Cobalt-templated radical processes: Inter- and intramolecular coupling of propargyl radicals

Article abstract of DOI:10.1016/S0022-328X(98)01105-X

A novel method for radical C-C bond formation in the α-position to Co₂(CO)₆-clusters has been elaborated. The two-dimensional exploration of the process resulted in the discovery of structurally diverse O- and S-containing organic molecules capable of acting as efficient mediators in reductive coupling reactions, and in the elaboration of the preparative synthesis of 3,4-disubstituted 1,5-alkadiynes and carbocycles with eight- and nine-membered rings.

Full text of DOI:10.1016/S0022-328X(98)01105-X

Journal of Organometallic Chemistry 578 (1999) 68–75
Cobalt-templated radical processes: inter- and intramolecular coupling
of propargyl radicals
Gagik G. Melikyan *, Asatour Deravakian, Steven Myer, Sarkhadoun Yadegar,
Kenneth I. Hardcastle, Joana Ciurash, Pogban Toure
Department of Chemistry, California State Uni6ersity Northridge, Northridge, CA 91330, USA
Received 1 July 1998
Abstract
A novel method for radical CꢀC bond formation in the a-position to Co2(CO)6-clusters has been elaborated. The two-dimen-
sional exploration of the process resulted in the discovery of structurally diverse O- and S-containing organic molecules capable
of acting as efficient mediators in reductive coupling reactions, and in the elaboration of the preparative synthesis of
3,4-disubstituted 1,5-alkadiynes and carbocycles with eight- and nine-membered rings. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Propargyl cation; Propargyl radical; Cobalt complex; 1,5-Cycloalkadiyne; Crystal structure
1. Introduction
intermediate radical. In continuation of our program
on the chemistry of transition-metal-altered reactive
intermediates [1l–n,4], we reported recently on the in-
ter- and intramolecular reductive coupling of Co-com-
plexed propargyl cations mediated by zinc [1m,n].
While readily soluble cationic intermediates afford
dimeric products in good yields, the experimental pro-
tocols are less feasible for monocations with lower
solubility in CH2Cl2 and also for bis-propargyl cations,
the starting materials for cyclic 1,5-alkadiynes. The
main reason is the heterogeneity of the reducing agent
that results in prolonged reaction times and substan-
tially compromises the chemoselectivity of the process.
Partial success was achieved with benzophenone ketyl
in benzene [1n], although attendant with the latter are
substantial experimental inconveniences. We report
herein a novel experimental method for inter- and
intramolecular radical CꢀC bond formation in the a-
position to a Co-cluster which is mediated by a variety
of homogeneous agents, O- and S-containing organic
molecules.
The chemistry of carbon-centered radicals p-bonded
to transition metals is an emerging area at the interface
of radical organic and organometallic chemistry. While
the mode of interaction of an unpaired electron with a
metal cluster is poorly understood, as well as the
configurational and conformational specifics involved,
the novel methods for radical generation and some
useful CꢀC bond forming reactions have been reported
[1]. Our interest in this area stems from manganese-ini-
tiated radical reactions of conjugated enynes [2] with
chemoselectivity refined by complexation of the triple
bond with a Co2(CO)6-protecting group [1l], as well as
from facile, regioselective reduction of Co-complexed
propargyl chloride [3], providing perhaps experimental
evidence for the stabilizing effect of a metal core upon
* Corresponding author. Tel.: +1-818-677-2565; fax: +1-818-677-
4068.
E-mail address: hcchm025@csun.edu (G.G. Melikyan)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01105-X

G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
69
Scheme 1.
