Communication pubs.acs.org/Organometallics
Cyclopentadienone and Hydroxycyclopentadienyl Cobalt Complexes from the Reaction of an Alkynylphenylsilane with Co2(CO)8 Florian Hoffmann, Jörg Wagler, and Gerhard Roewer* Institut für Anorganische Chemie, Technische Universität Bergakademie Freiberg, Leipziger Straße 29, 09596 Freiberg, Germany S Supporting Information *
ABSTRACT: Reaction of dimethyl(pent-1-yn-1-yl)phenylsilane n-PrC CSiMe2Ph (1) with Co2(CO)8 in n-heptane or 1,4-dioxane led to the formation of cobalt complexes with carbon−carbon bond coupling and insertion of a carbonyl ligand ([2 + 2 + 1] cycloaddition). In both solvents, the compounds (n-PrCCSiMe2Ph)·Co2(CO)6 (2), {[nPr2(PhMe2Si)2C4CO]Co(CO)2}2 (3), and [n-Pr2(PhMe2Si)2C5OH]Co(CO)2 (4) were obtained. In n-heptane, a fourth cobalt complex (5) was isolated as well but could not be identified, while in 1,4dioxane, metallic cobalt formed instead. The yields of 3 and 4 in 1,4-dioxane were distinctly higher than those in n-heptane (11 and 21% vs 7.5 and 6%). All compounds were characterized by spectroscopic methods (NMR, IR, UV/vis), and the molecular structures of 3 and 4 were determined.
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indication of cyclotrimerization products of 1.9 Apparently, the steric demand of the substituents at the alkyne is too large.6 Instead, formation of several cobalt complexes was observed if stoichiometric amounts of Co2(CO)8 in n-heptane were used. Otherwise the conversion was incomplete and unreacted silane 1 remained in the reaction mixture. Workup yielded four product batches which were comprised of different compounds: wellformed scarlet red crystals (separated by decantation), an emerald green microcrystalline powder (obtained by filtration, main product by mass), orange crystals (by crystallization from the filtrate), and a dark brown residual solution. The dark brown compound in the mother liquor of the crystallization was identified by its color as well as its IR and 29Si NMR data as hexacarbonyl{μ-dimethyl[(1,2-η:1,2-η)-pent-1-yn1-yl]phenylsilane}bis[cobalt(0)](Co−Co) ((n-PrCCSiMe2Ph·Co2(CO)6, 2) (Scheme 1). As compound 2 is the first and simplest product to be expected from the reaction of alkynylsilane 1 with Co2(CO)8 and belongs to a well-known class of complexes, it was not isolated for further characterization.10 The scarlet red crystals were identified as bis(μ-carbonyl)dicarbonylbis{(2,3,4,5-η)-2,5-bis[dimethyl(phenyl)silyl]-3,4-dipropylcyclopenta-2,4-dien-1-one}bis[cobalt(0)](Co−Co) ({[nPr2(PhMe2Si)2C4CO]Co(CO)2}2, 3) by X-ray crystallography (Figure 1). This complex is derived from the carbonylbridged isomer of Co2(CO)8, where at each cobalt atom two carbonyl groups have been replaced by a cyclopentadienone moiety, which had formed from two molecules of 1 and a carbonyl ligand (Scheme 1). Compound 3 is assumed to have formed from 2 by stepwise replacement of the carbonyl ligands, which are easy to substitute.11 Reports of this type of dinuclear
n a series of publications, we recently reported the syntheses of some alkynylsilanes, their transition-metal complexes, and the reactivity patterns of their alkyne triple bonds, including carbon−carbon bond coupling reactions.1 This type of reaction constitutes one of the most widespread applications of alkynes in the field of organic chemistry. Among these, an important synthetic route is the catalyzed cyclotrimerization of (hetero)alkynes to (hetero)aromatic compounds such as benzene2 and pyridine3 derivatives. Numerous compounds, especially complexes of cobalt, act as a catalyst in this reaction, the most widespread being dicobaltoctacarbonyl (Co2(CO)8) and cyclopentadienyl cobalt complexes CpCoL2.2b,4 Various types of cobalt compounds are known to form as intermediates, byproducts, and stoichiometric products if Co2(CO)8 is employed as a catalyst.2b,5 Under certain conditions other ring sizes than six-membered cycles can be formed or carbon monoxide is included into the ring, leading for example to [2 + 2 + 1] cycloaddition