Organometallics 1995,14, 4559-4569
4559
Metallacycles with Stereogenic Metal Centers: Synthesis and Characterization of Diastereomeric Cycloruthenated Chiral Amines Saeed Attar and John H. Nelson* Department of Chemistry I 216, University of Nevada, Reno, Nevada 89557
Jean Fischer and Andre de Cian Laboratoire de Cristallochimie (URA424 du CNRS), Universitk Louis Pasteur, 4, rue Blaise Pascal, F-67070 Strasbourg Cedex, France
Jean-Pascal Sutter and Michel Pfeffer Laboratoire de Synthkses Mdtallo-induites (URA 416 du CNRS), Universitd Louis Pasteur, 4, rue Blaise Pascal, F-67070 Strasbourg Cedex, France Received May 31, 1995@ n The transmetalation reaction of enantiomerically pure (R)c- or (S)c-{HgCl[C6H&H-
I
(Me)NMez]}with [(y6-arene)RuC1212dimers, where rf-arene = C6H6, (benzene l),C&l&H3 I
-
(toluene, 2), and l-Me-4-PI.l-C6H4 (cymene, 31, in CH3CN at ambient temperatures forms in each case a mixture of two diastereomeric ruthenacycles [(R)c(S)R,-a, major, and (R)c(R )Ru-a’, minor, or (s)c(R)~,-b,major, and (S)C(S)Ru-b,minor] of the type {(@n arene)RuCl[CsH4CH(Me)NMe2]) (la-a’, 2a-a‘, 3a-a’ or lb-b, 2 b - b , 3b-b) in good
chemical and optical ylelds. The diastereoselectivity of these reactions, which molecular mechanics calculations suggest to be under kinetic control, varies as a function of the nature of the arene ligand (de is the diastereomeric excess): benzene (90% de) > toluene (87% de) > cymene (67% de). These complexes are characterized in solution by a variety of spectroscopic techniques (lH- and 13C-NMR,W, CD) and in the solid-state by single-crystal X-ray crystallography and far-IR. X-ray crystallographic studies, carried out on l b , 2 b , and 3a, establish the absolute configuration at the chiral Ru center in each. Both l b and 2 b crystallize in the orthorhombic space group P212121, 2 = 4, but 3a crystallizes in the b = monoclinic space group P21, 2 = 2. Unit cell parameters for: l b : a = 12.886(3) 17.254(5) 8,c = 6.726(2) 8,V = 1495.4 A3. For 2 b : a = 12.523(3) 8, b = 18.679(5) c = 6.765(2), V = 1582.6 Hi3. For 3a: a = 14.050(4) 8, b = 6.633(1) 8,c = 10.131(3), V = 942.5 A3. The latter data show that of the two possible diastereomeric envelope conformations of the five-membered chelate ring in each compound only the one with a minimum C-CH3- r6arene interaction is formed. NMR (CDC13) studies show that the Ru-N bond is preserved in solution, leading to configurationally-stable(at Ru), optically-active species. Moreover, lH difference NOE spectroscopy reveals the existence of a dynamic equilibrium between the two limiting conformations of the five-membered chelate ring in each diastereomeric ruthenacycle.
A, A,
Introduction Stereoselective synthesis of organic compounds, promoted or catalyzed by optically-activetransition-metal complexes, remains a very active area of research in organometallic chemistry.l In this respect, complexes containing either Ru(I1) o r Rh(1) have figured prominently.1c-eB2In the case of ruthenium(I1) complexes, much of the success achieved in recent years has been obtained using often expensive optically-active diphosphines such as BINAP [bis(diphenylphosphino)-1,l’bi~mphthylI,~-~ or aminophosphine phosphinite ligands.2d The synthesis of a complex containing a chiral derivative of the p-cymene ligand has been reported recently.2e Among the complexes of the latter group, those contain@
Abstract published in Advance ACS Abstracts, September 1,1996.
ing an arene ligand are especially effective, mainly due to special stability of the arene-metal bonds. Thus, in addition to classic hydrogenation of olefins, they are useful and specific catalyst precursors for C-H bond (1)(a) Kolb, H. C.; van Nieuwenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994,94,2483. (b) Kagan, H. B.; Riant, 0. Chem. Reu. 1992,92, 1007. (c) Noyori, R. Chem. SOC.Rev. 1989,18,187. (d) Brunner, H. Top. Stereochem. 1988,18,129. (e) Brunner, H. Synthesis 1988,645. (0 Brunner, H. Acc. Chem. Res. 1979, 12, 250. ( g ) Brookhart, M.; Timmers, D.; Tucker, J. R.; Williams, G. D.; Husk, G. R.; Brunner, H.; Hammer, B. J . Am. Chem. SOC.1983,105, 6721. (h) Sokolov, V. I. Chirality and Optical Activity In Organometallic Compounds; Gordon and Breach Science Publishers: New York, 1990. (i) Morrison, J. D., Ed. Asymmetric Synthesis; Academic Press: Orlando, 1985; Vol. 5 (Chiral Catalysis). (j) Bosnich, B., Ed. Asymmetric Catalysis; Martinus
Nijhoff Publishers: Dordrecht, The Netherlands, 1986. (k) Davies, S. G. Pure Appl. Chem. 1988,60, 40. (1) Davies, S. G. Aldrichimica Acta 1990,23, 31. (m) Ito, Y.; Sawamura, M.; Hayashi, T. J . Am. Chem. Soc. 1986, 108, 6405. (n) Hayashi, T.; Uozumi, Y.; Yamasaki, A.; Sawamura, M.; Hamashima, H.; Ito, Y. Tetrahedron Lett. 1991,32, 2799.
