Alkynyl-Dicobalt Hexacarbonyl Complexes of Menthyl Cations

May 11, 2010 - Synopsis. The reaction of Fe(CO)5 with either axial- or equatorial-(phenylethynylmenthol)Co2(CO)6 yields a neutral iron−cobalt cluste...
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Organometallics 2010, 29, 4882–4892 DOI: 10.1021/om100107x

Alkynyl-Dicobalt Hexacarbonyl Complexes of Menthyl Cations: Isolobal Substitution of [Co(CO)3]þ by Fe(CO)3 as a Structural Model† Angela Moore, Yannick Ortin, Helge M€ uller-Bunz, and Michael J. McGlinchey* School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland Received February 11, 2010

The addition of phenylethynyl- or trimethylsilylethynyl-lithium to (-)-menthone yields primarily the alkynyl-menthols in which the alkynyl substituent occupies an equatorial site. The stereochemistry of both the axial and equatorial isomers was established unequivocally by an X-ray crystallographic study of their dicobalt hexacarbonyl derivatives. Treatment of these alkynol-Co2(CO)6 complexes with iron pentacarbonyl yields neutral iron-cobalt clusters whose structures provide excellent models for cobalt-stabilized propargyl cations found as intermediates in the Nicholas reaction. Although the replacement of a cationic Co(CO)3 vertex by a neutral Fe(CO)3 moiety was originally classified as a dehydroxylation, the isolation of bridged hydride species suggests that the mechanism more likely involves a decarboxylation process. When this reaction was applied to enantiomerically pure dicobalt hexacarbonyl complexes of axial or equatorial phenylethynylmenthols, the mixed iron-cobalt products that resulted always positioned the planar phenyl substituent, rather than the much bulkier cobalt tricarbonyl moiety, adjacent to the isopropyl substituent of the menthyl ring. Mechanisms to account for these observations are proposed, and all new metal clusters were characterized by X-ray crystallography.

Introduction The ability of alkyne-dicobalt clusters to stabilize an R-cationic site is the basis of the Nicholas reaction,1 a synthetically valuable procedure used, for example, in the preparation of enediynes,2a (þ)-begamide E,2b blastinomycine,2c cyclocolorenone,2d tetrapyrroles,2e insect pheromones,2f vita† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth in recognition of his many outstanding contributions to organometallic chemistry. *Corresponding author. E-mail: [email protected]. Fax: (þ353)-1-716-1178. (1) (a) Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207–214. (b) O'Boyle, J. E.; Nicholas, K. M. Tetrahedron Lett. 1980, 21, 1595–1598. (c) Caffyn, A. J. M.; Nicholas, K. M. In Comprehensive Organometallic Chemistry II; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 12, Chapter 7.1, pp 685-702. (d) McGlinchey, M. J.; Girard, L.; Ruffolo, R. Coord. Chem. Rev. 1995, 143, 331–381. (e) El Amouri, H.; Gruselle, M. Chem. Rev. 1996, 96, 1077– 1103. (2) (a) Magnus, P.; Lewis, R. T.; Huffman, J. C. J. Am. Chem. Soc. 1988, 110, 6921–6923. (b) Mukai, C.; Moharram, S. M.; Kataoka, O.; Hanaoka, M. J. Chem. Soc., Perkin Trans. 1 1995, 2849–2854. (c) Mukai, C.; Katuoka, O.; Hanaoka, M. J. Org. Chem. 1993, 58, 2946–2952. (d) Saha, M.; Bagby, B.; Nicholas, K. M. Tetrahedron Lett. 1986, 27, 915–918. (e) Jacobi, P. A.; Rajeswari, S. Tetrahedron Lett. 1992, 33, 6231–6234. (f) Henrick, C. A. Tetrahedron 1977, 33, 1845–1889. (g) Kienzle, F. Pure Appl. Chem. 1976, 47, 183–190. (h) Makin, S. M. Pure Appl. Chem. 1976, 47, 173–181. (i) Soleilhavoup, M.; Maurette, L.; Lamirand, C.; Donnadieu, B.; McGlinchey, M. J.; Chauvin, R. Eur. J. Org. Chem. 2003, 1652–1660. (j) Tyrell, E.; Millet, J.; Tesfa, K. H.; Williams, N.; Mann, A.; Tillett, C.; Muller, C. Tetrahedron 2007, 63, 12769–12778. (3) (a) D’Agostino, M. F.; Frampton., C. S.; McGlinchey, M. J. J. Organomet. Chem. 1990, 394, 145–166. (b) D'Agostino, M. F.; Frampton., C. S.; McGlinchey, M. J. Organometallics 1990, 11, 2972–2984. (c) Gruselle, M.; El Hafa, M.; Nikolski, M.; Jaouen, G.; Vaissermann, J.; Li, L.; McGlinchey, M. J. Organometallics 1993, 12, 4917–4925.

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min A derivatives,2g carotenes,2h pyrones,2i benzofurans,2i and benzopyrans.2j Moreover, such stabilization by a metal allows the convenient generation and isolation of otherwise inaccessible species such as the nonclassical bornyl3 and fenchyl4 cations, antiaromatic indenyl or fluorenyl cations,5 and cations bearing a strongly electron-withdrawing substituent such as trifluoromethyl.6 Interestingly, in several recent reports it has been shown that protonation of the precursor alcohols in diethyl ether or tetrahydrofuran leads to products derived from radicals rather than cations.7,8 This stabilization can be attributed to partial delocalization of the charge onto a cobalt atom through overlap of a filled metal d orbital with the vacant p orbital on the electrondeficient R-carbon3a,9 and is facilitated by a structural (4) Kondratenko, M.; El Hafa, H.; Gruselle, M.; Jaouen, G.; Vaissermann, J.; McGlinchey, M. J. J. Am. Chem. Soc. 1995, 117, 6907–6913. (5) Dunn, J. A.; Hunks, W. J.; Ruffolo, R.; Rigby, S. S.; Brook, M. A.; McGlinchey, M. J. Organometallics 1999, 18, 3372–3382. (6) Kondratenko, M. A.; Malezieux, B.; Gruselle, M.; BonnetDelpon, D.; Begue, J. P. J. Organomet. Chem. 1995, 487, C15–C17. (7) (a) Melikyan, G. G.; Villena, F.; Florut, A.; Sepanian, S.; Sarkissian, H.; Rowe, A.; Toure, P.; Mehta, D.; Christian, N.; Myer, S.; Miller, D.; Scanlon, S.; Porazik, M.; Gruselle, M. Organometallics 2006, 25, 4680– 4690. (b) Melikyan, G. G.; Sepanian, S.; Villena, F.; Jerome, J.; Ahrens, B.; McClain, R.; Matchett, J.; Scanlon, S.; Abrenica, A.; Paulsen, K.; Hardcastle, K. I. J. Organomet. Chem. 2003, 683, 324–330. (c) Toure, P.; Myer, S.; Melikyan, G. G. J. Phys. Chem. A 2001, 105, 4579–4584. (8) Kaldis, J. H.; Brook, M. A.; McGlinchey, M. J. Chem.;Eur. J. 2008, 14, 10074–10084. (9) Schilling, B. E. R.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 6274–6274. (10) (a) Padmanabhan, S.; Nicholas, K. M. J. Organomet. Chem. 1984, 212, C23–C27. (b) Schreiber, S. L.; Sammakia, T.; Crowe, W. E. J. Am. Chem. Soc. 1986, 108, 3128–3130. (c) Schreiber, S. L.; Klimas, M. T.; Sammakia, T. J. Am. Chem. Soc. 1987, 109, 5749–5759. (d) Edidin, R. T.; Norton., J. R.; Mislow, K. Organometallics 1981, 1, 561–562. r 2010 American Chemical Society

