Cobalt-complexed propargyl cations: Generation under neutral conditions and spontaneous, high-temperature conversion to propargyl Radicals

Article abstract of DOI:10.1021/om900448j

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 Co ₂(CO)₆-complexed prop

Full text of DOI:10.1021/om900448j

Organometallics 2009, 28, 5541–5549 5541
DOI: 10.1021/om900448j
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 cluster-
to-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
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,
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-
*Corresponding author. E-mail: gagik.melikyan@csun.edu.
(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,
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Organic Synthesis; Bateson, J. H., Mitchell, M. B., Eds.; Academic Press:
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(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.
(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.
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Kapon, M.; Cais, M.; Herbstein, F. H. Angew. Chem., Int. Ed. 1972, 11,
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De Benedetto, L.; McGlinchey, M. J. Organometallics 1993, 12, 4545. (d)
Melikyan, G. G.; Bright, S.; Monroe, T.; Hardcastle, K. M.; Ciurash, J.
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2009 American Chemical Society
Published on Web 08/28/2009
pubs.acs.org/Organometallics

5542 Organometallics, Vol. 28, No. 18, 2009
Scheme 1. Generation of the Propargyl Triflate 3 under Acidic Conditions: Method A
Melikyan et al.
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, develop-
ment 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 compat-
ibility of the reagents with peripheral functionalities becomes
pivotal. Herewith we report on the novel method for genera-
tion of cobalt-complexed propargyl cations under neutral
conditions and their spontaneous intermolecular dimerization
reactions. It stems from our long-standing interest in devel-
oping 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
by-product. The latter, being a superacid, creates a highly
acidic environment and makes the whole process inapplic-
able 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 high-
temperature 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-tetrachlor-
oethane 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 rela-
tively brief. Further attempts to lower the reaction tempera-
ture (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 diastereoselec-
tivity.
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. Organome-
tallics 2007, 26, 3173. (i) Melikyan, G. G.; Wild, C.; Toure, P. Organome-
tallics 2008, 27, 1569. (j) Melikyan, G. G.; Mikailian, B.; Sepanian, R.;
Toure, P. J. Organomet. Chem. 2009, 694, 785.
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
component, CF3SOþ2 vs Me3Siþ. 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.

Article
Organometallics, Vol. 28, No. 18, 2009 5543
Table 1. Spontaneous, High-Temperature Radical Coupling of Cobalt-Complexed Propargyl Triflates
reactant
composition
crude 6
T, °C reaction time, min d,l:meso yield,a %
isolated 6
d,l:meso
experimental
medium
acidic (TfOHb)
1
2
3
4
method A
method A
method B
method C
alcohol 1 þ 2 equiv Tf2O
alcohol 1 þ 1 equiv Tf2O
147
147
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
acidic (TfOHb)
alcohol 1 þ 1 equiv Me3SiOTf acidic (TfOHb/Me3SiOHb)
Me ether 10 þ 1 equiv Tf2O
neutral
83
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
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 equimo-
lar 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
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 condi-
tions 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.

5544 Organometallics, Vol. 28, No. 18, 2009
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 low-
temperature 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 neu-
tral 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-diastereoselec-
tion. 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), trifluor-
oacetic 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-diastereoselec-
tivity (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-com-
plexed propargyl trifluoroacetate 14 and methyl trifluoro-
acetate (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; 1,1,2,2-
tetrachloroethane). 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 com-
pounds (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 dis-
parity could be best accounted for by invoking a steric factor;
that is, a phenyl group happens to be much larger than a CF3
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 ,
3
62.46 A ), 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 conden-
sation product of sodium acetylide with the respective ben-
zaldehyde9 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,l-
89-99%; 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-com-
plexes 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-com-
plexed propargyl alcohols were converted to the respective
triflates when treated with Tf2O; to neutralize triflic acid
3
(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.;
˚
group, by 60 A (PCModel). Given the bulkiness of a di-
nuclear metal core positioned in the immediate vicinity of a
MeO group, a nucleophile, it is conceivable that even the
Markby, R.; Wender, I. J. Am. Chem. Soc. 1956, 78, 120.