products.2c,d Herein we report about the complexes resulting from the reaction of an alkynylphenylsilane with Co2(CO)8. As our initial intention was the cyclotrimerization of an alkynylphenylsilane, dimethyl(pent-1-yn-1-yl)phenylsilane (nPrCCSiMe2Ph, 1) was chosen from the silanes which were used for our study of transition-metal complexes containing alkynylsilyl groups.1 It has less steric demand than the phenyl group (and no conjugation of the CC bond) with regard to the organic substituent and the lowest reactivity of the silyl group (compared to hydrogenodimethylsilyl and cyclopentadienyldimethylsilyl) with regard to transition-metal complexation.6 In a preliminary study, the cyclotrimerizability of 1 was examined using the catalysts Co2(CO)8 in n-heptane or 1,4dioxane,2b,4 palladium chloride/chlorotrimethylsilane (PdCl2/ Me3SiCl) in tetrahydrofuran (THF),7 and aluminum chloride (AlCl3) in hexane.8 Different catalytic mechanisms are effective in these systems: homogeneous transition metal, heterogeneous transition metal, Lewis acid. However, we could not find any © 2014 American Chemical Society
Special Issue: Catalytic and Organometallic Chemistry of EarthAbundant Metals Received: May 8, 2014 Published: July 3, 2014 5622
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diastereotopicity by hindered rotation. A 59Co NMR signal could not be observed in the range 0 to −3000 ppm. The IR spectra of 3 in both solution and the solid state featured three carbonyl bands each for the terminal, bridging, and cyclopentadienone carbonyl groups (CH2Cl2, 2045, 1830, and 1615 cm−1; KBr, 2042, 1830, and 1622 cm−1). The UV/vis spectrum contained four medium-intensity absorptions at 300, 330, 375, and 470 nm, the last of which was responsible for the bright red color of the compound. In the solid crystalline state, the molecules of 3 possess inversion symmetry, the center of inversion being located at the middle of the Co−Co bond (Figure 1). Thus, all equivalent ligands are in positions trans to each other, in contrast to the case for the similar complex bis(μ-carbonyl)dicarbonylbis[(2,3,4,5η)-2,5-bis(trimethylsilyl)-cyclopenta-2,4-dien-1-one]bis[cobalt(0)](Co−Co) ({[H2(Me3Si)2C4CO]Co(CO)2}2),12a in which they are in positions cis to each other.14 The asymmetric unit contains a half of each of two molecules, i.e., in total one molecule. The two half-molecules differ somewhat in their conformation; one of them shows disorder at a low percentage. The bond lengths of the former CC bonds correspond to an intermediate state between single and double bonds. The Co−C bond lengths of the former triple-bond carbon atoms are in the range 209−216 pm, which is relatively long in comparison to those for other cyclopentadienone complexes of cobalt (200−206 pm) as well as all Co−C bonds (180−220 ppm).15 However, they correspond well to the equivalent bonds in {[H2(Me3Si)2C4CO]Co(CO)2}2 (206− 218 pm).12a The Co−C distance to the cyclopentadienone carbonyl group (236 pm/238 pm) is clearly above the range of “regular” Co−C bond lengths. The orange crystals were identified as dicarbonyl{η5-1hydroxy-2,5-bis[dimethyl(phenyl)silyl]-3,4-dipropylcyclopentadienyl}cobalt(I) ([n-Pr2(PhMe2Si)2C5OH]Co(CO)2, 4) by X-ray crystallography (Figure 2). This complex is assumed to have formed from half a molecule of 3 on addition of a hydrogen atom (considerations about the source of this hydrogen can be found in the Supporting Information) at the cyclopentadienone carbonyl group (Scheme 1). Compound 4 is a hydroxycyclopentadienyl complex, which is a very rare type of cobalt compound. Only one further example has been reported to date:
Scheme 1. Formation of Compounds 2−5 and Metallic Cobalt from 1 and Co2(CO)8 at Reflux Temperature in n-Heptane or 1,4-Dioxane, Respectively
Figure 1. Molecular structure of 3. Only the nondisordered halfmolecule in the asymmetric unit is shown (replenished to a complete molecule); hydrogen atoms are omitted for clarity. Bond lengths and angles can be found in the Supporting Information.
