Ruthenium diphosphine substituents

Reductively induced alterations from cumulenic to alkynyl resonance structures have been observed for mononuclear and dinuclear ruthenium allenylidene complexes. The half-wave potentials for the one-electron reduction of allenylidene complexes [ Ru = C = C = C(ER )(R )] ( Ru = trun.s-Cl(L2)2Ru L2 = chelating diphosphine ER = NR2, SR, SeR, aryl, alkyl R = aryl, alkyl) strongly depends on the nature of the ER substituent. Amino- and aryl-substituted congeners with reduction potentials of ca. -2.2 V and -1.0 V, respectively, constitute the two extremes within this series. These sizable potential... 

Since the diphosphine is appreciably more electron-rich than is BINAP, the major ruthenium complex is a more active hydrogenation catalyst than the parent. Increased electron-rich ligation may be the reason for the success of heterocyclic analogues of BINAP in which the binaphthalene is replaced by a bi(ben-zothiophene) or biindolyl the resulting Ru complexes are effective both in terms of enantioselectivity and reactivity [139]. Readers of the related Chapter 6.1 on the asymmetric hydrogenation of carbonyl compounds will encounter the Ru complexes of ligands in the DUPHOS family, where the ease of modification of the alkyl substituents of the phospholane enhances the power of the system, since it permits the easy optimization of ee for any substrate [140]. 

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