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Scheme 1. Generation of Cobalt-Stabilized Propargyl Cations

perturbation such that the carbocationic center leans toward a metal vertex, as illustrated in Scheme 1. Although there is overwhelming evidence from molecular orbital calculations9 and variable-temperature NMR data10 that a cationic center can interact directly with a tricarbonylcobalt vertex in propargyl-Co2(CO)6 or carbynyl-Co3(CO)9 tetrahedral clusters, we are aware of only a single example of a crystallographically characterized cobalt-stabilized carbocation, 1, whereby the carbon bearing the formal positive charge is apparently cooperatively stabilized by two cobalt clusters.11 However, this geometric distortion has been observed crystallographically in a wide variety of mixed-metal tetrahedral systems, whereby the R-carbon is clearly bonded to a metal cluster vertex, as discussed in more detail below. It is also relevant to summarize very briefly the dynamic NMR behavior of propargyl cations complexed to a dimetallic moiety, as exemplified by a number of [(RCtCCR1R2)Co2(CO)6]þ and [(RCtC-CR1R2)Mo2(CO)4Cp2]þ clusters.1d,10c As shown in Scheme 2, antarafacial migration of the CR1R2 moiety from one cobalt (or molybdenum) vertex to the other maintains the identity of the syn and anti substituents at the β-carbon, but racemizes the complex by equilibrating the metal sites. In contrast, rotation about the C(R)-C(β) linkage interconverts the syn and anti groups, thus effecting diastereomerization; this is equivalent to a suprafacial migration process. In the dimolybdenum systems, the barrier to the former process can be evaluated by monitoring the coalescence behavior of the cyclopentadienyl rings in either the 1H or 13C NMR regimes. In dicobalt clusters, the racemization process has been followed by variable-temperature 31P NMR after incorporation of a chelating diphosphine to label the metal vertices.5,12 The fluxional behavior of homometallic cluster cations [(RCtC-CR1R2)(MLn)2]þ, where MLn=Co(CO)3, Mo(CO)2Cp, W(CO)2Cp, contrasts with that of mixed-metal complexes, which are, in many cases, nonfluxional.13 In the bornyl cluster 2,3c the cation interacts with the molybdenum rather than the cobalt vertex; in 3,14 cymantrenyl competes with a tricarbonylcobalt vertex. However, when the cation has the choice of Co(CO)3 , CpMo(CO)2 , and (C 5H 5)Fe(C 5H4 ) (in 4), or Co(CO)3, CpMo(CO)2, and (OC)3Mn(C5H4) (in 5), it preferentially leans toward molybdenum in both cases.14 Other examples reveal that incorporation of a phosphine enhances (Ph3P)Co(CO)2 over Co(CO)3.15 (11) Melikyan, G. G.; Bright, S.; Monroe, T.; Hardcastle, K. I.; Ciurash, J. Angew. Chem. 1998, 110, 170–172. Angew. Chem., Int. Ed. 1998, 37, 161-164. (12) (a) D’Agostino, M. F.; McGlinchey, M. J. Polyhedron 1988, 7, 807–825. (b) Sutin, K. A.; Kolis, J. W.; Mlekuz, M.; Bougeard, P.; Sayer, B. G.; Quilliam, M. A.; Faggiani, R.; Lock, C. J. L.; McGlinchey, M. J.; Jaouen, G. Organometallics 1987, 6, 439–447. (13) El Hafa, H.; Cordier, C.; Gruselle, M.; Besace, Y.; Jaouen, G.; McGlinchey., M. J. Organometallics 1994, 13, 5149–5156. (14) Kondratenko, M. A.; Rager, M.-N.; Vaissermann, J.; Gruselle, M. Organometallics 1995, 14, 3802–3809. (15) Bradley, D. H.; Khan, M. A.; Nicholas, K. M. Organometallics 1992, 11, 2598–2607.

Figure 1. Bis(cobalt-cluster)-stabilized carbocation 1.

Moreover, a recent study on the protonation of acetylferrocene, (C5H5)Fe[C5H4-C(dO)CH3], and acetylcymantrene, (OC)3Mn[C5H4-C(dO)CH3], revealed the ferrocenyl-stabilized system to be favored.16 From these and other examples,17 it is evident that there is a hierarchy of cation-stabilizing organometallic moieties: (C5H5)W(CO)2 ≈ (C5H5)Mo(CO)2 > (C5H5)Fe(C5H4) > Co(CO)2PPh3 > Co(CO)3 ≈ (C5H4)Mn (CO)3. We here report the preparation, NMR spectra, and X-ray crystal structures of several mixed-metal clusters arising from the reaction of chiral (propargyl alcohol)-hexacarbonyldicobalt complexes of the type [(RCtC-CR1R2OH)Co2(CO)6] with Fe(CO)5 to yield neutral iron-cobalt clusters that provide excellent models of the analogous cobaltstabilized cations.

Results and Discussion The reaction of an alkynyl-lithium with (-)-menthone yields a mixture of axial- and equatorial-alkynols.18 In the original study, the major isomers were assigned on the basis of NMR chemical shifts of their derived allenyl phosphine oxides as possessing an equatorial alkynyl substituent.19 The direction of attack by alkynyl anions on alkyl-substituted five- and six-membered-ring systems (initially investigated by Cadiot and Chodkiewicz20) cannot be predicted simply from Cram’s rule.21 The torsional strain transition model proposed by Felkin,22 and subsequently supported by Ahn and Eisenstein,23 has been used to rationalize the experimentally (16) Ogini, F. O.; Ortin, Y.; Mahmoudkhani, A. H.; Cozzolino, A. F.; McGlinchey, M. J.; Vargas-Baca, I. J. Organomet. Chem. 2008, 693, 1957–1967. (17) (a) Troitskaya, L. L.; Sokolov, V. I.; Bakhmutov, V. I.; Reutov, O. A.; Gruselle, M.; Cordier, C.; Jaouen, G. J. Organomet. Chem. 1989, 364, 195–206. (b) Cordier, C.; Gruselle, M.; Vaissermann, J.; Troitskaya, L. L.; Bakhmutov, V. I.; Sokolov, V. I.; Jaouen, G. Organometallics 1992, 11, 3825–3832. (c) Gruselle, M.; Kondratenko, M. A.; El Amouri, H.; Vaissermann, J. Organometallics 1995, 14, 5242–5250. (d) Gruselle, M.; Malezieux, B.; Vaissermann, J.; El Amouri, H. Organometallics 1998, 17, 2337–2343. (e) Golovko, V. B.; Mays, M. J.; Solan, G. A. J. Organomet. Chem. 2008, 693, 2683–2692. (f) Seyferth, D. Adv. Organomet. Chem. 1975, 14, 97–144. (18) Strictly speaking, axial and equatorial designations should refer to the position of the hydroxyl group since it has priority over the alkynyl substituent in the Cahn-Ingold-Prelog rules. (19) (a) Phillipe, J.; Capmau, M.-L.; Chodkiewicz, W. Bull. Soc. Chim. Fr. 1971, 2248–2255. (20) Battioni, J.-P.; Chodkiewicz, W.; Cadiot, P. C. R. Seances Acad. Sci. Ser. C 1967, 264, 991–994. (21) Cram, D. J.; Elhafez, F. A. A. J. Am. Chem. Soc. 1952, 74, 5828– 5835. (22) (a) Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968, 9, 2199–2204. (b) Cherest, M.; Felkin, H. Tetrahedron Lett. 1968, 9, 2205– 2208. (23) (a) Anh, N. T.; Eisenstein, O. Nouv. J. Chim. 1977, 1, 61–70. (b) Anh, N. T. Top. Curr. Chem. 1980, 88, 145–162.

