Cobalt-Complexed Propargyl Cations: Generation under Neutral

Cobalt-Complexed Propargyl Cations: Generation under. Neutral Conditions and Spontaneous, High-Temperature. Conversion to Propargyl Radicals. Gagik G...
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Organometallics 2009, 28, 5541–5549 DOI: 10.1021/om900448j

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Cobalt-Complexed Propargyl Cations: Generation under Neutral Conditions and Spontaneous, High-Temperature Conversion to Propargyl Radicals Gagik G. Melikyan,* Ruth Sepanian, Ryan Spencer, Aaron Rowe, and Pogban Toure Department of Chemistry and Biochemistry, California State University Northridge, Northridge, California 91330-8262 Received May 26, 2009

A novel method for the generation of Co2(CO)6-complexed propargyl cations under neutral conditions is developed. The optimized experimental protocol involves treatment of the respective Co2(CO)6-complexed propargyl methyl ethers 10 and 16-19 with equimolar quantities of triflic anhydride (2) or trifluoroacetic anhydride (13) at 83 °C for 3-6 min. The conversion to propargyl triflates, such as 3, occurs in two steps, via the oxonium-triflate ionic pair 11. The transition of diamagnetic propargyl triflates to the respective propargyl radicals allegedly takes place via a clusterto-cluster reduction, followed by a cluster-to-ligand single-electron transfer. Radical dimeric products, polysubstituted 3,4-diaryl-1,5-alkadiynes 6 and 20-23, are formed in high yields (73-82%) and excellent d,l-diastereoselectivity (89-99%). Decomplexation with ceric ammonium nitrate affords topologically diverse d,l-3,4-diaryl-1,5-alkadiynes 26-30 in good to high yields (54-90%). The generation of Co2(CO)6-complexed propargyl cations under neutral conditions substantially expands the scope of both ionic and radical reactions, allowing for involvement of substrates with acid-sensitive peripheral functionalities.

Introduction Transition metals are well known for their ability to stabilize π-bonded organic carbocations located alpha to the metal core.1 Contrary to their organic counterparts, metal-coordinated carbocations can be readily isolated, stored, and spectrally characterized,1,2 even by means of X-ray crystallography.1,3 Over the last several decades, their development has substantially enriched synthetic chemistry by providing for novel approaches to a wide array of other-

wise hardly accessible organic molecules, including those of natural origin and medicinal relevance.1,4 The conventional protocol for generating carbocations involves treatment of the respective alcohols, ethers, esters, epoxides, aldehydes, acetals, or halides with Lewis or Bronsted acids, such as HBF4, BF3, CF3COOH, H2SO4, HBr, TiCl4, Bu2BOTf, MeAl(OR)2, Me2AlCN, TfOH, TsOH, AgBF4, and FSO3H. 1,4,5 While efficient with chemically robust substrates, the use of strong acids, usually in an excess, imposes severe limits on the substrate base. The existing protocols could hardly apply to the compounds bearing the acid-sensitive-benzyloxy,

*Corresponding author. E-mail: [email protected]. (1) (a) Melikyan, G. G.; Nicholas, K. M. In Modern Acetylene Chemistry; Stang P. J., Diederich F., Ed.; VCH Publishers: Weinheim, 1995; Chapter 4. (b) McGlinchey, M. J.; Girard, L.; Ruffolo, R. Coord. Chem. Rev. 1995, 143, 331. (c) Amouri, H. E.; Gruselle, M. Chem. Rev. 1996, 96, 1077. (d) Went, M. Adv. Organomet. Chem. 1997, 41, 69. (e) Green, J. R. Curr. Org. Chem. 2001, 5, 809. (f) Muller, T. J. J. Eur. J. Org. Chem. 2001, 2021. (g) Teobald, B. J. Tetrahedron 2002, 58, 4133. (h) Omae, I. Appl. Organomet. Chem. 2007, 21, 318. (2) 1H/13C NMR data (representative examples): (a) Padmanabhan, S.; Nicholas, K. M. J. Organomet. Chem. 1983, 268, C23. (b) Bradley, D. H.; Khan, M. A.; Nicholas, K. M. Organometallics 1992, 11, 2598. (c) Meyer, A.; McCabe, D. J.; Curtis, M. D. Organometallics 1987, 6, 1491. (d) El-Amouri, H.; Vaissermann, J.; Besace, Y.; Vollhardt, K. P. C.; Ball, G. E. Organometallics 1993, 12, 605. (e) Ortin, Y.; Ahrenstrof, K.; O'Donohue, P.; Foede, D.; Muller-Bunz, H.; McArdle, P.; Manning, A. R.; McGlinchey, M. J. J. Organomet. Chem. 2004, 689, 1657. (3) X-ray crystal structures (representative examples): (a) Lupan, S.; Kapon, M.; Cais, M.; Herbstein, F. H. Angew. Chem., Int. Ed. 1972, 11, 1025. (b) Barinov, I. V.; Reutov, O. A.; Sokolov, V. I. Zh. Org. Chem. 1986, 22, 2457. (c) Osella, D.; Dutto, G.; Jaouen, G.; Vessieres, A.; Raithby, P. R.; De Benedetto, L.; McGlinchey, M. J. Organometallics 1993, 12, 4545. (d) Melikyan, G. G.; Bright, S.; Monroe, T.; Hardcastle, K. M.; Ciurash, J. Angew. Chem., Int. Ed. 1998, 37, 161. (e) Maas, G.; Rahm, R.; Mayer, D.; Baumann, W. Organometallics 1995, 14, 1061.

(4) (a) Closser, K. D.; Quintal, M. M.; Shea, K. M. J. Org. Chem. 2009, 74, 3680. (b) Perez-Castells, J. Top. Organomet. Chem. 2006, 19, 207. (c) Palazon, J. M.; Martin, V. S. Tetrahedron Lett. 1995, 36, 3549. (d) Magnus, P.; Fortt, S. M. J. Chem. Soc., Chem. Commun. 1991, 544. (e) Mukai, C.; Sugimoto, Y. I.; Ikeda, Y.; Hanaoka, M. J. Chem. Soc., Chem. Commun. 1994, 1161. (f) Magnus, P. In Organometallic Reagents in Organic Synthesis; Bateson, J. H., Mitchell, M. B., Eds.; Academic Press: London, 1994; p 1. (5) Representative examples: HBF4: (a) Varghese, V.; Saha, M.; Nicholas, K. M. Org. Synth. 1989, 67, 141. BF3: (b) Saha, M.; Nicholas, K. M. J. Org. Chem. 1984, 49, 417. CF3COOH: (c) Nicholas, K. M.; Siegel, J. J. Am. Chem. Soc. 1985, 107, 4999. H2SO4: (d) Top, S.; Jaouen, G. J. Org. Chem. 1981, 46, 78. HBr: (e) Descoins, C.; Samain, D. Tetrahedron Lett. 1976, 745. TiCl4: (f) Magnus, P.; Pitterna, T. J. Chem. Soc., Chem. Commun. 1991, 541. Bu2BOTf: (g) Schreiber, S. L.; Klimas, M. T.; Sammakia, T. J. Am. Chem. Soc. 1987, 109, 5749. MeAl(OR)2: (h) Nakamura, T.; Matsui, T.; Tanino, K.; Kuwajima, I. J. Org. Chem. 1997, 62, 3032. Me2AlCN: (i) Stuart, J. G.; Nicholas, K. M. Synthesis 1989, 454. TfOH: (j) Tanaka, S.; Isobe, M. Tetrahedron 1994, 50, 5633. TsOH: (k) Hosokawa, S.; Isobe, M. J. Org. Chem. 1999, 64, 37. AgBF4: (l) Vizniowski, G. S.; Green, J. R.; Breen, T. L.; Dalacu, A. V. J. Org. Chem. 1995, 60, 7496. FSO3H: (m) Reutov, O. A.; Barinov, I. V.; Chertkov, V. A.; Sokolov, V. I. J. Organomet. Chem. 1985, 297, C25.

