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Inorganica Chimica Acta 361 (2008) 2785–2791

Homogeneous �C@O hydrogenation and transferhydrogenation catalyzed by ruthenium(II) complexescontaining optically active Ph2PCH(Ph)CH(Me)NH2

and 1,2-C5H8(PPh2)2 ligands

Lutz Dahlenburg *, Christian Kuhnlein

Institut fur Anorganische Chemie der Universitat Erlangen-Nurnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany

Received 11 December 2007; received in revised form 30 January 2008; accepted 31 January 2008Available online 7 February 2008

Abstract

Combination of (1S,2S)-cyclopentanediylbis(diphenylphosphine) with [Ru(g4-C8H12){g3-(CH2)2CMe}2] afforded the chelate complex[Ru{g3-(CH2)2CMe}2{(1S,2S)-C5H8(PPh2)2}] (1), which gave (OC-6-13)-[RuCl2{(1S,2S)-C5H8(PPh2)2}{(1S,2S)-Ph2PCH(Ph)CH(Me)-NH2}] (2) upon reaction with methanolic HCl in acetone, followed by the addition of the b-aminophosphine in DMF. The (P \ N)2-che-lated complexes (OC-6-13)-[RuCl2{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}2] (3) and (OC-6-13)-[RuCl2{(1R,2S)-Ph2PCH(Ph)CH(Me)-NH2}2] (4) resulted from RuCl3 � 3H2O and the P,N ligands under reducing conditions. The crystal structures of 3 and 4 were determinedby single-crystal X-ray diffraction. Following activation by KOBu-t in isopropanol, compounds 2–4 catalyzed the enantioselective trans-fer hydrogenation of acetophenone with i-PrOH as the hydrogen source as well as the direct hydrogenation of the ketone by H2 in low tomoderate e.e. (up to 67%).� 2008 Elsevier B.V. All rights reserved.

Keywords: Chirality; Ruthenium; P,N ligands; X-ray structure analysis; Hydrogenation

1. Introduction

Noyori’s outstanding ruthenium(II) complexes withenantiopure P,P and N,N donors – in particular, 2,20-bis-(diphenylphosphino)-1,10-binaphthyl (binap), 1,2-dipheny-lethylenediamine, and modifications thereof – have beendominating the field of homogeneous �C@O hydrogena-tion for several years because of their ability to catalyzethe asymmetric reduction of prochiral ketones to second-ary alcohols with consistently high activity and enantiose-lectivity [1].

In continuation of our investigations on Rh- and Ir-cen-tered �C@C� and �C@O hydrogenation catalysts contain-

0020-1693/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2008.01.042

* Corresponding author. Tel.: +49 9131 8527353; fax: +49 91318527387.

E-mail address: dahlenburg@chemie.uni-erlangen.de (L. Dahlenburg).

ing the structurally versatile chiral cyclopentane-1,2-diylbis(phosphines) 1,2-trans-C5H8(PR2)2 (R: variousC- or O-bonded alkyl and aryl groups) [2] and b-amino-phosphines Ph2PCH(Ph)CH(Me)N(H)R (R: H or alkyl)[3] that were previously developed by our group, we heredescribe some enantiomerically pure RuII complexes ofsuch chelate ligands and give an account of their applica-tion as catalysts for the stereoselective reduction of thestandard test substrate acetophenone. In two different waysdo these compounds contrast with the advanced Noyorisystems, where the central metal is always coordinated tothe two phosphorus and nitrogen atoms of one bis(phos-phine) and one diamine ligand: either their coordinationspheres are made up of one chelating bis(phosphine) andone aminophosphine to form P \ P/P \ N derivatives ortheir structural motif features the pairwise (P \ N)2 coordi-nation of two aminophosphine ligands.