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Scheme 2. (a) Antarafacial Migration Leads to Enantiomerization; (b) Suprafacial Migration, or Rotation about the Cluster-CR2þ Linkage, Leads to Diastereomerization

Chart 1. Mixed-Metal Cationic Clusters

observed stereoselectivities of nucleophilic additions to cyclic ketones. In this model, it is suggested that the direction of nucleophilic attack is controlled not only by steric effects but also by the torsional strain imposed on the system in the transition state. In the case of cyclohexanones, this involves a distorted chair conformation whereby the axial transition state is perfectly staggered, whereas equatorial attack proceeds through a transition state that affords partial eclipsing. However, in the particular case of the reaction of (-)-menthone, (2S,5R)-2-isopropyl-5-methylcyclohexanone, with phenylethynyllithium, it has been reported that the presence of the sterically demanding isopropyl group adjacent to the ketone favors attack on the less hindered face, thus generating the isomers possessing equatorial alkynyl substituents. When the reaction was carried out at -78 °C, an equatorial:axial ratio of 20:1 was reported.24,25 In our hands, treatment of (-)-menthone with phenylethynyllithium at 0 °C furnished a 1:2 mixture of alkynols 6 and 7; subsequent treatment with Co2(CO)8 led to two epimeric clusters, 8 and 9, respectively, which were each purified chromatographically. As shown in Scheme 3, the major isomer, 9, yielded X-ray quality crystals (of 10) after incorporation of a bis(diphenylphosphino)methane (dppm) ligand; the structure of 10 was established by X-ray crystallography (see Figure 2) and revealed that the alkynyl substituent was sited equatorially, thus confirming the original findings of the French group.19 (24) (a) Spino, C.; Beaulieu, C. J. Am. Chem. Soc. 1998, 120, 11832– 11833. (b) Spino, C.; Beaulieu, C.; Lafreniere, J. J. Org. Chem. 2000, 65, 7091–7097. (c) Dimitrov, V.; Panev, S. Tetrahedron: Asymmetry 2000, 1513–1516. (25) Very recently, the reaction of (-)-menthone with tert-BuMe2SiO-CtCLi at 0 °C has also been reported to yield the corresponding alkynols with a 2:1 preference for the axial alcohol: Kim, M.-s.; Lee, J. W.; Lee, J. E.; Kang, J. Eur. J. Inorg. Chem. 2008, 2510–2513. (26) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc. 1982, 104, 3858–3875.

The dppm bridges the two cobalts and adopts the classic “sawhorse” structure of perpendicular alkyne-Co2L6 systems26 such that the two phosphorus atoms occupy pseudoequatorial positions furthest away from the menthyl substituent. The plane of the phenyl ring that caps the cluster is aligned almost parallel with the cobalt-cobalt vector, thus minimizing steric interactions with both the menthyl and the phenyls of the dppm ligand. Likewise, the structure of the minor isomer, the mentholdicobalt cluster, 8, containing an axial alkynyl substituent, appears as Figure 3a. The trimethylsilyl analogue, 11, in which the alkynyl group is also axial, is shown as Figure 3b. The reaction of (propargyl alcohol)Co2(CO)6 clusters, or their corresponding diols (HOCR2CtCCR2OH)Co2(CO)6, with Fe(CO)5 in refluxing acetone to yield iron-cobalt or diiron clusters, respectively, was originally reported by Aime and Osella in 1983 and was described as a “dehydroxylation process” (Scheme 4).28 Since that time, a number of other examples of this metal exchange process have been reported.29-33 These mixed-metal clusters provide excellent structural models of the isoelectronic [(R0 CtC-CR2)Co2(CO)6]þ cations, whereby a neutral Fe(CO)3 moiety functions as an isolobal replacement for the cationic [Co(CO)3]þ (27) Mercury 1.4.2, available from http://www.ccdc.cam.ac.uk/ mercury/. (28) Aime, S.; Osella, D.; Milone, L.; Tiripicchio, A. Polyhedron 1983, 2, 77–81. (29) Osella, D.; Dutto, G.; Jaouen, G.; Vessieres, A.; Raithby, P. R.; De Benedetto, L.; McGlinchey, M. J. Organometallics 1993, 12, 4545– 4552. (30) Ruffolo, R.; Brook, M. A.; McGlinchey, M. J. Organometallics 1998, 17, 4992–4996. (31) Gerlach, J. N.; Wing, R. M.; Ellgen, P. C. Inorg. Chem. 1976, 15, 2959–2964. (32) (a) Eigemann, S.-E.; F€ ortsch, W.; Hampel, F.; Schobert, R. Organometallics 1996, 15, 1511–1513. (b) Bright, D.; Mills, O. S. J. Chem. Soc., Dalton Trans. 1972, 2465–2469. (33) Rohde, W.; Fendesak, G. Z. Naturforsch. B 1991, 46, 1169–1176.

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Scheme 3. Syntheses of Alkynyl-Menthyl Cluster Complexes

27

Figure 2. Mercury representation of the molecular structure of 10, emphasizing the axial character of the OH group. Selected distances (A˚) and angles (deg): Co(1)-Co(2) 2.4807(5), Co(1)C(11) 1.981(3), Co(1)-C(12) 1.967(3), Co(2)-C(11) 1.975(3), Co(2)-C(12) 1.967(3), C(1)-C(11) 1.531(4), C(11)-C(12) 1.337(4), C(1)-C(11)-C(12) 140.3(3).

unit.29 To this end, the (phenylethynylmenthol)Co2(CO)6 clusters 8 and 9 were each treated with Fe(CO)5 in refluxing acetone and the products separated by chromatography on a silica column. The reaction of iron pentacarbonyl with the cluster 9, in which the alkynyl-dicobalt tetrahedron is equatorial and the

Figure 3. Mercury representations of the molecular structures of (a) [1-(axial-phenylethynyl)menthol]Co2(CO)6, 8, and (b) [1-(axial-trimethylsilylethynyl)menthol]Co2(CO)6, 11. Selected distances (A˚) and angles (deg) for 8: Co(1)-Co(2) 2.4449(5), Co(1)-C(11) 2.008(3), Co(1)-C(12) 1.964(3), Co(2)-C(11) 1.980(3), Co(2)-C(12) 1.999(3), C(1)-C(11) 1.513(4), C(11)C(12) 1.342(4), C(1)-C(11)-C(12) 140.5(3). Selected distances (A˚) and angles (deg) for 11: Co(1)-Co(2) 2.4633(5), Co(1)C(11) 2.011(3), Co(1)-C(12) 2.012(3), Co(2)-C(11) 1.991(3), Co(2)-C(12) 2.007(3), C(1)-C(11) 1.520(4), C(11)-C(12) 1.332(4), C(1)-C(11)-C(12) 148.9(2).

hydroxyl substituent occupies the axial site, furnished two iron-cobalt clusters, 12 and 13, in the ratio 70:30. Treatment with triphenylphosphine led to replacement of a cobalt

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Scheme 4. Replacement of a Co(CO)3 Vertex by Fe(CO)3 with Concomitant Loss of OH

Figure 4. Mercury representation of the molecular structure of 14(S), in which the iron is situated on the less hindered face of the menthyl ring. (For clarity, only the ipso carbons of the phenyls on phosphorus are shown.) Selected distances (A˚) and angles (deg): Co(1)-Fe(1) 2.5293(8), Fe(1)-C(1) 2.277(4), Fe(1)-C(11) 2.014(4), Fe(1)-C(12) 2.001(4), Co(1)-C(11) 1.935(4), Co(1)C(12) 1.999(4), C(1)-C(11) 1.404(5), C(11)-C(12) 1.362(6), C(1)-C(11)-C(12) 135.5(4), C(1)-C(11)-Fe(1) 81.5(2).

Figure 5. Mercury representation of the molecular structure of 15(R), in which the iron is situated on the more hindered face of the menthyl ring. (For clarity, only the ipso carbons of the phenyls on phosphorus are shown.) Selected distances (A˚) and angles (deg): Co(1)-Fe(1) 2.5213(8), Fe(1)-C(1) 2.360(4), Fe(1)-C(11) 2.011(4), Fe(1)-C(12) 1.992(4), Co(1)-C(11) 1.914(4), Co(1)-C(12) 1.984(4), C(1)-C(11) 1.403(6), C(11)C(12) 1.343(6), C(1)-C(11)-C(12) 139.4(3), C(1)-C(11)Fe(1) 85.5(3).

