[42516-72-3] · C16H22Ru · (1,5-Cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium · (MW 315.42)
Physical Data: mp 92-94 °C (dec); 1H NMR (C6D6) d 0.9 (m, 2H), 1.64 (m, 2H), 2.22 (m, 8H), 2.92 (m, 4H), 3.79 (m, 2H), 4.78 (m, 2H), 5.22 (dd, 2H).
Solubility: sol toluene and chloroform; insol alcohols.
Form Supplied in: yellow crystalline solid.
Preparative Method: to a solution of RuCl3.3H2O (0.34 g, 1.3 mmol) in 10 mL of ethanol is added 1,5-cyclooctadiene (8.8 g, 81 mmol) and zinc dust (3.5 g); the mixture is heated under reflux with stirring for 3 h and then filtered; the resulting brown-yellow solution is evaporated in vacuo, extracted with 20 mL of pentane and filtered through a plug of alumina (20 cm); the volume is reduced to ca. 5 mL and the yellow solution cooled to -78 °C to give yellow crystals of (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium (0.2 g, 50%).1
Handling, Storage, and Precautions: the compound rapidly decomposes when exposed to air.
While little is known about the chemistry of bis(h5-cyclooctadienyl)ruthenium, the catalytic activities of (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium, Ru(cod)(cot), and its derivatives in organic synthesis are well established. This article will attempt to cover the chemistry of all these low valent ruthenium compounds.2
Allylsilanes are highly versatile synthetic intermediates3 which can be prepared selectively by the Ru(cod)(cot) catalyzed dehydrogenative silylation of alkenes (eq 1).4 In all reactions reported, only minor amounts of vinylsilanes and alkanes (derived from hydrogenation of the starting alkene)5 were detected. Conversely, reactions employing Ru3(CO)12 as the catalyst source afforded vinylsilanes as the major product.
In a related study, (benzene)(1,3-cyclohexadiene)ruthenium was found to be a moderately active and selective catalyst precursor for the hydrosilation of allyl chlorides with trimethoxysilane.6
A number of low valent ruthenium complexes catalyze the intermolecular hydroacylation of alkenes with various aromatic and heteroaromatic aldehydes.7 Unsymmetrical ketones are produced in moderate to high yields in reactions run at 180-200 °C for 24-48 h under 20 kg cm-2 of carbon monoxide. For example, the reaction of 2-thiophenecarbaldehyde with cyclohexene affords the corresponding cyclohexyl 2-thienyl ketone in 62% yield (eq 2). Treatment of cyclohexene with aliphatic aldehydes gives products arising from a transhydroformylation reaction, where the formyl group of the aldehyde has transferred to the alkene with concomitant formation of Tishchenko-type reaction products.8
Ru(cod)(cot) is a moderately effective catalyst precursor for the carbonylation of amines, affording N-substituted formamides. In the presence of alkenes, hydroamidation affords N-substituted alkanamides (eq 3).9 In both reactions, addition of phosphine reduces catalytic activity. Indeed, Ru3(CO)12 was found to be the most active and selective catalyst precursor for these transformations.
Secondary amines react with carbon dioxide and terminal alkynes in the presence of a catalytic amount of Ru(cod)(cot)/tertiary phosphine to give enol carbamates with high regio- and stereoselectivity (Z/E ca. 9:1).10 2-Acetoxyallyl derivatives are produced with high selectivity and in high yield by reaction of acetic acid with propargyl alcohol derivatives at 80 °C using a bis(h5-cyclooctadienyl)ruthenium/PCy3/maleic anhydride catalyst system.11
A number of C-C bond forming reactions catalyzed by low valent ruthenium complexes have been developed, including the oligomerization of alkenes and alkynes,12 [2 + 2] cross addition of norbornenes with alkynes13 and allylamines with acrylic compounds (eq 4),14 aldol condensations, and Michael addition reactions.15
More recently, ruthenium complexes such as Ru(cod)(cot) were found to catalyze the dehydrohalogenative coupling of vinyl halides with electron deficient alkenes to give substituted dienes (eq 5). In some cases, ruthenium complexes were more active than the well established palladium catalysts. For instance, even the sp2 carbon-chlorine bond in b-chlorostyrene is activated by Ru(cod)(cot).16 Monoallylated carbonucleophiles are produced in high yields and with high regioselectivity in the presence of a catalytic amount of Ru(cod)(cot). Regioselectivities are quite different from products obtained from palladium catalyzed reactions. Furthermore, allylations using allyl methyl carbonate give diallylated carbonucleophiles selectively and in high yields. Diallylations are not easily accomplished using other metal catalysts.17
Stephen A. Westcott
University of North Carolina, Chapel Hill, NC, USA