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Inorganica Chimica Acta 319 (2001) 176–182

Chiral bisphosphanes�

Part XIII. Nickel, palladium and platinum complexes containingoptically active cyclopentane-based P2 ligands with phosphorinane

and dicyclohexylphosphanyl donor functions,[(1S,2S)-C5H8(PC5H10-cyclo)2MCl2] and

[(1S,2S)-C5H8{P(C6H11)2}2MCl2]: synthesis, structure andapplication to catalytic Grignard cross-coupling

Lutz Dahlenburg *, Volker KurthInstitut fur Anorganische Chemie der Uni�ersitat Erlangen-Nurnberg, Egerlandstraße 1, D-91058 Erlangen, Germany

Received 24 February 2001; accepted 2 April 2001

Abstract

The chelate complexes [(1S,2S)-C5H8(PR2)2MCl2] [PR2=PC5H10-cyclo : M=Ni (1), Pd (2), Pt (3); PR2=P(C6H11)2: M=Ni(4), Pd (5), Pt (6)] were prepared from NiCl2·6H2O, [(�4-1,5-C8H12)PdCl2] or [(Me2SO)2PtCl2] and the required optically activebisphosphane ligand, (1S,2S)-C5H8(PC5H10-cyclo)2 or (1S,2S)-C5H8[P(C6H11)2]2, in order to serve as catalysts for the synthesis ofnon-racemic 2,2�-dimethyl-1,1�-binaphthyl by asymmetric cross-coupling of 1-bromo-2-methylnaphthalene with its derived Grig-nard reagent. The coupling reactions generally suffered from low conversion. Nickel complex 4, the crystal structure of which wasdetermined by X-ray diffraction, gave the (R)-(− ) form of the C–C coupling product in modest enantiomeric excess (26% e.e.).© 2001 Elsevier Science B.V. All rights reserved.

Keywords: Crystal structures; Chiral P ligands complexes; Group 10 metals complexes; Cross coupling

1. Introduction

The development of new chiral ligands for applica-tions in transition metal-based asymmetric catalysiscontinues to play an important role in synthetic organicand organometallic chemistry. Starting from the re-solved (1S,2S) and (1R,2R) enantiomers of the chiralcyclopentane-based, trans-disubstituted bis(primaryphosphane) C5H8(PH2)2 [2a–c] and bis(phosphonousdichloride) C5H8(PCl2)2 [2d,2e,3], respectively, we haverecently worked out several useful procedures for the

synthesis of optically active bidentate phosphorus lig-ands. These allow the preparation of a wide rangeof enantiopure cyclopentane-1,2-diyl-bisphosphanes(1R,2R)- and (1S,2S)-C5H8(PR2)2 with P–C-, P–N-, orP–O-bonded residues R featuring different electronic,spatial and stereochemical requirements [2b–g].

In this report we describe some dihalo complexes ofthe Group 10 metals Ni, Pd, and Pt of the moreelectron-rich bis(phosphorinane)- and bis(dicyclo-hexylphosphanyl)-substituted chelate ligands (1S,2S)-C5H8(PC5H10-cyclo)2 [2f] and (1S,2S)-C5H8[P(C6H11)2]2[2d], which were prepared to serve as catalysts for thetransition metal-mediated synthesis of non-racemic, at-ropisomeric 2,2�-dimethyl-1,1�-binaphthyl by asymmet-ric cross-coupling of 1-bromo-2-methylnaphthalenewith its derived Grignard reagent [4]. Optically activeP2 ligands of the cyclopentane-1,2-diyl-bisphosphane

� Part XII: [1].* Corresponding author. Tel.: +49-9131-85 27353; fax: +49-

9131-85 27387.E-mail address: dahlenburg@chemie.uni-erlangen.de (L. Dahlen-

burg).

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 -1693 (01 )00451 -0

L. Dahlenburg, V. Kurth / Inorganica Chimica Acta 319 (2001) 176–182 177

type, previously employed in enantioselective cross-cou-pling reactions, include the enantiomers of C5H8(PPh2)2

[5] and o- or p-methoxy-substituted derivatives thereof[6a–c]. Their dihalo nickel complexes were used tocatalyze Grignard-type alkylations of vinyl halides[6a,6c], cyclic allyl phenyl ethers [6b,6d,6e], and crotylalcohol derivatives [7], respectively.