carbonyl ligand and formation of X-ray quality crystals of 14(S) and 15(R), respectively. The structure of the major diastereomer, 14(S), is shown in Figure 4 and reveals that the hydroxyl substituent has been eliminated and a tricarbonylcobalt vertex has been replaced by an Fe(CO)3 moiety (Scheme 3). Likewise, the iron-cobalt cluster, 13, was characterized crystallographically as its triphenylphosphine derivative, 15(R),

in which the tricarbonyliron unit is now sited on the more hindered face of the menthyl ring (Figure 5). In both 14(S) and 15(R), it is evident that the carbon originally bearing a hydroxyl substituent now clearly leans toward the iron vertex such that the Fe(1)-C(1) distances are in the range 2.28 to 2.36 A˚, and the angle C(1)-C(11)-Fe(1) is reduced to ∼85°. The C(1)-C(11) and C(11)-C(12) bond lengths in the alkyne-dicobalt clusters, 8 and 10, are ∼1.52 and ∼1.33 A˚, respectively. In contrast, in the iron-cobalt complexes the alkyne-derived unit exhibits allenelike character;34 the C(1)-C(11) and C(11)-C(12) bond distances are ∼1.40 and ∼1.36 A˚, respectively. Analogously, the reaction of Fe(CO)5 with mentholdicobalt cluster 8, which possesses an equatorial hydroxyl substituent and an axial cluster, yields an approximately 50:50 mixture of 12 and 13, as determined by NMR spectroscopy. When we compare the structures of the known alkylidene clusters in which the capping group leans toward an Fe(CO)3 vertex, it is clear that the Fe 3 3 3 CR2 distance is a sensitive probe of the strength of this interaction. To put this in perspective, we note that a comprehensive survey35 of the Mo 3 3 3 Cþ distances in clusters of the type [Cp2Mo2(CO)4(RCtCR0 R00 )]þ reveals that the molybdenum-tocarbocation distance is in the range 2.44-2.55 A˚ for primary cations, but increases to 2.61-2.63 A˚ for secondary cations and can reach 2.74-2.92 A˚ for tertiary carbocations. Moreover, there is a very clear inverse relationship between this distance parameter and the NMR-derived activation energies for migration of the cationic site between the molybdenum centers: the longer the Mo 3 3 3 Cþ bond, the lower the barrier!35 Several relevant X-ray crystal structures containing (OC)3Fe 3 3 3 CR2 moieties have been reported. In Cp2W2(CO)4Fe(CO)3(CdCH2),36 (MeCdCdCH2)Fe(CO)3Co(CO)2PPh3,29 and (CH2dCdC=CH2)Fe2(CO)5PPh3,31 the iron-carbon distances are 2.21, 2.195, and 2.208 A˚, respectively. Taking an extreme view, these relatively short bonds can be considered as arising from the interaction of CH2þ (i.e., primary) cations with [Fe(CO)3]- vertices. In contrast, in (Me3SiCdCdCMe2)FeCo(CO)6,30 (Me2CdCdCdCMe2)Fe2(CO)6,32 and [Me3SiCdCd (fluorenylidene)]FeCo(CO)6,5 the Fe-C distances of 2.335, 2.401, and 2.626 A˚, respectively, are a manifestation of the weaker bonding between a nominal tertiary cation and the formally anionic Fe(CO)3 moiety. Thus, the Fe-C distances of 2.28 to 2.36 A˚ observed in 14 and 15 lie in the expected range for tertiary cations. Another pertinent observation concerns the absolute configurations of the clusters 14(S) and 15(R). In the precursor (34) Banide, E. V.; Grealis, J. P.; M€ uller-Bunz, H.; Ortin, Y.; Casey, M.; Mendicute-Fierro, C.; Lagunas, M. C.; McGlinchey, M. J. J. Organomet. Chem. 2008, 693, 1759–1770. (35) Girard, L.; Lock, P. E.; El Amouri, H.; McGlinchey, M. J. J. Organomet. Chem. 1994, 478, 189-196, and references therein. (36) Delgado, E.; Jeffrey, J. C.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1986, 2105–2112.

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Figure 6. “Pseudo-centrosymmetric” arrangement of the chiral clusters 14(S) and 15(R) in the solid state, where S and R indicate in each case the absolute configuration of the tetrahedral CoFeC2 cluster core. Chart 2a

a

M1 = Co(CO)3; M2 = Fe(CO)3.

dicobalt complexes 8 and 9, the presence of the chiral menthyl substituent renders the two cobalt vertices diastereotopic. Hence, replacement of a particular tricarbonylcobalt moiety by an Fe(CO)3 unit generates one specific diastereomer. Of course, in these cases, the (-)-menthyl configuration is predetermined, and the absolute configuration of the tetrahedral cluster can be designated by assuming the presence of a dummy atom in the center of the cluster13 and then applying the usual Cahn-Ingold-Prelog rules. For clusters of the type (R1CtCR2)[Fe(CO)3][Co(CO)3], the priority sequence is Co(CO)3 > Fe(CO)3 > R1 > R2. In 14 and 15, R1 is phenyl and R2 is menthylidene, thus leading to the assignments of their cluster configurations as S and R, respectively (Chart 2). The X-ray crystal structures of 14(S) and 15(R) merit discussion beyond the mere establishment of the atom connectivity and the bond distances and angles. In particular, we note that the reaction of the alkynol-Co2(CO)6 cluster, 9, with Fe(CO)5 yielded a 70:30 mixture of 12:13, as shown by 13 C NMR spectroscopy. However, after treatment with triphenylphosphine to obtain X-ray quality crystals, the resulting structure revealed a 50:50 combination of 14 and 15. As shown in Figure 6, the unit cell is pseudocentrosymmetric, although the presence of the homochiral (-)-menthyl fragment renders true inversion symmetry nonviable. The packing in the unit cell is dominated by the presence of the two (alkyne)[Fe(CO)3][Co(CO)2PPh3] tetrahedral clusters of opposite chirality, 14(S) and 15(R). The “pseudo-equivalent” menthyl groups differ only in the positioning of their methyl and isopropyl substituents, which make only a minor contribution to the overall scattering of the X-rays relative to that by the iron, cobalt, and phosphorus atoms.

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In terms of a model for a cobalt-stabilized cation, the Fe(CO)3 group in 12, characterized crystallographically as the phosphine complex 14, is now in the position that would have been occupied by the Co(CO)3þ unit in 16 if the hydroxyl had originally been sited axially (Scheme 5). Analogously, in the mixed metal cluster 13, characterized crystallographically as its triphenylphosphine derivative, 15, the iron vertex occupies the position that would have been occupied by the Co(CO)3þ unit in 17 if the hydroxyl had originally been sited equatorially. The mechanistic implications of this observation are discussed below. Proposed Mechanism of Metal Vertex Substitution. As noted above, the reaction of (propargyl alcohol)Co2(CO)6 clusters with Fe(CO)5 in refluxing acetone to yield ironcobalt clusters was originally reported by Aime and Osella in 1983 and was described as a “dehydroxylation process”;28 since that time, a number of other examples of this metal exchange process have been reported.29-33 However, the reactions of Fe(CO)5 with (alkynylcyclopentadienol)Co2(CO)6 complexes yielded unexpected products, 18, in which an (η5-cyclopentadienyl)Fe(CO)2 moiety was linked to a tricarbonylcobalt cluster vertex via a bridging hydride.37 As depicted in Scheme 6, it was proposed that initial attack by the hydroxyl oxygen on an iron carbonyl, migration of hydrogen to form a metal hydride, and loss of two iron carbonyls allowed complexation to the cyclopentadiene ring.36 Subsequent decarboxylation furnished the observed products, 18. It is relevant to note that Osella obtained the carboxylate-containing cluster Fe2(CO)6[(EtCdCEt)C(dO)O] from hex-3-yne.38 Moreover, Sappa convincingly demonstrated that “deoxygenation” of the propargylic alcohol HCtCC(Me)(Ph)OH by Fe3(CO)12 is really a decarboxylation and proceeds with elimination of carbon dioxide.39,40 As a corollary, it was proposed37 that in the absence of a substituent to which the iron carbonyl could bond, one could instead invoke expansion of the alkynyl-dicobalt tetrahedral cluster to a square-based pyramidal structure, 19, as in Scheme 7. While the apical and basal plane cobalts in 19 would formally be assigned 19 and 17 electrons, respectively, the overall electron count is appropriate for a nido octahedral cluster,41 and numerous examples of such cluster expansion42 or metal vertex replacements are known.43-46 Subsequent loss of HCo(CO)x and CO2 can yield the observed products. (37) Dunn, J. A.; Britten, J. F.; Daran, J.-C.; McGlinchey, M. J. Organometallics 2001, 20, 4690–4694. (38) Milone, L.; Osella, D.; Ravera, M.; Stanghellini, P. L.; Stein, E. Gazz. Chim. Ital. 1992, 122, 451–454. (39) Gervasio, G.; Sappa, E. Organometallics 1993, 12, 1458–1461. (40) For other related reactions, see: (a) Gervasio, G.; Sappa, E. J. Organomet. Chem. 1985, 498, 73–80. (b) Aime, S.; Milone, L.; Osella, D. Chem.Commun. 1979, 704–705. (41) Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1–66. (42) Adams, R. D. In The Chemistry of Metal Cluster Complexes: Shriver, D. F.; Kaesz, H. D.; Adams, R. D., Eds.; VCH: New York, 1989; Chapter 3, pp 121-170. (43) Vahrenkamp, H. Adv. Organomet. Chem. 1983, 22, 169–208. (44) (a) McGlinchey, M. J.; Mlekuz, M.; Bougeard, P.; Sayer, B. G.; Marinetti, A.; Saillard, J.-Y.; Jaouen, G. Can. J. Chem. 1983, 61, 1319– 1331. (b) Mlekuz, M.; Bougeard, P.; Sayer, B. G.; Peng, S.; McGlinchey, M. J.; Marinetti, A.; Saillard, J.-Y.; Naceur, J. B.; Mentzen, B.; Jaouen, G. Organometallics 1985, 4, 1123–1130. (c) Mlekuz, M.; Bougeard, P.; Sayer, B. G.; Faggiani, R.; Lock, C. J. L.; McGlinchey, M. J.; Jaouen, G. Organometallics 1985, 4, 2046–2053. (45) Churchill, M. R.; Bueno, C.; Wasserman, J. J. Inorg. Chem. 1982, 21, 640–644. (46) Busetto, L.; Green, M.; Hessner, B.; Howard, J. A. K.; Jeffrey, J. C.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1983, 519–525.