2786 L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791

In earlier work with ruthenium complexes of racemic1,2-trans-C5H8(PR2)2 and achiral Ph2PCH2CR2NH2

(E = H, Me) chelate ligands we had shown that one cata-lytic system derived from these compounds, viz.,[RuCl2(Ph2PCH2CMe2NH2)2]-KOBu-t/i-PrOH, was activefor both the direct hydrogenation of acetophenone withmolecular H2 as reducing agent and the transfer hydroge-nation of the substrate with i-PrOH as source of H+ and H�

equivalents [4]. Hence, one particular aspect of the presentstudy was to probe enantiomerically pure (pre)catalystssuch as, e.g., [RuCl2{(1S,2S)-C5H8(PPh2)2}{(1S,2S)-Ph2

PCH(Ph)CH(Me)NH2}] and [RuCl2{(1S,2S)-Ph2PCH-(Ph)CH(Me)NH2}2] in cross experiments for their stereo-selectivities in direct and transfer hydrogenation reactions.As summarized in a current in-depth paper dealing withthis specific topic [5], just a very few comparative studieshave been carried out until recently on the catalyticapplication of one and the same type of coordination com-pounds to the two modes of ketone reduction.

While this study was underway, some RuII complexeswith chiral and achiral b-aminophosphine ligands madefrom amino acids and alcohols were introduced by theMorris group as moderately selective (pre)catalysts forthe direct hydrogenation of simple ketones [6].

2. Experimental

2.1. General remarks

All manipulations were performed under nitrogen orargon using standard Schlenk techniques. Solvents weredistilled from the appropriate drying agents prior to use.NMR: Bruker DPX 300 (300.1 MHz for 1H, 75.5 MHz for13C, and 121.5 MHz for 31P) with SiMe4 as internal orH3PO4 as external standards (downfield positive; ‘‘m”:deceptively simple multiplets) at ambient temperature. Gaschromatography: Shimadzu GC-17A (FID). Publishedprocedures were used for the syntheses of the starting mate-rials (1S,2S)-C5H8(PPh2)2 [7], (1S,2S)-Ph2PCH(Ph)CH-(Me)NH2, (1R,2S)-Ph2PCH(Ph)CH(Me)NH2 [3a,3c], and[(g4-1,5-C8H12)Ru{g3-(CH2)2CMe}2] [8a]. Ruthenium(III)chloride hydrate (43% Ru) was purchased from PressureChemical Co. (Pittsburgh).

2.2. Ruthenium complexes

2.2.1. [Ru{g3-(CH2)2CMe}2{(1S,2S)-C5H8(PPh2)2}] (1)

A mixture of 0.734 g (2.29 mmol) of [(g4-1,5-C8H12)Ru{g3-(CH2)2CMe}2] and 1.000 g (2.29 mmol) of(1S,2S)-cyclopentanediylbis(diphenylphosphine) in 40 mLof hexane was stirred at reflux temperature for 5 h, whichcaused the product to separate from solution as a yellowprecipitate; yield (after thorough washing with hexaneand drying under vacuum) 1.205 g (81%). 1H NMR(CD2Cl2): d = 0.55, 0.57, 1.26, 1.78 (all br, 2H each, allallyl CH2), 2.02 (s, 6H, allyl CH3), 1.92–2.04, 2.20–2.22,2.39–2.44 (all m, 2H each, all ring CH2), 3.48–3.61 (m,

2H, ring CH), 6.77–7.94 (m, 20H, C6H5). 13C{1H} NMR(CD2Cl2): d = 26.94 (AA0X-‘‘t”,

PJ(P,C) = 10.90 Hz, ring

C-3,5), 26.25 (s, CH3), 32.02 (‘‘s”, ring C-4), 41.54 (AA0X-‘‘t”,

PJ(P,C) = 18.17 Hz, ring C-1,2), 43.57 (br ‘‘s”, allyl

CH2 trans P), 58.42 (t, J(P,C) = 23.99 Hz, allyl CH2 cis

P), 95.42 (s, allyl CMe), 127.75–139.58 (m, C6H5).31P{1H} NMR (CD2Cl2): d = 62.40 [s, (D-S,S) diastereo-mer; 65%], 63.07 [s, (K-S,S) diastereomer; P95%]. Anal.Calc. for C37H42P2Ru (649.72): C, 68.40; H, 6.52. Found:C, 67.85; H, 6.49%.

