Ruthenium furan synthesis

In 2003, the Uemura group reported a diversity-oriented approach to highly substituted furans based on ruthenium- and platinum-catalyzed intramolecular annulation of propargylic alcohols 59 and ketones 60 (Scheme 19.28) [22]. This reaction is related to the aforementioned pyrrole synthesis (Scheme 19.13). Based on the time profile of the reaction, in which rapid formation/consumption of the alk-4-yn-l-one 63 was seen but only gradual formation/consumption of the diketone 122 was observed, they proposed a mechanism including alkyne hydration and subsequent cyclization of the resulting diketones. However, this reaction can be considered as a furan synthesis by formal reaction between the C=0 and alkyne groups. 

The complex forms purple air-stable crystals and has good solubility in both polar and nonpolar solvents (e.g., acetone, dichloromethane, tetrahydro-furan, benzene, and hexane). It has been characterized crystallographically.7 The structure is an open, but folded, ladder-like array of six metal atoms with the two platinum atoms in the center. The IR spectrum (hexane) exhibits V(co> bands at 2085 (m), 2062 (vs), 2035 (vs), 2016 (w). Pt2Ru4(CO)18 has been found to be very useful for the synthesis of new platinum-ruthenium cluster complexes.10,12,13... 

Some other natural compounds have been transformed for their use in the synthesis of polymers via olefin metathesis processes. As mentioned in the introduction, furans, which are obtained from carbohydrates, are perfect precursors of monomers for ROMP via simple Diels-Alder cycloadditions (n) (Scheme 25) [26]. In this regard, the first example of the ROMP of 7-oxabicyclo[2.2.1]hept-5-ene derivatives was reported by Novak and Grubbs in 1988 using ruthenium- and osmium-based catalysts [186]. The number of examples of ROMP with monomers with this generic structure is vast, and it is out of the scope of this chapter to cover all of them. However, it is worth mentioning here the great potential of a renewable platform chemical like furan (and derived compounds), which gives access to such a variety of monomers. 

The recent synthesis of furans via isomerization of terminal epoxyalkynes catalysed by RuCl(Tp)(MeCN)2 in the presence of a base at 80 °C in 1,2-dichloroethane is explained by a related intramolecular nucleophilic addition of the oxygen atom of the epoxide onto the a-carbon atom of a ruthenium vinylidene intermediate (Scheme 8) [33]. This reaction is specific of terminal alkynes and tolerates a variety of functional groups (ether, ester, acetal, tosyl-amide, nitrile). 

Gunnoe has also reported examples of catalytic aromatic alkylation using a ruthenium complex and olefins. With propylene and other terminal olefins, a 1.6 1 preference for anti-Markovnikov addition was seen. The proposed mechanism involved olefin insertion into the metal-aryl bond followed by a metathesis reaction with benzene to give the alkylated aromatic and a new metal-phenyl bond (Equation (26)). DFT calculations supported the proposed non-oxidative addition mechanism. The work was extended to include catalytic alkylation of the a-position of thiophene and furan. With pyrrole, insertion of the coordinated acetonitrile into the a-C-H bond was observed. Gunnoe has also summarized recent developments in aromatic C-H transformations in synthesis using metal catalysts. ... 

Mclnturff EL, Nguyen KD, Krische MJ (2014) Redox-triggered C-C coupling of diols and alkynes synthesis of P, y-unsaturated a-hydroxyketones and furans by ruthenium-catalyzed hydrohydroxyalkylation. Angew Chem Int Ed 53 3232-3235... 

Ku9ukbay, H., Cebnkaya, B., Guesmi, S., Dixneuf, P.H., New (carbene)ruthenium-arene complexes preparation and uses in catalytic synthesis of furans, Organometallics 1996, 15 2434-2439. 

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