(η2-Alkene)(μ-alkyne) - ACS Publications - American Chemical Society

Sep 30, 2009 - Sarah A. Brusey,† Emilie V. Banide,† Steffen Dörrich,† Paul ... Helge Müller-Bunz,† Conor Long,‡ Paul Evans,*,† and Micha...
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Organometallics 2009, 28, 6308–6319 DOI: 10.1021/om900615m

X-ray Crystallographic and NMR Spectroscopic Study of (η2-Alkene) (μ-alkyne)pentacarbonyldicobalt Complexes: Arrested Pauson-Khand Reaction Intermediates Sarah A. Brusey,† Emilie V. Banide,† Steffen D€ orrich,† Paul O’Donohue,† Yannick Ortin,† † ‡ Helge M€ uller-Bunz, Conor Long, Paul Evans,*,† and Michael J. McGlinchey*,† †

School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland, and ‡ School of Chemical Sciences, Dublin City University, Dublin 9, Ireland Received July 14, 2009

The hexacarbonyldicobalt complexes of a range of 5-alkynyl-5H-dibenzo[a,d]cycloheptenes, 11a-e, readily undergo loss of a carbonyl ligand with concomitant formation of pentacarbonyldicobalt clusters, 12a-e, in which the vacant coordination site on cobalt is now occupied by the C(10)-C(11) double bond of the central seven-membered ring. These (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes provide structural models of the first step of the proposed mechanism of the Pauson-Khand process for the formation of cyclopentenones via the coupling of an alkene, an alkyne, and a source of carbon monoxide. X-ray crystallographic data reveal that the distance between the alkene carbons and the nearest alkyne carbon is approximately 2.85 A˚, slightly shorter than the theoretically predicted value of ∼2.95 A˚. VT NMR data for the interconversion of 11b and 12b yielded activation energies for the forward and reverse processes, 29 and 15 kcal mol-1, respectively, and the enthalpy change for the endothermic process (14 kcal mol-1); these match very well the earlier predictions from DFT calculations. Although the ethynyl-hexacarbonyldicobalt complex, 11f, does not suffer facile elimination of CO, it does participate in an intermolecular PKR with norbornadiene. Likewise, attempts to extend the reach of the alkynyl moiety through incorporation of an additional methylene group yield only an intermolecular PKR product. It is suggested that the pentacarbonyl complexes, 12a-e, do not continue along the PKR pathway because of the unfavorable relative orientation of the cobalt-coordinated alkyne and alkene. Introduction The Pauson-Khand reaction (PKR), the formal [2þ2þ1] cycloaddition of an alkyne, 1, an alkene, 2, and carbon *Corresponding authors. Phone: (þ353)-1-716-2880. Fax: (þ353)-1716-1185. E-mail: [email protected]; [email protected]. (1) For review literature see: (a) Pauson, P. L. Tetrahedron 1985, 41, 5855–5860. (b) Schore, N. E. Chem. Rev. 1988, 88, 1081–1119. (c) Schore, N. E. Org. React. 1991, 40, 1–90. (d) Schore, N. E. In Comparative Organic Syntheses; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 5, pp 1037-1062. (e) Schore, N. E. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Elsevier: New York, 1995; Vol. 12, pp 703-739. (f ) Fr€uhauf, H.-W. Chem. Rev. 1997, 97, 523–596. (g) Geis, O.; Schmalz, H. G. Angew. Chem., Int. Ed. 1998, 37, 911–914. (h) Chung, Y. K. Coord. Chem. Rev. 1999, 188, 297–340. (i) Fletcher, A. J.; Christie, S. D. R. J. Chem. Soc., Perkin Trans. 1 2000, 1657– 1668. ( j) Brummond, K. M.; Kent, J. L. Tetrahedron 2000, 56, 3263–3283. (k) Sugihara, T.; Yamaguchi, M.; Nishizawa, M. Chem.;Eur. J. 2001, 7, 1589–1595. (l) Blanco-Urgoiti, J.; A~norbe, L.; Perez-Serrano, L.; Domínguez, G.; Perez-Castells, J. Chem. Soc. Rev. 2004, 33, 32–42. (m) Laschat, S.; Becheanu, A.; Bell, T.; Baro, A. Synlett 2005, 2547–2570. (n) Rodriguez Rivero, M.; Adrio, J.; Carretero, J. C. Synlett 2005, 26–41. (o) Ingate, S. T.; Marco-Cantelles, J. Org. Prep. Proc. Int. 1998, 30, 121–143. (p) Buchwald, S. L.; Hicks, F. A. In Comparative Asymmetric Catalyses I-III; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer-Verlag: Berlin, 1999; Vol 2, pp 491-510. (q) Gibson, S. E.; Stevenazzi, A. Angew. Chem., Int. Ed. 2003, 42, 1800–1810. (r) Gibson, S. E.; Mainolfi, N. Angew. Chem., Int. Ed. 2005, 44, 3022–3037. (s) Alcaide, B.; Almendros, P. Eur. J. Org. Chem. 2004, 3377–3383. (t) Bo~naga, L. V. R.; Krafft, M. E. Tetrahedron 2004, 60, 9795–9833. (u) Jeong, N. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2006; Vol. 11, pp 335-366. (v) Scheuermann, C. J.; Ward, B. D. New J. Chem. 2008, 32, 1850–1880. pubs.acs.org/Organometallics

Published on Web 09/30/2009

monoxide, is a highly convergent method for the preparation of the cyclopentenone ring, 3 (see Scheme 1).1 The process was initially reported in 1973,2 and early examples focused on the use of dicobalt octacarbonyl as both the reaction mediator and the source of the carbonyl functional group. Since that time, several variants of the original thermal protocol have been introduced that have enhanced the scope and utility of the process.3 However, the mechanism continues to attract discussion.4 The chemically reasonable reaction pathway first proposed in 1985 by Magnus5a-c and by Schore5d invokes initial formation of the wellcharacterized (μ-alkyne)hexacarbonyldicobalt tetrahedral cluster, 4, which loses a carbonyl ligand to generate the pentacarbonyl cluster, 5, thus allowing incorporation of the alkene moiety as in 6. Subsequent coupling of the alkyne and alkene to form the metallocycle, 7, alkyl migration to incorporate the carbonyl group, as in 8, and reductive elimination of the cyclopentenone from the final intermediate, 9, completes the process. In the first step, the loss of a (2) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman, M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977–981. (3) (a) Shambayti, S.; Crowe, W. E.; Schreiber, S. L. Tetrahedron Lett. 1990, 31, 5289–5292. (b) Jeong, N.; Chung, Y. K.; Lee, B. Y.; Lee, S. H.; Yoo, S.-E. Synlett 1991, 204–206. (4) (a) Cabot, R.; Lled o, A.; Reves, M.; Riera, A.; Verdaguer, X. Organometallics 2007, 26, 1134–1142. (b) Pallerla, M. K.; Yap, G. P. A.; Fox, J. M. J. Org. Chem. 2008, 73, 6137–6141. r 2009 American Chemical Society

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Scheme 1. Proposed Mechanism of the Pauson-Khand Reactiona

a

eq = equatorial; ax = axial.

carbonyl ligand is typically achieved thermally or upon treatment with a mild oxidant such as an amine N-oxide.3 Since 1985 several reports concerning this mechanism have appeared; for example 5 has been detected using IR spectroscopy6 and is isolable when the alkyne contains an S atom capable of coordinating to the vacant site.7 Similarly, photolysis of 4 at 532 nm brings about loss of a carbonyl ligand, and the resulting pentacarbonyl can be trapped by pyridine or a phosphine.8 More recently, electrospray ionization mass spectrometry has been employed to detect, in one example, an alkene association complex of the type 6.9 However, apart from the initially formed (μ-alkyne)hexacarbonyldicobalt complex, 4, none of the proposed intermediates along this pathway have previously been isolated and characterized as stable entities, although Fox has identified side-products from unproductive pathways in the PKR involving cyclopropenes.4b We here report the syntheses, X-ray crystallographic structural characterizations, and a variabletemperature NMR investigation of a series of (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes, of the type 6a/6b, which represent the first isolated examples of the first step in the proposed mechanism of the PKR.10

Results and Discussion Hexa- and Pentacarbonyl Dicobalt Complexes of (5-Alkynyl-5H-dibenzo[a,d]cyclohepten-5-ols). Our involvement in this area arose somewhat serendipitously as a continuation of earlier studies on metal-cluster-stabilized nonclassical or (5) (a) Magnus, P.; Principe, L.-M. Tetrahedron Lett. 1985, 26, 4851– 4854. (b) Magnus, P.; Exon, C.; Albaugh-Robertson, P. Tetrahedron 1985, 41, 5861–5869. (c) Magnus, P.; Principe, L.-M.; Slater, M. J. J. Org. Chem. 1987, 52, 1483–1486. (d) La Belle, B. E.; Knudsen, M. J.; Olmstead, M. M.; Hope, H.; Yanuck, M. D.; Schore, N. E. J. Org. Chem. 1985, 50, 5215–5222. (6) Gordon, C. M.; Kiszka, M.; Dunkin, I. R.; Kerr, W. J.; Scott, J. S.; Gebicki, J. J. Organomet. Chem. 1998, 554, 147–154. (7) (a) Krafft, M. E.; Scott, I. L.; Romero, R. H.; Feibelmann, S.; van Pelt, C. E. J. Am. Chem. Soc. 1993, 115, 7199–7207. (b) Verdaguer, X.; Moyano, A.; Pericas, M. A.; Riera, A.; Bernardes, V.; Greene, A. E.; AlvarezLarena, A.; Piniella, J. F. J. Am. Chem. Soc. 1994, 116, 2153–2154. (c) Werz, D. B.; Schulte, J. H.; Rausch, B. J.; Gleiter, R.; Rominger, F. Eur. J. Inorg. Chem. 2004, 2585–2593. (8) Draper, S. M.; Long, C.; Myers, B. M. J. Organomet. Chem. 1999, 588, 195–199. (9) Gimbert, Y.; Lesange, D.; Milet, A.; Fournier, F.; Greene, A. E.; Tabet, J.-C. Org. Lett. 2003, 5, 4073–4075. (10) Preliminary communication: Banide, E. V.; M€ uller-Bunz, H.; Manning, A. R.; Evans, P.; McGlinchey, M. J. Angew. Chem., Int. Ed. 2007, 46, 2907–2910.