2.2.2. (OC-6-13)-[RuCl2{(1S,2S)-

C5H8(PPh2)2}{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}]

(2)A solution of 0.682 g (1.05 mmol) of allyl complex 1 in

20 mL of acetone was stirred with 2.42 mL of 1 M aqueousHCl, dissolved in 10 mL of methanol, for 3 h at room tem-perature. The orange yellow residue remaining after removalof all volatile material in vacuo was re-dissolved in 20 mL ofDMF and treated with 10 mL of a solution of 0.366 g(1.15 mmol) of (1S,2S)-[(2-amino-1-phenyl)propyl]diphen-ylphosphine in the same solvent. Stirring for 12 h at 70 �Cfollowed by evaporation to dryness left a semi-solid whichwas crystallized at �20 �C from a toluene/pentane mixture;yield: 0.390 g (40%) of orange crystals. 1H NMR (CDCl3):d = 0.99 (d, 3J(H,H) = 5.13 Hz, 3H, CH3), 1.14–1.77,2.33–2.54 (both m, 8H, C5H8), 3.12–3.17 (m, 1H, CHMe),3.37–3.48 (m, 1H, CHPh), 3.74–3.78, 4.26–4.29 (both br,1H each, NH2), 6.52–7.74 (m, 35H, C6H5). 31P{1H} NMR(CDCl3): ABX system with d(PA) = 33.66, d(PB) = 49.41(both PPh2 in mutual trans position), d(PX) = 46.54 (PPh2

trans NH2), J(PA,PB) = 309.32, J(PA,PX) = 28.97, andJ(PB,PX) = 26.73 Hz. Anal. Calc. for C50H50Cl2NP3Ru(929.16): C, 64.59; H, 5.42; N, 1.51. Found: C, 64.85; H,5.48; N, 1.60%.

2.2.3. (OC-6-13)-[RuCl2{(1S,2S)-

Ph2PCH(Ph)CH(Me)NH2}2] (3)A mixture of 0.233 g (1.00 mmol) of RuCl3 � xH2O,

0.262 g of zinc dust (4.00 mmol), and 0.654 g (2.05 mmol)of the P,N ligand in 25 mL of THF was heated at refluxtemperature for 3 h. The residue obtained after filtrationover Al2O3, elution with 40 mL of THF, and evaporationof all volatile material was triturated with methanol/diethylether (1:1), filtered off, thoroughly washed pentane, anddried under vacuum; yield: 0.187 g (23%) of orange micro-crystals. 1H NMR (CDCl3): d = 1.26 (d, 3J(H, H) =4.92 Hz, 6H, CH3), 3.57, 3.82, 4.20 (all br m, 4/2/2H,CH/NH2), 6.43–7.31 (m, 30H, C6H5). 31P{1H} NMR(CDCl3): d = 78.27 (s). Anal. Calc. for C42H44Cl2N2P2Ru(810.70): C, 62.22; H, 5.47; N, 3.46. Found: C, 62.02; H,5.47; N, 3.34%.

2.2.4. (OC-6-13)-[RuCl2{(1R,2S)-Ph2PCH(Ph)CH-(Me)NH2}2] (4)

The complex was prepared as outlined before from1.480 g (6.30 mmol) of RuCl3 � 3H2O, 1.700 g (26.00 mmol)

L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791 2787

of zinc dust, and 4.155 g (13.00 mmol) of (1R,2S)-[(2-amino-1-phenyl)propyl]diphenylphosphine in 150 mL ofTHF; yield 2.505 g (49%) of orange crystals. 1H NMR(CDCl3): d = 1.25 (d, 3J(H,H) = 6.06 Hz, 6H, CH3), 3.86,3.99, 4.28, 4.65 (all br m, 2H each, CH/NH2), 6.46–7.27(m, 30H, C6H5). 13C{1H} NMR (CDCl3): d = 23.09(s, CH3), 55.30 (s, CHMe), 57.00 (ABX-‘‘t”,

PJ(P,C) =

18.17 Hz, CHPh), 126.21–131.33 (m, C6H5). 31P{1H}NMR (CDCl3): d = 75.19 (s). Anal. Calc. for C42H44Cl2-N2P2Ru (810.70): C, 62.22; H, 5.47; N, 3.46. Found: C,62.33; H, 5.06; N, 3.14%.