2. Results and discussion
hol 2 without a tedious isolation of intermediate cation
1 (Scheme 1, path B). The latter was achieved by a
tandem action of equimolar quantities of HBF4, a
Lewis acid, and radical mediators. Among the latter,
THF and 1,3-dioxolane exhibited the highest
diastereoselectivity of 90%. From the synthetic view-
point, the most attractive feature of novel protocols is
the utilization of homogeneous and easy-to-handle radi-
cal mediators which stands in sharp contrast to the
inconveniences associated with choosing heterogeneous
[1m] or air-sensitive [1n] reducing agents. The compati-
bility of novel radical mediators with practically every
functional group is another positive feature favorably
distinguishing them from highly reactive species, such
as benzophenone ketyl [1n]. It is noteworthy that the
stabilizing effect of the Co2(CO)6-moiety upon the de-
veloping cationic center in 1 is crucial: the reaction does
not take place with uncomplexed propargyl alcohols due
to significant retardation of the heterolysis step.
We found that treatment of the cation 1, derived
from Co-complexed propargyl alcohol 2 [5], with topo-
logically diverse O- and S-containing compounds af-
fords isomeric bis-clusters 3 and 4 (Scheme 1, path A).
The propargyl radical 5 is the most plausible precursor
to the latter, indicating that cation 1 undergoes a
one-electron reduction along the reaction coordinate. A
variety of structures triggering reductive dimerization is
exhibited in Scheme 1 comprising the five- and six-
membered cyclic compounds (tetrahydrofuran, 1,3-
dioxolane, tetrahydrothiophene, 1,3-dithiane) and their
acyclic analogs with up to three heteroatoms (formalde-
hyde dimethyl acetal, trimethyl orthoformate). The
standard protocol includes isolation of cation 1 [6] and
subsequent treatment with a twofold excess of radical
mediators at ambient temperature. The diastereoselec-
ti6ity is remarkably high and varies in the range of
68–94%, with the D, L-isomer 3 as a major product.
These results exceed by far the highest selectivity of
50% reported for Co-propargyl derivatives [1m], as well
as for other organometallic species [1]. The chemoselec-
ti6ity is determined by the concentration of the side-
product, a hydrocarbon (HCꢁCCH2Ph)Co2(CO)6 (6),
which reaches the highest mark of 14% with tetrahy-
drothiophene as a radical mediator. Its formation is not
practically observed with most of compounds tested
(51%) thus making recombination the major conver-
sion pathway for transient radicals 5.
The mechanism of radical mediation by O- and S-
containing compounds, in particular, the genesis of an
The method represents a two-step—heterolysis, one-
electron reduction—conversion of propargyl alcohol 2
to dimeric bis-clusters 3 and 4 with high diastereo- and
chemoselectivities. A further refinement of the parent
procedure resulted in the elaboration of an even more
attractive synthetic alternative, a one-step protocol,
which allows for a direct reducti6e dimerization of alco-
Scheme 2.

70
G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
Table 1
Crystal data and structure refinement for 8, 9 and 13
Identification code
8
9
13
Empirical formula
Formula weight
Temperature (K)
C20H10Co4O12
678.00
173(2)
C
678.00
293(2)
0.71073
20H10Co4O12
C33H18Co4O12
842.19
198(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Space group
Triclinic
Triclinic
Triclinic
(
(
(
P1 (No. 2)
P1 (No. 2)
P1 (No. 2)
Unit cell dimensions
˚
a (A)
7.669(4)
11.485(3)
13.944(3)
94.68(3)
91.77(3)
101.77(3)
6.936(4)
7.997(4)
11.941(4)
77.64(3)
80.57(4)
74.16(4)
10.043(3)
10.229(6)
17.515(9)
95.86(4)
94.41(3)
106.59(4)
˚
b (A)
˚
c (A)
h (°)
i (°)
k (°)
3
˚
Volume (A ), Z
1196.9(7), 2
1.881
2.785
618.5(5), 1
1.820
2.695
1704.8(14), 2
1.641
1.973
Density (calc.) (mg m−3
)
Absorption coefficient (mm−1
F(000)
)
668
334
840
Crystal size (mm)