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antiaromatic carbocations.11 We chose to prepare (5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)hexacarbonyl dicobalt, 11a, with the aim of investigating the extent of the interaction between the 14π aromatic dibenzosuberenyl cation and the cobalt vertices.12 However, when a solution of the alkynol ligand in THF was allowed to react with Co2(CO)8 at room temperature for 15 h, two products were isolated: the major product (73%) was the anticipated hexacarbonyl cluster, 11a, and the minor product (15%), separable by flash column chromatography, proved to be the pentacarbonyldicobalt complex 12a, whereby the third ligand site on one of the cobalt centers was now occupied by the C(10)-C(11) double bond of the seven-membered ring (Scheme 2). Moreover, further investigation of this previously unprecedented decarbonylation/alkene-coordination process demonstrated it to be rather facile. Typically, a mixture of 11a and 12a (85:15) in deuterated chloroform gradually metamorphosed into a mixture containing equal amounts of 11a and 12a after 24 h at room temperature. The IR spectrum of the hexacarbonyl cluster 11a exhibits ν(CO) peaks at 2090, 2056, and 2029 cm-1, whereas, in the pentacarbonyl complex, 12a, the ν(CO) absorptions are found at 2075, 2023, and 1978 cm-1. These data compare well with those reported for the photolysis of (PhCtCH)Co2(CO)6 in an argon matrix that revealed strong infrared peaks at 2099, 2062, 2036, and 2031 cm-1 for the hexacarbonyl cluster and at 2081, 2035, 2016, and 1981 cm-1 after loss of a CO ligand.6 The molecular structures of 11a and 12a, determined by X-ray crystallography, appear in Figures 1 and 2, respectively. The (phenylethynyl)Co2(CO)6 moiety adopts a pseudoaxial position and is oriented such that the phenyl ring lies directly below the C(10)-C(11) double bond, thus shielding the attached protons, which resonate at 6.27 ppm (7.23 ppm in the free ligand). The carbonyl ligands in the Co2(CO)6 fragment in 11a are close to the normally observed eclipsed sawhorse conformation. In the pentacarbonyl complex, 12a, it is evident that rotation of the dicobalt-alkyne cluster about the C(5)-C(12) bond has occurred, thus placing one metal proximate to the C(10)-C(11) linkage. The boat conformation is now more pronounced, thus generating a distinctly avian-type structure. The alkyne carbon-carbon linkage (1.340 A˚) and the C-CtC-C “bend-back” angles (142.6° and 144.7°) in 12a lie within the usual ranges; as is normal, complexation to cobalt increases the C(10)-C(11) double bond distance (from 1.336(3) A˚ in 11a to 1.403(7) A˚ in 12a). This reaction has now been generalized to include a range of alkynyl substituents. Thus, the (μ-alkyne)hexacarbonyldicobalt complexes in which R = SiMe3 (11b), pC6H4CN (11c), m-C6H4F (11d), p-C6H4CF3 (11e), and H (11f) have also been prepared. Of these, all except 11f (R=H) gradually undergo decarbonylation at ambient temperature (11) (a) Harrington, L. E.; Reginato, N.; Vargas-Baca, I.; McGlinchey, M. J. Organometallics 2003, 22, 663–669. (b) Dunn, J. A.; Hunks, W. J.; Ruffolo, R.; Rigby, S. S.; Brook, M. A.; McGlinchey, M. J. Organometallics 1999, 18, 3372–3382. (c) Gruselle, M.; El Hafa, H.; Nikolski, M.; Jaouen, G.; Vaissermann, J.; Li, L.; McGlinchey, M. J. Organometallics 1993, 12, 4917– 4925. (d) Kondratenko, M.; Gruselle, M.; El Hafa, H.; Vaissermann, J.; Jaouen, G.; McGlinchey, M. J. J. Am. Chem. Soc. 1995, 117, 6907–6913. (e) Kaldis, J. H.; Brook, M. A.; McGlinchey, M. J. Chem.;Eur. J. 2008, 14, 10074–10084. (12) (a) McGlinchey, M. J.; Girard, L.; Ruffolo, R. Coord. Chem. Rev. 1995, 143, 331–381. (b) Caffyn, A. J. M.; Nicholas, K. M. Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 12, pp 685-702. (c) El Amouri, H.; Gruselle, M. Chem. Rev. 1996, 96, 1077–1104.

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Scheme 2. Sequential Formation of Hexacarbonyls 11 and Pentacarbonyls 12

to yield the corresponding (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes, 12b through 12e. Moreover, the identities of several of these clusters have been unequivocally established by X-ray crystallography, as shown in Figure 3. The molecular structures of the (μ-alkyne)dicobalt-hexacarbonyls (11a, 11b, 11e, 11f) and (η2-alkene)(μ-alkyne)dicobalt-pentacarbonyls (12a, 12b, 12c, 12d) illustrate clearly the boat conformations of the seven-membered rings, which are defined by C(4A)-C(5)-C(5A) [plane 1], C(4A)C(5A)-C(9A)-C(11A) [plane 2], and C(9A)-C(10)C(11)-C(11A) [plane 3]. The interplanar angles [plane 1]/[plane 2] (j1) and [plane 2]/[plane 3] (j2) and also the angles between the two benzo ring planes (j3), as defined in Figure 4, are listed in Table 1. These data reveal that, in each case, replacement of a carbonyl ligand, in 11, by the C(10)-C(11) double bond, in 12, leads not only to enhanced bending of the seven-membered ring, i.e., reduced values of j1 and j2, but also a decrease in the interplanar angle, j3, between the two benzo “wing-like” moieties. Although the carbonyls of the Co(CO)3 fragment in 12a-d deviate only slightly from the conventional orientation whereby one axial ligand is aligned with the midpoint of the alkyne bond (see, for example, the structure of the (trimethylsilyl)alkynyl-Co2(CO)6 cluster 11b in Figure 5a), the ligands of the Co(CO)2(alkene) fragment are staggered rather than eclipsed with respect to the Co(CO)3 tripod; in particular, the alkene is clearly in a pseudoequatorial site, as exemplified by the structure of 12b in Figure 5b. In this respect, these complexes provide structural models of the proposed (η2-alkene)(μ-alkyne)Co2(CO)5 intermediate 6a (Scheme 1). The thermal instability of the hexacarbonyl clusters, 11a-e, is remarkable; even at room temperature decarbonylation and formation of the pentacarbonyl complexes, 12a-e, is a facile process, possibly as a consequence of its intramolecular character whereby loss of CO is compensated by formation of the cobalt-alkene complex as a chelate. This result is important, as the clusters 12a-e represent the first examples of structurally characterized (η2-alkene)-(alkyne)pentacarbonyldicobalt complexes, a crucial structural type (13) Mercury 1.4.2, available from http://www.ccdc.cam.ac.uk/mercury/. (14) (a) Steiner, T.; Starikov, E. B.; Tamm, M. J. Chem. Soc., Perkin Trans. 2 1996, 67–71. (b) Steiner, T.; Mason, S. A.; Tamm, M. Acta Crystallogr. B 1997, B53, 843–848. (c) In the corresponding bis(1,4-dibenzo[a, d]cyclohepten-5-ol)buta-1,3-diyne, both alkynyl groups are positioned axially: Jacobs, A.; Masuku, L. N. Z.; Nassimbeni, L. R.; Taljaard, S. L. CrystEngComm 2008, 10, 322–326. (15) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2001, 123, 1703–1708. (16) (a) Robert, F.; Milet, A.; Gimbert, Y.; Konya, D.; Greene, A. E. J. Am. Chem. Soc. 2001, 123, 5396–5400. (b) de Bruin, T. J. M.; Milet, A.; Robert, F.; Gimbert, Y.; Greene, A. E. J. Am. Chem. Soc. 2001, 123, 7184– 7185. (c) de Bruin, T. J. M.; Milet, A.; Greene, A. E.; Gimbert, Y. J. Org. Chem. 2004, 69, 1075–1080.

in the widely invoked Magnus mechanism5 of the PausonKhand cyclopentenone synthesis. High-level density functional theory studies by Yamanaka and Nakamura,15 by Milet, Gimbert, and co-workers,16 and also by Peric as, Riera, Verdaguer, and co-workers,17 in which the relative energies of the proposed Magnus intermediates have been calculated, suggest that the most energetically demanding step is the loss of a carbon monoxide ligand from 4 to form the pentacarbonyl complex 5. These studies also indicate that the two pseudoaxial carbonyl groups are more strongly bound than their four pseudoequatorial counterparts, thereby implying that the alkenyl complex 6a will be preferentially formed.15,16,18 Subsequently, irreversible bond migration within 6a generates metallocycle 7a. Milet and co-workers have considered this key carbon-carbon bond-forming step, and they proposed an alternative scenario whereby systems capable of undergoing rotation of the ML3 vertex generate complex 6b, from which position olefin insertion, forming 7b, is comparatively facile energetically.19 The calculations of Yamanaka and Nakamura15 indicated that the carbonyl ligands in (HCtCH)Co2(CO)5(η2-C2H4) should be only slightly displaced from the eclipsed orientation generally observed in alkyne-Co2(CO)6 systems; this contrasts with the experimentally observed staggered orientation in the pentacarbonyl complexes 12a-d, as shown in Figures 2, 3, and 5. However, one must recall that these experimental data are derived from an intramolecular rather than an intermolecular process. In all the calculated geometries it was found that the incoming ethylene initially favors a pseudoequatorial site such that the distance between an alkene carbon and the nearer alkyne carbon is approximately 2.95 A˚.16 As listed in Table 1, and exemplified in Figure 6, the crystallographically determined structures of the η2-alkene-pentacarbonyldicobalt-alkyne complexes 12a-d reveal cobalt-alkene-carbon distances of 2.133 to 2.184 A˚. More importantly, the cobaltcomplexed alkene carbons are sited within the range 2.817 to 2.880 A˚ from the nearer alkyne carbon (average of the 10 values in 12a-d is 2.845 A˚); that is, they are somewhat shorter than the calculated value.16 However, despite the proximity of these carbons, the pentacarbonyl complexes 12a-d are reasonably stable and do not continue along the PKR pathway toward the corresponding cyclopentenones, 13, the formation of which may be precluded by severe ring strain. However, this observation may also support the (17) Pericas, M. A.; Balsells, J.; Castro, J.; Marchueta, I.; Moyano, A.; Riera, V.; Vazquez, J.; Verdaguer, X. Pure Appl. Chem. 2002, 74, 167–174. (18) Derdau, V.; Laschat, S.; Jones, P. G. Eur. J. Org. Chem. 2000, 681–689. (19) de Bruin, T. J. M.; Michel, C.; Vekey, K.; Greene, A. E.; Gimbert, Y.; Milet, A. J. Organomet. Chem. 2006, 691, 4281–4288.