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Scheme 5. Isolobal Relationships between Cationic Dicobalt Clusters and Neutral Iron-Cobalt Clusters

Scheme 6. Proposed Mechanism of Formation of a Bridged Hydride Complex

Scheme 7. Proposed Mechanism of Metal Vertex Substitution

Origin of the Diastereoselectivity of Metal Exchange. We can now pose the question as to the origin of the diastereoselectivity observed in these metal vertex replacement processes. Inspection of the relevant structures, 14 and 15, reveals that in both cases the planar phenyl substituent is aligned adjacent to the bulky isopropyl group on the menthyl ring, as depicted in the space-filling representations in Figures 7 and 8. The other possibility would have positioned the much more sterically demanding tricarbonylcobalt unit proximate to the isopropyl substituent, thus engendering significant steric strain. Now, in light of the mechanistic discussion above, one can infer that in each case the iron carbonyl approaches

the hydroxylated face of the alkynyl-menthol. However, as explicated above, dependent on the identity of the cobalt vertex that is replaced, the absolute configuration of the Fe-Co-C2 tetrahedral cluster so generated will be either R or S. We now offer an explanation to account for the observations that, when the iron binds to the less hindered face of the menthyl ring, the cluster has the S configuration, but the cluster adopts only the R configuration when the Fe(CO)3 unit is attached to the more hindered face of the menthyl ring. Moreover, the formation of both diastereomers by reaction of Fe(CO)5 with either of the epimeric alkynyl-menthols may also be rationalized.

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Taking the metal-carboxylate system 20, derived originally from the dicobalt cluster 8, in which the hydroxyl substituent is equatorial, one can envisage replacement of either of the tricarbonylcobalt vertices by an Fe(CO)3 fragment and subsequent loss of carbon dioxide, to generate the zwitterionic intermediates 21 and 22, possessing S and R

Figure 7. Space-filling and capped stick views of 14(S), where the Fe(CO)3 group lies on the less hindered face of the menthyl ring.

Figure 8. Space-filling and capped stick views of 15(R), where the Fe(CO)3 group lies on the more hindered face of the menthyl ring. The phenyls on phosphorus have been removed for clarity.

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cluster configurations, respectively (Scheme 8). In the former case, immediate formation of a bond between the iron and the formally cationic carbon, C(1), of the menthyl ring yields the observed product, 12, in which the planar phenyl substituent is aligned adjacent to the isopropyl group (Figure 7). In contrast, if such an immediate closure were to occur in 22, it would place the bulky tricarbonylcobalt vertex proximate to the isopropyl substituent, as in 23. Instead, rotation about the C(1)-C(11) linkage, to generate 24, positions the iron on the opposite face of the menthyl ring and so again places the phenyl ring adjacent to the isopropyl group, as in 13 (Figure 8). In the case just described, the replacement of a cobalt vertex by a tricarbonyliron moiety shows very little discrimination, and the ratio of 12 to 13 is approximately 50:50. In contrast, starting from the dicobalt-alkynylmenthol 8, in which the hydroxyl group is axial, led to a 12:13 ratio of 70:30, presumably indicating that the presence of the bulky isopropyl substituent enforces greater discrimination between the diastereotopic cobalt vertices in terms of replacement by iron.

Conclusions Treatment of (propargyl alcohol) dicobalt hexacarbonyls with iron pentacarbonyl yields neutral mixed-metal clusters whose structures provide excellent models for cobaltstabilized propargyl cations found as intermediates in the Nicholas reaction. Although originally classified as a dehydroxylation, the isolation of bridged hydride species suggests that the mechanism more likely involves a decarboxylation process. When this reaction was applied to enantiomerically pure dicobalt hexacarbonyl complexes of axial or equatorial phenylethynylmenthols, the mixed iron-cobalt products that resulted always positioned the planar phenyl substituent, rather than the much bulkier cobalt tricarbonyl moiety, adjacent to the isopropyl substituent of the menthyl ring. Although these neutral iron-cobalt clusters are excellent

Scheme 8. Proposed Origin of the Diastereoselectivity of Metal Exchange

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structural models of the dicobalt cations, the mechanism of their formation is quite different. In the dicobalt case, protonation of the hydroxyl substituent is followed by nucleophilic attack by cobalt from the opposite face; subsequent quenching by an external nucleophile results in overall retention of configuration via a double inversion process. In contrast, it is proposed that iron carbonyl is attacked by the hydroxyl group directly and, after a proton migration, undergoes decarboxylation to yield an iron carbonyl hydride and, ultimately, a neutral iron-cobalt analogue of the cobalt-stabilized cation. Extension of these reactions to other alkynyl-terpenols will be the subject of a future report.