Article
Organometallics, Vol. 28, No. 18, 2009 5545
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
NR2 group is positioned alpha to the propargyl carbon.3e
Scheme 6
Topologically complex, fused cyclooctynes were synthe-
sized, via R-alkoxy propargyl triflates, by the interaction
of cobalt-complexed unsymmetrical acetals with trialkyl
silyl triflates, such as Me3SiOTf, TBDMSiOTf, and Et3-
SiOTf.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,
formed in the course of the reaction, 2,6-di-tert-butyl-4-
such as tosylates5k,11f,g and mesylates.11h
methylpyridine was utilized.4f,11c Presynthesized propargyl
triflate can also be complexed with a Co core, if a donating
(12) (a) Curran, D. P. In Comprehensive Organic Synthesis; Trost, B.
(11) Co2(CO)6-complexed propargyl triflates: (a) Tanaka, S.; Isobe,
M. Tetrahedron Lett. 1993, 34, 5757. (b) Jacobi, P. A.; DeSimone, R. W.
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5546 Organometallics, Vol. 28, No. 18, 2009
Melikyan et al.
It is worthy to mention that intermolecular radical dimer-
ization 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 Unfortu-
nately, 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 re-
present a synthetic challenge. The “classical” propargyl-
propargyl coupling reaction exhibits a poor regioselectivity
due to acetylene-allene rearrangement.15a Catalytic pro-
cesses (Ru, Pd) are inherently limited in scope.15b,c Their
yields and diastereoselectivities drastically decrease in the
presence of electron-withdrawing (CF3) and electron-donat-
ing substituents (Me; OMe), as well as the bulky substituents
(naphthyl group).15b Also, metal-induced dimerizations ex-
hibit a low regioselectivity because of the formation of
isomeric allene-ynes15c and could potentially have reprodu-
cibility problems. The alternative mediation of intermolecu-
lar coupling of propargyl alcohols with a Ti(OiPr)2Cl2/Mg
suffers from the low conversions (∼70%), lack of diastereos-
electivity, and a poor regioselectivity, with target 1,5-alka-
diynes being accompanied by comparable quantities of
acetylenic allenes (45-50%).15d
propargyl ethers, generation of propargyl triflates, forma-
tion of propargyl radicals, and dimerization thereof;sub-
stantially enhances the synthetic potential of cobalt-
mediated radical dimerization reactions. It also allows the
use of substrates containing acid-sensitive peripheral func-
tionalities (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 13
C
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 tetra-
methylsilane. Spin-spin coupling constants (J) are given in
hertz. Elemental analyses were performed by Desert Analytics
(Tucson, AZ). Melting temperatures (uncorrected) were mea-
sured 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 com-
plexation with dicobaltoctacarbonyl.10
Conclusion
Generation of cobalt-complexed propargyl cations can be
carried out under neutral conditions by employing, as sub-
strates, 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 cobalt-
complexed propargyl triflates, which, in turn, undergo spon-
taneous conversion to propargyl radicals via cluster-to-
cluster and cluster-to-ligand single-electron transfers. Radi-
cal 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-1,5-
alkadiynes can be produced in good to high yields, as pure d,
l-diastereomers. Ionic propargyl triflates can also be synthe-
sized 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
General Procedure for the Synthesis of Methyl Propargyl
Ethers [RCtCCH(OMe)Ar]Co2(CO)6 (Protocol A: Acidic Con-
ditions). Under an atmosphere of nitrogen, at -20 °C, a solution
of HBF4 Me2O (6.0 mmol) was added dropwise (5 min) to a
3
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 pres-
sure, and the crude mixture was fractionated by column chro-
matography.
(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
(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.; Else-
vier: 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.
[μ-η2-3-Methoxy-3-(40-methoxyphenyl)-1-propyne]dicobalt Hexa-
carbonyl (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.
1H NMR (400 MHz, CDCl3): δ 3.43 (3H, s, OMe), 3.80 (3H, s,

