Stoichiometric Regio- and Stereoselective Oxidative Coupling

After recrystallization of the crude product from cold Et2O/MeOH, the η3:η3-2 ... (28) One of the η3-allyl groups of 2 displays a supine orientatio...
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Organometallics 2011, 30, 768–777 DOI: 10.1021/om100956f

Stoichiometric Regio- and Stereoselective Oxidative Coupling Reactions of Conjugated Dienes with Ruthenium(0). A Mechanistic Insight into the Origin of Selectivity Masafumi Hirano,*,† Yumiko Sakate,† Nobuyuki Komine,† Sanshiro Komiya,*,† Xian-qi Wang,‡ and Martin A. Bennett*,‡ †

Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan, and ‡Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia Received October 2, 2010

Treatment of [Ru(η6-C10H8)(η4-1,5-COD)] (1) with excess butadiene at room temperature produces supine,prone-[Ru(η3:η3-2,6-octadiene-1,8-diyl)(η4-1,5-COD)] (supine,prone-2). Similar treatment of 1 with isoprene and (E)- and (Z)-1,3-pentadiene also gives the corresponding analogues of supine,prone-2, while 2,3-dimethylbutadiene does not react. A low-temperature NMR study of the reaction of 1 with butadiene shows initial formation of [Ru(η4-cisoid-butadiene)(η2-transoidbutadiene)(η4-1,5-COD)] (5) as an intermediate, which is spontaneously converted into supine, prone-2 upon warming to room temperature. Similarly, treatment of 1 with (E)-1,3-pentadiene gives two intermediates, [Ru{η4-(E)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5COD)] (E-6) and [Ru{η4-(Z)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (Z-6), in 44/56 ratio at -50 °C. These intermediates are converted into anti-supine,syn-prone-[Ru(η3:η3-3,7-decadiene-2,9-diyl)(η4-1,5-COD)] (anti-supine,syn-prone-4). The observed stereochemistries can be explained by a mechanism that involves oxidative coupling between η4-(Z)-cisoid-1,3pentadiene and η2-(E)-transoid-1,3-pentadiene in Z-6.

Introduction The catalytic oligomerization of conjugated dienes has been extensively used in the production of cyclic and linear hydrocarbons since the pioneering discoveries by Wilke’s group.1 It is also widely used in many industrial processes such as Kuraray’s 1,9-nonanediol process.2 These studies have shown the selectivity of product formation to be strongly dependent on the catalytic species present. For example, so-called “naked Ni(0)” catalyzes cyclotrimerization of butadiene to 1,5,9-cyclododecatriene. However, in the presence of PPh3, the main products become the cyclic dimers of butadiene, namely, 1,2-divinylcyclobutane, 4-vinylcyclohexene, and 1,5-COD.3 Although these Ni(0)-based catalysts mainly produce cyclic products, catalytic linear dimerization of conjugated dienes has been achieved by a variety of *Corresponding authors. E-mail: [email protected] (M.H.), bennett@ rsc.anu.edu.au (M.A.B.). (1) (a) Wilke, G. Angew. Chem., Int. Ed. 1963, 2, 105. (b) Tsuji, J. Adv. Organomet. Chem. 1979, 17, 141. (c) Wilke, G. J. Organomet. Chem. 1980, 200, 349. (d) Jolly, P. W. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: New York, 1982; Vol. 8, p 671. (2) Yoshimura, N.; Tokito, Y.; Matsumoto, M.; Tamura, M. Kogyo Kagaku Zasshi (J. Chem. Soc. Japan Ind. Chem. Sect.) 1993, 119. (3) (a) Bogdanovic, B.; Heimbach, P.; Kr€ oner, M.; Wilke, G.; Hoffmann, E. G.; Brandt, J. Liebigs Ann. Chem. 1969, 727, 143. (b) Brenner, W.; Heimbach, P.; Hey, H.; M€ uller, E. W.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 161. (c) Heimbach, P.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 183. (d) Brenner, W.; Heimbach, P.; Wilke, G. Liebigs Ann. Chem. 1969, 727, 194. pubs.acs.org/Organometallics

Published on Web 02/03/2011

Ziegler-type transition metal systems and by precious metal salts or complexes, usually in the presence of a protic substrate.4,5 Recently, a Pd(0) complex having an imidazole carbene ligand was reported to show quite high activity toward linear dimerization of butadiene in the presence of 2-propanol.6 In the presence of disilane or distannane, the Pd(0) complexes Pd(dba)2 and Pd2(dba)3 catalyze the formation of linear dimerization products such as 1,8-disila- or 1,8-distanna-2,6-octadienes in high yield.7 Since the metal species, in particular, the valency of the metal atom and the nature of its ancillary ligands, control the nature of the product, the structures and chemical reactivity of putative intermediates is of continuing interest. Most of the mechanistic studies of butadiene dimerization have concerned Ni(0),8 and 2,6-octadiene-1,8-diyl complexes have been implicated as intermediates in the cyclodimerization of butadiene. For example, [Ni(η1 :η3 -2,6-octadiene-1, (4) Keim, W.; Behr, A.; Ro¨per, M. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 8, p 371, and references therein. (5) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; Wiley: New York, 1992; pp 72- 82. (6) Harkal, S.; Jackstell, R.; Nierlich, R.; Ortmann, D.; Bella, M. Org. Lett. 2005, 7, 541. (7) (a) Tsuji, Y.; Kakehi, T. J. Chem. Soc., Chem. Commun. 1992, 1000. (b) Obora, Y.; Tsuji, Y.; Kawamura, T. Organometallics 1993, 12, 2853. (8) Graham, C. R.; Stephenson, L. M. J. Am. Chem. Soc. 1977, 99, 7098. r 2011 American Chemical Society

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Organometallics, Vol. 30, No. 4, 2011 Chart 1

