Selective Oxidation of Alcohols Catalyzed by a Hemicryptophane-Ruthenium Complex

Abstract

Hemicryptophane, a covalent molecular capsule that is composed of a cyclotriveratrylene (CTV) host unit, a tris(2-aminoethyl)amine (tren) ligand, and three rigid phenyl spacers, is a useful ligand for preparing synthetic transition metal complexes containing well-defined cavities that mimic the active sites of metalloenzymes. A method for the selective oxidation of primary alcohols using 5 mol % hemicryptophane-ruthenium(II) dichloride as the catalyst and cerium(IV) ammonium nitrate (CAN) as the terminal oxidant is described. The corresponding aldehydes of both benzylic and aliphatic alcohols are afforded in good to excellent yields under mild conditions without significant over-oxidation to produce the corresponding carboxylic acids. Keywords Cyclotriveratrylene • Cerium(IV) ammonium nitrate (CAN) © Versita Sp. z o.o.

Emergence of Hemicryptophanes: From Synthesis to Applications for Recognition, Molecular Machines, and Supramolecular Catalysis

“…143 Following this method, Makita et al synthesized Zn(II)@22 with ClO 4 − or CH 3 COO − as counterion 54,144 and the RuCl 2 @ 22 hemicryptophane complexes. 145 The tren unit easily binds Cu(II) ions, and a series of Cu(II) @hemicryptophane complexes has been described. Different tren-hemicryptophane ligands reacted with stoichiometric amounts of Cu(ClO 4 ) 2 in a CH 2 Cl 2 /MeOH mixture to produce Cu(II)@15a, Cu(II)@15b, Cu(II)@15i, and Cu(II)@ 125 complexes with high yields (Scheme 32).…”

“…tren-Hemicryptophane 15i reacted with Zn­(ClO 4 ) 2 (H 2 O) 6 to yield the Zn­(II)@ 15i complex in 63% yield (Scheme ). Following this method, Makita et al synthesized Zn­(II)@ 22 with ClO 4 – or CH 3 COO – as counterion , and the RuCl 2 @ 22 hemicryptophane complexes …”

“…RuCl 2 @ 22 was tested as catalyst in the oxidation of alcohols into aldehyde and carboxylic acid using cerium­(IV) ammonium nitrate (Ce­(NH 4 ) 2 (NO 3 ) 6 ) as stoichiometric oxidant (Table ). The cage catalyst was found to be much more selective than the model compound based on the tris­(2-benzylaminoethyl)­amine ligand 132 (RuCl 2 @ 132 ). Indeed, a complex mixture of aldehyde and carboxylic acid was obtained when catalyst RuCl 2 @ 132 was used for oxidation of primary alcohols, whereas the hemicryptophane complex RuCl 2 @ 22 strongly favored formation of the aldehyde and suppressed overoxidation to the carboxylic acid.…”

Synthesis and Structural Studies of Gallium(III) and Iron(III) Hemicryptophane Complexes

“…In these structures, the metal atom is located inside the molecular cavity, leading to an endohedral complex. Some of these complexes present interesting and original catalytic properties, and the inner space of the molecular receptor behaves like a chemical reactor at the nanoscale. Similarly, proazaphosphatrane superbases and their conjugate acid azaphosphatranes have been incorporated into the cavity of a tren-hemicryptophane ligand, where the highly reactive center is encapsulated in the molecular cavity.…”

Counteranions at Peripheral Sites Tune Guest Affinity for a Protonated Hemicryptophane

“…39 One particularly promising example is L, which has been reported to mimic the active sites of metalloenzymes when complexed with metals, and also catalyze the hydrolysis of activated alkyl carbonates 40,41 and the selective oxidation of primary alcohols. 42 Inspired by the host−guest properties of calix [6]arenes reported by Reinaud and Jabin, 43,44 we sought to enhance the binding affinity of L for polar guests by protonating the TREN moiety and thus polarizing the cavity. In this process, we discovered serendipitously that the resulting conjugate base counteranions cap the apertures of the cavity and regulate small-molecule access and guest binding affinity.…”