Q276-7333l95/2314-4559$Q9.OQfQ 0 1995 American Chemical Society
Attar et al.
4560 Organometallics, Vol. 14,No. 10, 1995
Scheme 2
Scheme 1 Me '%,.,,
1 : Y = H ($-benzene) 2 : Y = Me ($.toluene) 3 : Y = 1 P i . 4 . Me ($-cymene)
/h
(a)-HgCI, (R)c: R' = H, RZ = Me (b)-HgCI. (S)C: R' = Me, RZ = H
.
diastereomers
(RM%u
mior product)
(R)C(R)R"
(minor product)
la', 2a', 3a'
la, 2a,3a
.HgCI2
+
activation and activation of alkynes.2f Furthermore, the ease and generality of their preparation and their thermodynamic stability have been additional factors in the recent rise of their use in catalytic studies. We have embarked on a project to prepare a series of cyclometalated, diastereomeric complexes of Ru(I1) of
-
diastereomers
(S)C(R)R.
(mor P
~
Ib', 2b', 3b'
W
(S)o(s)Ru
(minor product)
Ib, 2b, 3b
n
the type ((~f-arene)RuCl[C6H4CH(Me)NMezl}, la,a'-
3b,b, where V6-arene = CsH6 (benzene, 11, CsHsMe (toluene, 21, or l-Me-4-Pf-CsH4 (cymene, 31, and either of the two bidentate, chiral ligands (R)c-(+)-or (S)C-(1- N,N-dimethyl-1-phenylethylamine, a or b, respectively (Schemes 1and 2). The "prime" in the structural designations of each pair of diastereomers symbolizes the fact that the two members are identical in the absolute configuration of their benzylic carbon atom but are different in that of the ruthenium center. Our interest in this project stems from the fact that the stability of the metal-arene bond in complexes containing this ligand often results in very interesting reactivity patterns and catalytic properties.2f In addition, metallacycles with stereogenic metal centers have been used or implicated in a number of catalytically or stoichiometrically important asymmetric organic transformations.ld-m It is generally believed that the chirality a t the metal, in addition t o that of the chiral ligand, plays an important role in such transformations. Nevertheless, the extent of such a role is not very well understood, mainly due to the lack of a large set of data on such complexes.lfs3g A literature search3reveals that there is only a handful of reports on resolved and wellcharacterized diastereomeric, monoarene ruthenacycles. Thus, further studies on these types of compounds are warranted. Compounds la,a'-3b,b', which are obtained in high (2) (a)Noyori, R.; Takaya, H. ACC. Chem. Res. 1990,23,345. (b) Wan, K.;Davis, M. E. Tetrahedron: Asymmetry 1993,4,2461.( c ) Krasik, P.; Alper, H. Tetrahedron: Asymmetry 1992,3, 1283.(d) Hapiot, F.; Agbossou, F.; Mortreux, A. Tetrahedron: Asymmetry 1994,5,515. ( e ) Petrici, P.; Pitzalis, E.; Marchetti, F.; Rosini, C.; Salvadori, P.; Bennett, M. A. J.Organomet. Chem. 1994,466,221. (0 Le Bozec, H.; Touchard, D.; Dixneuf, P. H. Adu. Organomet. Chem. 1989,29,163.(g) Faller, J. W.; Chase, K. J. Organometallics 1995,14,1592.
chemical and optical yields, represent a novel series of stable and configurationally-rigid (under mild conditions) ruthenacycles with a stereogenic Ru(I1) center. Furthermore, we have recently shown4 that internal alkynes insert into the Ru-C bonds of complexes of this type, leading to the formation of novel Ru(0) sandwich complexes with good chemo- and regioselectivities; the organic moiety is subsequently isolated by oxidative demetalation induced by CuBrz. Thus, considering the well-defined geometry and configurational stability of these compounds (vide infra) in addition t o their demonstrated reactivity: our long-term goals in undertaking this project are (i) to carry out studies on the stereochemical courses of simple reactions which they undergo and (ii) t o utilize such information in asymmetric transformations of organic molecules. Herein we report the synthesis of ruthenacycles la,a'-3b,b' and their characterization via a variety of spectroscopic methods PH- and 13C-NMR,IR, W, CD). In addition, X-ray crystallographic characterization of three complexes ( l b , 2b,3a) has established the absolute configuration of the Ru(I1) center in each compound. Finally, a possible explanation for the observed stereoselectivity in the reactions leading to the formation of these complexes is offered. (3)(a)Brunner, H.;Oeschey, R.; Nuber, B. h o g . Chem. 1995,34, 3349.(b) Brunner, H.; Oeschey, R.; Nuber, B. Angew. Chem., Znt. Ed. ( c ) Mandal, S. R;Chakravarty, A. R. Inorg. Chem. Engl. 1994,33,866. 1993,32,3851.(d) Mandal, S.K.; Chakravarty, A. R. J. Chem. SOC., Dalton Trans. 1992, 1627.( e ) Mandal, S. K.; Chakravarty, A. R. J. Organomet. Chem. 1991,417, C59. (0 Martin, G. C.; Boncella, J. M. Organometallics 1989,8,2863. (g) Consiglio, G.;Morandini, F. Chem. Rev. 1987,87,761. (h) Brunner, H. Adu. Organomet. Chem. 1980,18, 151. (4)(a) Abbenhuis, H. C. L.; Pfeffer, M.; Sutter, J.-P.; de Cian, A,; Fischer, J.; Ji, H. L.; Nelson, J. H. Organometallics 1993,12, 4464. (b) Pfeffer, M. Pure Appl. Chem. 1992,64,335.
