In view of the depletion of petroleum oils, new synthetic routes for the sustainable production of chemicals, fuels, and energy from renewable biomass sources are currently widely investigated. In particular, nonedible sugars and polyols are promising starting materials to produce olefins by dehydration, deoxygenation, or deoxydehydration (DODH) of these poly vicinal alcohols. In this perspective, we highlight the recent evolution of rhenium-catalyzed dehydration and DODH of biomass-derived alcohols and polyols to obtain olefins. Improving over the classical acid-catalyzed dehydration reaction, rheniummediated systems are very selective and more active to provide high yields of olefin products, but dehydration alone cannot be used to fully defunctionalize sugars. This issue is addressed by a growing research effort in the field of Re-catalyzed DODH, which allows complete dehydroxylation to form olefins in high yield. Recent developments in this field include the development of new molecular rhenium catalysts, the application of cheaper and more available reductants, and a growing mechanistic understanding owing to both experimental and computational studies. Finally, recent efforts to move beyond rhenium toward cheaper metals (Mo, V) are discussed.

A bulky cyclopentadienyl (Cp)-based trioxorhenium compound was developed for the catalytic deoxydehydration of vicinal diols to olefins. The 1,2,4-tri(tert-butyl)cyclopentadienyl trioxorhenium (2) catalyst was synthesised in a two-step synthesis procedure. Dirhenium decacarbonyl was converted into 1,2,4-tri(tert-butyl)cyclopentadienyl tricarbonyl rhenium, followed by a biphasic oxidation with H2 O2 . These two new three-legged compounds with a 'piano-stool' configuration were fully characterised, including their single crystal X-ray structures. Deoxydehydration reaction conditions were optimised by using 2 mol % loading of 2 for the conversion of 1,2-octanediol into 1-octene. Different phosphine-based and other, more conventional, reductants were tested in combination with 2. Under optimised conditions, a variety of vicinal diols (aromatic and aliphatic, internal and terminal) were converted into olefins in good to excellent yields, and with minimal olefin isomerisation. A high turnover number of 1400 per Re was achieved for the deoxydehydration of 1,2-octanediol. Furthermore, the biomass-derived polyols (glycerol and erythritol) were converted into their corresponding olefinic products by 2 as the catalyst.

The cyclopentadienyl-based trioxo-rhenium complexes Cp ttt ReO 3 (Cp ttt = 1,2,4-tritert-butylcyclopentadienyl) 1b and Cp*ReO 3 (Cp* = pentamethylcyclopentadienyl) 4b) are known to be active catalysts for the deoxydehydration (DODH) of vicinal diols to olefins. Here, we report on the preparation of a series of complexes of the general formula Cp′ReO 3 (Cp′ = 1,3-di-tert-butylcyclopentadienyl (Cp tt , 2b, 18%), 1,2,3-triisopropylcyclopentadienyl (3b, 4%), 1,2,3-trimethyl-4,5,6,7-tetrahydroindenyl (6b, 36%), and tetramethylcyclopentadienyl (7b, 33%)) in which the electronic and steric properties of the Cp′ ligand are varied. These complexes were synthesized via oxidative decarbonylation from the corresponding Cp′Re(CO) 3 complexes with either H 2 O 2 or tBuOOH. An in situ NMR investigation revealed that complexes 2b, 3b, and 6b are unstable under the oxidizing reaction conditions. This information was used to determine the optimal reaction time to isolate the complexes 2b, 3b, and 6b. These "piano-stool" configurated Cp′ReO 3 complexes were characterized spectroscopically and by single crystal X-ray diffraction. The new complexes 2b, 3b, 6b, and 7b were found to be generally less thermally stable than Cp ttt ReO 3 (1b) or Cp*ReO 3 (4b). It appeared that 2b and 6b were better catalysts for the DODH of 1,2-octanediol to octene with PPh 3 as an oxygen-atom acceptor. Remarkably, the less substituted Cp′ReO 3 complexes (1b, 2b, and 3b) afforded significantly less 2-octene (presumably from isomerization of the primary 1-octene product) in comparison to those containing per-alkylated Cp-moieties (4b and 6b).

Mononuclear, coordinatively unsaturated rhenium(V) dioxo species of the type XReO2 (X = Me, substituted cyclopentadienyl) have long been postulated as intermediates in rhenium-catalyzed deoxydehydration, but their characterization was precluded because of aggregation into dimeric or oligomeric structures. Using the bulky 1,2,4-tri-tert-butylcyclopentadienyl (Cp(ttt)) ligand, the rhenium(V) dioxo species (Cp(ttt))ReO2 could now be observed, in equilibrium with the dimeric form [(Cp(ttt))Re(O)μ-O]2, and characterized by NMR, IR, and UV-vis spectroscopies, as well as electrospray ionization mass spectrometry. (Cp(ttt))ReO2 is shown to be the primary product of reduction of the rhenium(VII) complex (Cp(ttt))ReO3 with PPh3 and demonstrated to react with ethylene glycol significantly faster than its dimeric counterpart, supporting its role as an intermediate in rhenium-catalyzed deoxydehydration reactions.

Ru being served? The reactions of propargylic carbonates with silyl diazo compounds in the presence of [Cp*RuCl(cod)] as a catalyst precursor led to the formation of dienyl carbonates in excellent yields under mild conditions (see scheme; Y = SiMe(3)).