cyclopentadienone cobalt complexes are very rare. Only three similar compounds are known, which were isolated as byproducts from the reaction of cobalt carbonyl or cobalt− mercury carbonyl complexes with an excess of sterically demanding alkynes (ethynylbenzene (PhCCH), 3,3-dimethyl-but-1-yne (t-BuCCH), ethynyltrimethylsilane (Me3SiC CH)).5a,12 The yield of 3 in n-heptane was 7.5% with reference to silane 1. The crystals were air-stable for at least 1 h. Unfortunately, recrystallization was not possible, as 3 was only soluble in halogenated hydrocarbons, forming solutions that were unstable even under argon. Thus, the elemental analysis results were acceptable but not within the ±0.4% limit commonly assumed to ensure satisfying purity.13a The low stability of 3 in solution hampered the acquisition of the NMR spectra as well (all signals broadened). Nevertheless, the spectra were in good agreement with the structure of 3. Two 1H NMR signals for the methyl substituents on silicon indicated their chemical inequivalence, which was interpreted as diastereotopicity by hindered rotation of the silyl groups. The integrals of the three 1 H NMR signals of the propyl substituents appeared in a ratio of 1:3:3 instead of the expected 2:2:3, thus indicating chemical inequivalence of the two methylene protons at the cyclopentadienone ring as well, which was again interpreted as
Figure 2. Molecular structure of 4 with intramolecular OH···π interaction. The S isomer is shown; hydrogen atoms except for H3, H8a, and H9b are omitted for clarity. Bond lengths and angles can be found in the Supporting Information. HOH···CPh (OOH···CPh) distances (pm): H3···Ph(plane) 234(2), H3···C25 259 (O3···C25 329), H3···C24 237 (O3···C24 309), H3···C29 293 (O3···C29 362); sum of the van der Waals radii 300 (350) pm.26 HMe···OOH (CMe···OOH) distances (pm): H8a···O3 251 (320), H9b···O3 309 (361); sum of the van der Waals radii 280 (350) pm.26 5623
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dicarbonyl(η5-1-hydroxy-2,5-diphenyl-cyclopentadienyl)cobalt(I) ([H2Ph2C5OH]Co(CO)2) was obtained in 15% yield from the reduction of a cobalt−mercury butadiene complex (prepared from mercury(II) tetracarbonylcobaltate(−I) (Hg[Co(CO)4]2) and PhCCH) with sodium amalgam and subsequent protolysis with trifluoroacetic acid.16 The yield of 4 was 6% with reference to silane 1. Its crystals were more air-sensitive than those of 3 and could be handled in air only for a short period of time. The elemental analysis results were acceptable but not within the ±0.4% limit commonly assumed to ensure satisfying purity.13b However, the spectroscopic data confirmed the identity of 4. As in compound 3, the 1H NMR spectra indicated chemical inequivalence of the silicon-bound methyl groups (two signals) as well as of the propyl CH2 protons adjacent to the cyclopentadienyl ring (by their coupling pattern), which was again interpreted as diastereotopicity by hindered rotation of these substituents. While steric hindrance is assumed to be the reason in complex 3, the cause in compound 4 seems to be an OH···π contact between the hydroxyl proton and the phenyl groups of the silyl substituents.17 This renders complex 4 conformationally planar chiral, is supported by a sharp 1H NMR signal and a narrow IR band of the hydroxyl group (indicating the absence of exchange processes in the respective time scales, which usually broaden these signals), and is finally corroborated by the solid-state structure.18 The 59Co NMR spectrum featured a signal at −2015 ppm (Δν1/2 = 10.5 kHz), which is shifted significantly to low field in comparison to unsubstituted dicarbonyl(η5-cyclopentadienyl)cobalt(I) (CpCo(CO)2) (δ −2675 ppm, Δν1/2 = 6.9, 10.5 kHz).19 The carbonyl bands in the IR spectrum (n-hexane, 2009 and 1951 cm−1; KBr, 2001 and 1941 cm−1) were in good agreement with those for similar complexes (CpCo(CO)2, 2033, 1972 cm−1; Cp*Co(CO)2, 2011, 1949 cm−1; [H2Ph2C5OH]Co(CO)2, 1997, 1942 cm−1).16,20 In the solid state, three additional weak bands appeared at 2041, 1830, and 1620 cm−1, which fit very well to those of 3. Because of its low solubility it is unlikely that 3 was included as an impurity in the crystal lattice of 4. In fact, it is more likely that it was formed from 4 by oxidation during sample preparation.21 In solution as well as in the solid state the band of the O−H stretching vibration was quite narrow and appeared at a rather low wavenumber (n-hexane, 3480 cm−1; KBr, 3449 cm−1), thus indicating the presence of an OH···π contact in both states.22 This conclusion is supported by a band at 1679 cm−1 (KBr) which was assigned to an O−H deformation vibration. It appears at a rather high wavenumber (normally 1200−1500 cm−1) and is comparable to the corresponding signal of associated hydroxyl groups (water, carboxylic acids).23 The molecular structure shows 4 to be planar chiral in the solid state (conformational chirality), caused by the aforementioned OH···π contact (Figure 2). However, it crystallizes as a racemate (space group P1̅, Z = 2). The hydroxyl hydrogen atom and one phenyl substituent of a silyl group are oriented toward each other, the hydrogen···phenyl plane and oxygen···phenyl plane distances being 234 and 306 pm, respectively. Both values are significantly lower than the sum of the corresponding van der Waals radii. The second silyl group shows an inverse orientation: its methyl substituents are directed toward the oxygen atom O3. Their distances are at or somewhat below the sum of the van der Waals radii, thus indicating a weak interaction.24 The bond lengths of the former CC bonds and the Co−C bonds are comparable to those of 3.