q range for data collection (°)
Limiting indices
0.43×0.30×0.18
1.47–24.97
−95h59
−135k513
−165l516
8850
4197 (Rint=0.0670)
c
0.9996, 0.9304
0.25×0.20×0.15
1.76–24.95
−85h58
−95k59
−145l514
4356
2178 (Rint=0.0882)
c
0.9978, 0.9061
0.30×0.18×0.10
1.18–21.98
−105h510
−105k510
−25l518
4427
No. of reflections collected
No. of independent reflections
Absorption correction
Max and min transmission
Refinement method
4169 (Rint=0.0060)
None
Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2
Data/restraints/parameters
4191/0/325
2.103
2178/0/163
1.058
4168/0/337
1.197
Goodness-of-fit on F2
Final R indices [I]2|(I)]
R indices (all data)
Largest difference peak and hole (e A
Rl=0.0487, wR2=0.1012
R1=0.0582, wR2=0.1212
) 0.917, −0.635
R1=0.0562, wR2=0.1443
R1=0.0670, wR2=0.1633
0.912, −0.697
R1=0.0686, wR2=0.1765
Rl=0.0978, wR2=0.1985
0.749, −0.521
−3
˚
electron that reduces cation 1, is not well established
[7]. Despite its deceptive simplicity, the two-component
composition—cation 1 and radical mediator—provides
a number of chemically conceivable pathways which
might account for the formation of dimeric products.
The most obvious one includes delivery of a single
electron from an heteroatom (O, S) towards a cationic
center (Scheme 2, path C), which can be ruled out
based on several lines of evidence. First, reaction does
not occur with diethyl ether as used in the standard
isolation procedure [6]; second, the reaction time is
different for all four O-containing mediators, ranging
from 10 min to 4 h; and third, no acceleration is
observed with tetrahydrothiophene relative to tetrahy-
drofuran (l.5 h vs. 1.5 h), the one that might be
expected based on the lower electronegativity of the
sulfur atom. It should also be mentioned that the
interaction of Co-complexed cations with O-, S-, P- and
N-nucleophiles is well documented to produce respec-
tive solvolysis products, while formation of dimeric
bis-clusters has not been observed [6,8]. An alternative
scenario is represented by path D (Scheme 2) with
initial attack of an electrophile, a p-orbital in cation A,
on the filled bond orbital of aꢀCꢀH bond in mediator
B. Such a coordination is analogous to that postulated
for boronꢀhydrogen bond hydrolysis [9], and was also
invoked to interpret the structural features of stable
organic cations [10]. The tentative transition state (C)
might include a cyclic three-membered molecular frag-
ment [10b,11] which could undergo either single-elec-
tron transfer (SET) or hydride-ion transfer (HIT, [12])
to radical D or hydrocarbon E, respectively. The latter
itself can act as an electron source because the metal
cluster supposedly has a higher electron density com-
pared to that in cation A (path E). The reaction path-
ways are not limited to those in Scheme 2; in fact, a
number of other interactive modes can be envisioned
[13] which are currently under detailed mechanistic
investigation [14]. From a practical viewpoint, an un-
derstanding of the stoichiometry is important for calcu-
lation of the actual yields of dimeric products [15].
The one-step method for reductive dimerization has
also been applied to the secondary alcohol 7 which
produces a cation with a low solubility in methylene

G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
71
Fig. 1. X-Ray determined molecular structure of D, L-8 with 30%
˚
probability ellipsoids. Selected interatomic distances (A) and angles
(°): C7ꢀC8 1.318(7), ColꢀCo2 1.769(6), H7ꢀC7ꢀC8 136.0(3), C7ꢀ
C8ꢀC9 144.1(5), H7AꢀC7AꢀC8A 135.9(3), C7AꢀC8AꢀC9A 143.6(5),
H7AꢀH7 2.29(1), H7AꢀC7AꢀC8AꢀC9A −3.4(2), H7ꢀC7ꢀC8ꢀC9 −
4.6(2), H9ꢀC9ꢀC9AꢀH9A 172.0(5), C10ꢀC9ꢀC9AꢀC10A 46.1(3),
C8AꢀC9AꢀC9ꢀC8 −62.6(3), C8AꢀC9AꢀC9ꢀC10 −172.0(5), C8ꢀ
C9ꢀC9AꢀC10A −171.8(5), C7C8ꢀC7AC8A 11.8(3). The numbering
scheme used illustrates the pseudo-C2 geometry and was chosen to
facilitate comparison of bond lengths and angles in this molecule.