r 2009 American Chemical Society

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Scheme 1. Generation of the Propargyl Triflate 3 under Acidic Conditions: Method A

Scheme 2

acetal, 1,3-dioxolane, enol ether-moieties and also the functional elements susceptible to protonation, such as carbonyl, cyano, amino, or imino groups. Thus, development of the novel method for cation generation under neutral conditions would drastically expand the substrate base and accommodate functionalities that would either be removed under acidic conditions or become protonated, and altered, structurally. It would also enhance the significance of the metal-complexed substrates, as key intermediates in the total syntheses of complex molecular assemblies when compatibility of the reagents with peripheral functionalities becomes pivotal. Herewith we report on the novel method for generation of cobalt-complexed propargyl cations under neutral conditions and their spontaneous intermolecular dimerization reactions. It stems from our long-standing interest in developing novel methodologies in the field of cobalt-complexed propargyl cations and radicals.6 Of immediate relevance are the triflic acid-induced spontaneous conversion of cations to radicals at ambient temperatures6f and high temperature (up to 147 °C) and rapid dimerization of preisolated requisite cations.6j

Results and Discussion Spontaneous generation of cobalt-complexed propargyl radicals occurs at ambient temperature when respective alcohols are treated with a 2-fold excess of triflic anhydride.6f The most attractive feature of this reaction is its ability to form propargyl triflates in situ, rendering the laborious isolation of cations unnecessary. Among the drawbacks are a low reaction rate (23 h)6f and formation of triflic acid, a (6) (a) Melikyan, G. G.; Vostrowsky, O.; Bauer, W.; Bestmann, H. J. J. Organomet. Chem. 1992, 423, C24. (b) Melikyan, G. G.; Combs, R. C.; Lamirand, J.; Khan, M.; Nicholas, K. M. Tetrahedron Lett. 1994, 363. (c) Melikyan, G. G.; Deravakian, A. J. Organomet. Chem. 1997, 544, 143. (d) Melikyan, G. G.; Deravakian, A.; Myer, S.; Yadegar, S.; Hardcastle, K. I.; Ciurash, J.; Toure, P. J. Organomet. Chem. 1999, 578, 68. (e) Melikyan, G. G.; Sepanian, S.; Riahi, B.; Villena, F.; Jerome, J.; Ahrens, B.; McClain, R.; Matchett, J.; Scanlon, S.; Abrenica, E.; Paulsen, K.; Hardcastle, K. I. J. Organomet. Chem. 2003, 683, 324. (f) Melikyan, G. G.; Villena, F.; Sepanian, S.; Pulido, M.; Sarkissian, H.; Florut, A. Org. Lett. 2003, 5, 3395. (g) 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. Organometallics 2006, 25, 4680. (h) Melikyan, G. G.; Floruti, A.; Devletyan, L.; Toure, P.; Dean, N.; Carlson, L. Organometallics 2007, 26, 3173. (i) Melikyan, G. G.; Wild, C.; Toure, P. Organometallics 2008, 27, 1569. (j) Melikyan, G. G.; Mikailian, B.; Sepanian, R.; Toure, P. J. Organomet. Chem. 2009, 694, 785.

by-product. The latter, being a superacid, creates a highly acidic environment and makes the whole process inapplicable for substrates with acid-sensitive functionalities. In the current study, the main objectives were to make the reaction faster by carrying it out at elevated temperatures and also to avoid an acidic medium altogether in order to enhance the synthetic potential of the parent reaction and to expand its substrate base. Borrowing the lessons from the recently reported hightemperature reaction of preisolated propargyl cations,6j cobalt-complexed propargyl alcohol 1 was treated with a 2-fold excess of Tf2O (2) and heated in 1,1,2,2-tetrachloroethane at 147 °C for 1 min (Scheme 1). An initial stage involves an in situ generation of propargyl triflate 3 and triflic acid (4). Given its ionic nature,7 the former is analogous to the cobalt-complexed propargyl tetrafluoroborate salts6j and undergoes spontaneous conversion to the radical 5, which, in turn, dimerizes to bis-cluster 6 (Scheme 2). While the d,l-diastereoselectivity was excellent (d,l:meso, 94:6), the isolated yield of bis-cluster 6 was only 21.0% (Table 1; entry 1). To establish if the presence of an excess of Tf2O (2) could be detrimental to the reaction outcome, only 1 equiv of the reagent was applied (Table 1, entry 2). Careful monitoring of the reaction, by NMR, revealed some retardation (6 min vs 1 min), accompanied by a significant decline in stereoselectivity of the radical coupling (d,l-6:meso-6, 86:14). Meanwhile, the yield increased noticeably, from 21.0% to 30.2%, clearly indicating that an excess of reagent affects the amount of product surviving a high-temperature treatment, albeit relatively brief. Further attempts to lower the reaction temperature (83 °C; 1,2-dichloroethane), to employ lesser quantities of the reagent, or to trap triflic acid in situ with triethylamine did not improve either the reaction yield or its diastereoselectivity. To minimize the impact of TfOH-induced acidity upon the main characteristics of the radical coupling reaction;yield, d,l-diastereoselectivity;triflic anhydride was replaced with trimethylsilyl triflate (7). The main difference between Tf2O (2) and Me3SiOTf (7) is the nature of the electropositive þ component, CF3SOþ 2 vs Me3Si . Combining the latter with a hydroxy group derives TfOH (4) and Me3SiOH (9), the (7) Throughout the paper, propargyl triflate 3 and its analogues are depicted as ionic pairs on the basis of the low-temperature NMR studies that will be reported in detail in the forthcoming account.

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Table 1. Spontaneous, High-Temperature Radical Coupling of Cobalt-Complexed Propargyl Triflates experimental 1 2 3 4

method A method A method B method C

reactant composition

medium

alcohol 1 þ 2 equiv Tf2O alcohol 1 þ 1 equiv Tf2O alcohol 1 þ 1 equiv Me3SiOTf Me ether 10 þ 1 equiv Tf2O

acidic (TfOHb) acidic (TfOHb) acidic (TfOHb/Me3SiOHb) neutral

T, °C

reaction time, min

crude 6 d,l:meso

yield,a %

isolated 6 d,l:meso

147 147 83 83

1 6 3 3

94:6 86:14 92:8 94:6

21.0 30.2 81.0 82.0

96:4 84:16 94:6 95:5

a The yields are calculated on the basis of the reaction stoichiometry that requires 2 equiv of propargyl cations to form an equivalent of respective radicals. b Compound determining the acidity of the reaction medium.

Scheme 3. Generation of the Propargyl Triflate 3 under Acidic Conditions: Method B

Scheme 4. Generation of the Propargyl Triflate 3 under Neutral Conditions: Method C

species drastically differing in their acidities (pKa -15 vs 11,8 respectively). By being less acidic, by some 26 orders of magnitude, Me3SiOH (9) could provide a more benign environment for radical reaction and its products, thus improving the yield and stereochemical outcome. The interaction of propargyl alcohol 1 with Me3SiOTf (7) affords silyl ether 8, along with triflic acid (4) (Scheme 3). The latter, analogous to HBF4, protonates a trimethylsiloxy group, giving rise to propargyl triflate 3 and Me3SiOH (9). The optimization of the experimental protocol;solvent, temperature, reaction time;allowed us to identify conditions far superior to that in method A (Table 1, entry 3). When carried out in 1,2-dichloroethane, at 83 °C, with equimolar quantities of Me3SiOTf (7), the reaction came to completion in 3 min, forming dimers 6 in high yield (81.0%) and enhanced d,l-diastereoselectivity (d,l:meso, 92:8). A significant increase in yield (81.0% vs 30.2%) serves as a (8) (a) Kagiya, T.; Sumida, Y.; Tachi, T. Bull. Chem. Soc. Jpn. 1970, 43, 3716. (b) Howells, R. D.; McCown, J. D. Chem. Rev. 1977, 77, 69.