The emerald green powder (5) was too microcrystalline for single-crystal X-ray diffraction. It was very air sensitive and changed color to yellow-brown on contact with oxygen. Even in a glovebox under argon it decomposed within some weeks. It was only soluble in halogenated hydrocarbons such as chloroform and 1,2-dichloroethane, forming emerald solutions, but these were not very stable, therefore recrystallization or even the acquisition of NMR spectra could not be achieved. The IR spectrum of solid 5 featured no carbonyl bands (except for small residual bands of 3). However, it was very similar to the IR spectra of 1, 3, and 4 (apart from the missing carbonyl bands), thus suggesting that 5 contains very similar or even identical organic ligands. The UV/vis spectrum featured two strong absorptions in the visible region at 421 and 612 nm, which are responsible for the green color of the compound. The elemental analysis gave values of 53.87% carbon and 6.16% hydrogen, resulting in a H/C ratio of 1.363, which is very close to that of silane 1 (1.385). On the basis of these limited data we assume that 5 is a cobaltcontaining compound, possibly a cluster complex. Further considerations about the structure of 5 can be found in the Supporting Information. To further investigate the reaction of 1 with Co2(CO)8, it was repeated in 1,4-dioxane as the solvent. The reaction took a similar course, again furnishing four products: complexes 2−4 (all identified spectroscopically) as well as a black powder instead of compound 5. It is worth noting that the yields of 3 and 4 were distinctly higher (11% and 21%, respectively) than those with nheptane as the solvent. The black powder was ferromagnetic and soluble in hydrochloric acid with a dark blue color, thus turning out to be metallic cobalt. Just as for 5 it was the main product of the reaction with a yield of 45%. A dark green color of the hot reaction mixture after some hours of reflux indicated that 5 (or a similar compound) was formed initially, as in n-heptane, but then decomposed, possibly as a consequence of the donor solvent properties of 1,4-dioxane. This fact strongly supports the assumed high cobalt content of compound 5, as there is only slight doubt that 5 and the metallic cobalt correspond to each other with regard to the otherwise very similar course of the reaction and distribution of the products. As 4 is most likely formed from 3 by hydrogenative cleavage and can be reconverted into 3 by atmospheric oxygen (see solidstate IR data above),21 a redox equilibrium between 3 and 4 is postulated (Scheme 1). Interestingly, cobalt is formally oxidized in the reduction step of the cyclopentadienone ligand (3 (Co0) → 4 (Co+)) and vice versa. This is possible because the change in its oxidation state is overcompensated by the change of the oxidation states of the cyclopentadienone/cyclopentadienyl ring carbon atoms. Compound 4 is related to some similar hydroxycyclopentadienyl iron and ruthenium complexes which are used as hydrogenation catalysts.25 The conversion of 3 into 4 even resembles the hydrogenative cleavage of a dimeric ruthenium compound into such a catalyst complex.25a,d In conclusion, alkynylsilane 1 does not cyclotrimerize with the catalyst systems AlCl3/hexane, PdCl2/Me3SiCl/THF, and Co2(CO)8/(n-heptane or 1,4-dioxane). Instead, while AlCl3 and PdCl2/Me3SiCl cause cleavage of the Si−CC bond, Co2(CO)8 reacts with 1 to form the dimeric cyclopentadienone compound 3 after initial formation of the simple alkyne complex 2. Compound 3 is then converted into the hydroxycyclopentadienyl complex 4 by hydrogenative cleavage. 5624
