DOI: 10.1002/adsc.201100672 Selective Acceptorless Conversion of Primary Alcohols to Acetals and Dihydrogen Catalyzed by the Ruthenium(II) Complex RuACHTUNGTRENNUNG(PPh 3 ) 2 ACHTUNGTRENNUNG(NCCH 3 ) 2 ACHTUNGTRENNUNG(SO 4 ) Elizaveta Kossoy, a Yael Diskin-Posner, b Gregory Leitus, b and David Milstein a, * a Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel Fax: (+ 972)-8-934-4142; e-mail: david.milstein@weizmann.ac.il b Chemical Research Support, the Weizmann Institute of Science, 76100 Rehovot, Israel Received: August 25, 2011; Revised: October 17, 2011; Published online: February 9, 2012 Abstract: The complex bis(acetonitrile)bis(triphenyl- phosphine)ruthenium(II) sulfate [RuACHTUNGTRENNUNG(PPh 3 ) 2 ACHTUNGTRENNUNG(NCCH 3 ) 2 ACHTUNGTRENNUNG(SO 4 )], fully characterized spectroscopical- ly and by a single crystal X-ray study, catalyzes at 110 8C the direct transformation of primary alcohols to the corresponding acetals with liberation of mo- lecular hydrogen. The formation of acetals proceeds via direct substitution of the hydroxy group of the hemiacetal intermediate by an alcohol molecule. The closely related bis(triphenylphosphine)ruthenium(II) acetate [RuACHTUNGTRENNUNG(PPh 3 ) 2 ACHTUNGTRENNUNG(OAc) 2 ] catalyzes the conversion of primary alcohols to the corresponding esters rather than acetals. Keywords: alcohols; dehydrogenation; homogeneous catalysis; ruthenium; transition metals Introduction The atom-efficient and environmentally benign cata- lytic oxidation of alcohols to various carbonyl com- pounds is of much industrial interest. [1–4] Particularly desirable is the homogeneous selective alcohol dehy- drogenation with evolution of molecular hydro- gen. [5–12] Acetals are usually prepared by condensation of al- dehydes with alcohols, catalyzed by various catalysts, including protic [13] or Lewis [14] acids. The direct con- version of alcohols to the corresponding acetals is an attractive reaction, due to its high atom economy and circumventing the need for aldehydes or aldehyde de- rivatives. To the best of our knowledge, there are only three reported homogeneous catalysts for the direct trans- formation of primary alcohols to acetals and molecu- lar hydrogen. [11,12,15] In 1987, Murahashi et al. [12] re- ported the RuACHTUNGTRENNUNG(PPh 3 ) 3 Cl 2 -catalyzed conversion of 1- hexanol to 1,1-bisACHTUNGTRENNUNG(hexyloxy)hexane in 8 turnovers at 180 8C after 4 h under Ar, together with the formation of hexyl hexanoate. In 2000, Thorp et al. [15] reported the conversion of ethanol to the acetal + acetaldehyde (total of 30 turnovers) catalyzed by Re(4- NC 6 H 4 Cl)Cl 3 ACHTUNGTRENNUNG(PPh 3 ) 2 + 8dppe, after 48 h reflux in 95% ethanol under Ar. An acridine-based pincer catalyst (Scheme 1) was reported by our group in 2009. [11] Heating 0.1 mol% catalyst solution in either neat 1- hexanol (157 8C, oil bath temperature) or 1-pentanol (bp 138 8C, reflux) under Ar for 72 h yielded 81.5% and 92% of the corresponding acetals, respectively. In this paper, we describe a ruthenium complex based on simple commercially available ligands, which efficiently and selectively catalyzes dehydro- genation of primary alcohols to acetals and H 2 in high turnover numbers and good selectivity under mild conditions. The simple catalyst structure and its easy synthesis, along with its efficiency, make it particularly attractive. Results and Discussion The new RuACHTUNGTRENNUNG(PPh 3 ) 2 ACHTUNGTRENNUNG(NCCH 3 ) 2 ACHTUNGTRENNUNG(SO 4 )(1) was prepared by reaction of RuACHTUNGTRENNUNG(PPh 3 ) 3 Cl 2 with Ag 2 SO 4 in a solution of acetonitrile and methanol (Scheme 2). The 31 P{ 1 H} NMR spectrum of 1 exhibits a singlet at 52.47 ppm. The methyl groups of acetonitrile give rise Scheme 1. Conversion of primary alcohols to acetals and H 2 catalyzed by an acridine-based pincer catalyst. Adv. Synth. Catal. 2012, 354, 497 – 504  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 497 FULL PAPERS