Experimental Section General Methods. All reactions were carried under a nitrogen atmosphere, and solvents were dried by standard procedures. 1H and 13C NMR spectra were recorded on Varian 300, 400, 500, or 600 MHz spectrometers. Assignments were based on standard two-dimensional NMR techniques (1H-1H COSY, 1H-13C HSQC and HMBC, NOESY). Electrospray mass spectrometry was performed on a Micromass Quattro micro instrument. Infrared spectra were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrometer and were calibrated with polystyrene. Merck silica gel 60 (230-400 mesh) or alumina was used for flash chromatography. Melting points were determined on an Electrothermal ENG instrument and are uncorrected. Elemental analyses were carried out by the Microanalytical Laboratory at University College Dublin. Synthesis of (1R,2S,5R)-2-Isopropyl-5-methyl-1-{2-phenyl-1ethyn-1-yl}cyclohexan-1-ol (6) and (1S,2S,5R)-2-Isopropyl-5methyl-1-{2-phenyl-1-ethyn-1-yl}cyclohexan-1-ol (7). In a typical procedure, nBuLi (27.8 mL of a 1.6 M hexane solution, 44.5 mmol) was added dropwise to a solution of phenylacetylene (4.9 mL, 44.6 mmol) in THF (250 mL) at -78 °C and stirred for 30 min. The solution was allowed to warm to room temperature, and after 15 min, (-)-menthone (7.0 mL, 40.5 mmol) was added dropwise over 5 min. The solution was stirred at room temperature for 12 h, quenched with water (150 mL), and extracted with dichloromethane (3  100 mL). The organic layers were combined, washed with brine, and dried over MgSO4, and the solvent was removed on a rotary evaporator. The crude product was purified on a silica column using pentane/ethyl acetate as eluent. The isomer with an equatorial alkynyl group, 7 (3.91 g, 15.25 mmol; 38%), was isolated as the major product with a small amount of ketone present. This was further treated with Co2(CO)8. Subsequent removal of the dicobalt hexacarbonyl using ceric ammonium nitrate (CAN) yielded the pure isomer 7. The second isomer, 6, was isolated as a pure white solid (2.38 g, 9.2 mmol; 23%). Data for 7: 1H NMR (500 MHz, CDCl3): δ 7.36-7.31 (m, 2H, phenyl o-H), 7.23-7.19 (m, 3H, phenyl m,pH), 2.39 (ds, J=7.0, J=2.0 Hz, 1H, H8), 1.98 (dt, J=14.0, J=3.0 Hz, 1H, H6eq), 1.78-1.72 (m, 1H, H5), 1.72-1.70 (m, 1H, H3), 1.70-1.67 (m, 1H, -OH), 1.49-1.43 (m, 1H, H3), 1.40 (dq, J= 13.0, J=3.0 Hz, 1H, H4), 1.38 (dd, J=13.5, J=12.5 Hz, 1H, H6ax), 1.32 (ddd, J=12.0, J=4.0, J=2.2 Hz, 1H, H2), 0.93 (d, J= 7.0 Hz, 3H, isopropyl CH3), 0.90 (d, J=6.9 Hz, 3H, isopropyl CH3), 0.86 (dq, J=13.0, 4.3 Hz, 1H, H4), 0.82 (d, J=6.4 Hz, 3H, CH3). 13C NMR (125 MHz, CDCl3): δ 131.6 (phenyl o-C), 128.2 (phenyl m-C), 128.1 (phenyl p-C), 123.1 (phenyl ipso-C), 94.1 (C11), 83.5 (C12), 72.2 (C1), 50.7 (C2), 50.1 (C6), 34.9 (C4), 28.5 (C8), 27.3 (C5), 23.9 and 21.9, (C9, C10), 20.8, (C3) 18.9 (C7). IR (CH2Cl2): 2930 (OH), 2360 (CtC) cm-1. Data for 6: 1H NMR (600 MHz, CDCl3): δ 7.41-7.43 (m, 2H, phenyl, o-H), 7.29-7.31 (m, 3H, phenyl, m,p-H), 2.27 (ds, J = 3 Hz, J = 7 Hz, 1H, H8), 2.19 (bs, 1H, OH), 2.07 (dq, J=12 Hz, J=2 Hz, 1H, H6eq), 1.85-1.79 (m, 1H, H5), 1.75 (doublet, pseudoquintet,

Moore et al. J=13 Hz, J=2.5 Hz, 1H, H4eq), 1.70 (dq, J=13 Hz, J=3 Hz, 1H, H3eq), 1.40 (dq, J=3.5 Hz, J=12 Hz, 1H, H3ax), 1.32 (dt, J=13 Hz, t, J=3 Hz, 1H, H2ax), 1.29 (t, J=12 Hz, 1H, H6ax), 1.05 (d, J=7 Hz, 3H, isopropyl CH3), 1.02 (d, 3H, J=7 Hz, isopropyl CH3), 0.94 (d, J=7 Hz, 3H, CH3), 0.90 (dq, J=4 Hz, J=12 Hz, 1H, H4ax). 13C NMR (150 MHz, CDCl3): δ 131.7 (phenyl o-C), 128.4 (phenyl m-C), 128.3 (phenyl p-C), 123.2 (phenyl ipso-C), 91.9 (C11), 86.5 (C12), 72.4 (C1), 53.4 (C2), 51.5 (C6), 35.0 (C4), 30.9 (C5), 26.7 (C8), 24.5 (C3), 24.1 and 19.4 (C9, C10), 22.0 (C7). IR (CH2Cl2): 2930 (OH), 2360 (CtC) cm-1. Synthesis of [(1R,2S,5R)-2-Isopropyl-5-methyl-1-{2-phenyl-1ethyn-1-yl}cyclohexan-1-ol]hexacarbonyldicobalt (8). To a solution of dicobalt octacarbonyl (2.64 g, 7.72 mmol) in THF (70 mL) was added a solution of 6 (1.80 g, 7.02 mmol) in THF (10 mL) via cannula. Upon stirring for 12 h at room temperature, the solution became deep red; this was concentrated under vacuum to give a maroon solid that was chromatographed on silica using pentane/dichloromethane as eluent to furnish 8 as a maroon solid (3.19 g, 5.88 mmol; 84%). A sample suitable for X-ray crystal structure determination was obtained by recrystallization from pentane/dichloromethane. Data for 8: 1H NMR (600 MHz, CDCl3): δ 7.56 (d, J=7.0 Hz, 2H, phenyl, o-H), 7.32 (t, J=7.0 Hz, 2H, phenyl, m-H), 7.27 (m, 1H, phenyl, p-H), 2.11 (dt, J=12 Hz, J=2.5 Hz, 1H, H6eq), 1.93 (s, 1H, OH), 1.82-1.78 (m, 1H, H5ax), 1.77-176 (m, 1H, H8), 1.75-1.74 (m, 1H, H4eq), 1.74-1.72 (m, 1H, H3eq), 1.56 (m, 1H, H2ax), 1.49 (td J=12 Hz, 1H, H6ax), 1.36 (dq, J=3.0 Hz, J=13 Hz, 1H, H3ax), 0.99 (d, J= 6.5 Hz, 3H, CH3), 0.95 (dq, 1H, J=3.0 Hz, J=12 Hz, H4ax), 0.91 (d, 3H, J = 7 Hz, isopropyl-CH3). 0.73 (d, 3H, J = 7 Hz, isopropyl-CH3). 13C NMR (150 MHz, CDCl3): δ 200.0 (CO), 139.1 (phenyl, ipso-C), 130.3 (phenyl, o-C), 128.4 (phenyl, m-C), 127.3 (phenyl, p-C), 103.6 (C11), 98.5 (C12), 79.7 (C1), 56.6 (C6), 55.2 (C2), 34.8 (C4), 29.5 (C8), 27.4 (C5), 27.0 (C3), 25.8 (C7), 21.5 and 18.4 (C9, C10). IR (CH2Cl2): 2956 (OH), 2025, 2050, 2089 (CO) cm-1. HRMS (ES-): m/z 541.0087; calcd for C24H23Co2O7 541.0108 [M - H]-. Anal. Calcd for C24H24Co2O7: C, 53.15; H, 4.46. Found: C, 53.02; H, 4.47. Preparation of [(1S,2S,5R)-2-Isopropyl-5-methyl-1-{2-phenyl1-ethyn-1-yl}cyclohexan-1-ol]hexacarbonyldicobalt (9). To a solution of dicobalt octacarbonyl (5.66 g, 16.55 mmol) in THF (150 mL) was added a solution of 7 (3.86 g, 15.06 mmol) in THF (10 mL) via cannula. Upon stirring for 12 h at room temperature, a deep red solution resulted; this was concentrated under vacuum to give a maroon oil, which was chromatographed on silica using pentane/dichloromethane as eluent to furnish 9 as a maroon oil (7.60 g, 14.0 mmol; 93%). Data for 9: 1H NMR (600 MHz, CDCl3): δ 7.64 (d, J=7.5 Hz, 2H phenyl, o-H), 7.33 (t, J= 7.5 Hz, 2H, phenyl, m-H), 7.28 (t, J=7.5 Hz, 1H, phenyl, p-H), 1.94-1.92 (m, 1H, H8), 1.92-1.90 (m, 1H, H6eq) 1.86-1.85 (m, 1H, H4eq), 1.85 (s, 1H, -OH), 1.80-1.76 (m, 1H, H5ax), 1.64 (dq, J=13 Hz, J=3.5 Hz, 1H, H3eq), 1.59 (t, 1H, J=12 Hz, 1H, H6ax), 1.55 (dq, J=3.0 Hz, J=13 Hz, 1H, H3ax), 1.36 (dd, J=13 Hz, J= 2.5 Hz, 1H, H2ax), 0.97 (d, 3H, J=6.5 Hz, CH3), 0.93 (dq, J=3.0 Hz, J=12 Hz, 1H, H4ax), 0.84 (d, J=7 Hz, 3H, isopropyl-CH3), 0.73 (d, J = 7 Hz, 3H, isopropyl-CH3). 13C NMR (150 MHz, CDCl3): δ 200.0 (CO), 138.4 (phenyl, ipso-C) 131.7 (phenyl, o-C), 128.6 (phenyl, m-C), 127.6 (phenyl, p-C), 107.3 (C11), 95.9 (C12), 79.3 (C1), 52.7 (C6), 51.7 (C2), 34.9 (C4), 28.9 (C5), 26.6 (C8), 23.2 and 18.4 (C9, C10), 22.4 (C7), 21.5 (C3). IR (CH2Cl2): 2926 (OH), 2023, 2050, 2089 (CO) cm-1. HRMS (ES-) m/z: 541.0107; calcd for C24H23Co2O7 541.0108 [M - H]-. Synthesis of [(1S,2S,5R)-2-Isopropyl-5-methyl-1-{2-phenyl-1ethyn-1-yl}cyclohexan-1-ol][bis(diphenylphosphino)methane]tetracarbonyldicobalt (10). To a solution of 8 (0.423 g, 0.782 mmol) in THF (20 mL) was added bis(diphenylphosphino)methane (0.300 g, 0.782 mmol). Stirring for 12 h under reflux gave a deep red solution, which was concentrated under vacuum to give a maroon solid (0.623 g, 0.721 mmol; 92%). A sample of 10 suitable for X-ray crystal structure determination was obtained by recrystallization from hexane/dichloromethane.