2. Experimental

2.1. General remarks

All manipulations were performed under nitrogenusing standard Schlenk techniques. Solvents were dis-tilled from the appropriate drying agents prior to use.NMR: Bruker DPX 300 (300.1 MHz for 1H, 75.5 MHzfor 13C and 121.5 MHz for 31P) at 20�2°C with SiMe4

as internal or with H3PO4 as external standard(downfield positive); ‘�J(P,C)’: separation of the twomost intense lines in the ‘virtually coupled’ 13C{1H}AA�X subspectra (A, A�=31P; X=13C) [8,9]. Massspectra: JEOL JMS 700; m/z values refer to the peak ofhighest relative intensity in the respective cluster ofisotopomeric ions. Polarimetry: Schmid & Haensch Po-lartronic E (1 dm cells at room temperature); [� ]� in 10°cm2 g−1, c in g 100 ml−1.

NiCl2·6H2O (Fluka) was used as supplied. 1-Bromo-2-methylnaphthalene (�90% technical grade, Aldrich)was freed from isomeric bromomethylnaphthalenes byremoving these impurities through a Grignard cross-coupling reaction with �15 mol % of methylmagne-sium bromide, in which 1-bromo-2-methylnaphthalenereacts much slower than its regioisomers [10] and canbe separated easily from the dimethylnaphthalenesformed by fractional distillation [4c,4f]. The enantio-pure ligands (1S,2S)-C5H8(PC5H10-cyclo)2 [2f] and(1S,2S)-C5H8[P(C6H11)2]2 [2d] were prepared as de-scribed previously. Published procedures were used forthe synthesis of [(�4-1,5-C8H12)PdCl2] [11] and[(Me2SO)2PtCl2] [12].

2.2. Metal complexes

2.2.1. [(1S,2S)-C5H8(PC5H10-cyclo)2NiCl2] (1)A solution of 0.20 g (0.74 mmol) of (1S,2S)-

C5H8(PC5H10-cyclo)2 in 15 ml of CH2Cl2 was addeddropwise to a stirred solution of 0.18 g (0.76 mmol) ofNiCl2·6H2O in 40 ml of ethanol. The mixture, whichimmediately changed color from green to reddishbrown, was stirred for 8 h at ambient conditions.Evaporation to a residual volume of ca. 30 ml causedthe product to precipitate as a reddish brown precipi-tate, which was filtered off, washed with 5 ml of ethanoland dried under vacuum; yield 0.27 g (91%). 1H NMR(CDCl3): �=0.8–3.1 (m; CH2, CH). 13C{1H} NMR

(CDCl3): �=51.04 (AA�X-t, �J(P,C)=46.9 Hz;2C5H8–C1,2H); partially overlapping CH2 singlets andAA�X multiplets at �=18.7, 22.6, 23.1, 23.6, 24.8–25.1,26.1, and 31.2 not analyzed. 31P{1H} NMR (CDCl3):�=25.53. MS (EI; 70eV, 180°C): m/z=400 (3%) [M+],363 (6%) [M+−Cl], 328 (4%) [M+−2Cl]. Anal.Found: C, 44.49; H, 7.11. C15H28Cl2NiP2 (399.95)Calc.: C, 45.05; H, 7.06%.

2.2.2. [(1S,2S)-C5H8(PC5H10-cyclo)2PdCl2] (2)A quantity of 0.25 g (0.93 mmol) of (1S,2S)-

C5H8(PC5H10-cyclo)2, dissolved in 20 ml of CH2Cl2,was added to a solution of 0.30 g (1.05 mmol) of[(�4-1,5-C8H12)PdCl2] in 30 ml of the same solvent.Stirring overnight induced the precipitation of 2 as anorange microcrystalline solid, which was isolated byfiltration, washed with dichloromethane (2×5 ml), anddried under vacuum; yield 0.31 g (74%). 1H NMR(CD2Cl2): �=1.4–3.1 (m; CH2, CH). 13C{1H,31P}NMR (CD2Cl2): �=20.18 (2C5H8–C3,5H2), 23.81,24.18 (both 2C5H10–C2,6H2), 25.71, 25.81 (both2C5H10–C3,5H2), 26.09 (2C5H10–C4H2), 32.44 (C5H8–C4H2), 52.56 (2C5H8–C1,2H); assignments of the C5H8–CH2 resonances by analogy to those of [C5H8(PR2)-PtCl2] (R=Ph, NC5H10-cyclo, OPh) [2g]; phosphori-nane signals were assigned tentatively on the basis ofthe 13C{1H} spectrum observed the free ligand [2f].31P{1H} NMR (CD2Cl2): �=29.91. MS (FD; CH2Cl2,50 mA, 2 kV): m/z=448 [M+], 413 [M+−Cl]. Anal.Found: C, 41.10; H, 6.37. C15H28Cl2P2Pd (447.64)Calc.: C, 40.25; H, 6.30%.