8-diyl)(PPh3)] (Ia), [Ni(η1:η3-2,6-octadiene-1,8-diyl)(PCy3)] (Ib), and [Pd(η1:η3-2,6-octadiene-1,8-diyl)(PMe3)] (II) have been isolated and characterized by NMR spectroscopy (Chart 1).9,10 There are numerous examples of stoichiometric coupling of 1,3-diene units at a transition metal center. A Pd(II) complex containing a trimer of butadiene, [Pd(η3:η3-C12H18)] (III), has been isolated as a thermally unstable crystalline compound, which decomposes above -20 °C in the absence of butadiene.11 A platinacyclopentane complex, [Pt(1,4-divinylbutane1,4-diyl)(η4-1,5-COD)] (IV), which was obtained by treatment of [Pt(η4-1,5-COD)2] with excess butadiene at room temperature, has been isolated and structurally characterized.12 The dimethyltitanium(II) compound [TiMe2(dmpe)2] gave an η3:η1-octa-2,6-diene-1,8-diyltitanium(IV) complex V on treatment with butadiene at room temperature.13 Treatment of [MoCp(η3-allyl)(η4-transoid-butadiene)] with excess butadiene and HBF4 produced supine,supine-[MoCp(η3:η3-2,6-octadiene-1,8-diyl)(acetone)]BF4 (VI).14-16 Heating of [Rh(η4-cisoid-butadiene)2(PiPr3)](OTf)] produced supine, prone-[Rh(η3:η3-2,6-octadiene-1,8-diyl)(PiPr3)(OTf-κ1O)] (VII), (9) Henc, B.; Jolly, P. W.; Salz, R.; Stobbe, S.; Wilke, G.; Benn, R.; Mynott, R.; Seevogel, K.; Goddard, R.; Kr€ uger, C. J. Organomet. Chem. 1980, 191, 449. (10) (a) Jolly, P. W.; Tkatchenko, I.; Wilke, G. Angew. Chem., Int. Ed. Engl. 1971, 10, 329. (b) Barnett, B.; B€ ussemeier, B.; Heimbach, P.; Jolly, P. W.; Kr€ uger, C.; Tkatchenko, I.; Wilke, G. Tetrahedron Lett. 1972, 1457. (11) Benn, R.; Jolly, P. W.; Mynott, R.; Schenker, G. Organometallics 1985, 4, 1136. (12) Barker, G. K.; Green, M.; Howard, J. A. K.; Spencer, J. L.; Stone, F. G. A. J. Am. Chem. Soc. 1976, 98, 3373. (13) Spencer, M. D.; Wilson, S. R.; Girolami, G. S. Organometallics 1997, 16, 3055. (14) Poli, R.; Wang, Li.-S. J. Am. Chem. Soc. 1998, 120, 2831. (15) The terms supine and prone were originally introduced to describe the limiting orientations (concave and convex, respectively) of two coordinated 1,3-diene units relative to a third ligand (L) in complexes of the type M(diene)L2. Yasuda, H.; Tatsumi, K.; Okamoto, T.; Mashima, K.; Lee, K.; Nakamura, A.; Kai, Y.; Kanehisa, N.; Kasai, N. J. Am. Chem. Soc. 1985, 107, 2410. (16) In this article, supine and prone are defined as concave and convex orientations, respectively, of the η3-allylic fragment with respect to the Ru center, and the terms exo and endo are used only to indicate the stereochemistry of the terminal atom (or group) in the η4-cisoid-diene. The terms syn and anti are defined as the stereochemistry of the atom (or group) of the allylic moiety with respect to the central proton as usual. However, since the bridging methylenes between two η3-allylic fragments are always in the position anti to the prone-η3-allylic fragment and syn to the supine-η3-allylic fragment, the expressions syn and anti in the compound numbers are used only to indicate the stereochemistry of the methyl groups in the η3-allylic fragments. (17) (a) Bosch, M.; Brookhart, M. S.; Ilg, K.; Werner, H. Angew. Chem., Int. Ed. 2000, 39, 2304. (b) Bosch, M.; Werner, H. Organometallics 2010, 29, 5646.

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Scheme 1

in which both of the η3-allylic fragments are bound to the bridging carbon chain at their syn positions.17 Stoichiometric coupling of 1,3-dienes at ruthenium centers seems to be particularly favorable, in both low and high oxidation states. For example, heating of the 1,3-cyclohexadiene complex [Ru(η4-1,3-C6H8)(CO)3] with excess 1,3cyclohexadiene gave the coupled product [Ru(η3:η3-C6H8C6H8)(CO)2], although in only 13% yield.18 Addition of CO to the complexes [M(CO)(η4-cisoid-2,3-dimethylbutadiene)2] induced diene coupling to give the 2,3,6,7-tetramethyl-2,6diene-1,8-diyl complexes [M(η3:η3-C12H20)(CO)2] (M = Ru, Fe).19 The analogous monosubstitution product [Ru(η3:η3C12H20)(CO){P(OMe)3}] was obtained when [Ru(η4-cisoid2,3-dimethylbutadiene)2(CO)] was heated with trimethylphosphite; in the presence of excess trimethylphosphite, under forcing conditions, one of the allyl groups was displaced to give, after hydrogen migration, the 2,3,6,7-tetramethyl-1,3,6-octatriene complex [Ru(η4-C12H20)(CO){P(OMe)3}2] (Scheme 1).20 Treatment of hydrated RuCl3 with excess isoprene in refluxing ethanol gave the dimeric Ru(IV) complex [Ru(η3:η32,7-dimethyl-2,6-octadiene-1,8-diyl)(μ-Cl)Cl]2,21 the molecular structure of which was determined by X-ray crystallography (eq 1).22

The reaction of the labile butadiene(triflato)ruthenium(II) complex [RuCp*(OTf)(η4-C4H6)] with butadiene caused linear dimerization, giving a rare example of a 1,3,7-octatriene complex of Ru(II) (Scheme 2).23 Interestingly, in the presence of CO, the 1,3,7-octatriene fragment underwent cyclization to 1,5-COD at the metal center. The resulting complex [RuCp*(CO)(η4-1,5-COD)]þ catalyzes the dimerization of butadiene to 1,5-COD;24 half-sandwich complexes such as [RuClCp(η4-1,5-COD)] and [RuCp(NCMe)3]þ catalyze a range of C-C coupling reactions of substituted alkenes and alkynes.25 (18) Whitesides, T. H.; Budnik, R. A. J. Chem. Soc., Chem. Commun. 1973, 87. (19) Cox, D. N.; Roulet, R. Organometallics 1985, 4, 2001. (20) Cox, D. N.; Roulet, R. J. Organomet. Chem. 1988, 342, 87. (21) Porri, L.; Gallazzi, M. C.; Colombo, A.; Allegra, G. Tetrahedron Lett. 1965, 47, 4187. (22) Colombo, A.; Allegra, G. Acta Crystallogr., Sect. B 1971, 27, 1653. (23) Itoh, K.; Masuda, K.; Fukahori, T.; Nakano, K.; Aoki, K.; Nagashima, H. Organometallics 1994, 13, 1020. (24) Masuda, K.; Nakano, K.; Fukahori, T.; Nagashima, H.; Itoh, K. J. Organomet. Chem. 1992, 428, C21. (25) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2001, 101, 2067, and references therein.

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Scheme 2

Scheme 3

Figure 1. Structure of one of the crystallographically independent molecules of supine,prone-[Ru(η3:η3-2,6-octadiene-1,8diyl)(η4-1,5-COD)] (supine,prone-2). Hydrogen atoms in the 1,5-COD fragment are omitted for clarity. Ellipsoids represent 50% probability.

Results

In spite of these pioneering studies, the mechanism of dimerization of conjugated dienes at a transition metal center and the factors that control the stereo- and regio-selectivity in the C-C bond-forming step are still largely unknown. We have reported26 that, in the presence of acetonitrile, naphthalene is displaced from the Ru(0) complex [Ru(η6C10H8)(η4-1,5-COD)] (1) by conjugated dienes, at or below room temperature, to give (η4-diene)ruthenium(0) complexes [Ru(η4-cisoid-1,3-diene)(η4-1,5-COD)(NCMe)]. Interestingly, in aromatic or aliphatic solvents containing no acetonitrile, stoichiometric dimerization of certain conjugated dienes took place and intermediates in the dimerization could be observed by low-temperature NMR spectroscopy. This seemed to offer an opportunity to answer some of the outstanding questions mentioned above, and we describe here the regio- and stereospecific dimerization of conjugated dienes on Ru(0), together with an assessment of the controlling factors. (26) Bennett, M. A.; Wang, X.-Q. J. Organomet. Chem. 1992, 428, C17.