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proposal that alkene insertion in this type of complex occurs preferentially from an axial position (i.e., 6b f 7b) whereby the alkene and alkyne are suitably aligned for coupling, and the existence of 12 as an arrested Pauson-Khand intermediate is a consequence of the rigidity of this class of compound.

This lack of rotational freedom manifests itself in the 1H and 13C NMR spectra of 11 and 12. Typically, in the (trimethylsilyl)alkynyl-hexacarbonyldicobalt cluster, 11b, the time-averaged Cs symmetry of the system renders the

Figure 1. Molecular structure of the hexacarbonyl cluster 11a (thermal ellipsoids at 50%).

Figure 2. Molecular structure of the pentacarbonyl cluster 12a (thermal ellipsoids at 50%).

Figure 3. Mercury13 representations of the molecular structures of 11b, 12b, 11e, 12d, 11f, and 12c (thermal ellipsoids at 50%).

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Figure 4. Definition of the interplanar angles j1, j2, and j3.

two benzo rings equivalent, and the protons at C(10) and C(11) appear as a singlet at 7.01 ppm. In contrast, in the pentacarbonyl complex 12b, the mirror symmetry is broken, the benzo rings are now inequivalent, and the protons at C(10) and C(11) give rise to two 9.5 Hz doublets at 5.34 and 5.19 ppm. By monitoring the intensities of these resonances as a function of time over a range of temperatures, the gradual conversion of 11b into 12b was conveniently followed by NMR spectroscopy and prompted a kinetic study that allowed the determination of the energetics of the process. In all cases (except for 11f, R = H), the hexacarbonyls, 11a-e, eventually reached an equilibrium with their alkene-complexed counterparts, 12a-e, at ambient temperature. In no case did the reaction go to completion even when the carbon monoxide was free to escape. However, when a sample of hexacarbonyl 11f (R = H) was thermolyzed in refluxing toluene, the corresponding pentacarbonyl, 12f, was detected by 1H NMR spectroscopy, but was not isolable chromatographically; the products were the free alkynol 10f, the hexacarbonyl 11f, and cobalt-containing decomposition products. Interestingly, in Gordon’s matrix infrared study6 of the 254 nm photolysis of (phenylacetylene)Co2(CO)6 to give (phenylacetylene)Co2(CO)5, it was noted that further irradiation at 390 nm resulted in regeneration of the original hexacarbonyl, presumably accompanied by decomposition products. Assuming that the loss of CO is irreversible, one can now evaluate the activation energies for the interconversion of the hexacarbonyl and pentacarbonyl species, 11b and 12b, by intermolecular migration of a carbonyl ligand. As precedent for such an exchange process we note Seyferth’s observation that the reaction of chlorocarbynyltricobalt nonacarbonyl, (OC)9Co3CCl, with AlCl3 to form the acylium salt [(OC)9Co3CCO][AlCl4] proceeds readily in the absence of CO, presumably by ligand transfer from the original cobalt carbonyl cluster.20 A standard kinetic treatment21 yielded kforward and kreverse at each temperature (Table 2), and the van’t Hoff plot of the temperature dependence of the Keq values gave the overall enthalpy change for the conversion of the (μ-alkyne)dicobalt hexacarbonyl, 11b, into the (η2-alkene)(μ-alkyne)dicobalt pentacarbonyl, 12b. As shown in Figure 7, the activation energy for the decarbonylation process was found to be ∼29 kcal mol-1, and the overall reaction was endothermic by ∼14 kcal mol-1. (20) Seyferth, D. Adv. Organomet. Chem. 1975, 14, 97–144. (21) Engel, T.; Drobny, G.; Reid, P. Physical Chemistry for the Life Sciences; Pearson-Prentice Hall: Upper Saddle River, NJ, 2008; pp 682-684.

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These experimental values correlate very well with the calculations of Yamanaka and Nakamura,15 who reported Eact and ΔH as 26.4 and 14.6 kcal mol-1, respectively, and with those of Peric as and co-workers,17 who calculated corresponding values of 33.5 and 12 kcal mol-1. We reiterate, however, that their calculations were based on an intermolecular reaction to form (HCtCH)Co2(CO)5(η2-C2H4), whereas the experimental data were obtained from a more heavily substituted intramolecular process. Nevertheless, the remarkably good agreement between the calculated and experimental values is gratifying. As noted above, molecules 11a-e were readily converted into 12a-e; in contrast, the least substituted complex, 11f (R = H), was much more stable and only suffered loss of a carbonyl ligand at elevated temperatures (PhMe, 110 °C) or when treated with N-methylmorpholine N-oxide monohydrate. In this instance the pentacarbonyl complex 12f proved to be unstable, and we were unable to isolate this species in pure crystalline form. However, 12f was detected in a low-temperature matrix when 11f was deposited in methane at 20 K. In this experiment it was necessary to raise the temperature to 353 K in vacuo to generate sufficient vapor pressure of 11f and to promote the deposition process. In addition to the expected IR peaks of the precursor hexacarbonyl, 11f (2097, 2062, and 2033 cm-1), peaks of the pentacarbonyl, 12f, at 2083, 2026, 2015, 2007, and 1982 cm-1, along with uncoordinated CO (2143 cm-1) were observed, thus confirming the decarbonylation of 11f at elevated temperatures and demonstrating that the pentacarbonyl 12f is sufficiently stable to survive in the gas phase until isolated in the frozen gas matrix. Preliminary photochemical experiments suggest that 557 nm irradiation of the hexacarbonyl, 11f, also promotes decarbonylation to form 12f. Nevertheless, under standard thermal conditions 11f smoothly underwent an intermolecular Pauson-Khand reaction with norbornadiene to furnish the expected cyclopentenone, 14, which was fully characterized spectroscopically and by high-resolution mass spectrometry. The structure of 14 was elucidated from its 1H and 13C NMR data, which indicated formation of the anticipated exo-diastereoisomer.2,22 The conformation of 14 was probed by NOE measurements, which indicated the proximity of the single proton in the cyclopentenone ring to the alkene protons at C(10) and C(11). Cobalt Carbonyl Complexes of (5-Propynyl-5H-dibenzo[a,d]cyclohepten-5-ols). With the aim of synthesizing a more flexible system to probe whether it is possible to complete the intramolecular Pauson-Khand reaction in this case, we chose to prepare [5-(3-propynyl)-5H-dibenzo[a,d]cyclohepten-5-ol], 15, which possesses an additional methylene group between the seven-membered ring and the alkyne unit. Propargyl bromide was treated with 2 equiv of butyllithium, and the resulting dianion23 was allowed to react with dibenzosuberenone, thus forming 15 after hydrolysis. The X-ray crystal structures of 15 and of its hexacarbonyldicobalt complex, 16, reveal that the alkyne moiety (free or complexed) adopts a position such that it is oriented maximally distant from the C(10)-C(11) double bond in the seven-membered ring (see Figure 8). Moreover, there was (22) It is widely reported that this type of intermolecular PKR process delivers high levels of exo-diastereoselectivity. (23) Cabezas, J. A.; Pereira, A. R; Amey, A. Tetrahedron Lett. 2001, 42, 6819–6822.

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Table 1. Interplanar Angles (deg) and Bond Distances (A˚) for Free Alkynes 10, (μ-Alkyne)Co2(CO)6 Complexes 11, and (η2-Alkene)(μ-alkyne)Co2(CO)5 Complexes 12 compound 10b (R = TMS) 10f (R = H)a 11a (R = C6H5)b 11b (R = TMS) 11e (R = p-C6H4CF3) 11f (R = H)b 12a (R = C6H5) 12b (R = TMS)b 12c R = p-C6H4CN) 12d (R = m-C6H4F) a

j1

j2

j3

Co1-C10

Co1-C11

C10 3 3 3 C12

C11 3 3 3 C12

126.9 123.2 130.2 129.6 135.3 131.7 131.3 129.0 122.4 122.8 123.5 122.3 121.6

153.1 157.9 157.8 156.5 161.3 155.5 155.1 155.6 150.8 152.4 152.8 152.9 151.6

125.8 126.0 132.5 129.7 142.1 133.6 130.3 127.2 120.6 127.3 127.2 123.4 119.9

2.184(5) 2.169(3) 2.178(4) 2.160(2) 2.165(1)

2.139(6) 2.145(3) 2.152(4) 2.146(2) 2.133(1)

2.858 2.877 2.880 2.863 2.830

2.817 2.829 2.840 2.831 2.827

Data from ref 14. b Two independent molecules in the unit cell.

Table 2. Kinetic Data for the Conversion of 11b into 12b temperature (K) 293 298 303 313

kforward (min-1) -4

5.18  10 1.15  10-3 2.24  10-3 1.27  10-2

kreverse (min-1) -3

1.11  10 1.50  10-3 2.20  10-3 6.05  10-3

Keq 0.46 0.69 1.02 2.10

Figure 5. View along the cobalt-cobalt bond (a) illustrating the eclipsed conformations of the Co(CO)3 vertices in 11b and (b) emphasizing the staggered conformation of the Co(CO)3 and Co(CO)2(alkene) moieties in the core of the pentacarbonyl complex 12b. Figure 7. Energy profile for the conversion of 11b to 12b.

