Aminotroponate/aminotroponiminate zinc complexes functionalized mesoporous silica catalysts for intramolecular hydroamination of non-activated alkenes with varied steric and electronic properties

Various aminotroponiminate-/aminotroponate-zinc complexes functionalized mesoporous silica catalysts were synthesized. These included [N-propyl-2-(propylamino)troponiminato] zinc methyl (ZnDPRT), [N-propyl-2-(phenylamino)troponiminato] zinc methyl (ZnDPHT), and [2-(propylamino)troponato] zinc methyl (ZnPRT) functionalized mesoporous silica catalysts. The catalysts were characterized by various structural (small angle XRD and gas sorption) and compositional (solid-state NMR, XPS, FTIR, and thermogravimetric and elemental analyses) techniques. Their catalytic activities for intramolecular hydroamination of nonactivated alkenes with varied steric and electronic properties were investigated. The catalysts' properties were generally found to differ from one another. This is mainly attributed to (1) fluorosilicate formation during the synthesis of the aminotroponiminate (ATI) ligands on the mesoporous silica, (2) the higher reaction efficiency of the nucleophilic substitution reaction of aminopropyl-functionalized mesoporous silica (API) with tropolone p-toluenesulfonate forming 2-(propylamino)troponato- [or aminotroponate (ATO) ligand]-functionalized sample (PRT), compared to that of the ATI ligands, where 2-(alkylamino)tropones or 2-(arylamino)tropones are activated with Et₃OBF₄ forming corresponding vinylgous ethers which then undergo aminolysis with the API precursor, and (3) the different compositional and structural characteristics of the amine/imine (ATI) or amine/carbonyl (ATO) chelation sites. To better understand the catalytic capabilities of the synthesized catalysts, a broader functional group compatibility study was also performed. It was found that decreased bite angles (Thorpe-Ingold effect) and increased electron donation of the p-substituted benzylic side arm of the substrates resulted in slightly increased conversion efficiencies in the hydroamination reaction. Overall, the 2,2-diphenyl cyclizing agents formed their corresponding N-containing 5-membered heterocycles at 74-94% conversion in 8 h under reflux conditions while the 2,2-dimethyl species exhibited 59-81% conversion of 12 h.