Article
Organometallics, Vol. 28, No. 18, 2009 5547
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.
(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.
[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 nitro-
gen, 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 re-
duced 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 pres-
sure, 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 capil-
lary; 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 -
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
1
mg, 85.4%) as a dark red oil. H NMR (400 MHz, CDCl3): δ
3.47 (3H, s, OMe), 3.80 (3H, s, 4-OMe), 3.88 (6H, s, 3-OMe, 5-
OMe), 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.
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) accord-
ing 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):
δ3.44(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.
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 chromatogra-
phy on Florisil (15 g, PE) and subsequent preparative TLC (2
plates, PE), affording 19 (120 mg, 63.0%) as a dark red oil. TLC
[μ-η2-3-Methoxy-3-(30,40,50-trimethoxyphenyl)-1-propyne]dico-
balt 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 satu-
rated 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
(PE:E, 10:1): Rf = 0.56. H NMR (400 MHz, CDCl3): δ 1.25
1
(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 re-
quires: C, 46.98; H, 3.07.
Synthesis of Methyl Propargyl Ethers [RCtCCH(OMe)-
Ar]Co2(CO)6 under Nonacidic Conditions (Protocol B). (μ-η2-
1-Methoxy-1-phenyl-2-pentyne)dicobalt Hexacarbonyl (19). Un-
der 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

5548 Organometallics, Vol. 28, No. 18, 2009
Melikyan et al.
overnight, concentrated under reduced pressure, and fractio-
nated on a silica gel column (200 g, PE) to give 19 (482 mg,
17.5% over 3 steps).
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,l-
127.6, 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), meso-
143.1 (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.
Interaction of Methyl Propargyl Ethers 10 and 16-19 with
Triflic Anhydride 2 (ProtocolC): d,l- and meso-(μ-η2-3,4-Diphen-
yl-1,5-hexadiyne)bis(dicobalthexacarbonyl) (6). Under an atmo-
sphere 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, [HCtCCHPh-
CHPhCtCH]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, andthe reactionmixturewas 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 pres-
sure, and the residue was fractionated on a preparative TLC plate
d,l- and meso-(μ-η2-3,4-Diphenyl-1,5-hexadiyne)bis(dicobal-
thexacarbonyl) (6). (a) By Interaction of Propargyl Alcohol 1
and Trimethylsilyl Triflate (7). Under an atmosphere of nitro-
gen, 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 Anhy-
dride (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-dichlor-
oethane (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:meso-
6:[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).
(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 meso-
20 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-hexa-
diyne]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, 3ꢀ1 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). 13
C
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.
Decomplexation Step (Protocol D): d,l-3,4-Diphenyl-1,5-hex-
adiyne (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
d,l- and meso-[μ-η2-3,4-Di(30,40,50-trimethoxyphenyl)-1,5-
hexadiyne]bis(dicobalthexacarbonyl) (22). According to proto-
col 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 charac-
terized 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, meso-
23, and [C2H5CtCCH2Ph]Co2(CO)6 in the ratio of 72:8:20 (d,l-
23: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,
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). 13
C

Article
Organometallics, Vol. 28, No. 18, 2009 5549
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.
(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-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; coevapora-
tion with benzene, 3ꢀ1 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). Ac-
cording 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 capil-
lary; coevaporated with benzene, 3 ꢀ 1 mL). TLC (E): Rf 0.48.
1H 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- 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 prepara-
tive 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,l-
7.45-7.51 (4H, m, arom. H). 13C NMR (100 MHz, C6D6): d,l-
30 þ meso-30 δ meso- 12.59 (C2, C9), d,l- 12.66 (C2, C9), meso-
13.99 (C1, C10), d,l- 14.10 (C1, C10), meso- 46.27 (C5, C6), d,l-
46.41 (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). 13
C
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.
d,l-3,4-Di(30,40,50-trimethoxyphenyl)-1,5-hexadiyne (29). Ac-
cording 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: 100-
106 °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
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 Inter-
national Programs, and University Corporation,
California State University Northridge, for their gen-
erous support.