2.3. General procedure for the catalytic hydrogenation of

acetophenone

Solutions of the precatalysts 2, 3, or 4 (typically 2–5 � 10�3 M) were prepared in a 1:1 benzene/isopropanolsolvent mixture (used for transfer hydrogenation) or – fordirect hydrogenation experiments – in neat isopropanol.Aliquots of these solutions and 0.02 M isopropanolicKOBu-t were mixed to generate typical catalyst-to-baseratios of 1:5 (transfer hydrogenation) or 1:100 (directhydrogenation). After stirring the mixtures for 30 minunder nitrogen, acetophenone was added in quantities cor-responding to substrate-to-catalyst ratios (s/c) of 200:1 and2000:1 for transfer hydrogenation experiments and 10 000:1for catalytic runs carried out under the conditions of direct�C@O hydrogenation. The final volumes of the reactionmixtures amounted to 10–15 mL.

In transfer hydrogenations, stirring under an inert atmo-sphere was continued at 60 �C. For monitoring the pro-gress of the reactions, small aliquots were removed atregular intervals, evaporated to dryness, re-dissolved indiethyl ether, and filtered over a short silica gel column.Volatile material was distilled off and the mixtures of prod-ucts were analyzed by 1H NMR. Conversions and productcompositions were determined on the basis of the integra-tions of the PhC(O)CH3 and PhCH(OH)CH3 signals.Enantiomeric excesses were measured by GC using aChrompack Chiralsil-Dex CB column.

For the direct hydrogenation of the substrate by H2, theinitial catalyst/substrate mixtures were transferred in smallSchlenk tubes equipped with a magnetic stirring bar to anautoclave. The autoclave was pressurized and vented sev-eral times with H2 (Messer-Griesheim; 99.999%), andfinally pressurized to 20 bar and kept at 30 or 60 �C for3 h. Work-up and determination of conversions and prod-uct compositions were done as outlined above for the mix-tures of products resulting from transfer hydrogenationexperiments.

2.4. X-ray structure determinations

The specimen used for the structure analysis of 4 wasgrown from THF/hexane. Recrystallization of 3 from a tol-uene/pentane solvent mixture afforded single crystals of thetoluene-containing addition compound 3 � C7H8. Diffrac-

tion measurements were made at 20 ± 2 �C on an Enraf–Nonius CAD-4 MACH 3 diffractometer; data correctedfor absorption using semi-empirical methods [9] (3 � C7H8:Tmin = 0.859, Tmax = 0.959; 4: Tmin = 0.821, Tmax = 0.917)methods. The structures were solved by direct methods andsubsequently refined by full-matrix least-squares proce-dures on F2 with allowance for anisotropic thermal motionof all non-hydrogen atoms employing the WINGX [10a]package with some of the relevant programs (SIR-97 [11],SHELXL-97 [12], ORTEP-3 [10b]) implemented therein.3 � C7H8 (0.42 � 0.10 � 0.07 mm): C42H44Cl2N2P2Ru,C7H8 (902.84); monoclinic, P21, a = 14.736(2), b =10.517(2), c = 14.819(1) A, b = 106.23(1)�, V = 2205.1(9) A3,Z = 2, dcalc = 1.360 g cm�3, l(Mo Ka) = 0.585 mm�1;2.30� 6 H 6 25.17�, 8742 reflections collected (�17 6h 6 +17, �12 6 k 6 +12, � 17 6 l 6 +17), 7936 unique(Rint = 0.0288); wR2 = 0.0735 for all data and 506 parame-ters, R1 = 0.0383 for 6913 intensities I > 2r(I); absolutestructure parameter x = �0.05(3) [13]. 4 (0.35 � 0.28 �0.13 mm): C42H44Cl2N2P2Ru (810.70); monoclinic, P21,a = 13.489(2), b = 10.724(3), c = 14.114(3) A, b =106.64(1)�, V = 1956.2(8) A3, Z = 2, dcalc = 1.376 g cm�3,l(Mo Ka) = 0.651 mm�1; 2.42� 6 H 6 25.17�, 7736 reflec-tions collected (�16 6 h 6 +16, � 12 6 k 6 +12, � 16 6l 6 +16), 7018 unique (Rint = 0.0334); wR2 = 0.0841 forall data and 444 parameters, R1 = 0.0484 for 5496 intensi-ties I > 2r(I); absolute structure parameter x = �0.04(3)[13].