Metallacycles with Stereogenic Metal Centers
Results and Discussion
-
Ruthenacycles la,a'-3b,b are prepared by the transmetalation reaction of 1 equiv of either optically pure
Organometallics, Vol. 14, No. 10,1995 4561
Table 1. Reaction Stereoselectivity for Ruthenacycles la,a'-3b,b
@-arene n (R)c or (S)C-{H~C~[C~H~CH(M~)NM~~~}~" a- or b-HgC1, benzene benzene respectively, w t h 0.5 equiv of each of the dimeric [($toluene toluene arene)RuCl& c o m p l e x e ~where , ~ ~ ~arene ~ = $-benzene cymene (11, $-toluene (21, or $-cymene (31, in acetonitrile at cymene The latter dimers
-
ambient temperatures (Scheme 1). provide a n ideal entryway into the synthesis of compounds of the type reported here due to their ease of preparation with a variety of arenes and also due to their remarkable air and moisture s t a b i l i t ~ . ~In~ , ~ addition, their transmetalation reactions with the Hg(11)derivatives of the chiral amine ligands are very clean and do not require extensive further workup. It should be noted here that attempts to prepare la,a'-3b,b via similar reactions with Zn(I1) derivatives of the type LzZn (L = a or b) met with failure mostly due to the air and moisture sensitivity of the latter which led to the formation of unidentified products and elemental ruthenium. The direct reaction of the ortholithiated derivatives of the two ligands a or b with the [ ( p arene)RuCl2]2dimers also failed as elemental ruthenium was the major product.4a It is noteworthy that these ruthenacycles can also be prepared via direct cyclometalation (intramolecular C-H bond activation) reaction starting from [(y6-arene)RuC1212and one of the chiral amines a or b.4a However, this reaction suffers from very poor chemical yields (-10%-20%) and from the fact that twice the amount of the often expensive chiral amine ligand is required. Performing the latter reaction in the presence of either Ag(C2H302) or AgBF4, following a report by Ryabov and Eldik5don the cycloplatination of an arylazide ligand, did not lead to any improvement of the previous results. Each of the transmetalations (depicted in Scheme 1) leads to the formation of only two out of four possible diastereomers. The ruthenium atom becomes a chiral center during the course of this reaction, in addition to the existing chirality at the benzylic carbon atom of ligands a orb. The stereochemistry at the latter atom remains fxed as it is not a reaction center. Thus, each pair of diastereomers, e.g., la,a', differs only in the absolute configuration at their Ru center. The stereochemical relationships between the members of each diastereomeric pair, as well as those with the other two pairs, are illustrated in Scheme 2 . Each pair is isolated from its crude reaction mixture as a homogeneous orange-red powder following removal of the formed HgCl2 by filtration chromatography on alumina. The chemical yields are good, ranging from 50% to 55%. Each diastereomeric mixture can be further enriched in the major species by fractional crystallization from a CHsClz:(hexane-EtzO, 1:l)mixture. Since the members in each diastereomeric pair show well-separated signals in the (CDC13) lH-NMR spectra of their product mixture and are stable toward interconversion in solution (vide infra), the stereoselectivity or optical yield of each reaction can be assessed from ( 5 ) (a)Attar, S.; Nelson, J. H.; Fischer, J. Organometallics, in press. (b) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974, 233.(c) Zelonka, R.A.; Baird, M. C . Can. J.Chem. 1972,50,3063. (d) Ryabov, A. D.; van Eldik, R. Angew. Chem., Int. Ed. Engl. 1994,33, 783.
products major minor la la' lb lb 2a 2a' 2b 2b 3a 3a' 3b 3b
major:minor ratio 20:l 20:l 14:l 14:1 5:1 5:1
% dea 90.4 90.4 86.7 86.7 66.7 66.7
Percentage of diastereomeric excess (de) = (% major - % minor), as determined from the 'H-NMR spectra of the crude reaction products.