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Pr cyclotrimerization is reported with moderate yields: Li, Y.; Butenschön, H. Chem. Commun. 2002, 2852−2853. (7) Jhingan, A. K.; Maier, W. F. J. Org. Chem. 1987, 52, 1161−1165. (8) (a) Schäfer, W. Angew. Chem. 1966, 78, 716−716; Angew. Chem., Int. Ed. Engl. 1966, 5, 669−669. (b) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654−7655. Cyclotrimerization is an unwanted side reaction in the AlCl3-catalyzed hydrosilylation of alkynes. (9) Details about the experiments with PdCl2/Me3SiCl/THF and AlCl3 can be found in the Supporting Information. (10) Traces of 4 and other substances were observed as well. (11) For example, replacement with phosphines at −10 °C: Iwashita, Y.; Tamura, F.; Nakamura, A. Inorg. Chem. 1969, 8, 1179−1183. (12) (a) Hong, F.-E; Wu, J.-Y.; Huang, Y.-Ch.; Hung, Ch.-K.; Gau, H.M.; Lin, Ch.-Ch. J. Organomet. Chem. 1999, 580, 98−107. (b) Hong, F.E; Chang, Y.-Ch; Chang, R.-E.; Lin, Ch.-Ch.; Wang, S.-L.; Liao, F.-L. J. Organomet. Chem. 1999, 588, 160−166. (c) Baxter, R. J.; Knox, G. R.; Pauson, P. L.; Spicer, M. D. Organometallics 1999, 18, 215−218. (d) Hong, F.-E; Lien, F.-Ch; Chang, Y.-Ch.; Ko, B.-Ts. J. Chin. Chem. Soc. 2002, 49, 509−515. (e) A synthesis of the Me3SiCCH-derived compound with a yield of 64% was reported, as well.12b (13) Although this elemental analysis result is outside the range viewed as establishing analytical purity (Δ ≤ 0.4%), it is provided to illustrate the best value obtained to date. No precautions to avoid the formation of carbides and carbonates were taken. The diminished carbon values are ascribed to such byproducts of the combustion during elemental analysis. (a) The carbon value is 3.3% too low. Small amounts of 5, which could not be removed, were visible between the crystals under the microscope. (b) The carbon value is 2.1% too low. (14) Cis/trans isomerism is also observed in the analogous diene complexes: (a) Winkhaus, G.; Wilkinson, G. J. Chem. Soc. 1961, 602− 605. (b) McArdle, P.; Manning, A. R. J. Chem. Soc. A 1970, 2123−2128. (15) All average values are search results from the Cambridge Structural Database (CSD) of the CCDC. (16) Tyler, S. J.; Burlitch, J. M. J. Organomet. Chem. 1989, 361, 231− 247. (17) A similar feature was reported in ref 16 and was discussed as a partial CO bond or hydrogen bridge. (18) Racemization seems to be possible in solution; otherwise, all NMR signals should appear in duplicate. However, only one 29Si NMR signal was observed, for example. (19) (a) Lucken, E. A. C.; Noack, K.; Williams, D. F. J. Chem. Soc. A 1967, 148−154 (calculated from 6.8 G at 14.197 MHz). (b) von Philipsborn, W. Pure Appl. Chem. 1986, 58, 513−528. (20) King, R. B.; Bisnette, M. B. J. Organomet. Chem. 1967, 8, 287−297. (21) A similar observation was reported for [H2Ph2C5OH]Co(CO)2.16 (22) (a) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π InteractionEvidence, Nature, and Consequences; Wiley-VCH: New York, 1998; p 81 (νO−H(MeOH in CCl4) 3645 cm−1, νO−H(PhOH in CCl4) 3610 cm−1, νO−H(PhOH in PhMe) 3547 cm−1, νO−H(PhOH in C6Me6) 3505 cm−1). (b) Reference 16: νO−H([H2Ph2C5OH]Co(CO)2) 3453 cm−1. (23) (a) Steger, E., et al. Lehrwerk Chemie, Arbeitsbuch 3, Strukturaufklärung-Spektroskopie und Röntgenbeugung; VEB Deutscher Verlag für Grundstoffindustrie: Leipzig, Germany, 1973. (b) Rauscher, K., et al. Chemische Tabellen und Rechentafeln für die analytische Praxis, 9th ed.; Harri Deutsch: Thun, Frankfurt am Main, Germany, 1993. (24) Interestingly, the phenyl substituent of this silyl group is directed toward the neighboring propyl substituent of the cyclopentadienyl ring. Although their distance is clearly above the sum of the van der Waals radii, the aromatic ring current may exert an influence on the propyl protons, thus explaining the chemical inequivalence of the propyl groups in the 1H NMR spectrum. (25) A list of selected references (25a−h) can be found in the Supporting Information. (26) (a) Holleman, A. F.; Wiberg, E.; Wiberg, N. Lehrbuch der Anorganischen Chemie, 101st ed.; Walter de Gruyter: Berlin, 1995; pp 1838−1841. (b) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction-Evidence, Nature, and Consequences; Wiley-VCH: New York, 1998; pp 34−35.
