In Situ Spectroscopic Investigation of the Rhenium- Catalyzed Deoxydehydration of Vicinal Diols Johannes R. Dethlefsen and Peter Fristrup* [a] Introduction Whether the motivation is economic, environmental, or geo- political, the realization of an economy completely independ- ent of fossil reserves requires not only the exploitation of alter- native energy sources but also the substitution of organic chemicals of fossil origin by biomass-derived compounds. Al- though it is realistic to produce biomass in an amount compa- rable to the current production of oil based on weight, [1] the lower energy density of biomass and the problems associated with the use of agricultural land for the production of fuels [2] make it desirable to use the entire biomass feedstock, which includes waste products. One such waste product is glycer- ol—the inevitable byproduct from the production of biodiesel (i.e., fatty acid methyl esters) from triglycer- ides—the production of which is estimated to become six times larger than its demand by 2020. [3] The already low value of glycerol could decrease even further, and most chemical transformations—oxidation, reduction, dehydration, and others—will, therefore, make it more valuable. [4] Petroleum, the primary feedstock for the chemical industry today, contains very little oxygen, and the C/O ratio of 1:1 for glycerol and other biomass-derived polyols (sugar alcohols) means that reactions that are able to reduce the oxygen con- tent are relevant. One such reaction is deoxydehydration (DODH), in which a vicinal diol is transformed into an alkene in the presence of a reductant and a Re catalyst. [5–21] Multiple re- ductants and Re-based catalysts have been tested and, in par- ticular, the CH 3 ReO 3 -catalyzed DODH driven by the oxidation of a secondary alcohol has received increasing attention over the past four years [10, 11, 17, 19, 20] and has recently been the topic of two reviews. [22, 23] In this work, we present an in situ-spectro- scopic investigation of a slightly modified version of this reac- tion (Scheme 1). The main difference between this modified re- action and those published previously is that it is conducted in dodecane instead of neat alcohol to allow the concentration of the alcohol to be changed. The use of the two reactants 1,2- tetradecanediol and 3-octanol, the two products 1-tetradecene and 3-octanone, and the solvent dodecane is justified by their high boiling points and the innocent nature of the aliphatic chains. The mechanism of DODH has been the subject of a number of studies, both experimental [10, 11, 17] and computational, [12, 14, 15] but the conclusions are conflicting, which in part may be be- cause of the use of different catalysts and reductants. There is agreement on the existence of three stages in the reaction: condensation of the diol and the Re center, reduction of the Re center, and extrusion of the alkene accompanied by oxidation of the Re center, but the two main points of dispute are 1) the sequence of the condensation and reduction and 2) the iden- tification of the rate-limiting step. The two fundamentally dif- ferent pathways that can be envisioned for the CH 3 ReO 3 -cata- lyzed DODH of 1,2-tetradecanediol driven by the oxidation of 3-octanol (Scheme 1) are shown in Scheme 2. A pathway similar to that shown on the left-hand side of Scheme 2, in which the rhenium(VII) center and the diol under- The mechanism of the CH 3 ReO 3 -catalyzed deoxydehydration of a vicinal diol to an alkene driven by oxidation of a secondary alcohol was investigated by time-resolved, in situ IR spectros- copy and was found to occur in three steps: 1) reduction of the catalytically active methyltrioxorhenium(VII) to a rhenium(V) complex (the rate-limiting step), 2) condensation of the diol and the rhenium(V) complex to a rhenium(V) diolate, and 3) extrusion of the alkene accompanied by oxidation of the Re center and thus regeneration of CH 3 ReO 3 . The reaction follows zero-order kinetics initially but, unexpectedly, accelerates to- wards the end, which is explained in terms of a deactivating pre-equilibrium, in which the catalytically active CH 3 ReO 3 con- denses reversibly with the diol to form an inactive rhenium(VII) diolate. This conclusion is supported by the direct observation of a catalytically inactive species as well as DFT calculations of the IR spectra of the relevant compounds. Scheme 1. Re-catalyzed DODH of 1,2-tetradecanediol driven by the oxidation of 3-octa- nol. [a] Dr. J. R. Dethlefsen, Dr. P. Fristrup Department of Chemistry Technical University of Denmark Kemitorvet 207, 2800 Kgs. Lyngby (Denmark) E-mail : pf@kemi.dtu.dk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201403012. ChemCatChem 2015, 7, 1184 – 1196 ⌫ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1184 Full Papers DOI: 10.1002/cctc.201403012