the ratio of their integrated intensities. The results are presented as percentages of diastereomeric excess (% de) in Table 1. As seen in this table, the major-to-minor ratio (hence, % de) remains constant with the changing chirality at the benzylic C atom, e.g., (1a:la') = (lb: lb) = 20. But, this ratio changes according to the nature of the arene ligand. Each diastereomeric ratio corresponds to the reaction mixture obtained immediately after filtration chromatography to eliminate HgC12. The same ratio is obtained from the crude reaction mixture in each case; thus, the workup does not alter this ratio. In addition, the lHNMR spectra obtained on CDCl3, acetone-&, and CD3NO2 solutions of these diastereomeric ruthenacycles are independent of time (days) and temperature (-20 t o 50 "C), indicating their configurational stability.3a Thus, each ratio reflects the difference in activation energies for the formation of two diastereomeric transition states from the same prochiral precursor, i.e., the [(y6-arene)RuC1212 dimer, in the kinetically-controlled transmetalation reaction which leads to the formation of the two products (vide inpa). It is noteworthy that, although optically-active organometallic compounds containing stereogenic metal centers were first reported more than a decade ago, there are not many clear examples of kinetic control of asymmetric induction in the formation of such compound^.^^^^ Furthermore, it has been possible to determine the optical yields of only a few of those reactions in which kinetic control has been indicated. This is mainly due to (i) interconversion of the two diastereomers caused by vigorous conditions often required to drive such reactions and (ii) the presence of paramagnetic species in crude reaction mixtures which prevents the obtainment of their lH-NMR spectra. Thus, the ruthenacycles reported here provide, in addition to their structural aspects, an opportunity to assess such stereoselectivities. The solid-state and solution-phase structures of la,a'3b,b were elucidated via a combination of X-ray crystallographic and NMR spectroscopic studies. We will first discuss the solid-state structures of three compounds as representatives: l b , 2b', and 3a. Suitable crystals were obtained by the slow diffision of a 1:l mixture of hexane-ether into a saturated CHzClz solution of each compound. Crystallographic data and details of structure determination and refinement for the three compounds are presented in Table 2 and in the Experimental Section, respectively. The atomic coordinates and equivalent isotropic displacement parameters are given in Tables 3 (lb),4 (2b),and 5 (3a). Table 6 presents a selected list of structural parameters for the three compounds. The molecular structures
4562 Organometallics, Vol.14,No. 10,1995
Attar et al.
Table 2. Crystallographic Data for Ruthenacycles l b , 2 b , and 3a chem formula
fw cryst syst a (A) b (A)
(A)
c
lb Cl6H2oNClRu 362.9 orthorhombic 12.886(3) 17.254(5) 6.726(2)
2b C17H22NClRu 376.9 orthorhombic 12.523(3) 18.679(5) 6.765(2)
1495.4 4 P212121 0.7107 1.612 11.951 0.80 / 1.00 0.023 0.024
1582.6 4 P212121 0.7107 1.582 11.323 0.94 / 1.00 0.027 0.039
3a C2oHmNClRu 419.0 monoclinic 14.050(4) 6.633(1) 10.131(3) 93.42(2) 942.5 2 p2 1 0.7107 1.476 9.582 0.91 / 1.00 0.021 0.025
t3 (deg)
v (A31
z
group (g C m 3 ) P (cm-l) abs min I max ecalcd
R(F)O RW(mb a
R(F) = U I F o I
IFc12]"2; w
- IFcl)E
(IFoI).
= l/u2(F)2 = u2(counts)
R w O = [&I (IF01 - l F c l ) 2 E ~
+ (pn2.
Table 3. Atom Coordinates and Equivalent Isotropic Displacement Parameters"for n
Table 4. Atom Coordinates and Equivalent Isotropic Displacement Parametersa for (S)C(R)R,n
{ (qs-C&CHs>RuC1[C~CH(Me)NMe21), 2b I
I
Ru C1 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 N C16 C17
0.96721(3) 0.83242(9) 1.0955(5) 1.0713(6) 0.9680(7) 0.8890(6) 0.9158(6) 1.0196(6) 1.043(1) 1.0650(3) 1.1535(3) 1.2164(4) 1.1904(4) 1.1023(4) 1.0400(3) 0.9383(3) 0.9296(5) 0.9262(3) 0.8148(4) 0.9974(5)
(S)C(R)R,-((~~-C~H~)R~C~[C~H~CH(~M~)~~Z~), Ib'
0.16211(2) 0.14822(6) 0.2369(3) 0.2041(3) 0.2068(3) 0.2441(4) 0.2741(3) 0.2727(2) 0.3090(3) 0.0944(2) 0.1212(2) 0.0726(3) 0.0011(3) -0.0219(2) 0.0258(2) 0.0045(2) -0.0758(3) 0.0540(2) 0.0518(3) 0.0299(3)
0.85111(4) 1.1090(2) 0.7976(9) 0.6198(8) 0.5489(8) 0.650(1) 0.834(1) 0.9067(8) 1.100(1) 1.0238(6) 1.1325(7) 1.2396(7) 1.2401(7) 1.1345(7) 1.0293(6) 0.9250(7) 0.8766(9) 0.7484(5) 0.6755(8) 0.5874(8)
2.043(4) 2.98(2) 4.0(1) 4.5(1) 6.4(2) 7.6(2) 6.3(2) 4.6(1) 8.1(2) 1.95(6) 2.69(8) 3.27(9) 3.35(9) 2.75(7) 2.23(6) 2.18(7) 3.9(1) 2.30(6) 3.63(9) 3.6(1)
See footnote to Table 3.
