The oxidation of allylic and benzylic alcohols to aldehydes oxygen, in the presence of the bifunctional rutheniumcopper system (nPr 4 N)(RuO 4 )-CuCl acting as the catalyst. can be carried out at room temperature with molecular
The oxidation of alcohols to aldehydes and ketones is a species [RuCl 2 (PPh 3 ) 3 ] [6] , RuO 2 •xH 2 O [7] , the binuclear 8] , the bimetallic system fundamental reaction in organic synthesis, and numerous oxidizing reagents are available to effect this key transfor-RuCl 3 ϪCo(OAc) 2 [9] and the triple catalytic system [Ru-Cl(OAc)(PPh 3 ) 3 ]ϪhydroquinoneϪ[Co(salophen)(PPh 3 )] [10] mation. [1] In most instances, these reagents are used in stoichiometric amounts and are very often toxic such as the where H 2 salophen ϭ N,NЈ-bis(salicylidene)-O-phenylenediamine. However, these catalysts are rarely selective and are chromium oxidants used in vast amounts in both the laboratory and industry. [1] However, with the ever-growing en-often limited as rather high reaction temperatures and high pressures of molecular oxygen are required and low turn-vironmental and economic concerns the development of catalytic processes for alcohol oxidation is becoming in-over numbers are observed.
Recently, during the preparation of our manuscript creasingly important.
A particularly noteworthy process has been described by Ley [11] and Marko ´[12] have communicated the use of tetran-propylammonium perruthenate, (nPr 4 N)(RuO 4 ), in the Griffith et al. [2] who reported the use of the high oxidation state oxoruthenium compound tetra-n-propylammonium presence of molecular oxygen, as a catalytic oxidant of both primary and secondary alcohols. Marko ´[12] has also ob-perruthenate (nPr 4 N)(RuO 4 ), TPAP, as a mild catalyst for the oxidation of alcohols with N-methylmorpholine N-ox-served that the presence of CuCl • phenanthroline (5 mol-%) and K 2 CO 3 (200 mol-%) produced a retarding effect in the ide, NMO, as the secondary oxidizing source. More recently, we reported the catalytic oxidation of primary al-oxidation of 4-chlorobenzyl alcohol. We wish to communicate some of our recent results on a similar (nPr 4 N)(RuO 4 ) cohols to aldehydes with cis-dioxymolybdenum(VI) complexes and sulfoxides as the co-oxidant. [3] Although both of ϪCuCl bimetallic system which is a selective and efficient catalytic system for the oxidation of alcohols with molecu-the above function well as catalytic systems, unpleasant and harmful by-products, amine and sulfide respectively, are lar oxygen under mild conditions (25°C), producing water as the only stoichiometric by-product, Scheme 1. produced. This limits the use of these catalysts on an industrial scale. The quest for catalytic systems that use inexpen-
The catalytic oxidation of various alcohols takes place smoothly at 25°C with (nPr 4 N)(RuO 4 ) (1 mol-%), CuCl (1 sive, nontoxic, molecular oxygen in the air as the secondary oxidant remains a particularly important goal for the devel-mol-%), and 2-aminopyridine (5 mol-%) in the presence of 1 atm of molecular oxygen in toluene, dichloromethane, or opment of a "green method" for converting alcohols to carbonyl products on an industrial scale. [4] acetonitrile. The results are presented in Table 1. The addition of powdered 4-A ˚molecular sieves results in slightly Relatively few ruthenium-based catalytic systems using air or molecular oxygen have been reported for the dehydro-increased rates, as previously observed in oxidations with (nPr 4 N)(RuO 4 ) and NMO. [2] This may be because the water genation of alcohols. These include the trinuclear ruthenium carboxylates [Ru 3 O(O 2 CR) 6 L 3 ] n (R ϭ CH 3 or formed as a by-product in the reaction degrades the catalyst. Allylic and benzylic alcohols are oxidized with nearly C 2 H 5 ; L ϭ H 2 O or PPh 3 ; n ϭ 0, ϩ1) [5] , the ruthenium(II) Scheme 1