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Table 1. Crystallographic Data for 8, 10, 11, 14, and 15

formula M cryst syst space group a [A˚] b [A˚] c [A˚] R [deg] β [deg] γ [deg] V [A˚3] Z Fcalcd [g cm-3] T [K] μ [mm-1] F(000) θ range for data collection [deg] index ranges reflns measd indep reflns Rint data/restraints/params final R values [I > 2θ(I)]: R1 wR2 R values (all data): R1 wR2 abs struct param GOF on F2

8

10

11

14(S)

14(S)/15(R)

C24H24O7Co2 542.29 triclinic P1 (#1) 9.4908(13) 10.4920(15) 12.4810(18) 81.668(3) 80.035(3) 79.356(2) 1194.8(3) 2 1.507 100(2) 1.429 556 1.67 to 32.03

C47H46O5P2Co2 3 C6H14 956.81 triclinic P1 (#1) 11.3946(9) 11.8811(9) 20.0210(16) 76.772(2) 76.484(2) 64.331(1) 2349.7(3) 2 1.352 100(2) 0.821 952 1.92 to 30.50

C21H28SiO7Co2 538.38 monoclinic P21 (#4) 12.7295(13) 10.6561(11) 17.9422(18) 90 90.574(2) 90 2433.6(4) 4 1.469 100(2) 1.449 1112 1.95 to 30.50

C41H38O5PFeCo 756.46 triclinic P1 (#1) 9.6326(10) 13.7749(14) 13.8541(14) 92.937(2) 95.456(2) 95.175(2) 1819.3(2) 2 1.381 100(2) 0.942 784 2.03 to 30.54

C41H38O5PFeCo 759.49 triclinic P1 (#1) 9.4743(12) 11.2694(14) 17.422(2) 80.733(2) 88.724(2) 77.161(2) 1789.8(4) 2 1.404 100(2) 0.957 784 1.88 to 26.43

-13 e h e 14 -15 e k e 15 -18 e l e 18 28 125 15 022 0.0250 15 022/3/603

-16 e h e 16 -16 e k e 16 -28 e l e 28 54 986 27 880 0.0284 27 880/3/1129

-17 e h e 18 -14 e k e 14 -25 e l e 25 28 233 14 041 0.0324 14 041/1/573

-13 e h e 13 -19 e k e 19 -19 e l e 19 42 509 21 881 0.0260 21 881/3/889

-11 e h e 11 -14 e k e 14 -21 e l e 21 31 564 14 563 0.0368 14 563/3/889

0.0363 0.0824

0.0412 0.0994

0.0398 0.0903

0.0382 0.0872

0.0454 0.1076

0.0429 0.0854 0.005(9) 0.992

0.0482 0.1034 0.008(7) 1.038

0.0460 0.0936 -0.017(9) 0.990

0.0436 0.0897 -0.010(6) 1.021

0.0522 0.1115 -0.007(11) 1.032

Data for 10: 1H NMR (600 MHz, CDCl3): δ 7.38-6.81 (m, 25H, phenyl), 3.28 (dt, J=13 Hz, 2JH-H=11 Hz, 2JH-P, 1H), 3.14 (dt, 2 JH-H =13 Hz, 2JH-P =11 Hz, 1H), 2.25-2.22 (m, 1H, H6eq), 1.97-2.02 (m, 1H, H8), 1.93-1.86 (m, 1H, H5ax), 1.84-1.82 (m, 1H, H4eq), 1.78 (t, J = 7 Hz, 1H, H6ax), 1.65 (s, 1H, -OH), 1.56-1.53 (m, 1H, H3), 1.48-1.50 (m, 1H, H2ax), 1.46-1.48 (m, 1H, H3), 1.00 (d, 3H, J=6.5 Hz, CH3), 0.94-0.97 (m,1H, H4ax), 0.58 (d, 3H, J=7 Hz, CH3), 0.52 (d, 3H, J=7 Hz, CH3). 13C NMR (150 MHz, CDCl3): δ 207.0 (d, CO), 203 (d, CO), 139.1, 125-142 (phenyls), 113.6 (C10), 95.3 (C11), 79.7 (C1), 2 53.2 (C6), 51.0 (C2), 35.1 (C4), 33.2 (CH2, t, 1J(31P-13C)=33.2 Hz), 28.8 (C5), 26.4 (C8), 22.9 and 17.8 (C9, C10), 22.5 (C7), 21.3 (C3). 31P NMR (202 MHz, CDCl3): δ 37.6 (d, 1P, 2JP-P=114 Hz) 34.4 (d, 1P, 2JP-P= 114 Hz). IR (CH2Cl2): 2956 (OH), 2089, 2050, 2025 (CO) cm-1. Anal. Calcd. for C47H46Co2O5P2 3 C6H14: C, 66.53; H, 6.32; P, 6.47; Co, 12.32. Found: C, 66.34; H, 6.29; P, 7.46; Co, 12.89. Synthesis of [(1R,2S,5R)-2-Isopropyl-5-methyl-1-{2-trimethylsilyl)ethynyl}cyclohexan-1-ol]hexacarbonyldicobalt (11). To a solution of dicobalt octacarbonyl (147 mg, 0.58 mmol) in THF (10 mL) was added a solution of (1R,2S,5R)-2-isopropyl-5methyl-1-{2- trimethylsilyl)ethynyl}cyclohexan-1-ol, (219 mg, 0.64 mmol) in THF (5 mL) via cannula. After stirring for 12 h at room temperature, the resulting deep red solution was concentrated under vacuum to give a maroon solid, which was chromatographed on silica using pentane/dichloromethane as eluent to yield 11 as a maroon solid (203 mg, 0.38 mmol; 65%). A sample suitable for X-ray crystal structure determination was obtained by recrystallization from hexane. Data for 11: 1H NMR (500 MHz, CDCl3): δ 1.91 (dq, 1H, J=13 Hz, J=3 Hz, H3eq), 1.86-1.82 (m, 2H, H6ax, H8), 1.85(s, 1H, OH), 1.80-1.77 (m, 1H, H4eq), 1.54-1.52 (m, 1H, H5ax), 1.52-1.49 (m, 1H, H6eq), 1.40 (dq, 1H, J=3 Hz, J=13 Hz, H3ax), 1.21 (d, 3H, J=6.5 Hz, CH3), 1.04-0.97 (m, 1H, H4ax), 0.87-0.84 (m, 6H, CH3), 0.38 (s, 9H, Si(CH3)3). 13C NMR (125 MHz, CDCl3): δ 200.8 (CO), 114.0 (C10), 80.1 (C11), 79.4 (C1), 57.2 (C6), 55.0 (C2), 34.8 (C4), 28.9 (C5), 27.8 (C3), 27.2 (C8), 26.0 (CH3), 22.1 (CH3), 20.8 (CH3), 18.0