Ru C1 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 N C15 C16
0.96653(2) 0.83363(7) 1.0917(3) 1.0118(4) 0.9127(4) 0.8935(3) 0.9714(4) 1.0726(4) 1.0606(2) 1.1473(3) 1.0284(3) 1.1860(3) 1.0997(3) 1.0362(3) 0.9369(3) 0.9235(4) 0.9264(2) 0.9975(3) 0.8182(3)
0.16695(1) 0.15514(6) 0.2479(3) 0.2864(2) 0.2880(2) 0.2526(2) 0.2121(2) 0.2095(2) 0.1002(2) 0.1242(2) 0.0725(3) -0.0055(3) -0.0314(2) 0.0206(2) -0.0023(2) -0.0887(2) 0.0494(2) 0.0220(2) 0.0473(2)
0.92541(4) 1.1818(1) 0.8750(7) 0.9734(6) 0.8894(8) 0.7100(7) 0.6179(6) 0.6981(7) 1.1036(5) 1.2140(6) 1.3177(6) 1.3154(6) 1.2131(6) 1.1107(4) 1.0088(5) 0.9667(7) 0.8280(4) 0.6695(6) 0.-7506(7)
2.008(3) 3.20(2) 4.10(9) 4.4(1) 4.5(1) 4.42(9) 4.46(9) 3.98(8) 2.33(6) 2.88(7) 3.53(8) 3.59(8) 3.33(8) 2.31(6) 2.50(6) 3.76(9) 2.26(5) 3.37(8) 3.71(8)
aAnisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as: (4/3) [a2/3(l,l) b2t3(2,2) c28(3,3) ab(cos y)8(1,2) ac(cos ,8)/?(1,3) bc(cos a)/3(2,3).
+
+
+
+
+
along with the adopted numbering schemes are shown as ORTEP drawings in Figures 1 ( l b ) , 2 (2b), and 3 (3a). For l b and 2b, the structure of each compound consists of four monomeric molecules arranged in an orthorhombicunit cell (space group P212121). However, for 3a only two monomeric molecules crystallize in a monoclinic unit cell (space group P21). As seen in Figures 1-3, the geometry around the Ru atom in each structure is that of a "three-legged piano stool" in which the y6-coordinated arene ligand occupies the "stool" position while the C and N atoms of the arylamino ligand, in addition to the C1 atom, occupy the three "leg" positions. Thus, the Ru atom in each structure is in a pseudotetrahedral environment with four Werent groups attached to it, rendering it a chiral center. Since the chiral benzylic C atom of the arylamino group is not a reaction center in the course of formation of these diastereomers, its absolute configuration is unambiguously assigned according t o that in the a- or b-HgC1 starting materiaFa (Scheme 1): (S)Cfor lb' and 2 b and
Table 5. Atom Coordinates and Equivalent Isotropic Displacement Parameters"for (R)c(S)R,n
{ (qs-~ - M ~ - ~ - F % % J H ~ R U C ~ [ C ~ C H ( M ), 3a ~)NM~~]
Ru c1 C1 c2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 N C19 C20 a
0.22154(1) 0.31402(6) 0.1669(2) 0.1111(2) 0.0685(2) 0.0866(2) 0.1463(2) 0.1837(2) 0.2066(2) 0.1389(3) 0.3069(3) 0.0435(3) 0.2323(2) 0.1603(3) 0.1765(3) 0.2643(4) 0.3366(3) 0.3222(2) 0.4005(2) 0.4870(2) 0.3548(2) 0.3362(3) 0.4195(2)
0.794 0.5226(1) 0.8836(6) 0.7300(5) 0.7580(6) 0.9374(6) 1.0818(6) 1.0585(6) 0.8451(6) 0.9289(9) 0.930(1) 0.9692(8) 0.6368(5) 0.5286(6) 0.4327(7) 0.4434(8) 0.5477(7) 0.6419(5) 0.7451(5) 0.807(1) 0.9139(4) 1.0887(6) 0.9829(6)
0.91309(2) 1.02009(8) 1.1140(3) 1.0480(3) 0.9192(3) 0.8480(3) 0.9113(3) 1.0447(3) 1.2546(3) 1.3526(4) 1.2830(4) 0.7090(4) 0.7389(3) 0.6696(3) 0.5499(4) 0.4983(4) 0.5655(4) 0.6846(3) 0.7673(3) 0.6918(4) 0.8404(3) 0.7513(4) 0.9530(4)
1.997(3) 3.20(1) 2.71(5) 2.98(6) 3.15(8) 3.17(6) 3.12(6) 2.94(6) 3.47(8) 5.2(1) 5.4(1) 4.84(9) 2.77(6) 3.38(7) 4.49(8) 5.2(1) 4.36(8) 2.99(6) 3.15(7) 5.26(8) 2.69(5) 3.90(7) 3.90(8)
See footnote to Table 3.