Figure 6. Cobalt-carbon bond lengths (- - -) and nonbonded carbon-carbon distances (- - -) within the core of the pentacarbonyl complex 12a; values given in A˚.

no evidence of facile decarbonylation of 16 to the corresponding alkene-pentacarbonyldicobalt 17, nor to the PKR product(s) 18 (Scheme 4). In contrast, as indicated in Scheme 5, an intermolecular PKR with norbornadiene proceeded smoothly, and the anticipated cyclopentenone product, 19, was isolated and characterized by X-ray crystallography. The Pauson-Khand product, 19, is shown in Figure 9 and clearly illustrates that the cyclopentenone ring adopts an exo position adjacent to the bridgehead methylene group. Moreover, one of the bridgehead hydrogens, H(22b), lies

directly above a benzo ring; the 1H NMR resonance of this proton is readily identified since it is shielded by the ring current of the proximate benzo moiety and is found at δ 0.10, while its partner, H(22a), is found at its normal chemical shift of 0.97 ppm. Although the crystal quality is such as to preclude unambiguous location of the hydrogens, the oxygens O(1) and O(2) are well positioned to support intramolecular hydrogen bonding between the ketone and the alcohol; this may hinder rotation about the C(12)-C(13) bond and help stabilize the observed conformation. Likewise, the conformation of 14, the PKR product from 11f and norbornadiene, may also be favored by hydrogen bonding between the ketone and the alcohol. In light of the previous failure of (5-ethynyl-5H-dibenzo[a, d]cyclohepten-5-ol)hexacarbonyldicobalt, 11f, to suffer loss of a carbonyl, the propynol 15 was doubly deprotonated and allowed to react with chlorotrimethylsilane, thus forming 5-[3-(1-trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene, 20, whose structure appears in Figure 10; the monosilylated analogues 21 and 22 were also formed. Treatment of 20 and 21 with Co2(CO)8 furnished the hexacarbonyl clusters 23 and 24, respectively; however, formation of the corresponding Co2(CO)5 derivatives was not observed, and work in this area is continuing. To conclude, the hexacarbonyldicobalt complexes of a range of 5-alkynyl-5H-dibenzo[a,d]cycloheptenes readily undergo loss of a carbonyl ligand with concomitant

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Figure 8. Mercury representations of the structure of [5-(3-propynyl)-5H-dibenzo[a,d]cyclohepten-5-ol], 15, and of its dicobalt hexacarbonyl complex, 16 (thermal ellipsoids at 50%). Scheme 3. Intermolecular PKR of Hexacarbonyl 11f with Norbornadiene

formation of pentacarbonyldicobalt clusters, 12a-e, in which the vacant coordination site on cobalt is now occupied by the C(10)-C(11) double bond of the central sevenmembered ring. One can surmise that the rigidity of the pentacarbonyl complexes, such that the complexed alkene cannot rotate into a pseudoaxial site, together with potential steric strain in the product, prevents continuation along the Pauson-Khand reaction pathway. Consequently, these complexes provide structural models of the first step of the proposed mechanism of the PKR process for the formation of cyclopentenones via the coupling of an alkene, an alkyne, and a source of carbon monoxide. X-ray crystallographic data reveal that the distance between the alkene carbons and the nearest alkyne carbon is approximately 2.85 A˚, somewhat shorter than the theoretically predicted value of ∼2.95 A˚. However, the activation energies for the forward and reverse processes, 29 and 15 kcal mol-1, respectively, and the enthalpy change for the endothermic process (14 kcal mol-1) match very well the earlier predictions from DFT calculations. Although the ethynyl-hexacarbonyldicobalt complex, 11f, does not suffer facile elimination of CO, it does participate in an intermolecular PKR with norbornadiene. Attempts to bring about an intramolecular PKR by extending the reach of the alkynyl moiety through incorporation of an additional methylene group were unsuccessful. Future work will involve the gradual diminution of rigidity and potential steric strain in these systems so as to probe the viability of arresting the PKR at subsequent stages of the process.

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, or 500 MHz spectrometers. Assignments were based on standard twodimensional 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. 5-Trimethylsilylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol (10b) and 5-ethynyl-5H-dibenzo[a,d]cyclohepten5-ol (10f) were prepared according to literature procedures.14 Synthesis of 5-Phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol (10a). In a typical reaction, n-BuLi (17.15 mL, 27.5 mmol) was added dropwise to a solution of phenylacetylene (3.45 mL, 30 mmol) in dry THF (250 mL) at -78 °C. After stirring the solution for 30 min at -78 °C, it was allowed to warm to room temperature and stirred for a further 30 min. The reaction mixture was cooled to -78 °C, and a solution of dibenzosuberenone (5.15 g, 25 mmol) in dry THF (20 mL) was added dropwise. The mixture was stirred for 1 h, then quenched with distilled water (150 mL) and extracted using diethyl ether, and the organic layers were combined. These were washed with brine and dried over magnesium sulfate, the solvent was removed, and the crude product was chromatographed on an alumina column using pentane/dichloromethane (80:20) as eluent to give 5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol (10a) (6.0 g, 19.5 mmol; 77%) as a pale yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.16 (d, 2H, J=8.0 Hz, H4, H6), 7.48-7.43 (m, 6H, H1, H3, H7, H9, phenyl-H), 7.37-7.32 (m, 5H, H3, H8, phenylH), 7.23 (s, 2H, H10, H11), 3.14 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 140.5 (C4a, C5a), 132.9 (C9a, C11a), 131.8 (phenyl o-CH), 131.6 (C10, C11), 129.3 (C1, C9), 128.7 (phenyl p-CH), 128.6 (C3, C7), 128.4 (phenyl m-CH, phenyl ipso-C), 127.3 (C2, C8), 123.9 (C4, C6), 122.6 (C13), 90.5 (C12). IR (CHCl3): 3598 (OH), 2227 (CtC) cm-1. Anal. Calcd for C23H16O: C, 89.58; H, 5.23. Found: C, 89.19; H, 5.33. Analogously, (5-(4-cyanophenyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5-ol), 10c, 5-(3-fluorophenyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5-ol, 10d, and 5-(R,R,R-trifluorotolyl)ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol, 10e, were prepared by reaction of dibenzosuberene with the appropriate lithio-alkynes. (5-(3-Fluorophenyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5-ol (10d). 1 H NMR (400 MHz, CDCl3): δ 8.07 (d, J=8.00 Hz, H4, H6), 7.42 (td, 2H, J = 7.2 Hz, J=1.6 Hz, H3, H7), 7.40 (d, 2H, J = 6.8 Hz, H1, H9), 7.31 (td, J=7.6 Hz, J=1.2 Hz, H2, H8), 7.28-7.14 (m, 3H, phenyl-H), 7.18 (s, 2H, H10, H11), 7.07 (d, 1H, J=8.4 Hz, phenyl-H), 6.99 (tdd, 1H, J = 7.6 Hz, J=2.4 Hz, J=0.8 Hz, phenyl-H). 13C NMR (100 MHz, CDCl3): δ 162.4 (d, J = 245.2 Hz, CF), 140.5 (C4a, C5a), 132.9 (C9a, C11a), 131.6 (C10, C11), 130.0 (d, J = 8.6 Hz, C13a), 129.9 (d, J= 8.6 Hz, C15), 129.5 (C1, C9),128.5 (C3, C7), 127.6 (d, J = 2.8 Hz, C14), 127.2 (C2, C8), 123.5 (C4, C6), 118.5 (d, J=22.6 Hz),

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Scheme 4. Unsuccessful Attempts to Carry Out an Intramolecular PKR with 16

Scheme 5. Formation of 19, the Pauson-Khand Product of 16 with Norbornadiene

115.8 (d, J=21.1 Hz), (C16, C18), 91.9 (br s, C13), 82.5 (C12), 78.3 (C5a). Anal. Calcd for C23H15OF: C, 84.64; H, 4.63. Found: C, 84.29; H, 4.78. 5-(r,r,r-Trifluorotolyl)-5H-dibenzo[a,d]cyclohepten-5-ol (10e). Yellow solid, 45% yield, mp 79-82 °C. 1H NMR (500 MHz, CDCl3): δ 8.10 (d, J=8.0 Hz, 2H, H4, H6), 7.56 (d, J = 8.0 Hz, 2H, phenyl m-H), 7.51 (d, J = 8.0 Hz, 2H, phenyl o-H), 7.47 (td, J=7.0 Hz, J = 1.5 Hz, 2H, H3, H7), 7.44 (d, J = 8.0 Hz, 2H, H1, H9), 7.35 (td, J=7.0 Hz, J = 1.5 Hz, 2H, H2, H8), 7.22 (s, 2H, H10, H11), 3.23 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 140.2 (C4a, C5a), 132.9 (C9a, C11a), 132.1 (phenyl o-C), 131.7 (C10, C11), 130.4 (q, J= 32.9 Hz, phenyl p-C), 129.4 (C1, C9), 128.7 (C3, C7), 127.4 (C2, C8), 126.5 (phenyl ipso-C), 125.3 (q, J = 3.9 Hz, phenyl m-C), 124.0 (q, J = 271 Hz, CF3), 123.5 (C4, C6), 93.3 (C12), 83.5 (C13), 72.5 (C-OH). Anal. Calcd for C24H15OF3: C, 76.59; H 4.02. Found: C, 76.23; H 4.03. Syntheses of (5-Phenylethynyl-5H-dibenzo[a,d]cyclohepten-5ol)hexacarbonyldicobalt (11a) and (5-Phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)pentacarbonyldicobalt (12a). A solution of 10a (720 mg, 2.34 mmol) and Co2(CO)8 (1.60 g, 4.67 mmol) in THF (25 mL) was stirred at room temperature for 15 h. The solvent was removed at low temperature, and the crude product was purified by chromatography on a silica gel column using pentane/dichloromethane as eluent, to give (5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)hexacarbonyldicobalt, 11a (73%), and (5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5ol)pentacarbonyldicobalt, 12a (15%), as brown solids, which were subsequently recrystallized from pentane/dichloromethane. 11a: 1H NMR (400 MHz, CDCl3): δ 8.19 (dd, 2H, 3 J = 8.0, 4J=1.2 Hz, H4, H6), 7.42 (ddd, 2H, 3J = 8.0 Hz, 3J = 7.2 Hz, 4J = 1.6 Hz, H3, H7), 7.29 (td, 2H, 3J=7.6 Hz, 4J=1.2 Hz, H2, H8), 7.27-7.19 (m, 3H, phenyl H’s), 7.17 (dd, 2H, 3J = 7.6 Hz, 4J=1.6 Hz, H1, H9), 6.80-6.76 (m, 2H, phenyl H’s), 6.27 (s, 2H, H10, H11), 2.84 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 198.7 (CO), 141.2 (C4a, C5a), 138.0 (C13a), 132.2 (C9a, C11a), 130.2 (C10, C11), 128.8 (C1, C9), 128.7 (Cphenyl), 127.6 (C3, C7), 126.9 (Cphenyl), 126.5 (C2, C8), 126.1 (Cphenyl), 123.7 (C4, C6), 109.1, 93.0 (C12, C13), 75.2 (C5a). IR (liquid, CDCl3): ν(CO) 2090, 2056, 2029 cm-1. 12a: 1H NMR (500 MHz, CDCl3): δ 7.95 (d, 1H, 3J = 8.0 Hz, H6), 7.89 (d, 1H, 3J= 8.0 Hz, H4), 7.58 (d, 1H, 3J = 7.5 Hz, H1), 7.44 (td, 1H, 3J = 7.5 Hz, 4J = 1.0 Hz, H3), 7.37 (d, 1H, 3J = 7.5 Hz, H9), 7.33 (t, 1H, 3 J = 7.5 Hz, H7), 7.32 (t, 1H, 3J = 7.5 Hz, H2), 7.20 (t, 1H, 3J = 7.5 Hz, H16), 7.18 (t, 1H, 3J=7.5 Hz, H8), 7.14 (t, 2H, 3J=7.5 Hz, H15, H17), 6.90 (d, 2H, 3J=7.5 Hz, H14, H18), 5.40 (d, 1H, 3J = 9.5 Hz, H11), 5.37 (d, 1H, 3J = 9.5 Hz, H10), 2.54 (s, 1H, OH).