3. Results and discussion

3.1. Synthesis and characterization of the complexes

The preparation of the P \ P/P \ N-coordinated com-plex (OC-6-13)-[RuCl2{(1S,2S)-C5H8(PPh2)2}{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}] (2) utilized the protocolpreviously reported for the synthesis of Noyori-typemixed-ligand compounds from diallyl rutheniumprecursors [Ru{g3-(CH2)2CMe}2{bis(phosphine)}] [8,14].Following that procedure, [Ru{g3-(CH2)2CMe}2{(1S,2S)-C5H8(PPh2)2}] (1) was first made by the addition of oneequivalent of the chelating bis(phosphine) to [(g4-1,5-C8H12)Ru{g3-(CH2)2CMe}2] [8a] in refluxing hexane.Complex 2 was then obtained in a one-pot reaction bycombining 1 in acetone with two equivalents of aqueousHCl in methanol followed by an equimolar amount of(1S,2S)-[(2-amino-1-phenyl)propyl]diphenylphosphine inDMF; Scheme 1.

Because of the chelate-like bonding of its two g3-allylligands complex 1 can exist as (D-S,S) and (K-S,S) diaste-reomers. The presence of the two isomeric forms in solu-tion (CD2Cl2) was indeed evident from the 31P{1H}NMR spectrum displaying singlets at d = 62.40 and63.07. The ratio of the signal intensities (which is less than5:95; see Section 2) indicates a high degree of diastere-oselectivity for the formation of 1 as expected, sincewe had already shown that the racemic analogue of 1,

Scheme 1.

2788 L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791

[Ru{g3-(CH2)2CMe}2{rac-1,2-C5H8(PPh2)2}], exists pre-dominantly (�98%) as (D-R,R)/(K-S,S) pair of enantio-mers [d(31P) = 63.42 in CDCl3] and only in inferiorportions (�2%) as (K-R,R)/(D-S,S) stereoisomers[d(31P) = 62.65 in CDCl3] [4]. In agreement with these ear-lier findings, the prevailing diastereomeric form of 1 isassigned as (K-S,S).

The meridional arrangement of the three –PPh2 donorgroups in the coordination sphere of (OC-6-13)-[RuCl2{(1S,2S)-C5H8(PPh2)2}{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}] (2) was easily deduced from the presence ofa large (309.4 Hz) trans P,P coupling in the ABX type31P{1H} NMR spectrum of this complex. Further classifi-cation of 2 as trans rather than cis Cl–Ru–Cl isomer wasmade by analogy to two closely related [RuCl2{rac-1,2-C5H8(PPh2)2}{Ph2PCH2CR2NH2}] compounds (R = H,Me) which have similar 31P spectra and were shown byX-ray structural analysis to possess trans-bonded chloridoligands [4].

Complexes (OC-6-13)-[RuCl2{(1S,2S)-Ph2PCH(Ph)CH-(Me)NH2}2] (3) and (OC-6-13)-[RuCl2{(1R,2S)-Ph2PCH-(Ph)CH(Me)NH2}2] (4) were prepared by reacting RuCl3 �xH2O (x � 3) with 2 equiv. of (1S,2S)- or (1R,2S)-[(2-amino-1-phenyl)propyl]diphenylphosphine in THF in thepresence of zinc dust (Scheme 2). Although P,N-coordi-nated compounds of the general type [RuCl2(P \ N)2] arealso accessible by combining substitutionally labile precur-sors such as [RuCl2(p-cumene)] [6a], [RuCl2(Me2SO)4]

Scheme 2.

[15,16], [RuCl2(PPh3)3] [17,18] or [(g4-1,5-C8H12)RuCl2]x[15] with the required b-aminophosphine, the use ofhydrated ruthenium(III) chloride as a starting material[4,18] is particular attractive because it avoids additionalsynthetic intermediates.

The (OC-6–13)-coordination geometry of 3 and 4 withthe chlorido ligands in trans positions and the two nitrogenand phosphorus atoms cis to each other was established bysingle crystal X-ray crystallography (Figs. 1 and 2). Thisarrangement which puts the strong trans influence P donorsopposite the weaker trans bond influencing NH2 groups isapparently favored for electronic reasons over the stericallyless crowded geometry having the bulky diphenylphos-phino substituents trans to each other. In the solid state,molecules 3 and 4 are assembled through intermolecularN–H� � �Cl hydrogen bonds. The bond lengths around thecentral metal of the two compounds – Ru–N, 2.152(3)–2.165(4) A, Ru–P, 2.272(1)–2.283(1) A, and Ru–Cl,2.418(1)–2.442(1) A – fall within the range reported forthe corresponding metal-to-ligand distances of several pre-viously characterized (OC-6-13)-[RuCl2(Ph2P \ NH2)2]and (OC-6-13)-[RuCl2(Ph2P \ NHR)2] complexes[4,15,17,18]. Similar to the latter, molecules 3 and 4 featureconsiderable distortions of their Cl–Ru–Cl axes from line-arity: 163.10(4)� and 163.72(5)�, respectively.

