The oxidation of alcohols into the corresponding aldehydes or ketones is one of the most important functional group transformations in organic synthesis. 1 Recently, the use of molecular oxygen as terminal oxidant has received great attention for both economic and environmental benefits, and many highly efficient systems have been developed for catalytic aerobic alcohol oxidation using copper, 2 palladium, 3 or ruthenium catalysts. 4 Of particular interest are the catalysis systems involving both transition metals and nitroxyl radicals (e.g., 2,2,6,6-tetramethyl-piperidyl-1-oxy TEMPO). 5,6 However, only a few catalyst systems, for example, small amounts of cheap metal salts, together with TEMPO, provided an efficient catalyst for aerobic oxidation of alcohols under mild conditions. 6c,h Therefore, attempting to obtain more efficient processes, chemists have paid much attention to screening various transition metals and designing new ligands while largely ignoring the advantages inherent in a nonmetal catalytic system. We are particularly interested in exploring the potential of such a transition-metal-free catalyst system for aerobic alcohol oxidations.Our research was inspired by the results of the TEMPO-Cl 2 oxidation system by Bjørsvik et al. 7a and the TEMPO-Br 2 /I 2 system by Miller et al. 7b In their procedures, NaHCO 3 or Na 2 CO 3 was used to neutralize the coproduct HX (X ) Cl, Br, I). We reasoned that if HX can be oxidized to regenerate X 2 in situ by molecular oxygen rather than being scavenged by inorganic base, a TEMPO-catalyzed process with a catalytic amount of X 2 could be established. In this communication, we report a highly efficient catalytic system without transition metal for the aerobic oxidation of a variety of alcohols.Initial investigation of TEMPO-catalyzed (1 mol %) aerobic oxidation was carried out using benzyl alcohol as substrate with 4 mol % of Br 2 and 0.5 MPa of oxygen under 100°C for 1 h. The preliminary result (8.36% of conversion) clearly indicated the role of Br 2 as an active catalyst. Prolonging the reaction time to 5 h increased the conversion to 20.4%. 8 Recognizing that the first incorporation of molecular oxygen into the reaction system has been the keystone for successful aerobic oxidations, we sought to find a cocatalyst to bridge the gap between O 2 activation and HBr reoxidation. The ready availability and unique redox property of NaNO 2 as a source of NO under acidic conditions attracted our attention. 9 Although NaNO 2 alone showed almost no activity in TEMPO-catalyzed aerobic oxidation, when 4 mol % of Br 2 and 8 mol % of NaNO 2 were both employed in TEMPO-catalyzed aerobic oxidation, a highly efficient catalyst system emerged (eq 1). 8 The quantitative oxidation of benzyl alcohol can be achieved without acid either under 0.5 MPa of oxygen at 100°C or under 0.2 MPa of oxygen at 60°C. 8 Indeed, the transition-metal-free catalyst system for aerobic alcohol oxidations exhibited extremely high selectivities and are remarkably easy to control. After systematic op...
A series of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides were synthesized and characterized using FT-IR and elemental analysis, and the molecular structures of complexes , and were confirmed to present a pseudo-square-pyramidal or trigonal-bipyramidal geometry around the cobalt center using single-crystal X-ray diffraction. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all cobalt precatalysts gave high activities up to the level of 10(7) gPE mol(-1) (Co) h(-1) toward ethylene polymerization, being one of most active cobalt-based precatalysts. In comparison with cobalt analogues, the title precatalysts generally possessed longer lifetime along with good thermo-stability; moreover, the resultant polyethylenes were highly linear and unimodal in most cases.
A series of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine derivatives was synthesized and fully characterized, and thereafter reacted with iron dichloride to form their corresponding iron(II) complexes. The single crystals of representative organic and iron complex compounds were obtained and analyzed by the X-ray diffraction analysis, indicating the distorted bipyramidal geometry around the iron core. Moreover, DFT calculations were performed on selected species to determine their structural features. On treatment with either MAO or MMAO, all iron complex pre-catalysts showed high activities (up to 1.56 × 10(7) gPE mol(-1)(Fe) h(-1)) toward ethylene polymerization. Regarding the nature of the ligands and reaction parameters, their catalytic activities and the characters of the obtained polyethylenes have been carefully investigated. The ring strain of the fused-cycloheptane of the ligands within iron complexes was considered to affect their catalytic performance in ethylene polymerization. The active species were activated and controlled by using a co-catalyst of MMAO preferred over MAO, and the obtained polyethylenes with MMAO showed narrower molecular polydispersity than the corresponding polyethylenes with MAO.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.