2.2.3. [(1S,2S)-C5H8(PC5H10-cyclo)2PtCl2] (3)The preparation of this compound was carried out as

described for 2 by reacting 0.40 g (0.95 mmol) of[(Me2SO)2PtCl2] with 0.24 g (0.89 mmol) of the P2

ligand in 50 ml of dichloromethane; yield 0.39 g (82%)of 3 as a pale yellow solid. 1H NMR (CD2Cl2): �=1.0–3.8 (m; CH2, CH). 13C{1H,31P} NMR (CD2Cl2):�=19.32 (2C5H8–C3,5H2), 23.34, 23.51, 23.88, 24.78(2×2C5H10–C2,6H2 together with 2×2C5H10–C3,5H2),25.46 (2C5H10–C4H2), 31.56 (C5H8–C4H2), 50.91(2C5H8–C1,2H). 31P{1H} NMR (CD2Cl2): �=3.86[195Pt-flanked s, 1J(Pt,P)=3490 Hz]. MS (EI; 70 eV,180°C): m/z=536 (4%) [M+], 463 (28%) [M+−2HCl].Anal. Found: C, 33.19; H, 4.87. C15H28Cl2P2Pt (536.33)Calc.: C, 33.59; H, 5.26%.

2.2.4. [(1S,2S)-C5H8{P(C6H11)2}2NiCl2] (4)This material was obtained from the reaction be-

tween 0.52 g (1.12 mmol) of (1S,2S)-C5H8[P(C6H11)2]2in 30 ml of CH2Cl2 and 0.30 g (1.26 mmol) of NiCl2 ·6H2O, dissolved in 30 ml of ethanol, as outlined forcomplex 1; yield (after washing with 10 ml of methanoland drying) 0.54 g (81%) of dark orange crystals. 1HNMR (CDCl3): �=0.8–3.5 (m; CH2, CH). 13C{1H,31P}

L. Dahlenburg, V. Kurth / Inorganica Chimica Acta 319 (2001) 176–182178

NMR (CDCl3): �=24.02, 25.49, 26.20, 26.88, 26.90,27.06, 27.13, 27.63, 28.40, 29.64, 30.76, 31.96 (all CH2),32.68, 34.07 (both C6H11–CH), 48.48 (C5H8–CH); ap-parently simple AA�X triplets were observed for thelatter three CH resonances in the 31P-coupled spectrum:�J(P,C)=20.8, 28.6, and 40.8 Hz, respectively.31P{1H} NMR (CDCl3): �=44.58. MS (FD; CH2Cl2,50 mA, 2 kV): m/z=592 [M+], 555 [M+−Cl]. Anal.Found: C, 58.48; H, 8.88. C29H52Cl2NiP2 (592.30)Calc.: C, 58.81; H, 8.85%.

2.2.5. [(1S,2S)-C5H8{P(C6H11)2}2PdCl2] (5)The preparation followed the procedure given for 2:

0.99 g (2.14 mmol) of (1S,2S)-C5H8[P(C6H11)2]2 and0.61 g (2.14 mmol) of [(�4-1,5-C8H12)PdCl2] in 100 mlof CH2Cl2 yielded 1.36 g (99%) of complex 5 as a darkorange solid. 1H NMR (CDCl3): �=0.8–3.3 (m; CH2,CH). 13C{1H,31P} NMR (CDCl3): �=23.85, 25.38,26.12, 26.81, 26.88, 26.94, 26.98, 27.27, 28.44, 30.57,30.70, 31.76 (all CH2), 33.38, 34.18 (both C6H11–CH),49.36 (C5H8–CH). 31P{1H} NMR (CDCl3): �=57.27.MS (FD; CH2Cl2, 50 mA, 2 kV): m/z=605 [M+−Cl].Anal. Found: C, 55.08; H, 8.31. C29H52Cl2P2Pd (640.01)Calc.: C, 54.42; H, 8.19%.