Figure 9. Mercury representation of the structure of 19, the Pauson-Khand product from 16 and norbornadiene (thermal ellipsoids at 50%).

Figure 10. Mercury representation of the structure of 5-[3-(1trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene, 20 (thermal ellipsoids at 50%). C NMR (125 MHz, CDCl3): δ 197.6 (CO), 142.6 (C5a), 142.3 (C4a), 136.4 (C13a), 135.3 (C9a), 134.4 (C11a), 129.8 (C14, C18), 129.7 (C1), 128.8 (C9), 128.5 (C15, C17), 128.1, 128.0 (C3, C16), 128.0 (C7), 127.4 (C2), 127.1 (C8), 123.6 (C12), 122.0 (C4), 91.3 (C13), 76.4 (C5), 74.2 (C11), 70.9 (C10). IR (CHCl3): ν(CO) 2075, 2023, 1978 cm-1. The hexacarbonyl clusters, 11b-f, and the pentacarbonyl complexes, 12b-e, were prepared analogously. (5-Trimethylsilylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)hexacarbonyldicobalt (11b). Yield: 64%. 1H NMR (500 MHz, CDCl3): δ 8.20 (d, 2H, J = 8 Hz, H4, H6), 7.50-7.46 (m, 2H, H3, H7), 7.38 (d, 4H, J=4 Hz, H1, H2, H8, H9), 7.01 (s, 2H, H10, H11), 2.93 (s, 1H, OH), 0.28 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 198.5 (CO), 142.5 (C4a, C5a), 132.9 (C9a, C11a), 132.2 (C10, C11), 130.3 (C1, C9), 128.9 (C3, C7), 127.8 (C2, C8), 125.3 (C4, C6), 78.7 (C13), 77.3 (C12), 76.2 (C5), 1.6 (Me3Si). IR (CH2Cl2): ν(CO) 2090, 2055, 2026 cm-1. [5-(4-Cyanophenyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5-ol]hexacarbonyldicobalt (11c). Yield: 57%. 1H NMR (400 MHz, CDCl3): δ 8.19 (d, 2H, J = 8 Hz, H4, H6), 7.53 (d, 2H, H14, H18), 7.47 (t, 2H, H3, H7), 7.34 (t, 2H, H2, H8), 7.2 (d, 2H, H1, H9), 6.87 (d, 2H, H15, H17), 6.28 (s, 2H, H10, H11), 2.89 (s, 1H, OH). 13C NMR (100 MHz CDCl3): δ 198.0 (CO), 145.6 (C13a), 142.0 (C4a, 13

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C5a), 132.9 (C9a, C11a), 131.6 (C14, C18), 131.3 (C10, C11), 130.0 (C15, C17), 129.9 (C1, C9), 128.9 (C3, C7), 127.8 (C2, C8), 124.7 (C4, C6), 118.8 (CdN), 110.2 (C16), 90.6 (C12), 76.4 (C13), 76.0 (C5). [5-(3-Fluorophenyl)ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol]hexacarbonyldicobalt (11d). Yield: 84%. 1H NMR (500 MHz, CDCl3): δ 8.18 (dd, 2H, J = 8.5 Hz, J = 1 Hz, H4, H6), 7.42 (td, 2H, J = 7.5 Hz, J = 1.5 Hz, H3, H7), 7.32 (td, 2H, J = 7.5 Hz, J =1.5 Hz, H2, H8), 7.20 (d, 2H, J = 7.5 Hz, H1, H9), 7.20 (q, 1H, J =7.5 Hz, H16), 6.99 (tdd, 1H, J=8.5 Hz, J = 2.5 Hz, J = 1 Hz, H15), 6.57 (dq, 1H, J = 8 Hz, J = 1 Hz, H18), 6.47 (dt, 1H, J = 9.5 Hz, J = 2 Hz, H14), 6.33 (s, 2H, H10, H11), 2.85 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 193.3 (CO), 162.2 (d, J = 245.2, CF), 142.0 (C4a, C5a), 133.2 (C9a, C11a), 131.2 (C10, C11), 129.9 (C1, C9), 129.8 (d, J = 8.5 Hz, C13a), 129.4 (d, J = 103 Hz), 128.8 (C3, C7), 127.6 (C2, C8), 125.4 (d, J = 2.8 Hz, C18), 124.7 (C4, C6), 116.4 (d, J = 22.7 Hz, C14), 113.9 (d, J = 21.2 Hz, C15), 109.1 (C12), 92.06 (C13), 76.3 (C5). [5-(r,r,r-Trifluorotolyl)ethynyl-5H-dibenzo[a,d]cyclohepten5-ol]hexacarbonyldicobalt (11e). Yield: 88%. 1H NMR (500 MHz, CDCl3): δ 8.19 (d, 2H, J = 8.5 Hz, H4, H6), 7.50 (d, 2H, J = 8.5 Hz, phenyl m-H), 7.45 (td, 2H, J = 8.5 Hz, J =1.5 Hz, H3, H7), 7.32 (td, 2H, J = 7.5 Hz, J = 1.5 Hz, H2, H8), 7.19 (d, 2H, J = 7.5 Hz, J = 1.5 Hz, H1, H9), 6.88 (d, 2H, J = 8.0 Hz, phenyl o-H), 6.30 (s, 2H, H10, H11), 2.84 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 198.2 (CO), 141.9 (C4a, C5a), 133.1 (C9a, C11a), 131.3 (C10, C11), 129.7 (C1, C9), 129.6 (q, J = 28 Hz, phenyl p-C), 129.5 (phenyl o-C), 128.9 (C3, C7), 127.7 (C1, C9), 124.9 (q, J = 3.75 Hz, phenyl m-C), 124.7 (C4, C6), 124.1 (q, J = 270 Hz, CF3), 110.0 (C-alkyne), 91.4 (C-alkyne), 76.4 (C5). Microanalytical data on the hexacarbonyls 11a-e are variable because of ready conversion to the corresponding pentacarbonyl species. (5-Ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)hexacarbonyldicobalt (11f). Yield: 60%. 1H NMR (500 MHz, CDCl3): δ 8.10 (d, 2H, J = 8 Hz, H4, H6), 7.40 (td, 2H, J = 7.0, J = 1.5 Hz, H3, H7), 7.31 (td, 2H, J = 8.0 Hz, J=1.5 Hz, H2, H8), 7.28 (d, 2H, J = 7.0 Hz, H1, H9), 7.06 (s, 2H, H10, H11), 6.05 (s, 1H, OH), 2.66 (s, 1H, H13). 13C NMR (125 MHz, CDCl3): δ 198.50 (CO), 142.6 (C9a, C11a), 132.5 (C4a, C5a), 131.5 (C10, C11), 129.6 (C2, C6), 128.5 (C3, C7), 127.2, (C1, C9), 124.2 (C4, C6), 73.6 (C12), 29.6 (C13). IR (CH2Cl2): ν(CO) 2092, 2057, 2026 cm-1. Anal. Calcd for C23H12O7Co2: C, 53.30; H, 2.33; Co, 22.76. Found: C, 53.12; H, 2.39; Co 22.49. (5-Trimethylsilylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)pentacarbonyldicobalt (12b). Yield: 6.5%. 1H NMR (500 MHz, CDCl3): δ 7.82 (d, 1H, J = 7 Hz, H4), 7.78 (d, 1H, J = 8 Hz, H6), 7.55 (d, 1H, J = 7.5 Hz, H1), 7.40 (td, 1H, J = 7.5 Hz, J = 1 Hz, H3), 7.30 (d, 1H, J = 8 Hz, H9), 7.29 (td, 1H, J = 6.5 Hz, J = 1 Hz, H2), 7.24 (td, 1H, J = 7.5 Hz, J = 1 Hz, H7), 7.11 (td, 1H. J = 7.5 Hz, J = 1 Hz, H8), 5.34 (d, 1H, J = 9.5 Hz, H11), 5.19 (d, 1H, J = 9.5 Hz, H10), 2.26 (s, 1H, OH), 0.0 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 198.8 (CO), 143.8 (C5a), 142.5 (C4a), 135.7 (C9a), 134.9 (C11a), 129.8 (C1), 129.0 (C9), 128.3 (C3), 128.0 (C7), 127.8 (C2), 127.2 (C8), 122.5 (C4), 121.7 (C6), 83.0 (C13), 76.6 (C5), 70.7 (C10), 0.0 (C-TMS). IR (CH2Cl2): ν(CO) 2073, 2015, 1972 cm-1. Anal. Calcd for C25H20O6SiCo2: C, 53.39; H, 3.58. Found: C, 53.27; H, 3.61. [5-(3-Fluorophenyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5-ol]pentacarbonyldicobalt (12d). Yield: 16%. 1H NMR (500 MHz, CDCl3): δ 7.93 (d, 1H, J = 7.5 Hz, H6), 7.87 (d, 1H, J = 7.5 Hz, H4), 7.56 (dd, 1H, J = 7 Hz, 1 Hz, H1), 7.45 (dd, 1H, J = 7.5 Hz, 1.5 Hz, H3), 7.37 (dd, 1H, J = 8 Hz, H9), 7.34 (m, 1H, H7), 7.31 (td, J = 7.5 Hz, J = 1 Hz, H2), 7.19 (td, 1H, J = 7.5 Hz, J = 1 Hz, H8), 7.10-7.06 (m, 1H, H15), 6.87 (td, 1H, J = 8 Hz, J = 2 Hz, H16), 6.62 (d, 1H, J = 8 Hz, H14), 6.45 (dt, J = 10 Hz, J = 2 Hz, H18), 5.40 (d, 1H, J = 9.5 Hz, H10), 5.35 (d, 1H, J = 9.5 Hz, H11), 2.49 (s, 1H, -OH). 13C NMR (125 MHz, CDCl3): δ 197 (CO), 162.2 (d, J = 247 Hz, C17), 142.1 (C5a), 142.09 (C4a), 138.8 (C13a), 134.9 (C9a), 133.9 (C11a), 129.5 (C1), 128.72 (C9), 128.0 (C3), 127.6 (C7), 127.3 (C2), 127.1

