[55669-36-8] · C84H84Cl4P6Ru2 · Tetrachlorotris[bis(1,4-diphenylphosphino)butane]diruthenium · (MW 1623.44)
(a regioselective hydrogenation and dehydrogenation catalyst for the formation of lactones)
Physical Data: mp 186 °C.1
Form Supplied in: light-green solid.1
Solubility: insol alcohol, ethyl ether; sol CH2Cl2, chlorobenzene.1
Preparative Methods: K2[RuCl5(H2O)] (1 mmol) in 2 mL of water is added to a hot ethanolic solution (100 mL) of 1,4-bis(diphenylphosphino)butane (dppb) (4 mmol), and the mixture refluxed for 3 h. The resulting light-green precipitate is washed with diethyl ether and extracted with chlorobenzene. The soluble fraction is crystallized repeatedly from chlorobenzene-diethyl ether.1
Handling, Storage, and Precautions: should be handled and stored under an inert atmosphere.
Catalytic amounts of tetrachlorotris[bis(1,4-diphenylphosphino)butane]diruthenium (1) have been used to regioselectively hydrogenate unsymmetrically substituted cyclic anhydrides to the corresponding lactones. These reactions have been carried out using as little as 2 mol % of (1) with both high regioselectivity and isolated yields (eq 1).2
Attempts to reduce the anhydride of eq 1 with Lithium Tri-t-butoxyaluminum Hydride resulted in low chemical yields and poor regioselectivity. Use of Lithium Aluminum Hydride and Sodium Borohydride resulted in reduction of the more sterically hindered carbonyl group.2
Compound (1) also catalyzes the dehydrogenation of a,o-diols to lactones with the use of alkenic hydrogen acceptors (eq 2).3 This system has several advantages over other reagents and catalysts that could be used in these reactions: it is highly regioselective, can be used catalytically, is less expensive than AgI, is less toxic than CrVI, and is safer than peroxides.
These hydrogenation and dehydrogenation reactions are sensitive to the size and location of substituents.3b With hydrogenations, bulky substituents a to the anhydride give the highest regioselectivity, while with dehydrogenations, bulky substituents b to an alcohol give the highest regioselectivity. Another study has shown that other ruthenium catalysts, e.g. Dihydridotetrakis(triphenylphosphine)ruthenium(II), can be comparable and in several cases exceed compound (1) in both regioselectivity and yield in catalytic dehydrogenations.3a
Christopher R. Sarko & Marcello DiMare
University of California, Santa Barbara, CA, USA