2.2.6. [(1S,2S)-C5H8{P(C6H11)2}2PtCl2] (6)Equimolar amounts of [(Me2SO)2PtCl2] (0.12 g, 0.28

mmol) and (1S,2S)-C5H8[P(C6H11)2]2 (0.13 g, 0.28mmol) were combined in 50 ml of dichloromethane andallowed to interact for 8 h at room temperature. Re-moval of all volatile material left 6 as a bright yellowsolid residue which was thoroughly washed with diethylether. Even by repeated re-precipitation from CH2Cl2/Et2O the product could not be freed from pertina-ciously adhering dimethyl sulfoxide so that noconclusive analytical results were obtained; yield 0.19 g.1H NMR (CDCl3): �=1.0–3.7 (m; CH2, CH).13C{1H,31P} NMR (CDCl3): �=23.02, 25.47, 26.28,26.47, 26.60, 26.85, 26.88, 27.06, 28.12, 30.31, 30.48,31.45 (all CH2), 32.32, 32.39 (both C6H11–CH), 47.99(C5H8–CH); in the 31P-coupled spectrum, the latterthree CH resonances were observed as ‘filled-in’-dou-blets [8,9]: �J(P,C)=37.6, 40.6, and 45.7 Hz, respec-tively; residual dimethyl sulfoxide was identified at�=44.66. 31P{1H} NMR (CDCl3): �=30.33 [195Pt-flanked s, 1J(Pt,P)=3550 Hz]. MS (FD; CH2Cl2, 50mA, 2 kV): m/z=728 [M+], 693, 692 [M+−Cl] super-imposed by [M+−HCl].

2.3. X-ray structure determination

Single crystals of 4 (size 0.38×0.30×0.20 mm) weregrown from dichloromethane, which gave the complexas a 1/1 CH2Cl2 solvate, 4·CH2Cl2. Diffraction mea-surements were made at 20�2°C on a Nonius MACH-3 diffractometer using graphite-monochromated Mo

K� radiation (�=0.71073 A� ): orientation matrix andunit cell parameters from the setting angles of 25 cen-tered medium-angle reflections; collection of the diffrac-tion intensities by � scans; data corrected forabsorption by � scans [13] (Tmin=0.7060, Tmax=0.8274). The structure was solved by direct methodsand subsequently refined by full-matrix least-squaresprocedures on F2 with allowance for anisotropic ther-mal motion of all non-hydrogen atoms employing theWINGX package [14] and the relevant programs (SIR-97[15], SHELXL-97 [16], ORTEP-3 [17]) implementedtherein. H atoms were included in the final structuralmodels assuming ideal geometry and using appropriateriding models. The residual electron density remainingafter refinement of the structural model suggested two-fold rotational disorder of one of the two crystallo-graphically independent solvate molecules about theCl–C–Cl bisector, which was accounted for by assign-ing the particular chlorine and hydrogen positions splitoccupancies of 0.5 each. Positional disorder over twosites with occupancies of 0.6 and 0.4 was indicated forthe second CH2Cl2 molecule of crystallization as awhole. 4·CH2Cl2: C29H52Cl2NiP2·CH2Cl2 (677.18);monoclinic, P21, a=11.853(2), b=13.926(3), c=21.225(3) A� , �=100.53(1)°, V=3444(1) A� 3, Z=4,dcalc=1.306 g cm−3, �(Mo K�)=0.985 mm−1; 2.60���23.99°, 11 600 reflections (−13�h� +13,−15�k� +15, −24� l� +24, including Friedelpairs) collected, 10774 unique (Rint=0.0638); wR=0.1465 for all data and 697 parameters, R=0.0696 for6217 intensities I�2(I), GOF=1.022, absolute struc-ture parameter [18]: −0.03(3).

2.4. Grignard cross-coupling reaction

A 0.37 M stock solution of 1-(bromomagnesio)-2-methylnaphthalene in diethyl ether/toluene was pre-pared by adding a solution of 40.45 g (183 mmol) of1-bromo-2-methylnaphtalene in 200 ml of diethyl etherto a suspension of magnesium turnings (7.50 g, 308mmol) in 50 ml of the same solvent under ultrasonicirradiation according to a published procedure [19].The Grignard reagent was obtained as a yellow viscousslurry, which was dissolved by dilution with toluene toexactly 500 ml.

