Synthesis and Catalytic Applications of Electronically Modified Ruthenium Complexes and Carbonyl Bridged Diruthenium Complexes
- Synthesis and Catalytic Applications of Electronically Modified Ruthenium Complexes and Carbonyl Bridged Diruthenium Complexes
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- Dynamic kinetic resolution (DKR) is an attractive and challenging reaction in which kinetic resolution (KR) is combined with in situ racemization of unreacted enantiomer. Up to now, various metal catalysts have been reported for racemization of secondary alcohols. To develop more efficient catalysts for racemization and DKR of secondary alcohols several ruthenium catalysts were synthesized by focusing on the tuning electronic properties of catalysts.
From the reaction of hexafluoro-2-butyne and Ru3(CO)12, three ruthenium complexes 9-11 were obtained which contains several electron withdrawing CF3 substituents. Sturucture of the complexes were confirmed by X-ray crystallography. However, due to the unstable nature of complexes, further transformation of complexes did not achevied. On the other hand, inhibition of C-C bond rotation was found which is orginated from the interaction between electron rich fluorine atom of CF3 group and electron defficient cabon atom of C=O ligand in complex 10. In addtion, several derivatives of [5-Ph4C4CO(C=O)Ph]Ru(CO)2Cl (5) were prepared with systematically varying the phenyl groups to investigate the electronic effects of substituents on the catalytic racemization of alcohols. Electron-donating substituents enhanced the racemization activity significantly. In particular, the catalyst 20 which has five 4-methoxyphenyl substituetns, is about 14 times more active than 5 in the racemization of 1-phenylethanol. Catalyst 20 is applicable to the DKR of various secondary alcohols at room temperature without concerning about aerobic conditions and can be reused several times without losing any significant activity.Previously, photoactive carbonyl bridged diruthenium complex (7) was reported. This complex is activated by irradition of 30 W fluorescent light and catalyzed several organic reactions such as racemization of seceondary alcohols, hydrosilylation of aldehydes and hydrostannation of alkynes. From the effort to develop analogue derivatives of 7, reaction of AIBN with ruthenium hydride monomers was found to give carbonyl bridged diruthenium complexes. Several ruthenium complexes were prepared according to the procedure.
Obtained complexes also could be activated by fluorescent light under mild condition. They exihibited different catalytic acivities in racemization of secondary alcohols, hydrosilylation of benzaldehydes and hydrostannation of alkynes. For racemization and hydrostannation, new catalysts showed good activities but some catalysts were less reactive due to the low solubility. For hydrosilylation of aldehydes, all the catalysts were not effective for the reaction.
Imines are an important class of compound in organic synthesis which can be utilized for intermediate of various organics. Various tansformations were known such as hydrogenation to amines, cycloaddition, aza-Diels alder reaction to give N-heterocycles, nucelophilic addition to C=N bond, Mannich reaction and aza-Balliys-Hilman reaction.
Imines were obtained by the reaction of organic azides and amines catalyzed by 7 with illumination of fluorescent light. In this novel system, various azides and amines can react to give imines without additives other than 7. The reaction proceeded through the generation of primary aldimines which is difficult to isolate due to its unstable nature by evlotion of nitrogen. In our catalytic system, primary aldimines could be utilized by in situ generation from azides.
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