Fig. 3. X-Ray determined molecular structure of D, L-13 with 30%
probability ellipsoids. Selected interatomic distances (A) and angles
˚
(°): C51ꢀC52 1.34(1), C55ꢀC56 1.32(1), ColꢀCo2 2.464(2), Co3ꢀCo4
2.467(2), C53ꢀCol 3.21(1), C53ꢀCo2 3.20(1), C54ꢀCo3 3.20(1),
C54ꢀCo4 3.18(1), C59ꢀCol 3.19(1), C59ꢀCo2 3.11(1), C57ꢀCo3
3.24(1), C57ꢀCo4 3.13(1), C70ꢀC61 3.08(1), C70ꢀC65 4.05(1),
C52ꢀC51ꢀC59 145.5(9), C51ꢀC52ꢀC53 143.9(8), C70ꢀC53ꢀC54
chloride; this, in turn, results in low conversion (58%)
and extended reaction time (17 h) even in the presence
of fivefold excess of heterogeneous reducing agent [1m].
Under standard procedure including the treatment of
alcohol 7 with twofold excess of HBF4 and tetrahydro-
furan, D, L- and meso-isomers 8 and 9 were isolated and
their structures established by means of X-ray crystal-
lography (Table 1). The former adopts a pseudo-C2
symmetric conformation (Fig. 1) with noticeably dis-
torted gauche-orientation of methyl groups (C10ꢀC9ꢀ
116.7(7),
C70ꢀC53ꢀH53
106.6(5),
C60ꢀC54ꢀC53
114.8(7),
C60ꢀC54ꢀH54 107.1(4), C56ꢀC55ꢀC54 143.2(8), C55ꢀC56ꢀC57
143.5(8),
C51ꢀC59ꢀC58
C70ꢀC53ꢀC54ꢀC60
C58ꢀC57ꢀC56
118.1(8),
H54ꢀC54ꢀC53ꢀH53
−28.5(3), C52ꢀC53ꢀC54ꢀC55
C57ꢀC58ꢀC59
116.0(8),
−150.8(4),
79.6(4),
115.8(8),
C55ꢀC54ꢀC53ꢀC70 −153.8(7), C52ꢀC53ꢀC54ꢀC60 −155.1(6),
C53ꢀC52ꢀC51ꢀC59 3.6(3), C54ꢀC55ꢀC56ꢀC57 6.3(3); angle between
the bonds: C55ꢀC56 and Co3ꢀCo4 88.7(6), C51ꢀC52 and ColꢀCo2
90.3(6), ColꢀCo2 and Co3ꢀCo4 42.6(5), C51ꢀC52 and C55ꢀC56
44.5(5); angle between phenyl rings 60.8(5).
˚
Fig. 2. X-Ray determined molecular structure of meso-9 with 30% probability ellipsoids. Selected interatomic distances (A) and angles (°): C7ꢀC8
1.323(3), ColꢀCo2 1.775(2), H7ꢀC7ꢀC8 139.9(1), C7ꢀC8ꢀC9 144.2(2), H7ꢀC7ꢀC8ꢀC9 6.7(1), H9AꢀC9AꢀC9ꢀH9 180, C8ꢀC9ꢀC9AꢀC10A 54.5(1).
Molecules of meso-9 contain a center of symmetry utilized and consistent with the special position symmetry of the lattice, e.g. there is only one
(
molecule per unit cell in space group P1(c2) and the molecule sits on the crystallographic center.