Dimerization of Conjugated Dienes on Ruthenium(0). On exposure of a benzene solution of [Ru(η6-C10H8)(η4-1,5-COD)] (1) to excess butadiene for 30 min at room temperature, the yellow solution gradually turned deep brown. After recrystallization of the crude product from cold Et2O/MeOH, the η3:η32,6-octadiene-1,8-diyl complex supine,prone-[Ru(η3:η3-C8H12)(η4-1,5-COD)] (supine,prone-2) was isolated as analytically pure, pale yellow crystals in 78% yield (Scheme 3). Complex 2 was also formed, more slowly, by treatment of the complex [Ru(η4-cisoid-1,3-butadiene)(η4-1,5-COD)(NCMe)]26,27b with butadiene. The bis(η3-allylic) formulation of 2 is based on spectroscopic data, elemental analysis, chemical reactions, and, most decisively, a single-crystal X-ray structural analysis, which, though of poor quality, rules out the alternative formulation as a Ru(0) complex, [Ru(η4-1,5-COD)(η6-1,3,7-C8H12)].27 The unit cell contains two crystallographically independent molecules, one of which is depicted in Figure 1; selected bond distances and angles are listed in Table 2. The metrical data for the second molecule do not differ significantly from those in Table 2. The bond distance C(4)-C(5) [1.53(1) A˚] shows unequivocally that there is a C-C single bond linking the two original butadiene units, which are present as a pair of η3-allyl groups in a 2,6-octadiene-1,8-diyl chain. Complex 2 is, therefore, an analogue of the well-known Ru(II) complexes [Ru(η3-allyl)2(η4-1,5-COD)].28 One of the η3-allyl groups of 2 displays a supine orientation with respect to the metal center, (27) (a) The Ru(0) formulation was suggested tentatively in a review: Bennett, M. A. Coord. Chem. Rev. 1997, 166, 225. (b) Wang, X.- Q. Ph. D. Thesis, Australian National University, Canberra, 1992. (28) (a) Smith, A. E. Inorg. Chem. 1972, 11, 2306. (b) Marsh, R. A.; Howard, J.; Woodward, P. J. Chem. Soc., Dalton Trans. 1973, 778. (c) MacFarlane, K. S.; Retting, S. J.; Liu, Z.; James, B. R. J. Organomet. Chem. 1998, 557, 213. (d) Six, C.; Gabor, B.; Goerls, H.; Mynott, R.; Phillipps, P.; Leitner, W. Organometallics 1999, 18, 3316. (e) Werner, H.; Fries, G.; Weberndorfer, B. J. Organomet. Chem. 2000, 607, 182. (f) Smith, D. C., Jr.; Cadoret, J.; Jafarpour, L.; Stevens, E. D.; Nolan, S. P. Can. J. Chem. 2001, 79, 626.

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Organometallics, Vol. 30, No. 4, 2011 Chart 2

and the other is orientated prone.16 The arrangement of the eight-carbon chain is similar to that present in the dimeric bis(η3-allylic) complex of Ru(IV) obtained from RuCl3 and isoprene (eq 1).22 The methylene groups C(4) and C(5) are located in the anti and syn positions, respectively, with respect to the central methine proton in the corresponding η3-allylic fragments. The bond distances from Ru to C(1), C(2), C(3), C(6), C(7), and C(8) fall in the range 2.18(2)-2.35(2) A˚, which is typical both of many RuII(η3-allyl)2L2 complexes (L = various monodentate tertiary phosphines; L2 = various bis(tertiary phosphines))28 and of the bis(η3-allylic)ruthenium(IV) complex (eq 1)21 and its ligand derivatives.29 Acidolysis of 2 with HPF6 gave a mixture of three isomeric octadienes in 97% yield. The EI-mass spectrum showed a parent-ion peak at m/z 318. The NMR spectroscopic data of 2 are consistent with a highly unsymmetrical structure: there are 16 signals in the 13C{1H} NMR spectrum, indicating inequivalence of all the carbon atoms of 1,5-COD and octadiene fragments, and the 1H NMR spectrum comprises 15 resonances, some overlapping, in the range δ ca. 0.9-5.3. On the basis of the X-ray structure and 1H-1H COSY, the 1H NMR spectrum has been completely assigned (see Chart 2 for numbering of carbon atoms and attached protons, together with coupling constants). The resonances at δ 1.0 (d, 1H), 1.4 (dd, 1H), 2.6 (d, 1H), and 3.5 (d, 1H) can be assigned to the terminal allylic protons at the 8-CHsyn, 8-CHanti, 1-CHanti, and 1-CHsyn positions, respectively. The central protons in the allylic fragments, 2-CH and 7-CH, appear at δ 4.27 [(dt, 3J(H2-H1anti) = 10, 3J(H2-H1syn) = 3J(H2H3syn) = 7 Hz, 1H] and 3.84 [dt, 3J(H7-H6anti) = 3J(H7H8anti) = 10, 3J(H7-H8syn) = 7 Hz, 1H], respectively, where the 3J coupling constant between the central and anti protons is 10 Hz and that between the central and syn protons is 7 Hz. The corresponding bis(η3-allylic)ruthenium(II) complex, supine,prone-[Ru(η3:η3-2,7-dimethyl-2,6-octadiene-1, (29) (a) Lydon, J. E.; Truter, M. R. J. Chem. Soc., A 1968, 362. (b) Preece, M.; Robinson, S. D.; Wingfield, J. N. J. Chem. Soc., Dalton Trans. 1976, 613. (c) Hitchcock, P. B.; Nixon, J. F.; Sinclair, J. J. Organomet. Chem. 1975, 86, C34. (d) Xu, Chang; Pullarkat, S. A.; Goh, L. Y. Aust. J. Chem. 2009, 62, 1537, and references therein.

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8-diyl)(η4-1,5-COD)] (supine,prone-3), was obtained similarly from 1 and isoprene in 55% yield as analytically pure brown crystals. Complex 3 was characterized by 1H and 1 H-1H COSY measurements. The 1H NMR spectrum of 3 is basically similar to that of 2, the most characteristic feature being two singlets at δ 1.73 (3H) and 1.90 (3H) assignable to the methyl groups. Two mutually coupled doublets at δ 0.9 (1H) and 1.1 (1H) and two singlets at δ 3.2 (1H) and 2.7 (1H) are assignable to the 8-CHsyn, 8-CHanti, 1-CHanti, and 1-CHsyn protons, respectively; the absence of spin coupling to a central proton in the allylic fragments in 3 clearly suggests that the two methyl groups in 3 are located at positions 2 and 7. The 13C{1H} NMR spectrum is also consistent with the proposed structure. When complex 1 was treated with (E)-1,3-pentadiene under the same conditions at room temperature, the corresponding complex 4 was obtained as a brown oil in 67% yield. Selected coupling constants derived from 1H NMR and 1H-1H COSY measurements are shown in Chart 2. The resonances due to the central protons in the allylic fragments, 3-CH and 8-CH, appeared at δ 4.93 [dd, 3J(H3-H2) = 8.3, 3 J(H3-H4) = 7.8 Hz, 1H] and 3.64 [t, 3J(H8-H9) = 3J(H8-H7) = 9.6 Hz, 1H], respectively, consistent with an anti-supine, syn-prone arrangement of the fragments. Moreover, these coupling constants also indicate that the methine protons at 4-CH and 7-CH are located in the syn and anti positions, respectively, by comparison with those in 2. Therefore, the diastereotopic methylene protons 4-CH2 and 5-CH2 must be at anti and syn positions, respectively, and this compound is characterized as anti-supine,syn-prone-[Ru(η3:η3-3,7decadiene-2,9-diyl)(η4-1,5-COD)] (anti-supine,syn-prone-4). On the other hand, when 1 was treated with (Z)-1,3pentadiene in benzene at room temperature, an inseparable mixture of compounds was obtained. However, the 1H NMR spectrum and 1H-1H COSY permitted the assignment of most of the signals of the principal product. The resonance due to 3-CH of the allylic fragment appeared at δ 4.89 [t, 3J(H3-H2) = 3J(H3-H4) = 8.5 Hz, 1H], suggesting that 2-CH, 3-CH, and 4-CH adopt a syn configuration. Although the resonance due to the 8-CH proton overlapped with an impurity resonance, the 9-CH signal showed the coupling constant 3J(H9-H8) to be 6.9 Hz, suggesting a syn arrangement of these protons. These data suggest the stereochemistry of the allylic fragment to be anti-supine, anti-prone; hence the main product is characterized as anti-supine, anti-prone-[Ru(η3:η3-3,7-decadiene-2,9-diyl)(η4-1,5-COD)] (anti-supine,anti-prone-4), formed in an estimated yield of 33%. Although we cannot rule out the possible presence of the stereoisomers in this mixture, anti-supine,syn-prone-4 was clearly absent and no free (E)-1,3-pentadiene was observed during the reaction of (Z)-1,3-pentadiene with 1. The complex [Ru(η4-cisoid-2,3-dimethylbutadiene)(η41,5-COD)(NCMe)] failed to react with excess 2,3-dimethylbutadiene, but it did react with butadiene to give a coupled product similar to complexes 2-4.27 Butadiene and isoprene also did not couple with the 1,3-diene units of the corresponding trimethylphosphite complexes [Ru(η4-1,3-diene)(η4-1,5-COD){P(OMe)3}]. Intermediates in the Dimerization on Ruthenium(0). When the reaction of 1 with butadiene was monitored by 1H NMR spectroscopy in toluene-d8, naphthalene was instantly liberated and very broad signals due to an intermediate were observed, which were gradually replaced by the resonances of supine,prone-2. On cooling to -60 °C, the broad signals