3.2. Catalytic hydrogenation of acetophenone

If activated by an alkoxide base, compounds 2–4 cata-lyzed both the direct and the transfer hydrogenation ofthe standard test substrate acetophenone (Table 1).

Transfer hydrogenation experiments were carried out at60 �C using the respective ruthenium precatalyst in thepresence of 5 equiv. of added KOBu-t in isopropanol/ben-zene (1:1). At a substrate-to-catalyst ratio (s/c) of 2000:1,base-modified mixed-ligand complex 2 gave 64% substrateconversion within 3 h, which represents at least 420 turn-overs/h by this catalytic system. The resulting productalcohol was enriched in the R enantiomer with a moderateenantiomeric excess (e.e.) of 59%. Compounds 3 and 4

C8C7

C6C5

C4

C9

C16

C17

C18

C19

C20

C21

C11

C12

C10C15

C14 C13

P1

N1C1

C2

C3Cl2

Cl1

Ru1

N2C24

C22

C23C25

C26C27

C28

C29C30

C32

C33C34

C35

C36C31

C37

C38

C39

C40

C41

C42

P2

Fig. 1. Perspective view of (OC-6-13)-[RuCl2{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}2] (3); phenyl hydrogen atoms are omitted for clarity; selected bondlengths (A) and angles (�): Ru1–Cl1, 2.4266(12); Ru1–Cl2, 2.4421(12); Ru1–P1, 2.2833(12); Ru1–P2, 2.2771(11); Ru1–N1, 2.161(3); Ru1–N2, 2.152(3).Cl1–Ru1–Cl2, 163.10(4); Cl1–Ru1–P1, 99.31(4); Cl1–Ru1–P2, 88.17(4); Cl1–Ru1–N1, 83.66(10); Cl1–Ru1–N2, 82.46(11); Cl2–Ru1–P1, 90.20(4); Cl2–Ru1–P2, 102.97(4); Cl2–Ru1–N1, 83.67(10); Cl2–Ru1–N2, 86.21(10); P1–Ru1–P2, 104.80(4); P1–Ru1–N1, 83.02(10); P1–Ru1–N2, 171.68(10); P2–Ru1–N1, 169.54(10); P2–Ru1–N2, 83.33(10); N1–Ru1–N2, 89.13(13). Hydrogen bonding interactions as D–H, H� � �A, D� � �H, D–H� � �A (A, �): N1–H1A� � �Cl2#1, 0.90, 2.70, 3.524(4), 152.4; N2–H2B� � �Cl2#1, 0.90, 2.91, 3.484(4), 122.9 (symmetry transformation #1 used to generate equivalent atoms:�x, y � 1/2, �z + 1).

C20

C19

C18

C21

C16

C17

C33

C32

C31C36

C35

C34

C30C29

C28

C27C26

C25C22

C23

C24

C39

C38

C37

P2C42

C40C41

N2

Ru1

Cl2

Cl1

C15

C14C13

C12C11

C10P1

N1

C2

C1C4

C5

C6

C7C8

C9

C3

Fig. 2. Perspective view of (OC-6-13)-[RuCl2{(1R,2S)-Ph2PCH(Ph)CH(Me)NH2}2] (4); phenyl hydrogen atoms are omitted for clarity; selected bondlengths (A) and angles (�): Ru1–Cl1, 2.4180(15); Ru1–Cl2, 2.4289(16); Ru1–P1, 2.2791(17); Ru1–P2, 2.2723(15); Ru1–N1, 2.157(4); Ru1–N2, 2.165(4).Cl1–Ru1–Cl2, 163.72(5); Cl1–Ru1–P1, 92.92(6); Cl1–Ru1–P2, 96.50(5); Cl1–Ru1–N1, 83.52(12); Cl1–Ru1–N2, 85.05(13); Cl2–Ru1–P1, 97.09(6); Cl2–Ru1–P2, 93.14(6); Cl2–Ru1–N1, 85.01(12); Cl2–Ru1–N2, 83.28(13); P1–Ru1–P2, 105.62(6); P1–Ru1–N1, 82.86(13); P1–Ru1–N2, 172.05(13); P2–Ru1–N1,171.49(13); P2–Ru1–N2, 82.26(13); N1–Ru1–N2, 89.27(17). Hydrogen bonding interactions as D–H, H� � �A, D� � �H, D–H� � �A (A, �): N1–H1B� � �Cl1#1,0.90, 2.62, 3.281(4), 131.3 (symmetry transformation #1 used to generate equivalent atoms: �x, y � 1/2, �z).

