Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

Abstract

ARTICLE This journal is © The Royal Society of Chemistry 2013J. Name., 2013, 00, 1-3 | 1 The one-pot reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde, (R)-2-aminoglycinol and Cu(OAc)2·2H2O in a 1:1:1 ratio in the presence of triethylamine led to the isolation of X-ray quality crystals of the chiral complex (R)-1 in high yield. The single crystal structure of (R)-1 reveals a tetranuclear copper(II) complex that contains a {Cu4(μ-O)2(μ3-O)2N4O4} core. A reaction using (1S,2R)-2-amino-1,2-diphenylethanol as precursor under the same conditions generated the chiral complex (S,R)-2; its structure was determined by single crystal X-ray crystallography and was found to contain a {Cu2(μ-O)2N2O2} core. Both (R)-1 and (S,R)-2 have been used for catalytic aerobic oxidation of benzylic alcohols in combination with the TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) radical. (R)-1 selectively catalyses the conversion of various aromatic primary alcohols to the corresponding aldehydes with high yields (99%) and TONs (up to 770) in the air, while (S,R)-2 exhibits less promising catalytic performance under the same reaction conditions. The role of the cluster structures in (R)-1 and (S,R)-2 in controlling the reactivity towards aerobic oxidation reactions is discussed. IntroductionThe oxidation of alcohols to carbonyl compounds is an important transformation in synthetic organic chemistry and extensive efforts have been focused on the development of atom-economic oxidation methodologies for such reactions. [1][2][3][4] Amongst the protocols developed to date, catalytic aerobic oxidation has proven to be especially efficient and valuable by virtue of the use of a low-loaded, low-cost catalyst and molecular oxygen from the air. [5][6][7][8][9][10] In the past two decades, copper-catalysed, TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) mediated aerobic oxidation has been the most popular choice of catalyst systems since the pioneering work of Semmelhack and coworkers in 1984. These authors utilized a CuCl/TEMPO system in DMF (N,N-dimethylformamide) to furnish the aerobic oxidation of benzylic and allylic alcohols. 11 Recent progress indicates that both Cu(I) and Cu(II) complexes with a N,N-or N,O-ligands in combination with TEMPO could be efficient catalysts or catalyst precursors for the aerobic oxidation of various alcohols, and of particular note is a highly efficient copper/2,2'-bipyridine/TEMPO system developed by the Stahl group. 12-17 However, the types of organic ligands exploited in the copper catalyst systems are still limited and the structures of copper catalysts involved in the catalytic process are rarely explored. [17][18][19][20] The construction of metal-organic cluster compounds through self-assembly of predesigned ligands with appropriate metal ions is a prime research topic [21][22][23][24][25] Herein, we report the one-pot syntheses and crystal structures of two tetranuclear and dinuclear copper(II) complexes resulting from the in situ synthesis of chiral Schiff base ligands (Scheme 1), and their catalytic ap...

Copper(II) complexes derived from bidentate N,O-ligands for catalytic aerobic oxidation

“…Slightly higher yields (21-46%) were obtained while the reactions were performed in water at room temperature, with the addition of inorganic bases such as potassium carbonate or potassium tert-butoxide (entries 2-8, Table 1). This is in agreement with our recent finding that water was a good solvent for multinuclear Cu II complexes catalyzed aerobic alcohol oxidation [35]. However, yields were not improved significantly when reactions were performed at elevated temperature (entries 9-11, Table 1).…”

New copper(II) species from the copper/2,2′-bypyridine and copper/4-dimethylaminopyridine catalyzed aerobic alcohol oxidations

“…1 contains a copper center that is uncoordinated with ligands, resulting from the decomposition the catalytically active Cu 4 (µ 4 -O)(DMAP) 4 Cl 6 , and therefore, no reactivity was observed. On the other hand, dinuclear complexes 2 and 3 were modestly active, being in agreement with our previous results observed during the course of the study of structure-activity relationship in Cu II complex catalysts with various nuclearities [10][11][12][13][14][15]. The fact that the presence of extra DMAP (5 mol%) remarkably improved the catalytic efficacy is also consistent with the results obtained by in-situ formed catalysts, where 2.0 equivalent of ligand was required.…”

“…Previously, monovalent Cu I salts were found to be more effective copper source for most of ligands studied, however, Cu II also showed comparable catalytic activity in many cases [6,7,[10][11][12][13][14][15]. Here, we conducted several catalytic experiments in order to disclose the reactivity difference between the Cu II /BPY/TEMPO and Cu II /DMAP/TEMPO catalyst systems, while using CuCl 2 as the copper source.…”

“…Recently, we were interested to investigate the influence of various organic ligands in the catalytic reactivity of Cu I -or Cu II -catalyzed aerobic alcohol oxidations, with a focus on the structural elucidation of possible Cu I or Cu II complexes formed during the catalytic reactions [10][11][12][13][14][15]. We have observed that tetranuclear copper clusters derived from N,O-Schiff base ligands catalyzed aerobic oxidations of benzylic alcohols under aqueous basic conditions, featuring higher activity than its dinuclear analogue [10].…”

“…We have observed that tetranuclear copper clusters derived from N,O-Schiff base ligands catalyzed aerobic oxidations of benzylic alcohols under aqueous basic conditions, featuring higher activity than its dinuclear analogue [10].…”