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Organometallics, Vol. 30, No. 4, 2011 Chart 3

sharpened and the 1H-1H COSY was measured at this temperature to reveal the spin correlations. Two pairs of correlated signals at δ -1.0 (d, 1H) and 0.83 (d, 1H) and δ 0.20 (d, 1H) and 1.3 (overlapped, 1H) showed spin correlation to a multiplet signal at about δ 4.83-4.90 (2H). They are assigned to the endo-1-CH2, exo-1-CH2, endo-4-CH2, exo-4CH2, and 2- and 3-CH protons, respectively, of a η4-cisoidbutadiene fragment in an asymmetric environment. In addition, a set of six correlated signals at δ 0.8 (m, 1H), 1.7 (overlapped, 1H), 2.3 (overlapped, 1H), 4.81 (dd, J = 10.0, 2.0 Hz, 1H), 5.24 (dd, J = 16.8, 2.0 Hz, 1H), and 5.52 (dt, J = 16.8, 10.0 Hz, 1H) are assigned as 5-CH2, 6-CH, 5-CH2, 8-CH2 (cis to 7-CH), 8-CH2 (trans to 7-CH), and 7-CH, respectively, belonging to a second butadiene moiety. The coupling constant between methine protons in butadiene is reported to be 4-6 Hz for cisoid-butadiene and 10 Hz for transoid-butadiene.14 Thus, the spin coupling constant of 10.0 Hz between 6- and 7-CH in this intermediate suggests that this butadiene fragment has a transoid configuration. Moreover, the chemical shifts for 7-CH (δ 5.52) and 8-CH2 (δ 4.81 and 5.24) are at relatively low field and are similar to the corresponding resonances of free butadiene resonances. On the basis of these data, we believe the intermediate to be the 18-electron complex [Ru(η4-cisoid-butadiene)(η2-transoid-butadiene)(η4-1,5-COD)] (5). There are two potential diastereomers depending on which prochiral face of the η2transoid-butadiene (re-5 and si-5) is coordinated (Chart 3).30 Although the observed diastereomer cannot be specified on the basis of the spectroscopic data alone, we suggest that it is si-5 (and its enantiomer) because this correlates with the molecular structure of the coupled product 2. All the signals due to 5 broadened at room temperature, as also did those of free butadiene, showing that free and coordinated butadiene were exchanging at room temperature. Similar observations were made when the reaction of 1 with (E)-1,3-pentadiene in toluene-d8 was monitored by 1H (30) The terms si and re in front of the compound number indicate the stereochemistry of the prostereogenic face of the η2-transoid-1,3-dienes.

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NMR spectroscopy. The very broad resonances that appeared at room temperature were gradually replaced by those due to anti-supine,syn-prone-4, indicative of the formation of an intermediate (or intermediates) in the dimerization. The low-temperature 1H NMR spectrum revealed the presence of two intermediates, each containing a pair of inequivalent 1,3-pentadiene ligands. At first sight, one might consider these species to be diastereoisomers arising from the prostereogenic31 property of η2-1,3-pentadiene, but this fails to explain why only one intermediate should be observed in the case of the likewise prostereogenic η2-butadiene. The solution to the problem was finally provided by low-temperature 1H-1H COSY analysis, which revealed that these two intermediates differ in containing either the E or Z isomers of the η4-cisoid-1,3-pentadiene; that is, they are formulated as [Ru{η4-(E)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (E-6) and [Ru{η4-(Z)cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η41,5-COD)] (Z-6) (Chart 3). The ratio of E-6/Z-6 was estimated to be 44/56 at -50 °C in toluene-d8. A characteristic feature of the 1H NMR spectrum of E-6 is the 3J(H8-H9) constant of 14.7 Hz, which is assignable to a trans coupling. Therefore, the methyl group in the η2-transoid-1,3-pentadiene has the E configuration. Moreover, signals at δ -0.46 (qui, 1H) and 0.14 (d, 1H) are assigned to 4-endo-CH and 1-endo-CH2 in the (E)-cisoid1,3-pentadiene fragment. A trans 3J(H8-H9) constant (14.4 Hz) is also evident in the 1H NMR spectrum of Z-6, indicating that the η2-transoid-1,3-pentadiene moiety in this species also has an E configuration. The signals at δ -1.24 (d, 1H) and 0.68 (qui, 1H) are assigned as 1-endo-CH2 and 4-exo-CH in the (Z)-cisoid-1,3-pentadiene fragment. Since the signals due to free (E)-1,3-pentadiene broadened at room temperature and free (Z)-1,3-pentadiene was not observed under these conditions, the η2-transoid-1,3-pentadiene in E-6 and Z-6 must exchange rapidly with free (E)-1,3-pentadiene at room temperature, while the η4-cisoid-1,3-pentadiene remains attached. Similarly, two intermediates in a 40/60 ratio were also observed during the reaction of 1 with (Z)-1,3-pentadiene. The 1H-1H COSY analysis of these intermediates at -60 °C in toluene-d8 showed that the major species has resonances at δ -1.2 (d, 1H) and 0.60 (t, 1H) assignable to the 1-endo-CH2 and 4-exo-CH, indicative of a (Z)-cisoid-1,3-pentadiene fragment. The minor species had resonances at δ -0.41 (t, 1H) and 0.26 (m, 1H) assignable to the 4-endo-CH and 1-endoCH2 protons, respectively, of an (E)-cisoid-1,3-pentadiene fragment. The signals arising from these intermediates could not be completely assigned because they overlapped with each other and with the signals of the final product, but we believe the major and minor intermediates to be [Ru{η4-(Z)cisoid-1,3-pentadiene}{η 2 -(Z)-transoid-1,3-pentadiene}(η4-1,5-COD)] (Z-7) and [Ru{η4-(E)-cisoid-1,3-pentadiene}{η2-(Z)-transoid-1,3-pentadiene}(η4-1,5-COD)] (E-7), respectively, by analogy with Z-6 and E-6, the products of reaction of 1 with (E)-1,3-pentadiene. E/Z Exchange Process between E-6 and Z-6. As outlined in Table 1, EXSY32 measurements at -50 °C clearly show (31) As chirality is a property of the whole molecule, we use “prostereogenic” for the chirality of the coordinated prochiral molecules in this article. (32) (a) Derome, A. E. In Modern NMR Techniques for Chemistry Research; Baldwin, J. E., Ed.; Organic Chemistry Series; Pergamon, Oxford, 1987; p 239. (b) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy: A Guide for Chemists, 2nd ed.; Oxford University; Oxford, 1993; pp 223- 230.