clear indication that the presence of superacid, as a side product, was, in fact, detrimental to the radical products and their isolability (Table 1, method A). The transient presence of TfOH (4) generated in situ and also a relatively high acidity (pKa 11)8 of Me3SiOH (9), both “co-existing” with reactive intermediates and products, might still jeopardize the declared objective of fully accommodating the acid-sensitive functional groups. An ideal case scenario would have been the generation of propargyl triflate 3 under truly neutral conditions, when the strong acids are not used, or generated, in the course of the reaction, either as final artifacts or intermediate products. This objective was achieved by replacing propargyl alcohol 1 with Me-ether 10 (Scheme 4). Its treatment with an equimolar amount of Tf2O (2), at 83 °C, brought the reaction to fruition in 3 min, providing for high yield (82.0%) and an excellent level of d,l-diastereoselection (d,l-6:meso-6, 94:6) (Table 1, entry 4). The mechanism of the process involves a nucleophilic attack upon electropositive sulfur in Tf2O (2), affording an ionic pair 11 (Scheme 4). An oxonium

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Melikyan et al. Scheme 5

counterpart then receives a nucleophilic attack by CF3SO3 anion, generating the requisite propargyl triflate 3 and methyl triflate (12), a nonacidic molecule. Its formation was established by the careful monitoring of the reaction by lowtemperature NMR. It is also conceivable that the oxonium salt 11 could undergo heterolysis, releasing methyl triflate (12), as a leaving group. Given the chemical nature of the latter, and also that of intermediates formed in the course of the reaction, the method is considered to be an efficient generation of cobalt-complexed propargyl cations under neutral conditions (Scheme 4). Among its attractive features are (1) an in situ formation of propargyl cation, bypassing the laborious isolation step; (2) compatibility of the reaction, and its intermediates, with practically every acid-sensitive functional group; and (3) a high level of d,l-diastereoselection. Replacement of traditional propargyl alcohols with propargyl ethers, as the cation precursors, also expands the substrate base of the spontaneous radical coupling reactions. To expand the reagent base, and also to establish if the spontaneous radical reaction is unique to Tf2O (2), trifluoroacetic anhydride (13) was employed, as an alternative (Scheme 5). Analogous to the parent reaction, the conversion of Me-ether 10 to bis-complex 6 occurred at 83 °C, in 6 min, providing for a high yield (87.8%) and d,l-diastereoselectivity (d,l-6:meso-6, 84:16). Given the structural similarities between both acid anhydrides used, it is conceivable that the reaction mechanism involves the formation of cobalt-complexed propargyl trifluoroacetate 14 and methyl trifluoroacetate (15), a nonacidic artifact. A bulkier reagent, p-toluenesulfonic anhydride, did not react with Me-ether 10 at 83 °C and even at higher temperatures (147 °C; 1,1,2,2tetrachloroethane). The reason for this might be electronic and/or steric in nature. The positive charge on the S-atom in p-toluenesulfonic anhydride could be lower than that in Tf2O, retarding a nucleophilic attack by the methoxy group, a weak nucleophile. The ab initio calculation data revealed that the charge in question is quite similar in both compounds (Tf2O δS þ2.53; Ts2O δS þ2.63; Titan, 3-21G*), suggesting that the inertness of Ts2O is not derived from an unfavorable charge distribution. Perhaps, the observed disparity could be best accounted for by invoking a steric factor; that is, a phenyl group happens to be much larger than a CF3 group, by 60 A˚3 (PCModel). Given the bulkiness of a dinuclear metal core positioned in the immediate vicinity of a MeO group, a nucleophile, it is conceivable that even the

subtle steric differences in the receiving party, Tf2O vs Ts2O, could retard, and even inhibit, the reaction. It is worthy to mention that no correlation was observed between the stereoselectivity of coupling reaction and the size of the counterion. Thus, for anions arranged in the order of decreas˚3 ˚3 ing volume, CF3SO3 >CF3COO >BF4 (102.93 A , 85.77 A , 62.46 A˚3), the ratio of d,l-6:meso-6 fluctuated randomly, from 94:6, via 84:16, to 92:8,6j respectively. The scope of the reaction was expanded by varying the topology and functionality of methyl propargyl ethers 16-19 (Table 2). These were synthesized either under acidic conditions from the respective propargyl alcohols (HBF4/ methanol) or under nonacidic conditions, when the condensation product of sodium acetylide with the respective benzaldehyde9 was alkylated in situ with methyl iodide, followed by the complexation with Co2(CO)8.10 Introducing the MeO substituents in the topologically diverse positions of the aromatic ring (16-18) as well as an Et group in the γ-position of the acetylenic moiety (19) did not interfere with the course of the reaction, indicating, in particular, the compatibility of Tf2O (2) with the substituents having a lone pair(s) of electrons. Under standardized conditions (83 °C, 3 min), radical dimerization products 20-23 were formed in high yields (73-82%) and excellent diastereoselectivity (d,l89-99%; Table 2). The stereochemical assignments were made on the basis of the configuration of authentic samples, X-ray crystallography,6b,d,e,h and the NMR signature of methyne hydrogen atoms.6 Decomplexation of bis-complexes 24 was carried out with ceric ammonium nitrate (8-10 equiv)1a,g within the temperature range of -78 to -40 °C, releasing organic counterparts 25 (Scheme 6). The 3,4-diaryl-1,5-alkadiynes were obtained in good to high yields (54-90%) as pure d,l-diastereomers (26-29) or as a diastereomeric mixture (30). Cobalt-complexed propargyl triflates were previously synthesized under acidic conditions by the interaction of cyclic ethers with TfOH5j,11a and propargyl methyl ethers with Bu2BOTf.5g,11b Under neutral conditions, cobalt-complexed propargyl alcohols were converted to the respective triflates when treated with Tf2O; to neutralize triflic acid (9) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes; Elsevier: New York; 1981; p 80. (10) Greenfield, H.; Sternberg, H. W.; Friedel, R. A.; Wotiz, J. H.; Markby, R.; Wender, I. J. Am. Chem. Soc. 1956, 78, 120.

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Table 2. Tf2O-Mediated Conversion of Propargyl Methyl Ethers to d,l-1,5-Alkadiynes

a Chromatographically separable. b Chromatographically inseparable. c Only major d,l-diastereomers were decomplexed with ceric ammonium nitrate unless indicated otherwise. d The diastereomeric ratio of d,l-30:meso-30, 71:29, was obtained with zinc acting as a reducing agent (d,l-23:meso-23, 73:27).6h,i

Scheme 6

formed in the course of the reaction, 2,6-di-tert-butyl-4methylpyridine was utilized.4f,11c Presynthesized propargyl triflate can also be complexed with a Co core, if a donating (11) Co2(CO)6-complexed propargyl triflates: (a) Tanaka, S.; Isobe, M. Tetrahedron Lett. 1993, 34, 5757. (b) Jacobi, P. A.; DeSimone, R. W. Tetrahedron Lett. 1992, 33, 6239. (c) Magnus, P.; Eisenbeis, S. A.; Fairhurst, R. A.; Iliadis, T.; Magnus, N. A.; Parry, D. J. Am. Chem. Soc. 1997, 119, 5591. (d) Jamison, T. F.; Shambayati, S.; Crowe, W. E.; Schreiber, S. L. J. Am. Chem. Soc. 1994, 116, 5505. (e) Jamison, T. F.; Shambayati, S.; Crowe, W. E.; Schreiber, S. L. J. Am. Chem. Soc. 1997, 119, 4353. Co2(CO)6-complexed propargyl tosylates: (f) Saeeng, R.; Isobe, M. Tetrahedron Lett. 1999, 40, 1911. (g) Soleilhavoup, M.; Saccavini, C.; Lepetit, C.; Lavigne, G.; Maurette, L.; Donnadieu, B.; Chauvin, R. Organometallics 2002, 21, 871. Co2(CO)6-complexed propargyl mesylates: (h) Mukai, C.; Yamashita, H.; Ichinyu, T.; Hanaoka, M. Tetrahedron 2000, 56, 2203.