Brusey et al. (C8), 125.4 (C14), 121.89 (C4), 121.85 (C6), 115.9 (C18), 114.8 (C16), 105.0 (C12), 88.98 (C13), 76.01 (C5), 74.3 (C10), 70.9 (C11). IR (CH2Cl2): ν(CO) 2076, 2022, 1980 cm-1. [(5-(r,r,r-Trifluorotolyl)ethynyl)-5H-dibenzo[a,d]cyclohepten-5ol)]pentacarbonyldicobalt (12e). Yield: 12%. 1H NMR (400 MHz, CDCl3): δ 7.92 (d, 1H, J = 8 Hz), 7.86 (d, 1H, J = 8 Hz, H4, H6), 7.58 (d, 1H, J = 8 Hz,), 7.50 (d, 1H, J = 8 Hz, H1, H9), 7.44 (br t, J = 8 Hz, H3 or H7), 7.38-7.30 (m, phenyl-H’s), 7.2-7.17 (m, phenyl-H’s), 6.90 (d, 1H, J = 8 Hz, H2 or H8), 6.89 (t, J = 8 Hz, H2 or H8), 5.42 (d, 1H, J = 9.6 Hz, H10 or H11), 5.37 (d, 1H, J=9.6 Hz, H10 or H11). IR (CH2Cl2): ν(CO) 2078, 2022, 1982 cm-1. Anal. Calcd for C29H15O6F3Co2 3 0.25CH2Cl2: C, 53.59; H, 2.38; F, 8.69; Co, 17.98. Found: C, 53.63; H, 2.58; F, 9.14; Co, 17.63. Synthesis of exo-2-(5-Hydroxy-5H-dibenzo[a,d]cyclohepten-5yl)-3a,4,7,7a-tetrahydro-4,7-methanoinden-1-one (14). A solution of the cobalt complex 11f (54 mg, 0.104 mmol, 1 equiv) in toluene (5 mL) was degassed for 10 min with nitrogen. Norbornadiene (0.11 mL, 1.02 mmol, 10 equiv) was added, and the mixture was heated to reflux for 1 h. On cooling, the title compound, 14 (35 mg, 96%) was isolated as a yellow oil by preparative TLC (cyclohexane/EtOAc, 9:1). Rf = 0.25 (cyclohexane/EtOAc, 9:1). IR (neat): 3467, 3065, 3022, 2976, 2947, 1681, 1484, 1436, 1311, 1021 cm-1. 1H NMR (400 MHz, CDCl3): δ 8.07 (1H, d, J = 7.5 Hz, ArH), 8.05 (1H, d, J=7.0 Hz, ArH), 7.50-7.46 (2H, m, ArH), 7.34-7.25 (4H, m, ArH), 6.92 (1H, d, J=12.0 Hz, CH), 6.86 (1H, d, J=12.0 Hz, CH), 6.74 (1H, d, J = 3.0 Hz, CH), 6.21 (1H, dd, J = 3.0, 5.5 Hz, CH), 6.09 (1H, dd, J = 2.0, 5.5 Hz, CH), 5.02 (1H, s, OH), 2.72-2.70 (1H, m, CH), 2.60-2.58 (1H, m, CH), 2.54-2.52 (1H, m, CH), 2.13 (1H, d, J=5.0 Hz, CH), 1.25 (1H, d, J = 9.0 Hz, CH2), 0.97 (1H, d, J = 9.0 Hz, CH2). 13C NMR (100 MHz, CDCl3): δ 210.4 (CO), 160.7 (CH), 145.0 (C), 139.6 (C), 139.2 (C), 138.6 (CH), 137.0 (CH), 132.5 (C), 132.3 (C), 131.5 (CH), 130.9 (CH), 129.1 (CH), 129.0 (2  CH), 128.9 (CH), 126.9 (CH), 126.8 (CH), 124.35 (CH), 124.3 (CH), 74.4 (C), 53.1 (CH), 46.7 (CH), 43.8 (CH), 43.1 (CH), 41.2 (CH2). MS m/z (ESþ): 353 (MH)þ, 40%, 335 (M - OH)þ, 100%. HRMS: m/z 353.1554; calcd for C25H21O2 353.1542. Synthesis of 5-(3-Propynyl)-5H-dibenzo[a,d]cyclohepten-5-ol (15). To a solution of n-BuLi (75 mL, 120 mmol) in THF at -78 °C was added freshly dried TMEDA (4.5 mL, 30 mmol), followed by dropwise addition of propargyl bromide (5.35 mL, 60 mmol), and the resulting mixture was stirred for 30 min. A solution of dibenzosuberenone (6.19 g, 30 mmol) in THF (50 mL) was added dropwise over 15 min, and the reaction mixture was allowed to warm to room temperature over 2 h. The reaction was quenched with distilled water (150 mL) and extracted with diethyl ether. The organic layers were combined, washed with aqueous HCl solution (150 mL), saturated NH4Cl solution, and then with brine, and dried using MgSO4, and the solvent was removed. The crude material was purified by chromatography on an alumina column using pentane/ethyl acetate (85:15) as eluent, and 5-(3-propynyl)-5H-dibenzo[a, d]cyclohepten-5-ol, 15 (2.96 g, 12 mmol, 40%), was isolated as a pale yellow oil. X-ray quality crystals, mp 113-114 °C, were obtained from dichloromethane. 1H NMR (300 MHz, CDCl3): δ 8.03 (d, 2H, 3JHH=8.00 Hz, H4, H6), 7.46 (dt, 2H, 3JHH = 7.48 Hz, 4JHH =1.02, H3, H7), 7.40-7.30 (m, 4H, H1, H2, H8, H9), 7.06 (s, 2H, H10, H11), 3.38 (s, 1H, OH), 2.98 (s, 2H, H12), 1.91 (s, 1H, H14). 13C NMR (75 MHz, CDCl3): δ 140.8 (C4a, C5a), 132.2 (C9a, C11a), 131.5 (C10, C11), 129.4 (C2, C8), 128.7 (C3, C7), 126.8 (C1, C9), 124.3 (C4, C6), 79.8 (C13), 74.7 (C5), 72.5 (C14), 27.2 (C12). IR (CHCl3): 3553 (OH), 3305 (alkyne CH), 2116 cm-1 (CtC). EI-MS: m/z 246 (M)þ 3%, 207.4 100%. Anal. Calcd for C18H14O: C, 87.78; H, 5.73. Found: C, 87.82; H, 5.92. Synthesis of [5-(3-Propynyl)-5H-dibenzo[a,d]cyclohepten-5ol]hexacarbonyldicobalt (16). A solution of Co2(CO)8 (1.18 g, 3.45 mmol) in dry THF (15 mL) was added slowly at room temperature to a solution of 15 (0.85 g, 3.45 mmol) in dry THF (35 mL). After stirring for 18 h, the solvent was removed and the