(R)c for 3a. The absolute configuration a t ruthenium is then assigned assuming the following priority numbers? 1(y6-areneligand), 2 (C1 atom), 3 (N atom), and 4 (phenyl C atom). Thus, each of the structures l b and R~ while 2 b is designated as the ( S ) C ( R ) diastereomer, that of 3a is designated as the (R)c(S)R,diastereomer. It is interesting to note that the lH-NMR spectra obtained on CDC13 solutions of the bulk samples from which the X-ray-quality crystals were isolated show the existence of two diastereomers in the same ratio as that observed before crystallization. Since the lack of any observable epimerization in these ruthenacycles in (6) (a) Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem., Int. Ed. Engl. 1966,5,385. (b) Stanley, K.;Baird, M. C. J. Am. Chem. Soc. 1975,97,6598. (c) Sloan, T.E. Top. Stereochem. 1981,12, 1.
Organometallics, Vol. 14,No. 10,1995 4563
Metallacycles with Stereogenic Metal Centers
Table 6. Selected Structural Parameters for Ruthenacycles l b , 2b,and 3a bond distances (A) compound/ abs config Ru-C(arene)o Ru-C(aw1) Ru-N Ru-Cl C-C(arene)b 2.057(3) 2.193(3)2.439(1) 1.389(7) 1b/[(s)c(R)~"] 2.201(4) 2.202(6) 2.058(4) 2.196(4)2.441(1) 1.389(4) 2b/[(S)&)R,] 2.064(3) 2.201(3)2.4373(8) 1.429(5) 2.225(4) 3al[(R)c(S)~,] 2.08(1) 2.148(8)2.430(2) 1.401(2) achirald 2.18 ~~
~
bond angles (deg) chelate ring C-C(aryl)b C(ary1)-Ru-N C(aryl)-Ru-Cl N-Ru-Cl puckering' 1.391(5) 78.2(1) 87.43(9) 88.19(9) 18.9" 1.391(7) 78.2(2) 86.9(1) 88.1(1) 18.5" 1.389(6) 77.8(1) 86.69(9) 88.50(8) 18.4" 1.372(3) 77.1(3) 86.0(3) 86.6(2) 18.2"
Mean value of the six Ru-C distances which range as follows: l b [2.164(4)- 2.274(4)A], 2 b [2.160(6)-2.272(7)A], 3a [2.169(3)2.269(3)AI, achiral analog [2.13(2)-2.22(2)AI. Mean value of the six C-C distances. The extent of this puckering is reflected in the value of the dihedral angle between the C(benzy1,chirall-(=(aryl, ortho)-C(ary1) and C(ary)-Ru-N planes. Achiral analog of the benzene compound, ((~6-C6H6)RuC1[C6H4cH~NMe~]}, from ref 4a.
C16
c10
Figure 1. Figure 1. ORTEP drawing of (S)C(R)R~-{(@-
Figure 2. ORTEP drawing of (S)C(R)R,-{($-C6H&H&
CsHs)Rucl[CsH4CH(Me)NMe21), lb', showing the atom-
RuCl[C~H4CH(Me)NMe~l}, 2b,showing the atom-number-
numbkng scheme (50%Lrobability ellipsoids); Hydrogen atoms have been omitted for clarity.
ihg scheme (50% drobability ellipsoids); hydrogen atoms have been omitted for clarity.
n
solution over a period of several days at ambient temperatures, hence, their configurational stability, has been established (vide supra), one has to assume that each bulk-crystallized sample is a solid mixture of the two diastereomers in a ratio equal to that observed in their solutions. Thus, we assign the crystal structures presented here to those of the major species in each diatereomeric mixture since it follows from the above discussion that the crystals of this species have a higher statistical chance of being isolated from a mixture. This argument is also supported by the reasonably high % de values obtained for these reactions, especially in the case of the la,a' and lb,b pairs (90% de). All attempts to separate the minor diastereomer by column chromatography and/or fractional crystallization have met with failure; the same solid mixture is obtained in every case. As evident from the data in Table 6, the bond lengths and angles do not change significantly among the three structures. The effect of the additional steric requirement of the benzylic methyl group on the conformation of these structures becomes evident when one compares the structural parameters of these three compounds with those of the achiral analog of the v6-benzene
-
n
CIO
c13
CI8
compound, { ( ~ 6 - C s H s ) R u C l [ c s H 4 C H ~a!~ e One ~ l } pa-
Figure 3. ORTEP drawing of ( R ) c ( S ) R ~ - { ( ~ ~ - ~ - M ~ ~ - P I J n C ~ H ~ ) R U C ~ [ C ~ H ~ C H ( MSa, ~ ) showing N M ~ ~ ~the ) , atomnumbiring scheme (50%Lrobability ellipsoids); Hydrogen atoms have been omitted for clarity.
rameter which is clearly indicative of this effect is the Ru-N bond distance in the four complexes. It is shorter for the achiral analog [2.148(8) AI than for the three
chiral compounds [2.193(3) A ( l b ) ; 2.196(4) A (2b); 2.201(3) A (sa)]. On the basis of a Ru covalent radius
Attar et al.