NR2 group is positioned alpha to the propargyl carbon.3e Topologically complex, fused cyclooctynes were synthesized, via R-alkoxy propargyl triflates, by the interaction of cobalt-complexed unsymmetrical acetals with trialkyl silyl triflates, such as Me3SiOTf, TBDMSiOTf, and Et3SiOTf.11d,e Propargyl triflates synthesized under acidic, or neutral, conditions undergo a slow heterolysis at ambient temperature, releasing respective propargyl cations, which, in turn, readily participate in ionic reactions. Analogous behavior was exhibited by other sulfonic acid derivatives, such as tosylates5k,11f,g and mesylates.11h (12) (a) Curran, D. P. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: London, 1992; Vol. 5, pp 716, 780. (b) Malacria, M. Chem. Rev. 1996, 96, 289. (c) Giese, B.; Kopping, B.; Gobel, T.; Dickhaut, J.; Thoma, G.; Kulicke, K. J.; Trach, F. In Organic Reactions; Paquette L., Ed.; John Wiley: New York, 1996; Vol. 48, p 301. (d) Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical Reactions; VCH: Weinheim, 1997. (e) Sibi, M. P.; Porter, N. A. Acc. Chem. Res. 1999, 32, 163. (f) Sibi, M. P.; Ternes, T. R. Stereoselective Radical Reactions. In Modern Carbonyl Chemistry; Otera, J., Ed.; Wiley-VCH: New York, 2000. (g) Radicals in Organic Synthesis; Sibi, M., Ed.; Wiley: New York, 2001; Vol. 1. (h) Zard, S. Z. Radical Reactions in Organic Synthesis; Oxford University Press, Inc.: New York, 2003. (i) Togo, H. Advanced Free Radical Reactions for Organic Synthesis; Elsevier: Amsterdam, 2004.

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It is worthy to mention that intermolecular radical dimerization reactions exhibit a low stereoselectivity in a purely organic setting.12 Complexation with transition metals, either in π- or σ-fashion,13 allows for generation of radicals otherwise hardly accessible and also provides access to a remarkable array of new organic products.13,14 Unfortunately, the stereoselectivity of intermolecular dimerization is hard to control even with π-bonded metal cores present (Mo, Fe, Cr),13 further highlighting the significance of the newly developed cobalt-mediated spontaneous reaction that exhibits a remarkable level of diastereocontrol (d,l 89-99%) and also bypasses a laborious cation isolation step. Besides this, the spontaneous reaction provides access to the class of organic compounds;d,l-3,4-diaryl-1,5-alkadiynes;that represent a synthetic challenge. The “classical” propargylpropargyl coupling reaction exhibits a poor regioselectivity due to acetylene-allene rearrangement.15a Catalytic processes (Ru, Pd) are inherently limited in scope.15b,c Their yields and diastereoselectivities drastically decrease in the presence of electron-withdrawing (CF3) and electron-donating substituents (Me; OMe), as well as the bulky substituents (naphthyl group).15b Also, metal-induced dimerizations exhibit a low regioselectivity because of the formation of isomeric allene-ynes15c and could potentially have reproducibility problems. The alternative mediation of intermolecular coupling of propargyl alcohols with a Ti(OiPr)2Cl2/Mg suffers from the low conversions (∼70%), lack of diastereoselectivity, and a poor regioselectivity, with target 1,5-alkadiynes being accompanied by comparable quantities of acetylenic allenes (45-50%).15d

Conclusion Generation of cobalt-complexed propargyl cations can be carried out under neutral conditions by employing, as substrates, methyl propargyl ethers π-bonded to a Co2(CO)6 core. Their interaction with Tf2O or (CF3CO)2O occurs rapidly (3-6 min) at 83 °C, converting to ionic cobaltcomplexed propargyl triflates, which, in turn, undergo spontaneous conversion to propargyl radicals via cluster-tocluster and cluster-to-ligand single-electron transfers. Radical dimerization placing a C-C bond alpha to the metal core affords bis-clusters of 3,4-diaryl-1,5-alkadiynes with high d, l-diastereoselectivity (89-99%). Under oxidative conditions (Ce4þ), topologically and functionally diverse 3,4-diaryl-1,5alkadiynes can be produced in good to high yields, as pure d, l-diastereomers. Ionic propargyl triflates can also be synthesized under acidic conditions by using cobalt-complexed propargyl alcohols, as substrates, and Tf2O and Me3SiOTf, as reagents, although, as synthetic methods, the latter are inferior to the parent process carried out under neutral conditions. The newly acquired ability to carry out all essential steps under neutral conditions;synthesis of methyl (13) (a) Melikyan, G. G. In Frontiers in Organometallic Chemistry; Cato, M. A.; Ed.; Nova Science Publishers: New York, 2006; p 155, and references therein. (14) (a) Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: Amsterdam, 1990; Chapters 3, 4, 9, 10. (b) Astruc, D. Acc. Chem. Res. 1991, 24, 36. (c) Astruc, D. Electron Transfer and Radical Processes in Transition-Metal Chemistry; VCH: New York, 1995; Chapters 3, 5, 6. (d) Torraca, K. E.; McElwee-White, L. Coord. Chem. Rev. 2000, 206-207, 469. (15) (a) Badanyan, Sh. O.; Voskanyan, M. G.; Chobanyan, Zh. A. Russ. Chem. Rev. 1981, 50, 1074. (b) Onodera, G.; Nishibayashi, Y.; Uemura, S. Organometallics 2006, 25, 35. (c) Ogoshi, S.; Nishiguchi, S.; Tsutsumi, K.; Kurosawa, H. J. Org. Chem. 1995, 60, 4650. (d) Yang, F.; Zhao, G.; Ding, Y.; Zhao, Z.; Zheng, Y. Tetrahedron Lett. 2002, 43, 1289.

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propargyl ethers, generation of propargyl triflates, formation of propargyl radicals, and dimerization thereof;substantially enhances the synthetic potential of cobaltmediated radical dimerization reactions. It also allows the use of substrates containing acid-sensitive peripheral functionalities (benzylic, acetal, 1,3-dioxolane, enol ether) and functional groups susceptible to protonation (carbonyl, cyano, amino, imino).