L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791 2789

catalyzed the hydrogen transfer from the solvent to theketone with distinctly diminished reactivity and selectivity:at s/c = 200:1, the yields of 1-phenylethanol (82% and 97%

after 3 h) corresponded to turnovers of only 55–65/h, andthe enantioselectivities achieved with these compoundswere as low as 10% (S) and 12% (R), respectively.

Table 1Enantioselective hydrogenation of acetophenone in the presence of base-modified ruthenium complexes 2–4

Catalyst s/c t (h) %PhCH(OH)Me

% e.e. Config.

Transfer hydrogenationa

2–KOBu-t (1:5) 2000:1 3 64 59 R

3–KOBu-t (1:5) 200:1 3 97 10 S

4–KOBu-t (1:5) 200:1 3 82 12 R

Direct hydrogenationb

2–KOBu-t(1:100)c

10000:1 3 >99 67 R

3–KOBu-t(1:100)d

10000:1 3 50 38 S

3–KOBu-t(1:100)c

10000:1 3 >99 29 S

4–KOBu-t(1:100)c

10000:1 3 50 5 R

a In isopropanol/benzene (1:1) at 60 �C.b p(H2) = 20 bar.c In isopropanol at 60 �C.d In isopropanol at 30 �C.

2790 L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791

Similar trends in stereoselectivity and, to a lesser degree,in activity were observed when the three complexes wereprobed for their behavior as catalysts for the direct hydro-genation of acetophenone. Precatalyst 3 – containing two[(2-amino-1-phenyl)propyl]diphenylphosphine ligands withlike (1S,2S) configuration of their backbone carbon atoms– furnished a 50% yield of the product alcohol with 38%e.e. (S) and �1670 turnovers/h, if activated by the additionof 100 equiv. of KOBu-t and left with the ketone in isopro-panol1 under H2 (20 bar) at 30 �C and s/c = 10000:1 for3 h. With the same catalytic system at 60 �C, the activityincreased to at least 3300/h (almost quantitative substrateconversion within 3 h) but the selectivity decreased to29% (S). Use of base-modified complex 4 – being chelatedby two Ph2PCH(Ph)CH(Me)NH2 bidentates with unlike(1R,2S) carbon configurations in the connecting ethylenechain – led with reduced rate (�1670 turnovers/h at60 �C) to a mixture of 1-phenylethanol enantiomers closeto racemic. The combined system 2-KOBu-t (1:100)showed a more satisfactory performance: in isopropanolat s/c = 10 000:1, the hydrogenation of acetophenone to(R)-1-phenylethanol occurred in >99% yield (i.e.,>3300 turnovers/h) with 67% e.e. when the reaction wasleft under 20 bar of H2 for 3 h.

It is of interest to compare these results with thoseobtained for the direct hydrogenation of acetophenone cat-alyzed by alkoxide-activated ruthenium complexes[Ru(H)(Cl){(R)-binap}(b-aminophosphine)] and [Ru(X)-

1 The likelihood of the ketone reduction proceeding by direct metal-assisted transfer of Hd�/Hd+ equivalents from the dihydrogen molecule tothe carbonyl dipole was substantiated by ruling out the alternativepathway involving Hd�/Hd+ transfer from the alcoholic solvent: whenacetophenone, dissolved in Me2CDOH, was hydrogenated with H2

employing the combined (2–4)-KOBu-t catalysts, no a-deuteratedPhCD(OH)Me was observed in the mixture of 1-phenylethanolenantiomers.

