassium carbonate as base, the dibromo compound 15 was treated under an inert atmosphere with 2.6 equivalents of (3-nitrophenyl)boronic acid (35) Applying microwave conditions to the Suzuki cross coupling reaction offered significantly better yields than the applied conventional conditions. [200][201] The same reagents, the same catalyst and the same purification steps were applied but the temperature was increased from 85°C to 120°C for the microwave reaction.…”

“…[191] Treatment of 2.6 equivalents of the commercially available 3-nitrophenyl-boronic acid (35) The first purification step was a column chromatography with silica gel and dichloromethane, the obtained crude product was further purified by different preparative thin layer chromatography techniques, the use of n-hexane instead of cyclohexane and the use of 0.5 mm prep TLC plates instead of 2.0 mm prep TLC plates, respectively, turned out to be crucial for the purification. The separated bands of the chromatography were analyzed be MALDI-TOF mass spectrometry to determine the molecular weight; this purification procedure was repeated until the pure macrocyclic compound was isolated.…”

“…[35] [186] For this approach a halogenide functionalized precursor is needed. In Scheme 33 the synthesis to an iodide functionalized biphenyl precursor is shown.…”