Experimental Section All manipulations of air-sensitive materials were carried out in flame-dried Schlenk-type glassware on a dual-manifold Schlenk line interfaced to a vacuum line. Argon and nitrogen (Airgas, ultrahigh purity) were dried by passing through a Drierite tube (Hammond). All solvents were distilled before use under dry nitrogen over appropriate drying agents (ether, THF, from sodium benzophenone ketyl; CH2Cl2, from CaH2; benzene, from sodium). All reagents were purchased from Sigma-Aldrich and Acros and used as received. Co2(CO)8 and Ce(NH4)2(NO3)6 were purchased from Strem. NMR solvents were supplied by Cambridge Isotope Laboratories. 1H and 13C NMR spectra were recorded on a Bruker DRX-400 (1H, 400 MHz) spectrometer. Chemical shifts were referenced to internal solvent resonances and are reported relative to tetramethylsilane. Spin-spin coupling constants (J) are given in hertz. Elemental analyses were performed by Desert Analytics (Tucson, AZ). Melting temperatures (uncorrected) were measured on a Mel-Temp II (Laboratory Devices) apparatus and Optimelt Automated Meltemp. Silica gel S735-1 (60-100 mesh; Fisher) was used for flash column chromatography. Analytical and preparative TLC analyses (PTLC) were conducted on silica gel 60 F254 (EM Science; aluminum sheets) and silica gel 60 PF254 (EM Science; w/gypsum; 20  20 cm), respectively. Eluents are ether (E) and petroleum ether (PE). Mass spectra were run at the Regional Center on Mass Spectroscopy, UC Riverside, Riverside, CA (FAB, ZAB-SE; CI-NH3, 7070EHF; Micromass; TOF Agilent 6210 LC-TOF instrument with a multimode source). Precursory Co2(CO)6-complexed propargyl alcohols were synthesized by condensation of metal acetylides (Na, Li) with respective benzaldehydes9 followed by the complexation with dicobaltoctacarbonyl.10 General Procedure for the Synthesis of Methyl Propargyl Ethers [RCtCCH(OMe)Ar]Co2(CO)6 (Protocol A: Acidic Conditions). Under an atmosphere of nitrogen, at -20 °C, a solution of HBF4 3 Me2O (6.0 mmol) was added dropwise (5 min) to a solution of [RCtCCH(OH)Ar]Co2(CO)6 (1.0 mmol) in dry ether (35 mL) and stirred for 1 h. The ethereal layer was removed, and the cation was washed with ether (2  20 mL) at -20 °C. An additional amount of ether (25 mL) was added at -20 °C, followed by dry methanol (1 mL). The reaction mixture was stirred for 30 min at 20 °C and then diluted with H2O (20 mL). The organic layer was washed with water (2  20 mL) and dried (Na2SO4). Ether was removed under reduced pressure, and the crude mixture was fractionated by column chromatography. (μ-η2-3-Methoxy-3-phenyl-1-propyne)dicobalt Hexacarbonyl (10). This complex was synthesized from [HCtCCH(OH)Ph]Co2(CO)6 (1.0 mmol) according to protocol A and isolated by column chromatography on silica gel (150 g; PE:E, 10:1) in 91.1% yield. Spectral and physicochemical data are analogous to those reported earlier.6d [μ-η2-3-Methoxy-3-(40 -methoxyphenyl)-1-propyne]dicobalt Hexacarbonyl (16). This complex was synthesized from [HCtCCH(OH)(40 -OMeC6H4)]Co2(CO)66e (1.5 mmol) according to protocol A and then isolated by filtering through a short bed of Florisil (1 in) as red crystals (534 mg, 77.1%). Mp: 52-53 °C (sealed capillary; coevaporation with benzene, 3  1 mL). TLC (PE:E, 5:1): Rf 0.53. 1 H NMR (400 MHz, CDCl3): δ 3.43 (3H, s, OMe), 3.80 (3H, s,

Article OMe), 5.24 (1H, s, CH), 6.02 (1H, s, HCt), 6.89 (2H, d, arom H, J = 8.8), 7.30 (2H, d, arom H). MS TOF: m/z calcd for C16H9O7Co2 [M-OMe]þ 430.9007, found 430.9016. Anal. Found: C, 44.09; H, 2.69. C17H12O8Co2 requires: C, 44.18; H, 2.62. [μ-η2-3-Methoxy-3-(30 ,40 -dimethoxyphenyl)-1-propyne]dicobalt Hexacarbonyl (17). Synthesis of Alcohol [HCtCCH(OH)(30 ,40 -(OMe)2C6H3)]Co2(CO)6. Under an atmosphere of nitrogen, a suspension of sodium acetylide in xylene (6.66 g, 18%; sodium acetylide 1.20 g, 25.0 mmol) was added dropwise to a solution of 3,4-dimethoxybenzaldehyde (2.49 g, 15.0 mmol) in dry THF (100 mL) at -50 °C (25 min). Upon addition, the reaction mixture was stirred at 20 °C for 70 h, then cooled to 0 °C and quenched with saturated NH4Cl aqueous solution (50 mL). An aqueous layer was extracted with ether (5  30 mL), and combined ethereal fractions were dried (Na2SO4). Under reduced pressure, the crude alcohol (2.88 g, 15.0 mmol; assuming 100% yield) was evaporated to dryness, dissolved in dry ether (100 mL), and added, under an atmosphere of nitrogen, to a solution of dicobaltoctacarbonyl (6.16 g, 18.0 mmol) in dry ether (100 mL) (90 min). The reaction mixture was stirred at room temperature for 16 h, concentrated under reduced pressure, and fractionated on the silica gel column (220 g, PE:E, 3:1) to afford [HCtCCH(OH)(30 ,40 -(OMe)2C6H3)]Co2(CO)6 (2.21 g, 30.8%) as dark red crystals. Mp: 118-124 °C (sealed capillary; dried by coevaporation with benzene, 3  1 mL). TLC (PE: E, 1:1): Rf 0.39. 1H NMR (400 MHz, CDCl3): δ 2.29 (1H, d, OH, J = 3.2), 3.87 (3H, s, OMe), 3.92 (3H, s, OMe), 5.86 (1H, d, CH), 6.07 (1H, s, HCt), 6.84 (1H, d, 50 -H, J = 8.0), 6.98 (1H, dd, 60 H, J = 2.0), 7.03 (1H, s, 20 -H). MS FABþ: m/z 461 (Mþ - OH), 450 (Mþ - CO), 433 (Mþ - OH - CO), 422 (Mþ - 2CO), 394 (Mþ -3CO), 377 (Mþ - 3CO - OH), 366 (Mþ- 4CO), 349 (Mþ 4CO - OH), 338 (Mþ - 5CO), 310 (Mþ - 6CO), 175 (Mþ 2Co-6CO-OH). Anal. Found: C, 42.65; H 2.72. C17H12O9Co2 requires: C, 42.68; H 2.51. Preparation of Me-ether 17. This complex was synthesized from [HCtCCH(OH)(30 ,40 -(OMe)2C6H3)]Co2(CO)6 (1.0 mmol) according to protocol A. After aqueous workup, volatile solvents were removed under reduced pressure to afford 17 (427 mg, 86.8%) as a dark red oil. TLC (PE:E, 5:1): Rf 0.20. 1H NMR (400 MHz, CDCl3): δ 3.44 (3H, s, OMe), 3.87 (3H, s, OMe), 3.91 (3H, s, OMe), 5.23 (1H, s, CH), 6.02 (1H, d, HCt, J=0.8), 6.84 (1H, d, arom H5, J = 8.0), 6.91 (1H, dd, arom H6, J = 1.8), 6.94 (1H, d, arom H2, J = 2.0). MS TOF: m/z calcd for C19H17O10Co2 [M þ OMe]- 522.9491, found 522.9501. Anal. Found: C, 43.91; H, 2.49. C18H14O9Co2 requires: C, 43.93; H, 2.87. [μ-η2-3-Methoxy-3-(30 ,40 ,50 -trimethoxyphenyl)-1-propyne]dicobalt Hexacarbonyl (18). Synthesis of Alcohol [HCtCCH(OH) (30 ,40 ,50 -(OMe)3C6H2)]Co2(CO)6. Under an atmosphere of nitrogen, a solution of 3,4,5-trimethoxybenzaldehyde (3.92 g, 20 mmol) in dry THF (20 mL) was added dropwise (15 min) to a suspension of sodium acetylide (2.88 g, 60 mmol; 16 g, 18% suspension in xylene) in THF (160 mL) at -50 °C. The reaction mixture was stirred overnight at 20 °C, neutralized with saturated aqueous ammonium chloride (100 mL) at 0 °C, and diluted with ether (100 mL). Upon extraction with ether (3  20 mL), the combined ethereal extracts were dried (Na2SO4), and the filtrate was evaporated to dryness. Under an atmosphere of nitrogen, a solution of crude alcohol in dry ether (50 mL) was added dropwise (3 h) to a solution of Co2(CO)8 (8.20 g, 24 mmol) in degassed ether (100 mL) at 20 °C. Upon stirring overnight, the solvent was stripped under reduced pressure, and the residue was fractionated on Florisil (650 g, PE:E, 1:5) to afford [HCtCCH(OH)(30 ,40 ,50 -(OMe)3C6H2)]Co2(CO)6 (3.96 g, 39.0%) as dark red crystals. Tdecomp: 100-105 °C (without melting; sealed capillary; coevaporated with benzene, 3  1 mL). TLC (PE:E, 1:3): Rf 0.51. 1H NMR (200 MHz, CDCl3): δ 2.33 (1H, d, OH, J = 3.2), 3.80 (3H, s, OCH3), 3.89 (6H, s, OCH3), 5.83 (1H, d, OCH), 6.07 (1H, s, HCt), 6.71 (2H, s, arom. H). MS FABþ: m/z 509 (MHþ), 491 (Mþ - OH), 480 (Mþ - CO), 463 (Mþ - OH - CO), 452 (Mþ - 2CO), 424 (Mþ - 3CO), 407