(Cl)(b-aminophosphine)2] (X = H, Cl), where the P,N

ligands are derived from alanine, proline, or norephedrineenantiomers [6]; viz., (R)-[(2-amino)propyl]diphenylphos-phine [‘‘(R)-Pala”], 2-(S)-diphenylphosphinomethylpyrroli-dine [‘‘(S)-Ppro”], and (1R,2R)-Ph2PCH(Ph)CH(Me)NH2

(an aminophosphine that was originally made in our group[3] as a further stereoisomer of the chelate ligands used inthe present study).

When catalyzed by [Ru(H)(Cl){(S)-Ppro}2], acetophe-none was hydrogentated to 1-phenylethanol in very lowe.e. (6%), which is similar to the poor selectivity displayedby [RuCl2{(1R,2S)-Ph2PCH(Ph)CH(Me)NH2}2] (4). Thesparse ability of the [(S)-Ppro]2-chelated complex toaccomplish the enantiofacial discrimination of the sub-strate on its molecular surface can be attributed to the exis-tence of that catalyst as two diastereomers differing in thenitrogen configurations of the coordinated pyrrolidine ring[6b]. The equally feeble capacity of 4 to differentiatebetween the two acetophenone enantiofaces, on the otherhand, obviously results from the mismatch of the unlikecarbon configurations in the chelate backbones, since(pre)catalyst 3 with two Ph2PCH(Ph)CH(Me)NH2 ligandshaving like (S,S) carbon configurations in the connect-ing C2 chains is seen to afford the product in dis-tinctly higher (albeit still modest) e.e.: up to 38% (S),depending on the reaction conditions (Table 1). Similarselectivities of 32–51% e.e. for (R)-1-phenylethanol werepreviously obtained with two trans-HRuCl and trans-RuCl2 complexes containing the opposite (R,R)-enantio-meric P,N ligand [6a]. Combination of optically active b-aminophosphines and enantiopure bis(phosphines) givesrise to P \ P/P \ N-chelated ruthenium(II) catalysts withimproved enantioselectivities, representative examplesbeing the binap-coordinated systems [Ru(H)(Cl){(R)-bin-ap}{(R)-Pala}] [e.e.: 40% (S)], [Ru(H)(Cl){(R)-bin-ap}{(1R,2R)-Ph2PCH(Ph)CH(Me)NH2}] [e.e.: 55–61%(S)], and [Ru(H)(Cl){(S)-binap}{(1R,2R)-Ph2PCH(Ph)-CH(Me)NH2}] [e.e.: 67–72% (S)] introduced by Morrisand co-workers [6] as well as the chelate complex 2 withone (1S,2S)-C5H8(PPh2)2 and one (1S,2S)-Ph2PCH(Ph)-CH(Me)NH2 ligand as described above [e.e.: 67% (R)].

4. Concluding remarks

This work has shown that base-activated dichloridoru-thenium complexes with optically active 1,2-trans-C5H8(PPh2)2 and Ph2PCH(Ph)CH(Me)NH2 ligands canoperate as catalysts for the asymmetric reduction of aceto-phenone to 1-phenylethanol under the conditions of directand transfer hydrogenation. While [RuCl2{(1R,2S)-Ph2PCH(Ph)CH(Me)NH2}2] and [RuCl2{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}2] display only little or, at best,modest enantioselectivities in the two modes of �C@Oreduction, the mixed-ligand complex [RuCl2{(1S,2S)-C5H8(PPh2)2}{(1S,2S)-Ph2PCH(Ph)CH(Me)NH2}] cata-lyzes the formation of the product alcohol with a higherdegree of enantioselctivity which closely matches that of

L. Dahlenburg, C. Kuhnlein / Inorganica Chimica Acta 361 (2008) 2785–2791 2791

related catalysts containing the binap ligand in addition toa b-aminophosphine [6a].

Acknowledgements

Support of this work by the Deutsche Forschungsgeme-inschaft (Bonn, SFB 583) and the Fonds der Chemischen

Industrie (Frankfurt/Main) is gratefully acknowledged.We are also indebted to Mrs. S. Hoffmann for her skilfulassistance.

Appendix A. Supplementary material

CCDC 670140 and 670141 contain the supplementarycrystallographic data for this paper. These data can beobtained free of charge from The Cambridge Crystallo-graphic Data Centre via www.ccdc.cam.ac.uk/data_re-quest/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2008.01.042.

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