Article crude material was purified by chromatography on a silica gel column using pentane/dichloromethane (50:50) as eluent to give [5-(3-propynyl)-5H-dibenzo[a,d]cyclohepten-5-ol]hexacarbonyldicobalt, 16 (1.47 g, 2.76 mmol, 80%), as a deep red solid, mp 100-101 °C. 1H NMR (500 MHz, CDCl3): δ 7.95 (d, 2H, 3JHH= 8.0 Hz, H4, H6), 7.47 (dt, 2H, 3JHH =7.53 Hz, 4JHH = 1.6 Hz, H3, H7), 7.38 (d, 2H, 3JHH = 7.6 Hz, 4JHH = 1.5 Hz, H1, H9), 7.33 (dt, 2H, 3JHH = 7.3 Hz, 4JHH = 1.10 H2, H8), 7.02 (s, 2H, H10, H11), 4.90 (s, 1H, H14), 3.66 (s, 2H, H12), 2.61 (s, 1H, OH). 13C NMR (125 MHz, CDCl3): δ 199.7 (CO), 141.4 (C4a, C5a), 132.5 (C9a, C11a), 131.7 (C10, C11), 129.6 (C1, C9), 128.9 (C3, C7), 127.0 (C2, C8), 124.3 (C4, C6), 89.5 (C13), 77.6 (C5), 73.5 (C14), 41.7 (C12). IR (CHCl3): 2091, 2052, 2024 cm-1 (CO). MS (ES-): m/z 531 (M H). Anal. Calcd for C24H14Co2O7: C, 54.16; H, 2.65. Found: C, 54.09; H, 2.76. Attempts to carry out an intramolecular PKR with 16, either by treatment with NMO at room temperature or by heating at reflux in toluene, were unsuccessful. Pauson-Khand Reaction of 16 with Norbornadiene. Norbornadiene (0.38 mL, 3.76 mmol) was added under nitrogen to a solution of 16 (0.20 g, 0.38 mmol) in dry toluene and heated at reflux for 2 h. The solvent was removed, and the crude material was purified by chromatography on silica gel column using cyclohexane/ethyl acetate (80:20) as eluent to give 19 (0.13 g, 0.35 mmol, 94%) as a colorless oil, which was recrystallized from pentane/chloroform. 1H NMR (300 MHz, CDCl3): δ 8.12 (d, 1H, 3JHH = 8.0 Hz, H4 or H6), 7.94 (d, 1H, 3JHH = 8.0 Hz, H4 or H6), 7.42 (dt, 1H, 3JHH = 7.6 Hz, 4JHH = 1.45, H3 or H7), 7.32 (dt, 1H, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, H3 or H7), 7.31-7.27 (m, 2H, H1, H9), 7.26-7.24 (m, 1H, H2 or H8), 7.18 (dt, 1H, H2 or H8), 7.04 (s, 1H, H10 or H11), 7.03 (s, 1H, H10 or H11), 6.80 (d, 1H, 3JHH = 2.2 Hz, H21), 6.15 (dd, 1H, 3JHH = 5.4 Hz, 3JHH = 3.0 Hz, H18), 6.08 (dd, 1H, 3JHH = 5.6 Hz, 3JHH = 2.9 Hz, H17), 5.83 (s, 1H, OH), 3.28 (d, 1H, 2JHH = 14.7 Hz, H12), 2.72 (s, 1H, H16), 2.60 (d, 1H, 2JHH=14.7 Hz, H12), 2.45 (s, 1H, H20), 2.32 (s, 1H, H19), 2.15 (d, 1H, 3JHH = 4.9 Hz, H15), 0.97 (d, 1H, 2JHH = 9.6 Hz, H22a), 0.10 (d, 1H, 2JHH = 9.6 Hz, H22b). 13C NMR (75 MHz, CDCl3): δ 213.4 (C14), 165.0 (C21), 147.0 (C13), 143.0 (C4a or C5a), 141.7 (C4a or C5a), 138.3 (C18), 136.7 (C17), 132.6 (C9a or C11a), 132.2 (C9a or C11a), 131.9 (C10 or C11), 131.5 (C10 or C11), 129.2 (C1 or C9), 128.8 (C2 or C8), 128.7 (C3 or C7), 128.6 (C3 or C7), 126.5 (C1 or C9), 126.4 (C2 or C8), 125.3 (C4 or C6), 124.9 (C4 or C6), 76.0 (C5), 52.4 (C15), 48.2 (C20), 43.4 (C16), 42.4 (C19), 40.7 (C22), 34.0 (C12). IR (CHCl3): 3353 (OH), 1677 (CO) cm-1. A sample suitable for X-ray crystallography was obtained by recrystallization from pentane/chloroform. However, although the atom connectivity was readily established, the crystal quality precludes the reporting of precise bond lengths and angles. Synthesis of 5-[3-(1-Trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene (20). n-BuLi (2.85 mL, 4.55 mmol) was added dropwise to a solution of 15 (0.56 g, 2.27 mmol) in dry THF (60 mL) at -78 °C and stirred for 1 h. A solution of freshly distilled chlorotrimethylsilylane (1.15 mL, 6.82 mmol) in THF (10 mL) was added slowly, and the reaction was allowed to warm to room temperature over a period of 3 h. The solution was quenched with distilled water (30 mL) and extracted with diethyl ether several times, the organic layers were combined, washed with distilled water and brine, dried over MgSO4, and filtered, and the solvent was removed. The crude material was purified by chromatography on silica gel using pentane/dichloromethane (80:20) as eluent to yield 5-[3-(1-trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene, 20 (0.59 g, 1.51 mmol, 67%), as a colorless oil. A sample, mp 68-69 °C, suitable for an X-ray crystal structure determination was obtained by recrystallization from pentane/ dichloromethane. 1H NMR (500 MHz, CDCl3): δ 7.88 (d, 2H, 3 JHH = 7.7 Hz, H4, H6), 7.41 (dt, 2H, 3JHH = 7.6 Hz, 4JHH = 1.3, H3, H7), 7.34 (d, 2H, 3JHH = 7.6 Hz, H1, H9), 7.26 (dt, 2H, 3 JHH = 7.4 Hz, 4JHH = 1.2 Hz, H2, H8), 7.05 (s, 2H, H10, H11), 3.02 (s, 2H, H12), 0.47 (s, 9H, OTMS), -0.08 (s, 9H, TMS). 13C

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NMR (125 MHz, CDCl3): δ 142.7 (C4a, C5a), 132.1 (C9a, C11a), 132.0 (C10, C11), 129.1 (C2, C8), 128.2 (C3, C7), 126.4 (C1, C9), 125.2 (C4, C6), 104.1 (C13), 88.9 (C14), 81.6 (C5), 27.7 (C12), 3.0 (OTMS), -0.3 (TMS). IR (CH2Cl2): 2176 cm-1 (CtC). EI-MS: m/z (%) 390 (2) [Mþ], 279.4 (100). Anal. Calcd for C24H30O1Si2: C, 73.79; H, 7.74. Found: C, 73.57; H, 7.58. 5-[3-(1-Trimethylsilylpropynyl)]-5H-dibenzo[a,d]cyclohepten5-ol (21). Yield: 100 mg, 0.31 mmol, 14%, isolated as a colorless oil during purification. 1H NMR (300 MHz, CDCl3): δ 7.96 (d, 2H, 3JHH = 7.88 Hz, H4, H6), 7.46-7.25 (m, 6H, H1, H2, H3, H7, H8, H9), 7.01 (s, 2H, H10, H11), 3.42 (s, 1H, OH), 2.92 (s, 2H, H12), -0.01 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 141.0 (C4a, C5a), 132.2 (C9a, C11a), 131.5 (C10, C11), 129.4 (C1, C9 or C2, C8 or C3, C7), 128.6 (C1, C9 or C2, C8 or C3, C7), 126.9 (C1, C9 or C2, C8 or C3, C7), 124.5 (C4, C6), 102.3 (C5 or C13 or C14), 89.6 (C5 or C13 or C14), 74.8 (C5 or C13 or C14), 28.9 (C12), -0.2 (TMS). 5-(3-Propynyl)-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene (22). Yield: 100 mg, 0.31 mmol, 14%, isolated as a colorless oil during purification. 1H NMR (300 MHz, CDCl3): δ 7.98 (d, 2H, 3JHH = 7.13 Hz, H4, H6), 7.52 (t, 2H, 3JHH = 7.49 Hz, H3, H7), 7.43 (d,2H, 3JHH = 7.24 Hz, H1, H9), 7.38 (t, 2H, 3JHH = 7.30 Hz, H2, H8), 7.15 (s, H, H10, H11), 3.12 (s, 2H, H12), 1.85 (s, 1H, H14), 0.56 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 142.1 (C4a, C5a), 132.2 (C9a, C11a), 131.6 (C10, C11), 129.2 (C2, C8), 128.4 (C3, C7), 126.5 (C1, C9), 125.1 (C4, C6), 81.2 (C5 or C13), 81.4 (C5 or C13), 72.3 (C14), 25.9 (C12), 3.0 (OTMS). Synthesis of {5-[3-(1-Trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene}hexacarbonyldicobalt (23). A solution of Co2(CO)8 (0.47 g, 1.38 mmol) in dry THF (10 mL) was added slowly at room temperature under nitrogen to a solution of 18 (0.54 g, 1.38 mmol) in dry THF (25 mL). After stirring for 18 h, the solvent was removed and the crude material was purified by chromatography on silica gel using pentane as eluent to give {5-[3-(1-trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene}hexacarbonyldicobalt (0.42 g, 0.62 mmol, 45%) as a deep red solid. 1H NMR (500 MHz, C6D6): δ 8.17 (m, 2H, H4, H6), 7.51-7.05 (m, 6H, H1, H2, H3, H7, H8, H9), 6.80 (s, 2H, H10, H11), 4.08 (s, 2H, H12), 0.50 (s, 9H, OTMS), 0.67 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 200.7 (CO), 200.1 (CO), 142.6 (C4a, C5a), 137.3 (C9a, C11a), 132.0 (C10, C11), 128.9 (C1, C2, C3, C7, C8, C9), 128.7 (C1, C2, C3, C7, C8, C9), 127.3 (C4, C6), 105.1 (C5), 85.6 (C13), 82.7 (C14), 46.2 (C12), 3.8 (OTMS), 0.7 (TMS). IR (THF): ν(CO) 2083, 2044, 2015 cm-1. Synthesis of {5-[3-(1-Trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene}hexacarbonyldicobalt (24). A solution of Co2(CO)8 (0.47 g, 1.38 mmol) in dry THF (10 mL) was added slowly at room temperature under nitrogen to a solution of 19 (0.54 g, 1.38 mmol) in dry THF (25 mL). After stirring for 18 h, the solvent was removed and the crude material was purified by chromatography on silica gel using pentane as eluent to give {5-[3-(1-trimethylsilylpropynyl)]-5-trimethylsiloxy-5H-dibenzo[a,d]cycloheptene}hexacarbonyldicobalt, 24 (0.42 g, 0.62 mmol, 45%), as a deep red solid. 1H NMR (500 MHz, C6D6): δ 8.17 (m, 2H, H4, H6), 7.51-7.05 (m, 6H, H1, H2, H3, H7, H8, H9), 6.80 (s, 2H, H10, H11), 4.08 (s, 2H, H12), 0.50 (s, 9H, OTMS), 0.67 (s, 9H, TMS). 13C NMR (125 MHz, CDCl3): δ 200.7 (CO), 200.1 (CO), 142.6 (C4a, C5a), 137.3 (C9a, C11a), 132.0 (C10, C11), 128.9 (C1, C2, C3, C7, C8, C9), 128.7 (C1, C2, C3, C7, C8, C9), 127.3 (C4, C6), 105.1 (C5), 85.6 (C13), 82.7 (C14), 46.2 (C12), 3.8 (OTMS), 0.7 (TMS). IR (THF): ν(CO) 2083, 2044, 2015 cm-1. Kinetic Measurements. A sample of the trimethylsilylethynyldicobalt hexacarbonyl complex, 11b, dissolved in CDCl3, was monitored by NMR spectroscopy at regular intervals (every minute for the first hour and every 5 min thereafter) for 12 h. The decreasing intensity of the singlet attributable to H(10,11) at δ 7.01 was evident, as was the increase in the total intensity of the corresponding pair of doublets at δ 5.34 and 5.19 in 12b. After

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Table 3. Crystallographic Data for 11a, 12a, 11b, 11e, 11f, and 12b 11a empirical formula fw cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg volume, A˚3 Z density (calc; g cm-3) temp, K abs coeff, mm-1 F(000) θ range, deg index ranges

