Pincer complexes, ruthenium

Amide formation plays a very important role in chemical synthesis [1, 46]. Preparation of amides under neutral conditions and without generation of waste is a challenging goal [46-48]. Applying our pyridine-based ruthenium pincer complexes as catalysts, amides were synthesized directly from amines and alcohols or esters polyamides were obtained from diols and diamines, as delineated in this section. 

The analogous iridium formate complex was synthesized by Kaska and coworkers by reacting CO2 with a Ir(lll) dihydride complex. However, in this case, the formate complex proved to be unstable, undergoing disproportionation to form the hydrogen carbonate complex and the carbonyl dihydride, overall corresponding to the reverse water gas shift reaction CO2 -H H2 - CO-P H20[45j. Reduction of CO2 to the methanol level has since been effected using an aromatic nickel pincer complex and a cascade reaction involving a ruthenium pincer complex in one step [46]. 

Tridentate bis(oxazolinyl)pyridinyl rhodium and ruthenium pincer complexes are useful as catalysts for hydrosilylations and cyclopropanations. These NNN-type inorganic pincer complexes are not as stable, however, as phosphine or salen-type pincer complexes. On the other hand, an organometallic tridentate bis(oxazolinyl) phenyl NCN-type complex is stable. These optically active NCN-type pincer complexes act as efficient catalysts for enantioselective hetero Diels-Alder reactions of Danishefsky s diene with glyoxylates [26]. 

Pyridines and quinolines are prepared via dehydrogenative coupling of y-aminoalcohols and secondary alcohols in the presence of a bipyridyl-based ruthenium pincer complex (Scheme 6) (13CC6632). A number of cyclic and acyclic secondary alcohols were tolerated under these conditions. 

Ye X, Plessow PN, Hofmann P (2014) Alcohol amination with ammonia catalyzed by an acridine-based ruthenium pincer complex a mechanistic study. J Am Chem Soc 136(16) 5923-5929... 

Agrawal S, Lenormand M, Martm-Matute B (2012) Selective alkylation of (hetero)aromatie amines with alcohols catalyzed by a ruthenium pincer complex. Org Lett 14(6) 1456 1459... 

Hydrogenation of Polar Bonds Catalysed by Ruthenium-Pincer Complexes... 

Gnanaprakasam B, Balaiaman E, Ben-David Y, Milstein D (2011) Synthesis of peptides and pyrazines from p-amino alcohols through exbusion of H2 catalyzed by ruthenium pincer complexes Ligands-ctuitrolled selectivity. Angew Chem Int Ed 50 12240... 

Finally, the most recent report regarding reactivity of ruthenium pincer complexes with carbon monoxide was published by Milstein and coworkers [51]. They informed... 

Fig. 5 Possible catalytic cycle for the direct synthesis of amides from alcohols and amines, catalyzed by the PNN ruthenium pincer complex 5...

Wang and Huang ° reported the synthesis of air-stable ligand 49 and its water-stable PN -type ruthenium pincer complex 50, containing an aminophosphine arm (Scheme 12), which was demonstrated to be an effective catalyst in hydrogenation of esters into the corresponding alcohols, under mild conditions. 

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