4564 Organometallics, Vol. 14, No. 10,1995
Scheme 3 Conformer A
Conformer B
and arene protons which may be assigned t o those of the major and minor diastereomeric pairs (Scheme 2) in varying ratios (Table 1). In our earlier report,4awe had stated that, in the case of la,a', only one diastereomer could be detected in solution. Closer examination of the 'H-NMR spectra of CDC13 solutions of both laa' and lb-b pairs has revealed that both diastereomers are present in each solution in a ratio of 95%:5%, a ratio high enough to have permitted them t o be overlooked earlier. Of particular aid in assigning the solutionphase structures is the observation of two N-CH3 singlets (syn and anti to the y6-arene ring) for each of the major and minor species, which are indicative of their diastereotopic nature in these structures. This can be caused only if the pyramidal inversion at the N atom is blocked by its coordination to Ru, suggesting that the five-membered chelate ring found in the solid-state is also preserved in solution. The latter conclusion is also confirmed by our earlier report4, on the structure of the achiral analog of the benzene compound, { (y6-C&)n
RuCl[C6H&H2NMe21) where the CH2-N group gives
u
of 1.42 Ala and an N (sp3) radius of 0.70 the estimated length of a Ru-N single bond is 2.12 A. Thus, the Ru-N distances reported here are not significantly longer than the expected values. Factors such as residual strain in the chelate ring, the steric crowding of the axial groups, and the trans influence of the C1 group may contribute to such observed bond lengthening. The bite of the bidentate benzylamino ligand, represented by the C(ary1)-Ru-N angle, is remarkably similar (-78") in the four complexes listed in Table 6. The five-membered chelate ring in each compound is puckered at its Ru-N-C(benzy1) portion. The extent of this puckering is reflected in the value of the dihedral angle between the Ru-C(ary1)-C(ary1, ortho)-C(benzy1, chiral) and C(ary1)-Ru-N planes, and is listed in the last column in Table 6. The puckering of the chelate ring makes possible the existence of two limiting diastereomeric envelope conformations.8aThis is illustrated in Scheme 3 for the major diastereomer of each of the four complexes compared here. As seen in Scheme 3, the 1,3-diaxial interaction is the most significant one in each of the two diastereomeric conformers A and B. The X-ray crystal structures presented in Figures 1-3 indicate that, in the solidstate, the 1,3-diaxial y6-arene. CH3 interaction in each compound is disfavored. Thus, for l b and 2b' only conformer A, and for 3a only conformer B, are observed. This (y6-arene) * -(group)interaction is disfavored even when the group is H, as is the case in the structure of the achiral analogla (Scheme 3, reaction iii) which crystallizes only as conformer A and its enantiomer. The solution structures of ruthenacycles la,a'-3b,b' were elucidated by 'H- and 13C{lH}-NMR spectroscopic studies (Tables 7 and 8, respectively). Each spectrum shows two distinct sets of resonances for the arylamine
-
(7) (a) Howard, J.; Woodward, P.J. J. Chem. SOC.,Dalton Trans 1976,59. (b) Bennett, M. J.; Mason, R. Nature 1966,205,760. ( 8 ) (a) Hawkins, C. J. Absolute Configuration of Metal Complexes; Wiley: New York, 1971; Chapter 1. (b) van Koten, G.; Noltes, J. G. J. Am. Chem. SOC.1976,98, 5393.
rise t o two temperature-independent anisochronous ~ Hz) ~ in signals (AX pattern; 6 4.32 and 2.82, 2 J= 13 its lH-NMR (CDC13)spectrum. The preservation of the five-membered chelate structure in solution points toward the special configurational stability (at Ru) of these ruthenacycles, a feature which is unique among the comparable Ru(I1) systems reported r e ~ e n t l y . ~We ,-~ will show in a subsequent publication that this chelate ring is not ruptured even by diphosphines. The conformation of the chelate ring in solution was probed via lH difference NOE spectroscopy. It was found that irradiation of the arene protons results in enhancement of the signals for both the benzylic C-CH3 and N-CH3 (syn) groups. As seen in reactions i and ii of Scheme 3, the NOE results can only be explained in terms of a dynamic equilibrium between conformers A and B in solution. We have not been able to freeze out this equilibrium; however, such equilibria have been established for comparable systems. For example, the two possible conformations for the six-membered chelate (R ring of { [(5-MeO-8-N(Me2)CH2)naphthyllMeRSnBr) = Me, Ph) can be frozen out on the NMR time scale below -30 oC.8b Configurationally-rigiddiastereomers such as la- l b (benzene), 2a-b' (toluene), and 3a-b' (cymene), which differ in the stereochemistry at their Ru centers, are expected to be optically active in solution and to give mirror-image CD ~ p e c t r a . Such ~~,~ spectra, ~ along with the corresponding W - v i s data have been obtained on CH2C12 solutions of each compound (in the three pairs mentioned above) in the 300-600 nm region (see Experimental Section). The d-d electronic transitions of the metal chromophore dominate the spectra obtained in the visible and the n e a r - W regions, with the chiral center(s1in the ligand making only minor contributions to the chiroptical properties.lh This latter statement is confirmed by the observation that the W - v i s and CD spectra of the Hg(I1) salts of the chiral arylamino ligands a and b are transparent in this region.5a With respect t o the W - v i s spectra, each of the compounds la,b' and 2a,b' shows two strong absorption maxima at roughly similar wavelengths but with different molar nm ( E , L mol-l cm-l), for la,b, 366 absoptivities [A,,
Organometallics, Vol. 14, No. 10,1995 4565
Metallacycles with Stereogenic Metal Centers
Table 7.