C29H16O7Co2 594.28 triclinic P1 (#2) 10.5878(11) 13.3942(13) 18.1754(18) 74.179(2) 81.926(2) 89.863(2) 2453.6(4) 4 1.609 100(2) 1.401 1200 1.58 to 28.00 -13 e h e 13 -17 e k e 17 -23 e l e 24 reflns measd 23 995 11 701 (0.0231) reflns used (Rint) parameters 813 final R values I > 2σ(I)]: R1, wR2 0.0396, 0.1083 R values (all data): R1, wR2 0.0474, 0.1142 1.035 goodness-of-fit on F2 -3 1.820, -0.331 largest diff peak and hole, e A˚ a

12a

11b

11e

11f

12b

C28H16O6Co2 566.27 monoclinic Cc (#9) 15.4109(17) 13.1859(14) 12.1179(13) 90 109.525(2) 90 2320.8(4) 4 1.621 100(2) 1.473 1144 2.09 to 30.00 -21 e h e 21 -18 e k e 18 -17 e l e 16 12 130 6423 (0.0290) 326 0.0547, 0.1496 0.0629, 0.1915 1.163 1.035, -0.964

C26H20O7SiCo2 590.37 monoclinic P21/c (#14) 15.6083(15) 9.8377(10) 17.1024(16) 90 108.570(2) 90 2489.3(4) 4 1.575 100(2) 1.425 1200 2.42 to 30.00 -21 e h e 21 -13 e k e 13 -23 e l e 24 27 084 7227 (0.0302) 369 0.0306, 0.0778 0.0359, 0.0802 1.043 0.787, -0.435

C30H15O7F3Co2 662.28 monoclinic P21/n (#14) 9.8271(10) 20.906(2) 13.1398(13) 90 99.147(2) 90 2665.2(5) 4 1.651 100(2) 1.314 1328 1.85 to 26.99 -12 e h e 12 -26 e k e 26 -16 e l e 16 48 743 5818 (0.0362) 439 0.0285, 0.0724 0.0310, 0.0740 1.053 0.612, -0.353

(C23H12O7Co2)4 3 CH2Cl2 2157.67 triclinic P1 (#2) 9.8943(7) 11.1286(8) 20.5053(15) 75.448(1) 87.776(1) 77.788(1) 2135.7(3) 1 1.678 100(2) 1.660 1082 1.93 to 28.29 -13 e h e 13 -14 e k e 14 -26 e l e 27 21 896 10 496 (0.0208) 614 0.0361, 0.0850 0.0478, 0.0910 1.022 0.778, -0.560

C25H20O6SiCo2 562.36 triclinica P1 (#2) 9.8221(15) 9.2641(14) 26.205(4) 90.136(2) 96.267(2) 90.261(2 2370.2(6) 4 1.576 100(2) 1.489 1144 0.78 to 26.00 -12 e h e 12 -11 e k e 11 -32 e l e 32 20 335 9270 (0.0263) 768b 0.0406, 0.0945 0.0535, 0.1003 1.064 0.668, -0.545

The unit cell is given in the pseudo-monoclinic setting. b The SAME instruction was used to fit the minor disorder to the shape of the major one.

Table 4. Crystallographic Data for 12c, 12d, 15, 16, and 20

empirical formula fw cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg volume, A˚3 Z density (calc; g 3 cm-3) temp, K abs coeff, mm-1 F(000) θ range, deg index ranges reflns measd reflns used (Rint) parameters final R values I > 2σ(I)]: R1, wR2 R values (all data): R1, wR2 goodness-of-fit on F2 largest diff peak and hole, e A˚-3 a

12c

12d

15

16

20

C29H15 NO6 Co2 3 CHCl3 710.65 monoclinic P21/c (#14) 10.7596(9) 12.5615(11) 20.7266(17) 90 93.813(2) 90 2795.1(4) 4 1.689 100(2) 1.520 1424 1.90 to 28.33 -14 e h e 14 -16 e k e 16 -27 e l e 27 27 930 6935 (0.0284) 443 0.0357, 0.0910 0.0410, 0.0937 1.075 1.010, -0.369

C28H15O6FCo2 584.26 triclinic P1 (#2) 7.8288(3) 8.3735(3) 17.7715(7) 84.900(1) 88.034(1) 79.116(1) 1139.35(7) 2 1.703 100(2) 1.509 588 2.30 to 35.84 -12 e h e 12 -13 e k e 13 -29 e l e 28 56 397 10 241 (0.0205) 400 0.0239, 0.0657 0.0253, 0.0664 1.061 0.685, -0.481

C18H14O 246.29 monoclinic P21/c (#14) 11.5912(14) 13.8714(16) 8.0454(9) 90 101.579(2) 90 1267.3(3) 4 1.291 100(2) 0.078 520 1.79 to 32.02 -17 e h e 17 -20 e k e 20 -11 e l e 11 30 199 4213 (0.0297) 228 0.0486, 0.1286 0.0534, 0.1321 1.041 0.589, -0.183

C24H14O7Co2 532.21 triclinic P1 (#2) 10.6121(15) 14.552(2) 15.424(2) 114.073(2) 90.156(3) 90.025(3) 2174.7(5) 4 1.625 100(2) 1.569 1072 1.45 to 31.83 -15 e h e 15 -21 e k e 21 -22 e l e 22 24 719 13 277 (0.0427) 606 0.0530, 0.1346 0.0584, 0.1371 1.188 0.959, -0.477

C24H30OSi2 390.66 triclinic P1 (#2) 8.3253(7) 15.4506(14) 18.9537(17) 74.760(2) 89.845(2 81.079(2) 2322.0(4) 4 1.118 100(2) 0.163 840 1.38 to 30.45 -11 e h e 11 -21 e k e 21 -26 e l e 26 27 101 13 645 (0.0205) 716a 0.0463, 0.1168 0.0547, 0.1225 1.032 0.517, -0.199

The two disorder parts were restrained to have the same shape using the SAME command.

12 h, no more changes were apparent, and so these values were used to evaluate the equilibrium constant at the given temperature. For an equilibrium situation, A a B, a plot of ln([A]t [A]eq) versus time yields a straight line graph with a negative slope equal to the sum of the forward and reverse reaction rate constants (kf þ kr).21 Using this value and the ratio kf /kr derived from the measured equilibrium constant, Keq, these two rate constants can be calculated. The process was subsequently repeated over a range of temperatures to allow evaluation of the activation energies for the process.

Matrix IR Spectroscopy. The matrix isolation apparatus consists of a closed-cycle helium refrigerator, sample window, shroud, deposition tube, gas mixing chamber, gas inlet, backing pump, diffusion pump, and temperature control unit. Matrixes were deposited on a CaF2 window cooled to 20 K, with matching outer windows on the vacuum shroud. A CS202 closed-cycle refrigerator was used to cool the sample plate to 20 K, and its temperature was maintained using a Lakeshore 330 autotuning temperature controller. Two-stage backing pumps and an oil diffusion pump fitted with a liquid nitrogen trap reduce the

Article shroud pressure to 8  10-4 Torr prior to cooling and achieve better than 10-6 Torr when the sample window reaches 20 K. Host gases (Cryo Service) are deposited from the gas mixing chamber via a needle valve connected to two inlet jets positioned on either side of the deposition tube. A ratio of sample to host matrix in the region 1:2000 was achieved. Typically the rate of gas deposition of 0.3-0.6 Torr/min provides an appropriate sample-host dilution. Both the sample and isolating gas were deposited simultaneously. The alkyne dicobalt hexacarbonyl, 11f, was sublimed from a right-angled tube that was electrically heated to 353 K. The sample deposition was monitored using infrared spectroscopy and was stopped when the absorbance of one of the metal carbonyl bands reached approximately 1 AU. A Perkin-Elmer Spectrum One FTIR spectrophotometer was used to record IR spectra at 1 cm-1 resolution from 16 interferograms. Matrixes were photolyzed using a 200 W Xe/Hg arc lamp in combination with a 10 cm water filter to remove the IR component and appropriate interference filters to select the desired Hg emission line. X-ray Crystallography. X-ray crystallographic data were collected using a Bruker SMART APEX CCD area detector diffractometer and are listed in Tables 3 and 4. A full sphere of reciprocal space was scanned by phi-omega scans. Pseudoempirical absorption correction based on redundant reflections was performed by the program SADABS.24 The structures were solved by direct methods using SHELXS-9725 and refined by full matrix least-squares on F2 for all data using SHELXL-97.26 Anisotropic displacement parameters were refined for all non(24) Sheldrick, G. M. SADABS; Bruker AXS Inc.: Madison, WI, 2000. (25) Sheldrick, G. M. SHELXS-97; University of G€ottingen, 1997. (26) Sheldrick, G. M. SHELXL-97-2; University of G€ottingen, 1997. (27) Cell parameters for 19: monoclinic, space group: Cc (#9), a = 6.682(1) A˚, b=17.857(2) A˚, c=31.470(4) A˚, β=94.873(2)°, V=3742(1) 3 A˚ , Z=8, Dc=1.301 g cm-1.

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hydrogen atoms. Hydrogen atom treatment varied from compound to compound, depending on the crystal quality. All hydrogen atoms in 11a, 11e, 12c, and 15, alkyne hydrogens in 11f and 16, and ordered hydrogens in 12d and 18 were located in the difference Fourier map and allowed to refine freely with isotropic thermal displacement parameters. In 18 C-H bond lengths were restrained to be their default values (0.99 A˚ for CH2 groups, 0.98 A˚ for methyl groups, 0.95 A˚ for aromatic and alkene hydrogens) using DFIX. All other hydrogen atoms were added at calculated positions and refined using a riding model. Their isotropic thermal displacement parameters were fixed to 1.2 times (1.5 times for methyl and OH groups) the equivalent isotropic displacement parameters of the atom to which they are attached. The crystals of 1927 were of insufficient quality to yield a publishable structure. The connectivity of non-hydrogen atoms could be established without a doubt, but high R values led to uncertainties in bond lengths and angles, and hydrogen atom positions could not be reliably located. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications nos. CCDC 629903 (11a), CCDC 735735 (11b), CCDC 735738 (11e), CCDC 735737 (11f), CCDC 629902 (12a), CCDC 735736 (12b), CCDC 735740 (12c), CCDC 735739 (12d), CCDC 735742 (15), CCDC 735741 (16), and CCDC 735743 (18).

Acknowledgment. We thank Science Foundation Ireland and University College Dublin for generous financial support. S.D. thanks the University of W€ urzburg for an Erasmus exchange fellowship. Supporting Information Available: Kinetic plots for the conversion of 11b to 12b. This information is available free of charge via the Internet at http://pubs.acs.org.