For the cross-coupling reaction, a mixture of 2.0 ml(2.86 g, 12.9 mmol) of 1-bromo-2-methylnaphthaleneand 1.0–2.6 mol % of the corresponding Ni(II) orPd(II) pre-catalyst (Scheme 1) in 5 ml of diethyl etherwas treated with 1.0 ml of a 1.0 M solution of methyl-magnesium bromide in di-n-butyl ether and gentlyheated for 10 min in order to reduce the added complexto the catalytically active metal(0) species [4c]. Aftercooling, a 32.0 ml aliquot of the 0.37 M stock solutionof the Grignard component (11.8 mmol) was added andthe reaction was allowed to proceed for 13–29 days at

L. Dahlenburg, V. Kurth / Inorganica Chimica Acta 319 (2001) 176–182 179

room temperature, depending on the metal complexused (Scheme 1). The mixture was then hydrolyzed byadding 10 ml of 2 M aqueous HCl and extracted withdiethyl ether. The combined organic phases were thor-oughly washed with saturated NaHCO3 solution andwater, dried over Na2SO4 and evaporated to leave thecrude products as brown oily residues, which wereanalyzed by 1H NMR spectroscopy. Conversions asgiven in Scheme 1 were determined on the basis of theintegrations of the methyl signals of the couplingproduct (�=2.15) and unchanged 1-bromo-2-methyl-naphthalene (�=2.71). The oils were subsequentlychromatographed on a column (30×1.5 cm) made upwith SiO2 (silica gel 60, Merck) in n-hexane. Elutionwith n-hexane, followed by elution with 2% ethyl ace-tate in n-hexane gave the product-containing fraction,which on evaporation of all volatile material yielded theformed 2,2�-dimethyl-1,1�-binaphthyl as a colorlesssyrup exhibiting 1H NMR data that corresponded in allrespects to those reported in the literature [20]. Fromthe reaction catalyzed by nickel complex 4, a quantityof 0.34 g (10%) of spectroscopically pure couplingproduct was obtained. The optical rotations [� ]589 −9.8, [� ]578 −10.2, and [� ]546 −12.2 (all c=1.5, CHCl3)showed the sample had R configuration and a 26% e.e.;Ref. [4c]: [� ]D −35.6, at 94% e.e.; Ref. [4f]: [� ]589

−31.3, [� ]578 −33.0, and [� ]546 −38.6, at 83% e.e. (allc=1, CHCl3).

3. Results and discussion

The chelate complexes [(1S,2S)-C5H8(PR2)2MCl2][PR2=PC5H10-cyclo : M=Ni (1), Pd (2), Pt (3); PR2=P(C6H11)2: M=Ni (4), Pd (5), Pt (6); see Scheme 1]were synthesized by reacting the required enantiopureP2 ligand (1S,2S)-C5H8(PC5H10-cyclo)2 [2f] or (1S,2S)-C5H8[P(C6H11)2]2 [2d] with NiCl2·6H2O in ethanol orwith [(�4-1,5-C8H12)PdCl2] [11] or [(Me2SO)2PtCl2] [12]in dichloromethane at ambient conditions, similar tothe procedures previously described for related Group10 metal(II) containing the methyl(cyclooctyl)-substi-tuted (1S,2S)-C5H8[P(CH3)C8H15]2 ligand [2b]. Due tothe lack of a molecular mirror plane bisecting theR–P–R angles of the –PR2 donor groups, the carbonatoms of the cyclohexyl substituents and the phosphori-nane halves become pairwise diastereotopic and arefound to be sufficiently different for their nonequiva-lence to be detected in the 75.5 MHz 13C{1H,31P} NMRspectra.