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Table 1. Exchanging Protons in Z-6 and E-6 Observed by EXSY at -50 °C

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Scheme 4

endo-1-CH (δ -1.24, d) T endo-1-CH (δ 0.14, d) exo-1-CH (δ 0.77, d) T exo-1-CH (δ 1.69, d) exo-4-CH (δ 0.68, qui) T endo-4-CH (δ -0.46, qui) endo-5-Me (δ 0.95, d) T exo-5-Me (δ 0.60, d)

Table 2. Selected Bond Distances (A˚) and Angles (deg) for supine, prone-2 Ru(1)-C(1) Ru(1)-C(3) Ru(1)-C(7) Ru(1)-C(9) Ru(1)-C(13) C(1)-C(2) C(3)-C(4) C(5)-C(6) C(7)-C(8) C(1)-C(2)-C(3) C(3)-C(4)-C(5) C(5)-C(6)-C(7)

2.232(4) 2.35(2) 2.18(2) 2.241(3) 2.212(5) 1.43(2) 1.531(9) 1.514(9) 1.430(6) 122.6(6) 111.5(6) 120.9(4)

Ru(1)-C(2) Ru(1)-C(6) Ru(1)-C(8) Ru(1)-C(10) Ru(1)-C(14) C(2)-C(3) C(4)-C(5) C(6)-C(7)

2.23(2) 2.273(10) 2.20(4) 2.25(1) 2.218(4) 1.410(5) 1.53(1) 1.426(7)

C(2)-C(3)-C(4) C(4)-C(5)-C(6) C(6)-C(7)-C(8)

125.3(3) 108.0(6) 119.5(4)

correlation between the intermediates E-6 and Z-6 that are observed by low-temperature 1H NMR spectroscopy in the reaction of 1 with (E)-1,3-pentadiene; hence these species are in equilibrium. The most significant features of the EXSY are as follows: (i) endo-4-CH in E-6 becomes exo-4-CH in Z-6, and exo-5-Me in E-6 becomes endo-5-Me in Z-6 in the course of the exchange, and (ii) the endo-1-CH2 and exo-1-CH2 groups do not interchange during the exchange process.

Discussion In view of the stereochemical information that it provides, we focus attention on the reaction of 1 with (E)-1,3-pentadiene to give, regio- and stereoselectively, anti-supine,synprone-4 and on the presence of an equilibrium mixture of two intermediates, E-6 and Z-6, at low temperature. The suggested course of the equilibration reaction for a representative (E)-1,3-pentadiene case is outlined in Scheme 4. Since compact two-electron donors (L) are known to induce a η6 to η4 haptotropic change (ring slippage) in the coordinated naphthalene of 1 to give isolable complexes [Ru(η4-C10H8)(η4-1,5-COD)(L)],33 a conjugated diene can be supposed to behave similarly. This step is likely to be sensitive to the steric bulk of the diene; hence the less hindered CdC bond of (E)-1,3-pentadiene should coordinate preferentially to the ruthenium(0) center. In agreement, 1 is inert toward 2,3-dimethylbutadiene and acetonitrile is not displaced from [Ru(η4-cisoid-2,3-dimethylbutadiene)(η4-1,5-COD)(NCMe)] by 2,3-dimethylbutadiene. In the next step, the η4-naphthalene is replaced by more (E)-1,3pentadiene to give E-6 and Z-6 in equilibrium. If we make the reasonable assumption that the stereochemistry of the bis(allylic) fragment in 4 correlates with the configuration of the precursor η4- and η2-1,3-pentadiene ligands, then syn-supine, (33) Bennett, M. A.; Lu, Z.; Wang, X.; Bown, M.; Hockless, D. C. R. J. Am. Chem. Soc. 1998, 120, 10409.

anti-prone-4 must be derived exclusively from Z-6 in a process of oxidative coupling between (Z)-cisoid-1,3-pentadiene and (E)-transoid-1,3-pentadiene.34 The alternative stereoisomer anti-supine,anti-prone-4, which would be generated by coupling of (E)-cisoid-1,3-pentadiene and (E)-transoid-1,3-pentadiene in E-6, is never observed, possibly because of steric repulsion between the two (E)-1,3pentadiene moieties in the transition state. The interconversion of E-6 and Z-6 is noteworthy because it requires facile rotation about a coordinated carboncarbon double bond. The NMR study also indicated that there was exchange between the coordinated and uncoordinated 1,3-pentadiene. However, since free (Z)-1,3-pentadiene is not observed, the η4-(Z)-cisoid-1,3-pentadiene must remain firmly bound during the reaction. This feature suggests that only the η2-(E)-transoid-1,3-pentadiene ligand in 6 is exchanging with the free (E)-1,3-pentadiene. In other words, in the equilibrium between E-6 and Z-6, η2transoid-1,3-pentadiene binds only weakly to each component. Therefore, we believe that the E/Z isomerization of η4-cisoid-1,3-pentadiene in 6 occurs in the coordinatively unsaturated 16-electron species. Four possible mechanisms for this rotation can be discussed, all of which require coordinative unsaturation of the transition metal center. The first possibility is a 1,5-hydrogen shift via an (η3-allyl)(hydrido)ruthenium(II) intermediate, which is generated by oxidative addition of the 5-methyl group, as shown in the first sequence of Scheme 5. Such a mechanism was put forward first by Green et al.35 to account (34) Recently, we have reported the isolation of ruthenacyclopentanes by oxidative coupling reactions of acrylates on Ru(0) as well as their catalytic behavior: (a) Hirano, M.; Sakate, M.; Komine, N.; Komiya, S.; Bennett, M. A. Organometallics 2009, 28, 4902. (b) Hirano, M.; Hiroi, Y.; Komine, N.; Komiya, S. Organometallics 2010, 29, 3690. (c) Hirano, M.; Arai, Y.; Komine, N.; Komiya, S. Organometallics 2010, 29, 5741.

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Scheme 5. Possible Mechanisms for Exchange between E-6 and Z-6

for the formation from (Z)-1,3-pentadiene of an isomeric mixture of (E)- and (Z)-1,3-pentadiene complexes of [RuCp(CO)]þ from [RuCp(CO)(NCMe)2]. However, the EXSY experiments (Table 1) establish that, in the present case, there is no exchange between the terminal methylene protons in the 1-CH2 group and the methyl protons of the 5-CH3 group; hence the 1,5-hydrogen shift mechanism seems to be excluded in this case. Faller and Rosan,36 who discovered the first example of E to Z interconversion in [MoCp(η4-cisoid-1,3-diene)(CO)2] cations, proposed the so-called “envelope shift” mechanism, i.e., an inversion in the five-membered metallacyclopentene leading to interchange of the exo and endo substituents on the terminal carbon atoms of the diene, as shown in the second sequence of Scheme 5. The same mechanism was invoked by Vollhardt et al.37 in a study of the photochemical isomeriza(35) (a) Crocker, M.; Green, M.; Morton, C. E.; Nagle, K. R.; Orpen, A. G. J. Chem. Soc., Dalton Trans. 1985, 2145. (b) Crocker, M.; Green, M.; Nagle, K. R.; Williams, D. J. J. Chem. Soc., Dalton Trans. 1990, 2571. (36) Faller, J. W.; Rosan, A. M. J. Am. Chem. Soc. 1977, 99, 4857. (37) Eaton, B. A.; King, J. A., Jr.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 1359.

tion of a specifically labeled [CoCp(η4-1,3-diene)] complex and by Benn and Schroth38 for the thermal isomerization of [M(η-C8H8)(η4-cisoid-butadiene)] (M = Zr, Hf). In our case, however, this mechanism is also excluded by the EXSY experiment, which shows no exchange between the endoand exo-1-CH2 protons during the E/Z isomerization. A third possible mechanism posits the formation of a η3allylic fragment by the metal-cabon bond rupture in the enediyl extreme, giving a zwitterionic intermediate, as shown in the third sequence of Scheme 5. There appears to be only one previous proposal of such a mechanism. Erker, Berke, et al.39 postulated a thermally induced interconversion between η4diene and η3-allyl in ZrCp2(η4-transoid-1,4-disubsituted 1,3butadiene), to account for the stepwise E/Z isomerization of 1,4-disubstituents in the 1,3-diene, and supported their proposal with theoretical calculations. Although the possibility of forming a methyl-substituted carbanion seems unappealing on energetic grounds, migration of the electrons from Ru to C might be favored because of the high Lewis basicity of (38) Benn, R.; Schroth, G. J. Organomet. Chem. 1982, 228, 71. (39) Erker, G.; Engel, K.; Czisch, P.; Berke, H.; Caubere, P.; Vanderesse, R. Organometallics 1985, 4, 1531.