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(Mþ - OH - 3CO), 396 (Mþ - 4CO), 379 (Mþ - OH - 4CO), 368 (Mþ - 5CO), 340 (Mþ - 6CO), 205 (Mþ - OH - 6CO 2Co). Anal. Found: C, 43.18; H, 3.00. C18H14O10Co2 requires: C, 42.52; H, 2.76. Preparation of Me-ether 18. This complex was synthesized from [HCtCCH(OH)(30 ,40 ,50 -(OMe)3C6H2)]Co2(CO)6 (0.315 mmol) according to protocol A. After aqueous workup, volatile solvents were removed under reduced pressure to afford 18 (140 mg, 85.4%) as a dark red oil. 1H NMR (400 MHz, CDCl3): δ 3.47 (3H, s, OMe), 3.80 (3H, s, 4-OMe), 3.88 (6H, s, 3-OMe, 5OMe), 5.21 (1H, s, CH), 6.03 (1H, d, HCt, J = 0.8), 6.62 (2H, s, aromatic H). MS TOF: m/z calcd for C18H13O9Co2 [M-OMe]þ 490.9218, found 490.9216. Anal. Found: C, 43.07; H, 3.11. C19H16O10Co2 requires: C, 43.70; H, 3.09. (μ-η2-1-Methoxy-1-phenyl-2-pentyne)dicobalt Hexacarbonyl (19). Synthesis of Alcohol [C2H5CtCCH(OH)Ph]Co2(CO)6. Under an atmosphere of nitrogen, 1-butyne (5.94 g, 110 mmol) was bubbled through a solution of n-BuLi (6.6 mmol, 4.1 mL/ 1.6 M) in dry THF (50 mL) at -10 °C, and the reaction mixture was stirred for 5 h at -10 °C. A solution of PhCHO (636 mg, 6 mmol) in dry THF (15 mL) was added dropwise (15 min), and the mixture was warmed to 20 °C and stirred for 16 h. The suspension was cooled to 0 °C and quenched with saturated NH4Claq (50 mL). An aqueous layer was extracted with ether (2  50 mL), and combined ethereal fractions were dried (Na2SO4). Upon concentration under reduced pressure (1/2 of the initial volume), under an atmosphere of nitrogen, the crude alcohol (960 mg, 6.0 mmol; assuming 100% yield) was added to a solution of dicobaltoctacarbonyl (2.26 g, 6.6 mmol) in dry ether (100 mL). The reaction mixture was stirred at room temperature for 2 h (TLC control), concentrated under reduced pressure, and fractionated on a silica gel column (225 g, PE:E, 10:1) to give [C2H5CtCCH(OH)Ph]Co2(CO)6 (1.15 g, 41.7%) as a dark red oil. TLC (PE:E, 2:1): Rf 0.56. 1H NMR (400 MHz, CDCl3): δ 1.28 (3H, t, CH3, J = 7.2), 2.31 (1H, d, OH, J = 3.6), 2.77 (2H, ABX3, CHAHB, J(HAHB) = 15.6), 5.93 (1H, d, CH, J = 3.2), 7.27-7.45 (5H, m, aromatic). MS TOF: m/z calcd for C17H11O7Co2 [M - H]- 444.9174, found 444.9177. Anal. Found: C, 45.20; H, 2.74. C17H12O7Co2 requires: C, 45.77; H, 2.71. Preparation of Me-ether 19. This complex was synthesized from [C2H5CtCCH(OH)Ph]Co2(CO)6 (0.4 mmol) according to protocol A. Isolation was carried out by column chromatography on Florisil (15 g, PE) and subsequent preparative TLC (2 plates, PE), affording 19 (120 mg, 63.0%) as a dark red oil. TLC (PE:E, 10:1): Rf = 0.56. 1H NMR (400 MHz, CDCl3): δ 1.25 (3H, t, CH3, J = 7.4), 2.73 (2H, ABX3, CHAHB, J = 16.0), 3.45 (3H, s, CH3), 5.30 (1H, s, CH), 7.27-7.40 (5H, m, arom. H). MS TOF: m/z calcd for C19H17O8Co2 [M þ OMe]- 490.9582, found 490.9593. Anal. Found: C, 47.25; H, 2.92. C18H14O7Co2 requires: C, 46.98; H, 3.07. Synthesis of Methyl Propargyl Ethers [RCtCCH(OMe)Ar]Co2(CO)6 under Nonacidic Conditions (Protocol B). (μ-η21-Methoxy-1-phenyl-2-pentyne)dicobalt Hexacarbonyl (19). Under an atmosphere of nitrogen, 1-butyne (5.94 g, 110 mmol) was bubbled through a solution of n-BuLi (6.6 mmol, 4.1 mL/1.6 M) in dry THF (50 mL) at -10 °C, and the reaction mixture was stirred for 5 h at -10 °C. The solution of PhCHO (636 mg, 6 mmol) in dry THF (15 mL) was added dropwise (15 min), and the mixture was warmed to 20 °C and stirred for 16 h. The reaction mixture was cooled to 0 °C, and CH3I (17.04 g, 120 mmol) was added in four portions and stirred for 19 h at 20 °C (NMR control). The mixture was cooled to 0 °C and quenched with water (50 mL). An aqueous layer was extracted with ether (2  50 mL), and combined ethereal fractions were dried (Na2SO4). Upon concentration under reduced pressure (1/2 of the initial volume), under an atmosphere of nitrogen, the crude alcohol (1.04 g, 6 mmol; assuming 100% yield) was added to a solution of dicobaltoctacarbonyl (2.46 g, 7.2 mmol) in dry ether (100 mL). The reaction mixture was stirred at 20 °C