The molecular structure of nickel complex 4, crystal-lizing from dichloromethane as the a 1/1 CH2Cl2 sol-vate 4·CH2Cl2, was determined by single-crystal X-raydiffraction and is shown in Fig. 1, selected bond lengthsand valence and torsion angles being collected in thelegend. The X-ray structure analysis revealed the pres-ence of two crystallographically independent tetracoor-dinate molecules showing slight tetrahedral distortionfrom ideal square-planar coordination geometry. Thus,the sum of the four interligand cis angles (360.3° formolecule 1 and 360.2° for molecule 2) is as required fora square-planar surrounding of the central metal, butthe angles between the normals to the planes defined bythe two cis-Cl–Ni–Cl and cis-P–Ni–P fragments(molecule 1: 6.3°; molecule 2: 8.0°) are larger than thelimiting value of 0° for square-planar coordination.These deviations from planarity, which can safely beattributed to some steric congestion at the centralmetal, are somewhat larger than the interplanar twistsobserved for three related [(1S,2S)-C5H8{P(CH3)-C8H15}2MX2] complexes (MX2=NiCl2, PdI2, PtI2), theMX2/MP2 dihedral angles of which amount to 4.0�0.5° [2b]. They are however small in comparison withother tetracoordinate Group 10 metal(II) complexesfeaturing enhanced steric crowding, such as the mor-pholino-substituted dineopentyl [C5H8{P[N(C2H4)2-O]2}2Pt(CH2CMe3)2], having a PtP2/PtC2 interplanarangle of 12.2° [21b], or cis-[(t-Bu2PhP)2PtCl2], showinga twist of 31.5° for its PtP2 and PtCl2 planes [22]. TheP–Ni–P bite angle, 89.8(1)° in both molecules, is atthe upper end of the range 82.6–89.0° that appearsto be typical for the C5H8(PR2)2 chelate systemScheme 1. Complexes and reactions studied.

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Fig. 1. Molecular structures of the two crystallographically independent molecules of compound 4. Selected bond lengths (A� ) and valence andtorsion angles (°) 1: Ni1–Cl1, 2.206(3), Ni1–Cl2, 2.206(3); Ni1–P1, 2.182(3); Ni1–P2, 2.174(3). Cl1–Ni1–Cl2, 92.4(1); Cl1–Ni1–P1, 175.1(1);Cl1–Ni1–P2, 90.1(1); Cl2–Ni1–P1, 88.0(1); Cl2–Ni1–P2, 175.3(1); P1–Ni1–P2, 89.8(1). P1–Ni1–P2–C18, +132.2(4); P1–Ni1–P2–C24,–106.4(4); P2–Ni1–P1–C6, +125.7(4); P2–Ni1–P1–C12, –111.8(4). –Ni2–Cl3, 2.212(3), Ni2–Cl4, 2.212(3); Ni2–P3, 2.178(3); Ni2–P4,2.174(3).Cl3–Ni2–Cl4, 92.6(1); Cl3–Ni2–P3, 175.6(1); Cl3–Ni2–P4, 88.0(1); Cl4–Ni2–P3, 89.8(1); Cl4–Ni2–P4, 176.4(1); P3–Ni2–P4, 89.8(1).P3–Ni2–P4–C47, –113.3(4); P3–Ni2–P4–C53, +123.4(4); P4–Ni2–P3–C35, +130.1(3); P4–Ni2–P3–C41, –106.7(4).

[1,2b,2f,2g,5a,21,23,24]. Both the Ni–Cl and the Ni–Pbond lengths are equal within experimental error, asexpected. Probably as a consequence of the steric bulkof the cyclohexyl substituents, the Ni–P distances (av-erage, 2.177�0.04 A� ) tend to be slightly longer thanthose in the less crowded nickel complexes [(1S,2S)-C5H8{P(CH3)C8H15}2NiCl2] (average d(Ni–P), 2.161�0.05 A� ) and [(1R,2R)-C5H8{P(C6H5)2}2NiBr2],displaying distinctly unequal metal-to-phosphorus bondlengths of 2.158(1) and 2.178(1) A� , respectively [5a].The perspective views of the two molecules shown inFig. 1 identify the puckering of the chelate rings in the�, � notation as �. As judged from the torsion anglesof the respective P–Ni–P–CH� chains, two of thefour cyclohexyl substituents of either molecule, viz. onphosphorus atoms P1 [torsion angles, −111.8(4) and+125.7(4)°] and P4 [torsion angles, −113.3(4) and+123.4(4)°], display little differentiation between anaxial or equatorial orientation with respect to the coor-dination plane. A more distinct ‘axial–equatorial’ dif-ferentiation is evident for the spatial arrangement of theremaining C6H11 groups, where the pertinent P–Ni–P–CH� torsion angles amount to 132.2(4) and 130.1(3)°for the more equatorially aligned rings bonded throughcarbon atoms C18 and C35 and (with opposite sign) to106.4(4) and 106.7(4)° for the more axially orientedsubstituents bonded through C24 and C41. The confor-mation of the cyclohexyl ring is chair-like and thuscorresponds to the lowest-energy conformation of cy-clohexane itself.