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Ru(0). In addition, reversible homolytic cleavage of the putative Ru-C bond in the ruthenacyclopentene resonance hybrid could be considered. In both cases, the conversion of the diene CdC bond to a C-C single bond could then allow rotation to occur about it in the resulting carbanion (or carbon radical), leading to E/Z isomerization. It should be recalled that such η4-diene and η3-allyl interconversions are also well-known to be mediated by electrophilic or nucleophilic reagents.40 This mechanism is consistent with the results of the EXSY experiments, which show that only the C(3)dC(4) bond rotates during the interconversion between E-6 and Z-6, while the C(1)dC(2) bond remains unchanged. The fourth possibility, which may be termed a 1,1-hydrogen shift mechanism, has never been reported to the best of our knowledge. It can be regarded as a modification of the 1,5-hydrogen shift mechanism in which C-H oxidative addition reaction of the 5-Me group occurs, leading to the same η3-allylic intermediate. It is followed by the rotation about the C(3)-C(4) single bond and migration of the resulting hydride back to the original carbon atom C(5), giving E-6.41 Although this mechanism is also consistent with all the EXSY results and seems to be a lower energy pathway than the third mechanism, it is not clear why the hydride should return exclusively to the same carbon atom. Among these possibilities, therefore, we believe the η3-allyl mechanism provides the most satisfactory explanation for the results, although the 1,1-hydrogen shift mechanism certainly cannot be excluded. Calculations of the various reaction pathways might be helpful.

Concluding Remarks In summary, the reactions reported here provide good models for the dimerization of conjugated dienes on lowvalent transition metal complexes. Complete liberation of η6-naphthalene generates vacant sites on Ru(0) that can be occupied by a η4-cisoid-diene and a η2-transoid-diene, donating in total six electrons. The η2-transoid-diene is labile and undergoes rapid exchange with free diene. The oxidative coupling takes place with retention of the stereochemistry of cisoid- and transoid-dienes to give supine,prone-bis(η3allylic)ruthenium(II) complexes. When asymmetric dienes such as (E)-1,3-pentadiene are employed in this reaction, both η4-(E)-cisoid-1,3-pentadiene and η4-(Z)-cisoid-1,3-pentadiene are produced in an equilibrium mixture, while the η2transoid-diene retains its stereochemistry. The E/Z equilibration requires rotation about the carbon-carbon double bond, a process that, we suggest, is initiated by a reversible η4 to η3 interconversion of the coordinated diene. The final coupling product anti-supine,syn-prone-4 arises from Z-6 with retention of configuration, and C-C bond formation occurs on the less hindered side of the dienes to avoid steric repulsion in the oxidative coupling reaction.

Experimental Section All manipulations and reactions were performed under dry nitrogen with use of standard Schlenk and vacuum line techniques. Benzene, hexane, pentane, and Et2O were distilled over sodium benzophenone ketyl, and acetonitrile was distilled from (40) (a) Gemel, C.; Kalt, D.; Mereiter, K.; Sapunov, V. N.; Schmid, R.; Kirchner, K. Organometallics 1997, 16, 427. (b) B€ackvall, J. E. Acc. Chem. Res. 1983, 16, 335. (41) We thank a reviewer for this suggestion.

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Drierite. Methanol was distilled over magnesium methoxide. Butadiene (99.0% pure), isoprene, and (E)- and (Z)-1,3-pentadiene were purchased from Takachiho Chemical Industry and used as received. [Ru(η6-naphthalene)(η4-1,5-COD)] (1) was prepared according to the reported procedure.42 The NMR spectra were measured on a Varian Gemini 300 spectrometer (1H at 300 MHz, 13C at 75.4 MHz) (Canberra) and on JEOL ECX400P (1H at 399.8 MHz, 13C at 100.5 MHz) and JEOL LA300 (1H at 300.4 MHz) spectrometers (Tokyo). The 1H and 13 C chemical shifts were referenced to tetramethylsilane. The reported 1H-1H coupling constants for the diene fragments refer to the observed peak separations; non-first-order analysis of the spectra was not attempted. The accuracy of coupling constants was estimated from the raw data by taking account of experimental error based on the line shape of each resonance. IR spectra were recorded on PE 683 and JASCO FT/IR4100 spectrometers (Canberra and Tokyo, respectively). Elemental analyses were performed on a Perkin-Elmer 2400 series II CHN analyzer (Tokyo) and by the staff of the Research School of Chemistry (Canberra). GLC analysis was performed on a Shimadzu GC-14B instrument with a TC-1 column equipped with a FID detector (0.25 mmf  30 m). GC-MS analysis was performed on a Shimadzu QP2100 instrument (Tokyo). supine,prone-[Ru(η3:η3-2,6-octadiene-1,8-diyl)(η4-1,5-COD)] (supine,prone-2). Butadiene was bubbled for 5 min into a solution of complex 1 (320 mg, 0.948 mmol) in benzene (30 mL) at room temperature. The bubbling head was replaced by a glass stopper, and the mixture was stirred for 30 min at room temperature. Volatile materials were removed under reduced pressure, and the residual black solid was dried by means of an oil diffusion pump to remove free naphthalene. The black solid was extracted with hexane (20 mL). The brown extract was concentrated and set aside at -80 °C overnight. The resulting light brown solid was separated and recrystallized from cold Et2O/MeOH to give pale yellow crystals of supine,prone-2, which were washed with cold pentane and dried under reduced pressure. The yield was 233.6 mg, 0.736 mmol (78%). 1H NMR (400 MHz, C6D6, rt): δ 0.87 (tdd, 2JH4-H4 = 3JH4-H5 = 14, 3 JH4-H3 = 11, 3JH4-H5 = 5 Hz, 1H, endo-4-CH), 1.01 (dd, 3 JH1-H2 = 7, 2JH1-H1 = 2 Hz, 1H, syn-1-CH), 1.45 (dd, 3 JH1-H2 = 10, 2JH1-H1 = 2 Hz, 1H, anti-1-CH), 1.7-1.8 (m, 5H, exo-4-CH, endo-5-CH and COD), 1.8-1.9 (m, 2H, COD), 1.99 (td, 3JH6-H7 = 10, 3JH6-H5 = 5 Hz, 1H, anti-6-CH), 2.13 (tt, J = 14, 6 Hz, 1H, COD), 2.20 (td, 3JH5-H4 = 2JH5-H5 = 14 Hz, 3JH5-H6 = 5 Hz, 1H, exo-5-CH), 2.38 (dtd, J = 15, 11, 5 Hz, 1H, COD), 2.60 (d, 3JH8-H7 = 10 Hz, 1H, anti-8-CH), 2.64 (m, 1H, COD), 2.71 (m, 1H, COD), 2.79 (dd, J = 15, 7 Hz, COD), 3.55 (d, 3JH8-H7 = 7 Hz, 1H, syn-8-CH), 3.80 (dd, J = 9, 5 Hz, COD), 3.84 (td, 3JH7-H6 = 3JH7-H8 = 10, 3JH7-H8 = 7 Hz, 1H, 7-CH), 4.15 (dd, J = 8, 5 Hz, COD), 4.27 (dt, 3JH2-H1 = 10, 3 JH2-H1 = 3JH2-H3 = 7 Hz, 1H, 2-CH), 5.23 (dt, 3JH3-H4 = 10 Hz, 3JH3-H4 = 3JH3-H2 = 7 Hz, 1H, syn-3-CH). 13C NMR [75.4 MHz, C6D6, rt; JCH (Hz) in parentheses]: δ 26.4 (t, 125), 27.5 (t, 128), 30.2 (t, 123), 37.2 (t, 129), 39.0 (t, 129), 39.3 (t, 129), 39.6 (t, 150), 43.8(t, 150), 67.9 (d, 153), 71.1 (d, 150), 83.0 (d, 153), 87.2 (d, 158), 87.4 (d, 151), 94.4 (d, 158), 96.1 (d, 153), 97.5 (d, 158). EI/MS: m/z 318 (Mþ, based on 102Ru). Anal. Calcd for C16H24Ru: C, 60.54; H, 7.62. Found: 59.57; H, 7.70. Only one of the crystals of 2 selected from cold Et2O/MeOH proved suitable for X-ray structural analysis and only 23 reflections could be used to determine the lattice parameters, thus limiting the accuracy of the analysis. supine,prone-[Ru(η3:η3-2,7-dimethyl-2,6-octadiene-1,8-diyl)4 (η -1,5-COD)] (supine,prone-3). A solution of complex 1 (284 mg, 0.843 mmol) in hexane (5 mL) was treated with isoprene (300 μL, 3.00 mmol) from a hypodermic syringe. The reaction mixture was stirred overnight at room temperature. Volatile (42) Bennett, M. A.; Neumann, H.; Thomas, M.; Wang, X.-Q.; Pertici, P.; Salvadori, P.; Vitulli, G. Organometallics 1991, 10, 3237.

