In parallel to the activation of alkynes to generate active vinylidene intermediates our Rennes team explored the selective transformation of alkynes by oxidative coupling with electron-rich ruthenium(II) catalysts, derivatives of Cp*RuXL2 complexes, (Cp* = C5Me5), which was revealed first to be an excellent route to various functional dienes.
The above innovations in catalytic syntheses involving alkynes, by rising questions about mechanisms greatly contributed to the improvement of ruthenium organometallic chemistry.
The power of CpRu(Cl)L2 catalysts for the generation of vinylidene–ruthenium or alkylidene–ruthenium intermediates in synthesis has been illustrated in several reviews [, ].
The easy synthesis of unsaturated ketones from alkynes allowed to perform sequential catalysis. Thus after formation of the ketone, addition of copper(II) catalyst in air atmosphere allowed its transformation into furan derivative (Eq. ) . Alternatively the ruthenium catalyzed formation of unsaturated ketone followed by silver catalysis allow the hydroarylation of the double bond with electron rich arenes .
Since its inception in 2006, a rapidly growing number of different metal-catalysed reactions have proven suitable for the active metal template synthesis of both rotaxanes and catenanes, including the copper(i)-catalysed terminal alkyne-azide cycloaddition (the CuAAC "click" reaction), palladium- and copper-catalysed alkyne homocouplings and heterocouplings, and palladium-catalysed oxidative Heck couplings and Michael additions.
During the study of the ruthenium catalyzed synthesis of β-oxoalkylcarbamates by reaction of CO2, HNR2 and propargylic alcohol with RuCl2(PBu3)(arene) (Eq. ), to understand the mechanism different ratios of Ru(II) catalyst and PBu3 were used and by changing the amine by NEt3 or H2NR we observed that (i) PBu3 alone catalyzed the reaction, (ii) with tertiary amine NEt3 cyclic carbonate was partially formed, and iii) with primary amine H2NR α-methylene oxazolidinones were produced. Thus, tertiary propargylic alcohols with CO2 (50 bar) in an inert autoclave with 8 mol % of PBu3 led to the isolation (>95 %) of α-methylene cyclic carbonates, in the absence of other solvent than the propargylic alcohol. PBu3 appeared to be more efficient than PPh3 and PCy3 (Eq. ) [, ].
Our interest in directly incorporating CO2 derivatives into alkynes, by routes avoiding the use of phosgene, led J. Fournier and C. Bruneau to discover an “organocatalytic” synthesis of cyclic carbonates and carbamates by a route different from the usual insertion of CO2 into oxiranes and oxetanes.
The catalytic synthesis of vinylcarbamates, via a vinylidene–ruthenium(II) active intermediate, resulted from alkyne activation with a 16 electron ruthenium(II) species for example of type [RuX(PR3)(arene)]+Y−. We decided to explore the activation of terminal alkynes with ruthenium(II) catalyst precursors capable to provide a coordinatively unsaturated 14 electron catalytic species, by displacement of weakly coordinating ligands, but at the same time more electron-rich ruthenium(II) sites to favour oxidative couplings between two functional unsaturated molecules.
In that case H. Doucet showed that the catalyst precursors react with carboxylic acids to give Ru(O2CR)2(diphosphine) and isobutene inhibiting ruthenium protonation and favouring activation of alkynes via vinylidene intermediate. The nature of the diphosphine was crucial as bis(diphenylphosphino)butane (dppb) favours formation of Z-enol esters at low temperature whereas bis(diphenylphosphino)ethane (dppe) leads to better selectivity with bulky terminal alkynes (Eq. ) [, , , ]. These Z-enol esters were found to be useful for the access to (E)-enamines and to α-cyanoesters on reaction with amines and KCN/HSiEt3, respectively .
B. M. Trost showed also that the same catalyst CpRuCl(COD) could offer the access to unsaturated ketones from terminal alkyne and substituted allylic alcohols. In that case the oxidative coupling preferentially afforded the linear isomer .
It is noteworthy that the ruthenium catalytic addition of carboxylic acids to terminal alkynes and propargylic alcohols actually inhibited ruthenium–vinylidene formation. It was suggested that this inhibition was brought by the concomitant protonation of the ruthenium center and external addition of the carboxylate to the coordinated triple bond [, ].
Ruthenium vinylidene intermediates were also proposed by M. Murakami using a similar catalytic system CpRu(PPh3)2Cl/NaPF6 for the coupling of unactivated alkenes with terminal alkynes to afford 1,3-dienes as a mixture of two isomers, the linear one being favoured (Eq. ) .
The regioselective synthesis of vinylcarbamates led us to evaluate the ruthenium(II) activation of alkynes towards carboxylic acids addition in an attempt to produce enol and dienol esters useful acylating reagents, such as vinylacetate or in peptide synthesis, or polymer precursors. According to the nature of both catalysts and substrates, both regioselective Markovnikov and anti-Markovnikov additions could be obtained.