Given that cross-couplings of organic halides withorganomagnesium compound are most often catalyzedby soluble nickel or palladium species [25], only the two

nickel compounds 1 and 4 as well as their palladiumhomologues 2 and 5 were chosen as pre-catalysts forthe C–C bond-forming reaction between 1-bromo-2-methylnaphthalene and its 1-bromomagnesio derivative(Scheme 1). The induction of the catalytic cycle,whether on a concerted metal(0)/metal(II) non-radicalpathway [26,27] or via a Ni(I)/Ni(III) radical chainprocess [28]1 requires initial reduction of the metal(II)complexes. This was achieved by treatment withmethylmagnesium bromide in gently refluxing diethyl/di-n-butyl ether (5:1), as described in the literature [4c].Despite deliberately low substrate-to-catalyst ratios be-tween �40:1 and �100:1, the formation of the bi-naphthyl product proceeded very slowly at ambientconditions and generally suffered from low conversions(Scheme 1). This sluggishness of the naphthyl–naphthylcoupling reaction has frequently been observed and isattributed [4f] to a high kinetic barrier for the forma-tion of the sterically overcrowded bis(2-methylnaph-thyl)metal key intermediates releasing the product byC–C bond-making reductive elimination. A reasonabledegree of conversion was solely achieved in the presenceof added [(1S,2S)-C5H8{P(C6H11)2}2NiCl2] (4), whichfurnished the desired 2,2�-dimethyl-1,1�-binaphthyl in47% spectroscopic and 10% isolated yield after 28 daysat ambient conditions. It was therefore only for thiscatalysis that the enantiomeric excess was determined.Polarimetry a �=589, 578 and 546 nm showed theproduct to be modestly enriched by 26% in the (R)-(− )enantiomeric form. This is, admittedly, far from the

1 For some remaining mechanistic ambiguity, see Ref. [29].

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very high enantioselectivities of 83–95% obtained previ-ously with nickel complexes of oxygen-containing ferro-cenyl phosphanes, which are thought to greatly enhancethe stereoselectivity by coordinating both the nickel andthe magnesium at the transmetalation step [4c,4e,4f]. Itshould be noted, however, that the optical yield af-forded by five-membered chelate complex 4 is higherthan the 4.6 or 12.5% e.e. that were achieved prior tothis work with the aid of chiral (bisphosphane)nickelcatalysts featuring seven- or nine-membered chelatesystems, viz. NiCl2/(− )-diop [4a] and NiCl2/(− )-naphos(1,1) [4b], where diop=2,2-dimethyl-4,5-bis(diphenylphosphanylmethyl)-1,3-dioxolane, andnaphos(1,1)=2,2�-bis(diphenylphosphanylmethyl)-1,1�-binaphthyl, respectively. It has been proposed that thestereoselectivity of the naphthyl–naphthyl coupling re-action is determined kinetically at the stage ofdiastereomeric diorganonickel intermediates, whichpossess a chiral bis(2-methylnaphthyl) propeller struc-ture in addition to their stereochemically defined phos-phane ligand and are optically stable due to a sterichindrance preventing rotation about the metal–carbonbonds [4c]. The differences in the enantioselectivitiesafforded by nickel catalysts bearing either the (1S,2S)-C5H8[P(C6H11)2]2 bisphosphane or (− )-diop or (− )-naphos(1,1) ligands may then be attributed to a higherrigidity of the five-membered chelate ring as comparedto a seven- or nine-membered metallacycle.

4. Supplementary material

Crystallographic data (excluding structure factors)for the structural analysis have been deposited with theCambridge Crystallographic Data Centre, CCDC no.158983. Copies of this information may be obtainedfrom The Director, CCDC, 12 Union Road, Cam-bridge CB2 1EZ, UK (fax: +44-1233-336 033; e-mail:deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

Financial support from the Deutsche Forschungsge-meinschaft (Bonn) and the Fonds der Chemischen In-dustrie (Frankfurt/Main) is gratefully acknowledged.We are also indebted to Degussa-Huls AG (Hanau) fora generous gift of platinum salts.

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