72
G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
C54ꢀC53 114.8°) and dihedral angles (H54ꢀC54ꢀC53ꢀ
H53 −150.8°; C70ꢀC53ꢀC54ꢀC60 −28.5°). The aro-
matic nuclei are twisted relative to each other forming
an angle of 60.8°; in accord with this deviation are the
following non-bonding interatomic distances: C70ꢀC61
˚
˚
3.08A and C70ꢀC65 4.05A. While cobalt-alkyne units
are essentially undistorted with angles of bent triple
bonds lying within the expected range (143–145°) and
nearly planar p-bonded ligands (3.6°, 6.3°), the nine-
membered ring is nevertheless highly strained. It is
substantiated by bond angles for sp3-hybridized ring
carbons (C58ꢀC57ꢀC56 118.1°, C57ꢀC58ꢀC59 116.0°,
C51ꢀC59ꢀC58 115.8°), and the relative twist of metal
cores quantified by the angle between metal–metal
vectors (Co1–Co2 and Co3–Co4 42.6°) or CꢀC triple
bonds (C51ꢀC52 and C55ꢀC56 44.5°). It is worthy to
mention that the molecule has C2-symmetry with the
C58 atom positioned on the pseudo twofold axis which,
in turn, bisects the C53ꢀC54 bond.
The novel approach to the synthesis of 1,5-cycloocta-
and 1,5-cyclononadiynes (12, 13) constitutes the major
improvement over the existing methods [1m–n,17]. The
exclusive D, L-diastereoselectivity can be explained
based on conformational analysis of reactive intermedi-
ates (Schemes 3 and 4). Diradical F, a precursor of
D, L-12 and D, L-13, represents the only conformation
with anti-disposition of H-atoms and two gauche inter-
actions between bulkier substituents. The rotamer G
has three destabilizing gauche interactions which make
it kinetically disfavored, and the cyclization process
itself highly stereoselective. In intermolecular coupling
reactions the steric hindrance is minimized for H, an
acyclic analog of conformer F, which produces D, L-iso-
mers 3 and 8. The major structural difference between
otherwise analogous species G and I is the tether:
sterically hindered intermediates I can convert to more
stable species J which are responsible for the formation
of meso-isomers 4 and 9. Such an explanation is sub-
stantiated by results of X-ray crystallographic analysis
(Figs. 1 and 2) revealing C2 and Ci-symmetry for D, L-8
and meso-9, respectively. The difference in diastereose-
lectivity for dimerization of alcohols 2 and 7 (68–94%
Scheme 3.
C9AꢀC10A 46.1°) and nearly ideal disposition of Co-
alkyne units (C8AꢀC9AꢀC9ꢀC8 62.6°). As molecular
modeling shows, the repulsion between axial CO-
groups pushes the metal clusters away from each other,
which in turn brings the CꢀC triple bonds in close-to-
parallel locations (C7C8ꢀC7AC8A 11.8°). Acetylenic
hydrogens (H7A, H7) are remarkably proximate to
˚
each other with a separation distance of 2.29 A, less
than the sum of the van der Waals radii (about 2.5 A).
˚
Other noteworthy structural features of 8 include: (1)
an essentially undistorted planarity of alkyne moieties
(H7AꢀC7AꢀC8AꢀC9A −3.4°, H7ꢀC7ꢀC8ꢀC9 −4.6°);
(2) a bent geometry for the coordinated alkyne unit
(C7ꢀC8ꢀC9 144.1°, C7AꢀC8AꢀC9A 143.6°) with sub-
stantially smaller angles for acetylenic termini (H7ꢀ
C7ꢀC8 136.0°, H7AꢀC7AꢀC8A 135.9°); and (3) a
˚
lengthened coordinated CꢀC triple bond (1.32–1.33 A
vs. 1.21 A for free ligand) attendant with complexation
˚
to transition metal. The meso-complex 9 has Ci-symme-
try with the center of symmetry positioned on the
C9ꢀC9A bond (Fig. 2). In contrast to the D, L-isomer,
the molecule crystallizes in an extended conformation
with metal clusters further apart from each other.