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materials were removed under reduced pressure, and the resulting dark brown solid was recrystallized from cold Et2O/MeOH to give brown crystals of supine,prone-3, which were washed with cold pentane and dried under reduced pressure. The yield was 159.4 mg, 0.461 mmol, (55%). 1H NMR (400 MHz, C6D6, rt): δ 0.8-0.9 (m, 1H, endo-4-CH), 0.93 (d, 3JH1-H2 = 3.9 Hz, 1H, syn-1-CH), 1.12 (d, 3JH1-H2 = 1.8 Hz, 1H, anti-1-CH), 1.74 (s, 3H, 2- or 7-CH3), 1.8 (m, 1H, 6-CH), 1.89 (s, 3H, 7- or 2-CH3), 1.6-2.1 (m, 5H, COD), 1.9-2.0 (m, 1H, 4- or 5-CH), 2.26-2.43 (m, 2H, COD), 2.6-2.8 (m, 3H, COD), 2.74 (s, 1H, anti-8-CH), 3.13 (s, 1H, syn-8-CH), 3.86 (dd, J = 9.2, 5.0 Hz, COD), 4.19 (dd, J = 8.1, 4.8 Hz, 1H, COD), 5.00 (t, 3JH3-H4 = 8.0 Hz, syn-3-CH). 13C NMR (75.4 MHz, C6D6, rt): δ 20.5 (q, 126), 25.6 (q, 126), 26.0 (t, 122), 26.9 (t, 122), 37.0 (t, 124), 37.5 (t, 125), 39.2 (t, 128), 44.0 (t, 154), 47.4 (t. 156), 67.1 (d, 156), 72.9 (d, 160), 83.1 (d, 158), 86.0 (d, 153), 88.2 (d,151), 90.9 (d, 154), 105.4 (s), 112.0 (s). Anal. Calcd for C18H28Ru: C, 62.58; H, 8.17. Found: C, 62.74; H, 8.20. anti-supine,syn-prone-[Ru(η3:η3-3,7-decadiene-2,9-diyl)(η4-1, 5-COD)] (anti-supine,syn-prone-4). In a 25 mL Schlenk tube, a solution of 1 (268.0 mg, 0.795 mmol) in benzene (10 mL) was treated with (E)-1,3-pentadiene (400 μL, 3.99 mmol) from a hypodermic syringe. After stirring the mixture at room temperature for 2 h, volatile materials were removed under reduced pressure, and the resulting brown oil was extracted with hexane. The extract was evaporated to dryness to give antisupine,syn-prone-4 as a brown oil. The yield was 183.9 mg, 0.532 mmol (67%). This compound was characterized by 1H NMR and COSY analyses. 1H NMR (400 MHz, C6D6, rt): δ 0.88 (br d, 3JH6-H7 = 6.6 Hz, 1H, exo-6-CH), 0.9 (overlapped, exo-5-CH), 0.95 (br d, 3JH1-H2 = 6.4 Hz, 3H, anti-1-CH3), 1.25 (br m, 1H, 2-CH), 1.5 (overlapped, endo-5-CH), 1.6 (br dd, 3JH7-H8 = 9.6, 3JH7-H6 = 6.6 Hz, 1H, anti-7-CH), 1.731.75 (overlapped, 4H, COD), 1.84 (br d, 3JH10-H9 = 6.0 Hz, 3H, syn-10-CH3), 2.02-2.16 (m, 2H, COD), 2.2 (m, 1H, endo6-CH), 2.31-2.42 (m, 2H, COD), 2.5-2.6 (m, 2H, COD), 2.96 (dd, J = 8.4, 5.2 Hz, 1H, COD), 3.10 (dq, 3JH9-H8 = 9.6, 3 JH9-H10 = 6.0 Hz, 1H, anti-9-CH), 3.63 (t, 3JH8-H7 = 3 JH8-H9 = 9.6 Hz, 1H, 8-CH), 4.18 (dd, J = 6.2, 5.0 Hz, 1H, COD), 4.25 (q, 3JH4-H5 = 3JH4-H3 = 7.8 Hz, 1H, 4-CH), 4.93 (dd, 3JH3-H2 = 8.3 Hz, 3JH3-H4 = 7.8 Hz, 1H, 3-CH). anti-supine,anti-prone-[Ru(η3:η3-3,7-decadiene-2,9-diyl)(η41,5-COD)] (anti-supine,anti-prone-4). The reaction was carried out as described above from complex 1 (236.0 mg, 0.699 mmol), benzene (10 mL), and (Z)-1,3-pentadiene (300 μL, 3.40 mmol). Workup gave a brown oil (210.4 mg), which, on the basis of 1H NMR and 1H-1H COSY, was estimated to contain antisupine, antiprone-4 (33% yield). 1H NMR (400 MHz, C6D6, rt): δ 0.89 (br d, J = 0.89 Hz, 1H, exo-6-CH), 0.94 (d, 3JH1-H2 = 6.4 Hz, 3H, anti-1-CH3), 1.4 (overlapped, 2-CH), 1.47 (d, 3JH10-H9 = 6.9 Hz, 3H, anti-10-CH3), 2.1 (m, 1H, 7-CH), 2.32-2.38 (m, 1H, 6-CH), 3.78 (overlapped, 8-CH), 3.95 (dd, J = 8.7, 5.5 Hz, 1H, COD), 4.33 (qui, 3JH9-H8 = 3JH9-H10 = 6.9 Hz, 1H, syn-9-CH), 4.89 (t, 3 JH3-H2 = 3JH3-H4 = 8.5 Hz, 1H, 3-CH). [Ru(η4-cisoid-butadiene)(η2-transoid-butadiene)(η4-1,5-COD)] (5). This intermediate was formed from complex 1 and butadiene similarly to its 1,3-pentadiene analogues 6 and 7 described below. 1H NMR (400 MHz, toluene-d8, -60 °C): δ -1.0 (d, 3JH1-H2 = 8.8 Hz, 1H, endo-1-CH), 0.20 (d, 3JH4-H3 = 8.0 Hz, 1H, endo -4-CH), 0.8 (m, 1H, 5-CH), 0.83 (d, 3JH1-H2 = 7.0 Hz, 1H, exo-1-CH), 1.3 (overlapped, exo-4-CH), 1.38-1.70 (m, 4H, COD), 1.7 (overlapped, 6-CH), 2.14-2.17 (m, 4H, COD), 2.3 (overlapped, 5-CH), 2.36-2.39 (m, 2H, COD), 3.18-3.23 (m, 2H, COD), 4.81 (dd, 3JH8-H7 = 10.0, 2JH8-H8 = 2.0 Hz, 1H, 8-CHcis to 7-CH), 4.83-4.90 (m, 2H, 2- and 3-CH), 5.24 (dd, 3 JH8-H7 = 16.8, 2JH8-H8 = 2.0 Hz, 1H, 8-CHtrans to 7-CH), 5.52 (dt, 3JH7-H8 = 16.8, 3JH7-H6 = 3JH7-H8 10.0 Hz, 1H, 7-CH). Reaction of 1 with (E)-1,3-Pentadiene in Toluene-d8. Complex 1 (5.7 mg, 0.018 mmol) was placed in an NMR tube into which dry toluene-d8 (0.6 mL) was introduced by vacuum distillation.