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overnight, concentrated under reduced pressure, and fractionated on a silica gel column (200 g, PE) to give 19 (482 mg, 17.5% over 3 steps). Interaction of Methyl Propargyl Ethers 10 and 16-19 with Triflic Anhydride 2 (ProtocolC): d,l- and meso-(μ-η2-3,4-Diphenyl-1,5-hexadiyne)bis(dicobalthexacarbonyl) (6). Under an atmosphere of nitrogen, Tf2O (73.3 mg, 0.26 mmol) was added dropwise (11 min) to a solution of methyl ether 10 (110 mg, 0.25 mmol) in dry 1,2-dichloroethane (5 mL) at -20 °C. The reaction mixture was stirred for 10 min at -20 °C, then for another 10 min at 20 °C, and finally refluxed at 83 °C for 3 min. The solution was cooled to 20 °C, diluted with methanol (0.5 mL; 5 min), and stirred for an additional 30 min. By NMR, the crude mixture contained d,l-6, meso-6, [HCtCCHPhCHPhCtCH]Co2(CO)6, and [HCtCCH2Ph]Co2(CO)6 in the ratio of 68:6:23:3 (d,l-6:meso-6, 92:8). Solid Co2(CO)8 (29 mg, 0.085 mmol) was added, and the reaction mixture was stirred for 2 h, diluted with H2O (15 mL), and then extracted with ether (5 mL). The organic layer was washed with water (2  10 mL) and dried (Na2SO4). By NMR, the ratio of d,l-6:meso-6 was equal to 94:6. Organic solvents were evaporated under reduced pressure, and the residue was fractionated on a preparative TLC plate (PE:CH2Cl2, 10:1; 2 runs) to afford d,l-6 and meso-6 (41.1 mg, 82.0%; d,l-6:meso-6, 95:5). Both diastereomers were fully characterized in the previous account.6g d,l- and meso-[μ-η2-3,4-Di(40 -methoxyphenyl)-1,5-hexadiyne]bis(dicobalthexacarbonyl) (20). According to protocol C, methyl ether 16 (116 mg, 0.25 mmol) was converted, upon recomplexation, to the mixture of d,l-20 and meso-20 in the ratio of 97:3 (NMR). Fractionation on preparative TLC plate (PE:E, 7:1) afforded d,l-20 (39.2 mg, 72.8%) and meso20 þ [HCtCCH2(4-OMe)C6H4]Co2(CO)6 (1 mg; 78:22). Both diastereomers were fully characterized in the previous account.6e d,l- and meso-[μ-η2-3,4-Di(30 ,40 -dimethoxyphenyl)-1,5-hexadiyne]bis(dicobalthexacarbonyl) (21). According to protocol C, methyl ether 17 (123 mg, 0.25 mmol) was converted, upon recomplexation, to a mixture of d,l-21 and meso-21 in the ratio of 99:1 (NMR). Fractionation on a silica gel column (20 g; PE:E, 1:2) afforded d,l-21 (46.1 mg, 80.0%) as a dark red solid. Tdec = 90-115 °C (sealed capillary; dried by coevaporation with benzene, 3  1 mL). TLC (PE:E, 1:1): Rf 0.31. 1H NMR (400 MHz, CDCl3): δ 3.76 (6H, s, 2OMe), 3.78 (6H, s, 2OMe), 4.29 (2H, s, 2CH), 6.29 (2H, s, HCt), 6.54 (2H, d, arom. H, J = 1.6), 6.65 (4H, ABX-spectrum, arom. H, J(HA-HB) = 8.0). 13C NMR (50 MHz, CDCl3): δ 54.652, 55.765, 55.858 (C3/C4, 30 OMe, 40 -OMe), 76.515 (C1/C6), 102.011 (C2/C5), 110.837, 112.622, 121.517, 135.741 (arom. C10 , C20 , C50 , C60 ), 148.172, 148.435 (arom. C30 , C40 ), 198.822, 200.0 (CO). MS FABþ: m/z 944 (Mþ þ Na - H), 838 (Mþ - 3CO), 810 (Mþ - 4CO), 782 (Mþ - 5CO), 754 (Mþ - 6CO), 726 (Mþ - 7CO), 698 (Mþ 8CO), 669 (Mþ - 9CO - H), 641 (Mþ - 10CO - H), 613 (Mþ 11CO - H), 582 (Mþ - 11CO - OMe), 551 (Mþ - 11CO 2OMe), 526 (Mþ - 12CO - Co - H), 496 (Mþ - 11CO - 2Co), 377 (Mþ - 11CO - 4Co - H). Anal. Found: C, 44.44; H, 2.77. C34H22O16Co4 requires: C, 44.25; H, 2.38. d,l- and meso-[μ-η2-3,4-Di(30 ,40 ,50 -trimethoxyphenyl)-1,5hexadiyne]bis(dicobalthexacarbonyl) (22). According to protocol C, methyl ether 18 (131 mg, 0.25 mmol) was converted, upon recomplexation, to the mixture of d,l-22 and meso-22 in the ratio of 97:3 (NMR). Fractionation on silica gel column (20 g; PE:E, 1:2) afforded d,l-22 (19.6 mg, 31.9%), which was fully characterized in the previous account.6f d,l- and meso-(μ-η2-5,6-Diphenyl-3,7-decadiyne)bis(dicobalt hexacarbonyl) (23). According to protocol C, methyl ether 19 (115 mg, 0.25 mmol) was converted, upon recomplexation (Co2(CO)8, 30 mg, 0.10 mmol), to the mixture of d,l-23, meso23, and [C2H5CtCCH2Ph]Co2(CO)6 in the ratio of 72:8:20 (d,l23:meso-23, 89:11). Fractionation on a preparative TLC plate (PE) afforded d,l-23 þ meso-23 (44.2 mg, 82.4%; d,l-23:meso-23,

Melikyan et al. 91:9) as brown-red crystals. TLC (PE:E, 5:1): Rf 0.66. 1H NMR (400 MHz, CDCl3): d,l-23 þ meso-23 δ meso- 0.93 (6H, t, 2CH3, J = 7.4), d,l- 1.23 (6H, t, 2CH3, J = 7.2), meso- 1.50 (4H, ABX3, 2CHAHB, J = 16.0), d,l- 2.69 (4H, ABX3, 2CHAHB, J = 16.0), meso- 4.58 (2H, s, 2CH), d,l- 4.83 (2H, s, 2CH), d,l- 6.86 (4H, br signal, aromatic H), d,lþmeso 7.16-7.68 (d,l- 6H, meso- 10H, m, aromatic H). 13C NMR (100 MHz, CDCl3): d,l-23 þ meso-23 δ d,l- 15.7 (C1, C10), meso- 16.2 (C1, C10), meso- 26.0 (C2, C9), d, l- 26.7 (C2, C9), meso- 54.2 (C5, C6), d,l- 60.8 (C5, C6), d,l- 101.0, 101.2 (C3, C4, C7, C8), meso- 104.7, 106.7 (C3, C4, C7, C8), d,l127.6, 127.8 (arom. C), meso- 128.9, 129.1 (arom. C), d,l- 130.8 (arom. C), meso- 131.4 (arom. C), d,l- 141.4 (arom. C), meso143.1 (arom. C), d,lþmeso 199.0, 201.0 (CdO). 13C NMR assignments are based on HSQC data. MS TOF: m/z calcd for C35H25O13Co4 [M þ MeO]- 888.8629, found 888.8604. Anal. Found: C, 47.49; H, 2.26. C34H22O12Co4 requires: C, 47.58; H, 2.58. d,l- and meso-(μ-η2-3,4-Diphenyl-1,5-hexadiyne)bis(dicobalthexacarbonyl) (6). (a) By Interaction of Propargyl Alcohol 1 and Trimethylsilyl Triflate (7). Under an atmosphere of nitrogen, at -20 °C, trimethylsilyl trifluoromethanesulfonate (7, 56.5 mg, 0.25 mmol) was added dropwise (11 min) to a solution of alcohol 1 (105 mg, 0.25 mmol) in dry 1,2-dichloroethane (5 mL). The reaction mixture was stirred for 10 min at -20 °C, then for 10 min at 20 °C, and finally refluxed at 83 °C for 3 min. The solution was cooled to 20 °C, and methanol (0.5 mL) was added dropwise (5 min). After stirring for 30 min, the reaction mixture (d,l-6:meso-6:monocomplex 6, 56:8:36; d,l-6:meso-6, 88:12) was treated with Co2(CO)8 (42 mg, 0.123 mmol) for 2 h, then diluted with H2O (15 mL) and extracted with ether (5 mL). The organic extract was washed with water (2  10 mL) and dried (Na2SO4). The organic solvents were evaporated under reduced pressure, and the residue (d,l-6:meso-6, 92:8) was then fractionated on a preparative TLC plate (PE:CH2Cl2, 10:1; 2 runs). Obtained were d,l- and meso-6 (40.6 mg, 81.0%; 94:6). (b) By Interaction of Me-ether 10 and Trifluoroacetic Anhydride (13). Under an atmosphere of nitrogen, (CF3CO)2O (52.5 mg, 0.25 mmol) was added dropwise (10 min) to a solution of methyl ether 10 (110 mg, 0.25 mmol) in dry 1,2-dichloroethane (5 mL) at -20 °C. The reaction mixture was stirred for 10 min at -20 °C, then for another 10 min at 20 °C, and finally refluxed at 83 °C for 6 min. The solution was cooled to 20 °C, diluted with methanol (0.5 mL; 5 min), and stirred for additional 30 min. By NMR, the crude mixture contained d,l-6:meso6:[HCtCCHP hCHPhCtCH]Co2(CO)6:[HCtCCH2Ph]Co2(CO)6 in the ratio of 77:14:5:4 (d,l-6:meso-6, 85:15). Solid Co2(CO)8 (30 mg, 0.088 mmol) was added, and the reaction mixture was stirred for 2 h, diluted with H2O (15 mL), and then extracted with ether (5 mL). The organic layer was washed with water (2  10 mL) and dried (Na2SO4). Organic solvents were evaporated under reduced pressure, and the residue was fractionated on a preparative TLC plate (PE, 2 runs) to afford d,l-6 and meso-6 (44.0 mg, 87.8%; d,l-6:meso-6, 84:16). Decomplexation Step (Protocol D): d,l-3,4-Diphenyl-1,5-hexadiyne (26). Under an atmosphere of nitrogen, a solution of Ce(NH4)2(NO3)6 (592 mg, 1.08 mmol) in dry acetone (9 mL; degassed) was added dropwise to a solution of d,l-6 (96 mg, 0.12 mmol) in dry acetone (6 mL; degassed) at -78 °C (7 min). Upon addition, the reaction mixture was warmed to 20 °C (30 min), stirred an additional hour (20 °C; TLC control), treated with a degassed saturated aqueous solution of NaCl (20 mL), and extracted with ether (3  15 mL). The combined ethereal layers were dried (4 A˚), the solvent, upon filtration, was stripped away under reduced pressure, and the residue was filtered (SiO2 5 g; PE) to afford d,l-26 (24.5 mg, 88.8%) as a light yellow solid.6b Mp: 166-168 °C (dec; sealed capillary; coevaporation with benzene, 3  1 mL). TLC (PE:E, 10:1): Rf 0.56. 1H NMR (400 MHz, CDCl3): δ 2.38 (2H, br s, HCt), 4.02 (2H, three lines, CH, J = 1.0), 7.22-7.32 (10H, m, arom H). 13C