The novel synthetic methodology has also been ap-
plied for the construction of eight- and nine-membered
carbocycles with 1,5-disposition of triple bonds. Synthe-
sized by conventional methods [16], bis-complexes 10
and 11 were exposed to a combined action of HBF4
and THF in optimized molar ratios of 1:10:10 and
1:4:20, respectively. In situ generation of bis-cationoid
species and subsequent double reduction to bis-radicals
affords cyclized products 12 and 13, each represented
by a single stereoisomer. Their configurations as D, L
-
were determined by spectral and chromatographic com-
parison with an authentic sample (12, [1n]), and by
means of X-ray diffraction (13, Fig. 3). The molecular
assembly around carbon atoms bearing phenyl groups
(C53, C54) is severely distorted, which is indicated by
values of bond angles (C70ꢀC53ꢀC54 116.7°; C60ꢀ
vs. 8% [18]) is likely determined by the larger bulkiness
3
˚
of the phenyl group relative to the methyl (104 A vs.
3
˚
33 A ) which might result in better kinetic differentia-
tion of the intermediates H and J.
Scheme 4.

G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
73
3. Experimental
in the ratio of 54:46 (de 8%). TLC (PE): Rf 0.52. M.p.
50–80°C (decomp. without melting). 1H-NMR (200
MHz, CDCl3): D, L-diastereomer 8 6.14 (d, 2H, HCꢁ,
J=0.4 Hz), 3.04 (m, 2H, CH), 1.35 (d, 6H, CH3,
J=6.7 Hz); meso-diastereomer 9 6.18 (s, 2H, HCꢁ),
2.64 (m, 2H, CH), 1.52 (d, 6H, CH3, J=6.2 Hz).
MS-FAB: 678 (M−, 2%), 622 (M−ꢀ2CO, 4%), 594
(M−ꢀ3CO, 11%), 566 (M−ꢀ4CO, 9%), 538 (M−ꢀ5CO,
5%), 510 (M−ꢀ6CO, 6%), 479 (M−ꢀ5COꢀCo, 19%),
451 (M−ꢀ6COꢀCo, 8%), 426 (M−ꢀ9CO, 3%). Single
crystals suitable for X-ray structure analysis (Figs. 1
and 2) were obtained by methanol vapor diffusion into
a solution of Co-complexes D, L-8 and meso-9 in
petroleum ether.
3.1. Syntheses
3.1.1. D, L- and meso-v-p2-(3,4-Diphenyl-1,5-hexa-
diyne)-bis-dicobalt hexacarbonyl (3, 4) and v-
p2-(3-phenyl-1-propyne)dicobalt hexacarbonyl (6)
3.1.1.1. With isolation of propargyl cation 1 (two-step
protocol). Under a nitrogen atmosphere, complex 2 (105
mg, 0.25 mmol) was placed in a flame-dried flask and
dissolved in dry diethyl ether (20 ml). A solution was
cooled (−20°C) and treated with HBF4 · Et2O (203
mg, 1.25 mmol). After stirring for 30 min, the ethereal
layer was removed, and cation 1 was washed with dry
ether (3×20 ml) at −20°C. A residual amount of
ether was stripped under reduced pressure to afford
cation 1 (104 mg, 85%) as a dark-red solid. Methylene
chloride (2.5 ml) and radical mediator (0.5 mmol) were
added at room temperature, and the reaction was mon-
itored by TLC. The crude mixture was filtered through
a short bed of Florisil (60–100 mesh, 700 mg), and the
ratio of products 3:4:6:14 [19] was determined by
NMR. The isolation of dimers was carried out on an
SiO2-column (70–230 mesh, 10 g, petroleum ether) to
afford dark-red crystals which were identical by
physico-chemical and spectral (TLC, NMR, FAB) data
with authentic samples synthesized according to the
known procedure [1m]. Radical mediator, reaction
time, ratio of 3, 4, 6 and 14 (NMR), yields of 3+4 (%),
de: tetrahydrofuran, 1.5 h, 93:5:1:1, 46 [15], 90; 1,3-
dioxolane, 4 h, 88:5:6:1, 38 [15], 90; formaldehyde
dimethyl acetal, 2 h, 95:3:0:2 [20a], 32 [15], 94; trimethyl
orthoformate, 10 min, 92:8:0:0 [20b], 27 [15], 84; te-
trahydrothiophene, 1.5 h, 70:13:14:3, 37 [15], 68; 1,3-
dithiane, 1 h, 90:5:0:5, 26 [15], 90.