This cleavage appears to be reversible by oxidation, thus establishing a hitherto unknown link between these two rare types of cobalt complexes. The formation of cyclopentadienederived rings, which is reminiscent of the Pauson−Khand reaction, instead of benzene derivatives is ascribed to the sterically demanding substituents at the alkyne triple bond.5a However, the main product of the reaction of silane 1 with Co2(CO)8 is a hitherto unidentified cobalt-rich compound in nheptane and metallic cobalt in 1,4-dioxane, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, schemes, tables, and CIF files comprising cyclotrimerizability study of 1, considerations about the source of the hydroxyl hydrogen in 4 and the structure of 5, NMR and IR data and spectra, experimental details, additional references, crystal structure data, and bond lengths, bond angles, and torsion angles for complexes 3, 4, and S5·1.75C6D6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*G.R.: tel, +49 (0) 3731/39 4345; fax, +49 (0) 3731/39 4058; email,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft and the Fonds der chemischen Industrie for financial support. REFERENCES
(1) (a) Hoffmann, F.; Wagler, J.; Roewer, G. Eur. J. Inorg. Chem. 2010, 1133−1142. (b) Hoffmann, F.; Wagler, J.; Böhme, U.; Roewer, G. J. Organomet. Chem. 2012, 705, 59−69. (c) Hoffmann, F.; Wagler, J.; Roewer, G. Eur. J. Inorg. Chem. 2012, 6018−6026. (d) Hoffmann, F.; Wagler, J.; Roewer, G. Organometallics 2013, 32, 4531−4542. (2) (a) Reppe, W.; Schweckendieck, W. J. Liebigs Ann. Chem. 1948, 560, 104−116. (b) Dickson, R. S.; Fraser, P. J. Adv. Organomet. Chem. 1974, 12, 323−377. (c) Schore, N. E. In Comprehensive Organic Synthesis-Selectivity, Strategy & Efficiency in Modern Organic Chemistry; Trost, B. M., Fleming, I., et al., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 5 (Paquette, L. A., Volume Ed.), Chapter 9.4.4.3, pp 1144− 1152 and Chapter 9.4.5.1, pp 1152−1155. (d) Grotjahn, D. B. In Comprehensive Organometallic Chemistry II-A Review of the Literature 1982−1994; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier Science: Amsterdam, 1995; Vol. 12 (Hegedus, L. S., Volume Ed.), Chapter 7.3, pp 741−770. (3) (a) Wakatsuki, Y.; Yamazaki, H. Tetrahedron Lett. 1973, 14, 3383− 3384. (b) Bönnemann, H. Angew. Chem. 1978, 90, 517−526; Angew. Chem., Int. Ed. Engl. 1978, 17, 505−515. (c) Bönnemann, H. Angew. Chem. 1985, 97, 264−279; Angew. Chem., Int. Ed. Engl. 1985, 24, 248− 262. (4) (a) Hübel, H.; Hoogzand, C. Chem. Ber. 1960, 93, 103−115. (b) Schore, N. E. Chem. Rev. 1988, 88, 1081−1119. (5) (a) Krüerke, U.; Hübel, W. Chem. Ber. 1961, 94, 2829−2856. (b) Sappa, E.; Tiripicchio, A.; Braunstein, P. Chem. Rev. 1983, 83, 203− 239. (c) El Amouri, H.; Gruselle, M. Chem. Rev. 1996, 96, 1077−1103. (d) Sugihara, T.; Yamaguchi, M.; Nishizawa, M. Rev. Heteroat. Chem. 1999, 21, 179−194. (e) Perez del Valle, C.; Milet, A.; Gimbert, Y.; Greene, A. E. Angew. Chem. 2005, 117, 5863−5865; Angew. Chem., Int. Ed. Engl. 2005, 44, 5717−5719. (6) The cyclotrimerizability of a certain alkyne is not easy to assess: for example, t-BuCCH cannot be cyclotrimerized, but the sterically more demanding Me3SiCCH can.4a Also for PhCCPh and i-PrCC-i5625
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