Generation, characterization, and diazo-coupling reaction of cycloheptatrienols

Three isomeric cycloheptatrienols, cyclohepta-1,3,5-trien-1-ol (1), cyclohepta-1,3,5-trien-2-ol (2), and cyclohepta-1,3,5-trien-3-ol (3), were generated by controlled acid-catalyzed hydrolysis or methanolysis of their corresponding trimethylsilyl enol ethers in DMSO. H-1 NMR spectroscopic studies indicate that ketonization of 1-3 all took place by cli-protonation to give cyclohepta-3,5-dien-1-one (4), cyclohepta-2,4-dien-1-one (5), and cyclohepta-2,5-dien-1-one (9), respectively. 9 is not stable at room temperature and isomerized to 4 via enol 1 as an intermediate. The diazo-coupling reaction of these enols with p-substituted-benzenediazonium ions 10 was also studied, and three different behaviors were observed, 1 reacted at the alpha-position(s) with 1 and 2 equiv of 10 to give intramolecularly H-bonded 2-(p-substituted-phenylhydrazono)cyclohepta-3,5-dien-1-ones 15 and 2,7-bis(p-substituted-phenylhydrazono) 3,5-dien-1-ones 16, respectively. Coupling of 2 with p-chlorobenzenediazonium ion (10d) proceeded through its valence tautomer bicyclo[4.1.0]hepta 2,4-dien-2-ol (20), to give 4-(p-chlorophenylhydrazono)bicyclo[4.1.0]hept-3-en-2-one (24d). Diazocoupling of enol 3 with 10a,b yielded both 15a,b and 16a,b, the same products as the coupling reaction of P. Our results suggest that the most probable reaction pathway of 3 is instead of undergoing direct coupling, it is isomerized first to enol 1 which then reacted. Kinetics for the three coupling reactions were also studied by conventional and stopped-flow UV/vis spectroscopy. The reaction constant (p = 1.15) for the diazo coupling reaction of Be, the enolate anion of 1, is much smaller than that for the same reaction of phenolate anion (p = 4.2), suggesting that the formation of the transition state in the reaction of le has a greater activation energy than that for the aromatic phenolate anion, This observation and the dramatic difference in reactivity toward electrophilic additions are considered as evidence for the lack of significant homoaromatic stabilization in the cycloheptatriene derivatives.