Hirano et al. Table 3. Crystallographic and Physical Data for supine,prone-2 chemical formula fw cryst syst space group a (A˚) b (A˚) c (A˚) β (deg) V (A˚3) Z temp (K) cryst dimens (mm) color absorp corr radiation diffractometer measurement method reflns (total) params GOF R (Rw)

C16H24PRu 317.44 monoclinic P21/c (No. 14) 7.2(1) 15.3(1) 24.8(1) 90.3(9) 2747(48) 8 200.0 0.29  0.15  0.10 pale yellow psi-scan Mo KR (λ = 0.7107 A˚) Rigaku AFC7R ω 6223 307 1.074 0.0289 (0.0384)

(E)-1,3-Pentadiene (3.4 μL, 0.034 mmol) was added into the solution by a hypodermic syringe at -40 °C. The 1H NMR spectrum showed that the reaction proceeded sluggishly at -40 °C. When the reaction system was warmed to þ10 °C, signals due to free naphthalene, significant broadening of the signals due to (E)-1,3-pentadiene, and very broad signals of intermediates as well as the final product 4 were observed. When the reaction mixture was cooled to -50 °C, the broad signals of the intermediates became sharp. These signals were characterized by the 1H NMR spectrum at -50 °C and the 1H-1H COSY at -60 °C. The ratio of E-6 and Z-6 was 44/56 at -50 °C. [Ru{η4-(Z)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (Z-6): 1H NMR (400 MHz, toluene-d8, -50 °C): δ -1.24 (d, 3JH-H = 8.7 Hz, 1H, 1-CHendo), 0.68 (qui, 3 JH-H = 6.7 Hz, 1H, 4-CHexo), 0.77 (d, 3JH-H = 6.9 Hz, 1H, 1-CHexo), 0.95 (d, 3JH-H = 6.4 Hz, 3H, 5-Meendo), 1.73 (d, 3 JH-H = 6.9 Hz, 3H, 10-CH3), 2.7-2.8 (m, 2H, 7- and 8-CH), 3.99 (d, 3JH-H = 8.4 Hz, 1H, 6-CHcis to 7-CH), 4.8 (overlapped, 2-CH), 4.9 (overlapped, 3-CH), 5.28 (br d, 3JH-H = 14.6 Hz, 1H, 6-CHtrans to 7-CH), 5.72 (dq, 3JH-H = 14.4, 3JH-H = 7.1 Hz, 9-CHtrans to 8-CH). [Ru{η4-(E)-cisoid-1,3-pentadiene}{η2-(E)-transoid-1,3-pentadiene}(η4-1,5-COD)] (E-6): 1H NMR (400 MHz, toluene-d8, -50 °C): δ -0.46 (qui, 3JH-H = 3JH-H = 7.1 Hz, 1H, 4-CHendo), 0.14 (d, 3JH-H = 6.9 Hz, 1H, 1-CHendo), 0.60 (d, 3JH-H = 6.0 Hz, 3H, 5-Meexo), 1.31 (br d, 3JH-H = 6 Hz, 1H, 1-CHexo), 1.69 (d, 3JH-H = 6.9 Hz, 3H, 10-CH3), 2.7-2.8 (m, 7- and 8-CH), 4.06 (d, 3JH-H = 8.2 Hz, 1H, 6-CHcis to 7-CH), 4.8 (overlapped, 2-CH), 4.9 (overlapped, 3-CH), 5.20 (br d, 3JH-H = 13.8 Hz, 6-CHtrans to 7-CH), 5.64 (dq, 3JH-H = 14.7, 3JH-H = 7.1 Hz, 9-CHtrans to 8-CH). Reaction of 1 with (Z)-1,3-Pentadiene in Toluene-d8. Complex 1 (10.4 mg, 0.0308 mmol) was placed in an NMR tube into which dry toluene-d8 (0.6 mL) was introduced by vacuum distillation. The solution was cooled in chilled alcohol (∼-40 °C), and then (Z)-1,3-pentadiene (6.2 μL, 0.063 mmol) was added to the solution from a hypodermic syringe. Complex 1 reacted sluggishly at -40 °C, but at 0 °C smoothly produced intermediates E- and Z-7, together with the final product 4 and unidentified compounds. Since this reaction gave complex, overlapping signals, E- and Z-7 could be characterized only by 1H-1H COSY. [Ru{η4-(Z)-cisoid-1,3-pentadiene}{η2-(Z)-transoid-1,3pentadiene}(η4-1,5-COD)] (Z-7): 1H NMR (400 MHz, toluene-d8, -50 °C): δ -1.2 (d, 3JH-H = 6.9 Hz, 1H, 1-CHendo), 0.60 (t, 3JH-H = 6.7 Hz, 1H, 4-CHexo), 0.89 (d, 3JH-H = 6.4 Hz, 3H, 5-Meendo), 0.9 (m, overlapped), 1.89 (d, 3JH-H = 6.9 Hz, 3H, 10-Me), 4.8 (overlapped, 2-CH and 3-CH). [Ru{η4-(E)cisoid-1,3-pentadiene}{η2-(Z)-transoid-1,3-pentadiene}(η4-1,5COD)] (E-7): 1H NMR (400 MHz, toluene-d8, -50 °C): δ -0.41 (t, 3JH-H = 5.5 Hz, 1H, 1-CHendo), 0.26 (m, 1H, 4-CHendo), 0.70

Article (d, 3JH-H = 6.0 Hz, 3H, 5-Mexo), 0.9 (m, overlapped), 4.8 (overlapped, 2-CH and 3-CH). X-ray Analysis of 2. A Rigaku AFC-7R diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71069 A˚) was used for data collection at 200.2 K. A selected crystal of 2 was mounted on a glass capillary by use of Paraton N oil. The collected data were solved by direct methods and refined by a full-matrix least-squares procedure using CrystalStructure programs.43 All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were treated as (43) CrystalStructure ver.3.8; Rigaku America and Rigaku Corporation, 2007.

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idealized contributions. The crystallographic and physical data are listed in Table 3.

Acknowledgment. We are grateful to Ms. S. Kiyota for elemental analyses. Supporting Information Available: Experimental details for treatment of 1 with 2,3-dimethylbutadiene, acidolyses of supine, prone-2, supine,prone-3, and anti-supine,syn-prone-4. Full description of crystallographic data for supine,prone-2. 1D and 2D NMR charts and the assignments for supine,prone-2, supine,prone-3, (anti-supine, syn-prone)-4, (anti-supine,anti-prone)-4, 5, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.