Organometallic chemistry of diphosphazanes. Part 26. Ruthenium hydride complexes of chiral and achiral diphosphazane ligands and asymmetric transfer hydrogenation reactions

Venkatakrishnan, Thengarai S and Mandal, Swadhin K and Kannan, Raghuraman and Krishnamurthy, Setharampattu S and Nethaji, Munirathinam (2007) Organometallic chemistry of diphosphazanes. Part 26. Ruthenium hydride complexes of chiral and achiral diphosphazane ligands and asymmetric transfer hydrogenation reactions. In: Journal of Organometallic Chemistry, 692 (10). pp. 1875-1891.

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Abstract

The half-sandwhich ruthenium chloro complexes bearing chelated diphosphazane ligands, [(eta(5)-Cp)RuCl{kappa(2)-P,P-(RO)(2)PN(Me)P(OR)(2)}] [R = C6H3Me2-2,6] (1) and [(eta(5)-Cp*)RuCl{kappa(2)-P, P-X2PN(R)PYY'}] [R = Me, X = Y = Y' = OC6H5 (2); R = CHMe2, X-2 = C20H12O2, Y = Y' = OC6H5 (3) or OC6H4'Bu-4 (4)] have been prepared by the reaction of CpRu(PPh3)(2)Cl with (RO)(2)PN(Me)P(OR)(2) [R = C6H3Me2-2,6 (L-1)] or by the reaction of [Cp*RuCl2](n) with X2PN(R)PYY' in the presence of zinc dust. Among the four diastereomers (two enantiomeric pairs) possible for the "chiral at metal" complexes 3 and 4, only two diastereomers (one enantiomeric pair) are formed in these reactions. The complexes 1, 2, 4 and [(eta(5)-Cp)RuCl {kappa(2)-P,P-Ph2PN((S)-*CHMePh)PPhY)] [Y = Ph (5) or N2C3HMe2-3,5 (SCSPRRu)-(6)] react with NaOMe to give the corresponding hydride complexes [(eta(5) -Cp)RuH {kappa(2)-P,P-(RO)(2)PN(Me)P(OR)(2)}] (7), [(eta(5)-Cp*)RuH {kappa(2)-P,P'-X2PN(R)PY2)] [R = Me, X = Y = OC6H5 (8); R = CHMe2, X-2 = C20H12O2, Y = OC6H4'Bu-4 (9)] and [(eta(5) -Cp)RuH(kappa(2)-P, P-Ph2PN((S)-*CHMePh)PPhY)][Y =Ph (10) or N2C3HMe2-3,5 (SCSPRRu)(11a) and (SCSPSRu)-(11b)]. Only one enantiomeric pair of the hydride 9 is obtained from the chloro precursor 4 that bears sterically bulky substituents at the phosphorus centers. On the other hand, the optically pure trichiral complex 6 that bears sterically less bulky substituents at the phosphorus gives a mixture of two diastereomers (11a and 11b). Protonation of complex 7 using different acids (HX) gives a mixture of [(eta(5)- Cp)Ru(eta(2)-H-2){kappa(2)-P, P-(RO)(2)PN(Me)P(OR)(2))]X (12a) and [(eta(5)-Cp)Ru(H)(2){kappa(2)-P, P-(RO)(2)PN(Me)P(OR)(2)}]X (12b) of which 12a is the major product independent of the acid used; the dihydrogen nature of 12a is established by T, measurements and also by synthesizing the deuteride analogue 7-D followed by protonation to obtain the D-H isotopomer. Preliminary investigations on asymmetric transfer hydrogenation of 2-acetonaphthone in the presence of a series of chiral diphosphazane ligands show that diphosphazanes in which the phosphorus centers are strong pi-acceptor in character and bear sterically bulky substituents impart moderate levels of enantioselectivity. Attempts to identify the hydride intermediate involved in the asymmetric transfer hydrogenation by a model reaction suggests that a complex of the type, [Ru(H)(Cl){kappa(2)-P,P-X2PN(R)PY2)(solvent)(2)] could be the active species in this transformation. (c) 2007 Elsevier B.V. All rights reserved.

Item Type:	Journal Article
Publication:	Journal of Organometallic Chemistry
Publisher:	Elsevier Science
Additional Information:	Copyright of this article belongs to Elsevier Science.
Department/Centre:	Division of Chemical Sciences > Inorganic & Physical Chemistry
Date Deposited:	10 Jun 2010 09:22
Last Modified:	19 Sep 2010 05:59
URI:	http://eprints.iisc.ac.in/id/eprint/26852

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