Article NMR (400 MHz, acetone-d6): δ 45.1 (C3/C4), 74.0 (C1/C6), 82.6 (C2/C5), 127.1, 127.9, 128.4, 138.8 (arom C). MS TOF: m/z calcd for C18H15 MHþ 231.1168, found 231.1172. d,l-3,4-Di(40 -methoxyphenyl)-1,5-hexadiyne (27). According to protocol D [molar ratio d,l-20:Ce(NH4)2(NO3)6, 1:10; d,l-20 86 mg (0.1 mmol)], following the filtration on a short bed of Florisil (10 g, CH2Cl2), d,l-27 (26 mg, 89.7%) was obtained as a white solid. Mp: 149-151 °C (dec; sealed capillary; coevaporation with benzene, 31 mL). TLC (PE:E, 2:1): Rf 0.36. 1H NMR (200 MHz, CDCl3): δ 2.37 (2H, t, HCt, J = 1.1), 3.80 (6H, s, OMe), 3.94 (2H, br s, CH), 6.77-6.84 (4H, six lines, aromatic H), 7.16-7.22 (4H, six lines, aromatic H). MS-DEI: m/z 290 (Mþ, 7.9%), 145 (100%). HR-MS/DEI: calcd for C20H18O2 290.130680, found 290.131569. d,l-3,4-Di(30 ,40 -dimethoxyphenyl)-1,5-hexadiyne (28). According to protocol D [molar ratio d,l-21:Ce(NH4)2(NO3)6, 1:10; d,l-21 92 mg (0.1 mmol)], following the trituration of the crude product with precooled ether (0 °C, 2  0.5 mL) and methanol (0 °C, 2  0.5 mL), d,l-28 (19 mg, 54.3%) was isolated as a yellowish-white solid. Mp: 137-139 °C (dec; sealed capillary; coevaporated with benzene, 3  1 mL). TLC (E): Rf 0.48. 1 H NMR (400 MHz, CDCl3): δ 2.399 (2H, three lines, HCt, J = 0.6), 3.794 (6H, s, 2OMe), 3.862 (6H, s, 2OMe), 3.953 (2H, br s, HC), 6.75-6.82 (6H, m, aromatic H). MS DEI: m/z Mþ 350 (10%), 319, 199, 190, 175 (100%), 161, 131. HR-MS/DEI: calcd for C22H22O4 350.151809, found 350.150730. d,l-3,4-Di(30 ,40 ,50 -trimethoxyphenyl)-1,5-hexadiyne (29). According to protocol D [molar ratio d,l-22:Ce(NH4)2(NO3)6, 1:10; d,l-22 98 mg (0.1 mmol)], following the filtration on a short bed of silica (5 in.  1/2 in. column; ether), d,l-29 (27 mg, 65.9%) was obtained as a yellowish-white solid. Mp: 100106 °C (dec; sealed capillary; coevaporated with benzene, 3  1 mL). TLC (E): Rf 0.42. 1H NMR (400 MHz, CDCl3): δ 2.436 (2H, three lines, HCt, J = 1.2), 3.770 (12H, s, 4OMe), 3.815

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(6H, s, 2OMe), 3.927 (2H, three lines, HC), 6.439 (4H, s, aromatic H). MS DEI: m/z Mþ 410, 205. HR-MS/DEI: calcd for C24H26O6 410.172939, found 410.172750. d,l- and meso-5,6-Diphenyl-3,7-decadiyne (30). According to protocol D [molar ratio (d,l-23 þ meso-23):Ce(NH4)2(NO3)6, 1:10; d,l-23 þ meso-23 34.3 mg, 0.04 mmol; d,l-23:meso-23, 73:27; zinc reduction6h], following the fractionation by preparative TLC (PE), a mixture of d,l-30 þ meso-30 (7.9 mg, 69.3%) was isolated as a clear oil. By NMR (C6D6), the diastereomeric ratio was equal to d,l-30:meso-30, 70:30. TLC (PE:E, 2:1): Rf 0.66 (visualized with phosphoromolybdic acid). 1H NMR (400 MHz, C6D6): d,l-30 þ meso-30 δ meso- 1.04 (6H, t, 2CH3, J = 7.6), d,l- 1.10 (6H, t, 2CH3, J = 7.4), meso- 2.09 (4H, br q, 2CH2, J = 7.6), d,l- 2.14 (4H, br q, 2CH2, J = 7.2), d,l- 4.15 (2H, s, 2CH), meso- 4.19 (2H, s, 2CH), d,lþmeso 7.12-7.28 (d,l- 6H, meso- 6H, m, arom. H), meso- 7.35-7.42 (4H, m, arom. H), d,l7.45-7.51 (4H, m, arom. H). 13C NMR (100 MHz, C6D6): d,l30 þ meso-30 δ meso- 12.59 (C2, C9), d,l- 12.66 (C2, C9), meso13.99 (C1, C10), d,l- 14.10 (C1, C10), meso- 46.27 (C5, C6), d,l46.41 (C5, C6), d,l- 79.1, 86.5 (C3, C4, C7, C8), meso- 79.8, 86.4 (C3, C4, C7, C8), d,lþmeso 126.9, 127.7 (arom. C), d,l- 127.8, 128.9, 140.2 (arom. C), meso- 128.1, 129.1, 139.9 (arom. C). 13C NMR assignments are based on Dept 45, Dept 135, and HSQC data. MS TOF: m/z calcd for C22H23 [MH]þ 287.1794, found 287.1801.

Acknowledgment. This material is based upon work supported by the National Science Foundation under CHE-0707865. The authors are also greatly indebted to the Office of Graduate Studies, Research and International Programs, and University Corporation, California State University Northridge, for their generous support.