(Si(CH3)3. IR (CH2Cl2): 2929 (OH), 2083, 2045, 2009 (CO) cm-1. Anal. Calcd. for C21H28Co2O7Si: C, 46.85; H, 5.24. Found: C, 46.99; H, 5.28. Preparation of 12 and 13 from 8. To a solution of the dicobalt cluster 8 (0.44 g, 0.81 mmol) in acetone (10 mL) was added Fe(CO)5 (0.85 mL, 6.48 mmol), and the mixture was heated under reflux for 24 h. After removal of solvent, the residue was subjected to flash chromatography on silica gel. Elution with pentane gave a light red solid (0.237 g, 0.324 mmol; 56%); 13C NMR revealed the presence of two diastereomers, 12 and 13, in a 50:50 ratio. Preparation of 12 and 13 from 9. To a solution of the dicobalt cluster 9 (0.861 g, 1.57 mmol) in acetone (15 mL) was added Fe(CO)5 (1.70 mL, 12.95 mmol), and the mixture was heated under reflux for 24 h. After removal of solvent, the residue was subjected to column chromatography on silica gel. Elution with pentane gave a light red, oily product (0.401 g, 0.762 mmol; 49%). 13C NMR revealed the presence of two diastereomers, 12 and 13, in a 70: 30 ratio. Data for 12: 1H NMR (500 MHz, CDCl3): δ 7.42 (d, J=7.5 Hz, 1H phenyl), 7.30 (t, J=7.0 Hz, 2H, phenyl), 7.26 (m, 1H, phenyl), 2.28-2.20 (m, 1H, H6ax), 2.01 (dq, J=10 Hz, J=2.5 Hz 1H, H2) 1.90-1.84 (m, 2H, H4ax, H3ax), 1.69-1.61 (m, 1H, H5), 1.48-1.41 (m, 1H, H8), 1.36-1.28 (m, 1H, H6eq), 1.03 (d, 3H, J= 7 Hz, CH3), 1.00-0.93 (m, 1H, H4ax), 0.72 (d, J = 7 Hz, 3H, isopropyl-CH3), 0.37 (d, J = 7 Hz, 3H, isopropyl-CH3). 13C NMR (125 MHz, CDCl3): δ 210.4, (Fe(CO)3) 140.0 (C1), 132.6 (phenyl, ipso-C), 130.0 (phenyl, m-C), 129.1 (phenyl, p-C), 127.3 (phenyl, o-C), 116.0 (C11), 86.4 (C12), 54.4 (C2), 49.4 (C6), 36.7 (C5), 33.6 (C4), 29.1 (C3), 28.2 (C8), 22.0 (CH3), 21.4 and 19.3 (C9, C10). IR (CH2Cl2): 2075, 2031, 2014, 1970 (CO) cm-1. Data for 13: 13C NMR (125 MHz, CDCl3) δ 210.6 (Fe(CO)3), 202.0 (Co(CO)3), 139.3 (C1) 132.6, 129.6, 129.1, 127.3 (phenyl), 114.4, 86.4, (CtC), 55.5 (CH), 49.9 (CH), 46.7 (CH2), 31.0 (CH3), 32.70 (CH2), 25.7 (CH2), 22.6 (CH3), 21.0 (CH), 20.8 (CH3). IR (CH2Cl2): 2075, 2031, 2014, 1970 cm-1 (CO).

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Organometallics, Vol. 29, No. 21, 2010

Synthesis of 14 and 15, Method A. The 70:30 mixture of the two diastereomers 12 and 13 (0.151 g, 0.291 mmol) and triphenylphosphine (0.76 g, 0.29 mmol) were dissolved in THF (10 mL) and stirred under reflux for 24 h. The resulting deep red solution was concentrated under vacuum and the residue subjected to column chromatography. Elution with a 95:5 pentane/ dichloromethane solvent mixture yielded a bright red solid (0.183 g, 0.241 mmol, 83%), for which 13C NMR showed the presence of two diastereomers in a 70:30 ratio. A sample suitable for an X-ray crystal structure determination was obtained by recrystallization from hexane/dichloromethane. It was shown to contain a 50:50 mixture of both diastereomers 14(S) and 15(R). Synthesis of 14 and 15, Method B. A 50:50 mixture of the two diastereomers 12 and 13 (0.051 g, 0.097 mmol) and triphenylphosphine (0.25 g, 0.097 mmol) were dissolved in THF (5 mL) and stirred under reflux for 24 h. The resulting deep red solution was concentrated under vacuum and the residue subjected to column chromatography. Elution with a 95:5 pentane/dichloromethane solvent mixture yielded 14 (0.052 g, 0.071 mmol, 71%) as a bright red solid, shown by NMR to be a single diastereomer, 14. A sample suitable for an X-ray crystal structure determination was obtained by recrystallization from hexane/dichloromethane. Data for 14: 1H NMR (500 MHz, CDCl3): δ 7.29-6.90 (m, 20H, phenyl), 2.51-2.49 (m, 1H, H6a), 2.27-2.32 (m, 1H, H6b), 2.00-1.95 (m, 1H, H2) 1.80-1.82 (m, 2H, H4, H3), 1.57-1.62 (m, 1H, H5), 1.40-1.45 (m, 1H, H8, H3), 1.36-1.28 (m, 1H, H4), 0.95 (d, 3H, J=6 Hz, CH3), 0.57 (d, J=6 Hz, 3H, isopropyl-CH3), 0.39 (d, J = 6 Hz, 3H, isopropyl-CH3).13C NMR (125 MHz, CDCl3): δ 211.9, (Fe(CO)3), 141.8 (C1), 135.1-125.5 (phenyls), 117.8 (C11), 87.0 (C12), 54.2 (C2), 47.9 (C6), 33.7 (C5), 32.9 (C3), 29.0 (C8), 27.3 (C4), 22.3 (C7), 21.6 and 20.2 (C9, C10). 31P NMR (202 MHz, CDCl3): δ 49.3 (s). IR (CH2Cl2): 2039, 1977 cm-1 (CO) cm-1. HRMS (ES-) m/z: 755.1034; calcd for C41H37CoFeO5 755.1060. Anal. Calcd for C41H38CoFeO5P 3 C6H14: C, 66.10; H, 5.06. Found: C, 66.60; H, (47) Sheldrick, G.M. SADABS; Bruker AXS Inc.: Madison, WI, 2000.

Moore et al. 5.24. Data for 15: 13C NMR (125 MHz, CDCl3): δ 212.0, (Fe(CO)3) 141.8 (C1) 135.1-125.5 (phenyls), 117.9 (C11), 88.9 (C12), 55.9 (CH), 49.9 (CH2), 36.7 (CH2), 32.5 (CH), 32.4 (CH2), 27.9(CH3), 25.4 (CH), 21.7(CH3), 19.0 (CH3). 31P NMR (202 MHz, CDCl3): δ 49.3 (s). IR (CH2Cl2): 2039, 1994, 1975, 1951 cm-1 (CO). X-ray Measurements for 8, 10, 11, 14(S), and 15(R). Crystallographic data were collected using a Bruker SMART APEX CCD area detector diffractometer equipped with a Bruker SMART 1K CCD area detector and a rotating anode, using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚), and are listed in Table 1. A full sphere of the reciprocal space was scanned by phi-omega scans. A semiempirical absorption correction, based on redundant reflections, was performed by the program SADABS.47 The structures were solved by direct methods and refined by full-matrix least-squares on F2 for all data using the program library SHELXTL.48,49 Hydrogen atoms were added at calculated positions and refined using a riding model. Their isotropic temperature factors were fixed to 1.2 times (1.5 times for methyl groups and OH groups) the equivalent isotropic displacement parameters of the atom to which the H atom is attached. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 762405 (8), 762408 (10), 762406 (11), 762409 (14), and 762407 (14/15).

Acknowledgment. We thank Science Foundation Ireland, University College Dublin, and the Centre for Synthesis and Chemical Biology funded by the Higher Education Authority’s Programme for Research in ThirdLevel Institutions (PRTLI) for generous financial support. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. (48) Sheldrick, G. M. SHELXS-97; University of G€ottingen, 1997. (49) Sheldrick, G. M. SHELXL-97-2; University of G€ottingen, 1997.