1 H
NMR Data for Ruthenacycles (la,a')-(3b,b)a 6 (ppm)blmultiplicity'/J (Hz)
compddl assgnmte H2
l a , l b (major) 8.24ldd 3JH2H3 = 7.5 4JH2H4 = 1.5 7.08ltdd 3JH3H2 = 7.5 3JH3H4 = 7.5 4JH3H5 = 1.5 6JH3H7 = 1.0 6.951td 3JH4H3 = 7.5 3 5 ~ 4= ~ 7.5 5 4JH4H2 = 1.5 6.771ddd 3JH5H4 = 7.5 4JH5H3 = 1.5 'JH5H2 = 1.5 4.371qd
H3
H4
H5
H7
3JH7H8 = 7.0 6 J ~ 7 H 3= 1.0 (n. r.)
H8
1.181d
la',b (minor) 7.751dd 3JH2H3 = 7.5 4 J ~ 2= ~ 1.5 4 7.1lltdd 3JH3H2 = 7.5 3JH3H4 = 7.5 4JH3H5 = 1.5 6JH3H7 = (n. r.) 6.94/td 3JH4H3 = 7.5 3JH4H5 = 7.5 4JH4H2 = 1.5 6.7Uddd 3JH5H4 = 7.5 4JH5H3 = 1.5 (n. r.) 5JH5H2 = 1.5 (n. r.) 3.831qd 3JH7H8 = 7.0 6 J ~ 7= ~ 1.0 3 (n. r.) 1.281d 3JH8H7 = 7.0 1.961s 3.361s 5.29/se(')
2a,b (major) 8.19ldd 3 J ~ 2= ~ 3 7.5 4 J ~ 2= ~ 1.0 4 7.081tt (app) 3 J ~ 3= ~ 2 7.5 3 J ~ 3= ~ 4 7.5 4 J ~ 3= ~ n. 5 0.8 ' J H ~=H 1.0~ 6.94/td (app) 3 J ~ 4= ~ 7.5 3 3 J ~ 4= ~ 5 7.5 4 J ~ 4= ~ 21.0 6.761d (app) 3 J ~ 5= ~ 4 7.5 4 J ~ 5= ~ 1.0 3 (n. 1.)
2a',b (minor) 7.701dd 3 J ~ 2= ~ 7.5 3 4 J ~ 2= ~ 1.0 4 7.111tt (app) 3 J ~ 3 = ~ 27.5 (n. r.Y 'JH3H4 = 7.5 (n. r.) 4 J H 3 ~ 5= n. 0 . ' J H ~=H 1.0~(n. r.) 6.931td (app) 3 J ~ 4= ~ 3 7.5 (n. r.) 3 J ~ 4= ~ 7.5 5 (n. r.) 4JH4H2 = 1.0 (n. r.) 6.731d (app) 3 J ~ 5= ~ 4 7.5 (n. r.) 4 J ~ 5= ~ 1.0 3 (n. r.)
4.42lqd
3.851qd
3 J ~ 7= ~ 7.0 8 6 J H 7 ~ 3= 1.0 (n. r.)
3tJH7H8 'JHlH3
3a,b' (major)
8.151dd 3JH2H3 = 4e.JH2H4 =
7.681dd
3JH2H3 = 7.5 4JH2H4 = 1.0
7.5 1.0
7.091t (app)
3JH3H2 = 7.5 3JH3H4 = 7.5 4e.JH3H5 = n. 0 . 6JH3H7' n. 0.
6.921td (app) 3JH4H3 = 7.5 3JH4H5 = 7.5 4e.JH4H2 =
1.0
6.721d 3JH5H4
3JH7H8
7.081t (app)
3JH3H2 = 7.5 3JH3H4 = 7.5 4e.JH3H5 = n. 0. 6 J H 3 H l = n. 0.
6.901td (app)
3JH4H3 = 7.5 3JH4H5 = 7.5 4JH4H2 = 1.0
n. 0. = 7.5
3.731q
= 7.0 = 1.0 (n. r.)
3a',b (minor)
3.4119 = 7.0
1.181d
1.281d
1.171d
3 J ~ s= ~ 7.0 7
3 J ~ 8= ~ 7 7.0
3JHsH7 = 7.0
3JH7H8
= 7.0
1.271d
= 7.0 2.44/s 3.061s 2.411s 1.941s 3.331s 3.161s 3.301s 3.301s e(iii), h e(iii), h e(ii), h e(ii), h a Obtained at 500 MHz on CDCl3 solutions (25 "C). Referenced to TMS. Multiplicity: (app) = apparent, d = doublet, dd = doublet of doublets, ddd = doublet of double-doublets; dt = doublet of triplets, q = quartet, qd = quartet of doublets, s = singlet, t = triplet, td = triplet of doublets, tdd = triplet of double-doublets, tt = triplet of triplets. d For structural designations see Schemes 1 and 2; for the major-to-minor ratios see Table 1. e Assignments are based on the following numbering scheme: 3JH8H7
H9 (syn) H10 (anti) ye-arene
= 7.0
2.471s 3.381s 5.34/~~(~)
$( I
y-
(i) Y = H, all six C and H nuclei are equivalent
\N
