We show that the dirhodium(II) tetraamidinate complex Rh 2 (Msip) 4 efficiently catalyzes the oxidation of activated secondary alcohols at only 0.1 mol% loading. In this approach, we oxidized various benzylic, allylic and propargylic alcohols to the corresponding carbonyl compounds under mild aqueous conditions using the inexpensive oxidant T-HYDRO® (70 wt% aqueous tert-butyl hydroperoxide). Copyright © 2015 John Wiley & Sons, Ltd.Additional supporting information may be found in the online version of this article at the publisher's web-site.Keywords: dirhodium(II); catalyst; oxidation; alcohols; tert-butyl hydroperoxide
IntroductionSelective oxidation of alcohols to the corresponding carbonyls is one of the most fundamental reactions in organic synthesis, [1] and hence a broad range of methods have been developed, including Swern oxidation, [2] Dess-Martin oxidation [3] and oxidation in the presence of various metal reagents. [4] Researchers have sought to replace the oxidizing reagents in these methods with ecologically friendly and safe catalytic systems based on such reagents as molecular oxygen, aqueous hydrogen peroxide and tert-butyl hydroperoxide (TBHP).[5] TBHP has been used as an alternative oxygen source [6] for alcohol oxidation catalyzed by transition metals such as V, [7] Cr, [8] Mn, [9] Fe, [10] Co, [11] Cu, [12] Zr, [13] Mo, [14] Ru, [15] Rh [16] and Au [17] among others. [18] These methods have various disadvantages, including high catalyst loading, harsh conditions, poor selectivity and/or tedious workup procedures. [8d, 19] This highlights the need to identify better catalyst systems.We recently reported the synthesis of dirhodium(II) tetraamidinate derivative complexes Rh 2 (sip) 4 ( Fig. 1) and demonstrated that they can be used to catalyze benzylic oxidation. [20] This success prompted us to investigate whether the Rh 2 (sip) 4 /T-HYDRO system (where T-HYDRO is 70 wt% aqueous TBHP) can efficiently and selectively oxidize alcohols. We are aware of only one previous report of dirhodium-catalyzed selective oxidation of alcohols. In that study, Moody and Palmer[ 16b ] used 1.0 mol% Rh 2 (OAc) 4 as catalyst and anhydrous TBHP in dichloromethane (DCM) to oxidize benzylic and allylic alcohols. They found that prolonged reaction times were required to achieve low to moderate yields. Here we describe a milder and more efficient approach to achieve regio-and chemoselective alcohol oxidation using only 0.1 mol% dirhodium(II) tetraamidinate as catalyst and T-HYDRO in aqueous solution.
Results and DiscussionFirst we used 1-(4-nitrophenyl)ethanol (1a) as a model substrate to screen various dirhodium(II) complexes for catalytic activity and to optimize reaction conditions. Initially reactions were performed at room temperature using 1.0 mol% dirhodium(II) complexes and 2.0 equiv. of T-HYDRO in various solvents ( [21] Closer inspection of aqueous oxidation reactions reveals that the substrate 1a and dirhodium catalysts show poor solubility. Therefore we switched to a biphasic system and were pleased ...