3.1.3. bis-(D, L-3,4-Diphenyl-1,5-cyclooctadiyne)-
dicobalt hexacarbonyl (12)
Under nitrogen atmosphere, a solution of complex 10
[1k,16] (48 mg, 0.056 mmol) and THF (40 mg, 0.56
mmol) in CH2Cl2 (0.56 ml, [10]=0.1 M) was treated
with HBF4 · Et2O (90 mg, 0.56 mmol) at −5°C. Reac-
tion mixture was warmed up to room temperature and
stirred additionally for another 7 h (TLC control).
Isolation by preparative TLC (SiO2, petroleum
ether:ether 3:1, 4°C) afforded 15 mg of 12 (39% [15], de
100%, conversion 84%) identical by physico-chemical
(m.p., TLC) and spectral (NMR, FAB) data with au-
thentic sample synthesized according to a known proce-
dure [1n].
3.1.4. bis-(D, L-3,4-Diphenyl-1,5-cyclononadiyne)-
dicobalt hexacarbonyl (13)
Under a nitrogen atmosphere, a solution of complex
11 [16] (98 mg, 0.112 mmol) and THF (161 mg, 2.24
mmol) in CH2Cl2 (11.2 ml, [11]=0.01 M) was treated
with HBF4 · Et2O (73 mg, 0.448 mmol) at −5°C. The
reaction mixture was warmed up to room temperature
and stirred for 5 h (TLC control). To compensate for
minor mono-decomplexation of 13, the reaction mix-
ture was treated with Co2(CO)8 (15 mg, 0.044 mmol)
for 2 h, concentrated under reduced pressure to 2.5 ml
and applied to preparative TLC plate (SiO2, pentane,
4°C, extraction with THF). Afforded was 41 mg of 13
(43% [15], de 100%) as a dark-red solid. TLC (PE:E
3.1.1.2. Without isolation of propargyl cation 1 (one-step
protocol). As described previously [5], a treatment of
complex 2 (105 mg, 0.25 mmol) with THF (36 mg, 0.5
mmol) and HBF4 · Et2O (81 mg, 0.5 mmol) in CH2Cl2
(2.5 ml, 0.1 M) for 4.5 h afforded 3 and 4 (de 90%) with
a yield of 47% [15]. Analogous reaction with 1,3-diox-
olane (37 mg, 0.5 mmol) in 7 h yields 3, 4 and 14 in the
ratio of 94.9:4.7:0.4 (46% [15], de 90%).
1
15:1): Rf 0.70. Tdecomp 40–45°C (without melting). H-
3.1.2. D, L- and meso-v-p2-(3,4-Dimethyl-1,5-hexa-
diyne)-bis-dicobalt hexacarbonyl (8, 9)
NMR (200 MHz, C6D6): 1.65–1.85 (m, 9 1ines, 2H,
CH2), 3.02 (ABX2 system, 4H, 2CH2, JH(A)–H(B)=15.1,
Under nitrogen atmosphere, a solution of complex 7
(89 mg, 0.25 mmol) and THF (36 mg, 0.5 mmol) in
CH2Cl2 (2.5 ml, 0.1 M) was cooled (−5°C) and treated
with HBF4 · Et2O (81 mg, 0.5 mmol). Reaction mixture
was allowed to warm up to room temperature (30 min),
stirred for 3 h (TLC control), filtered through Florisil
(100–200 mesh, 14 g, pentane) under anaerobic condi-
tions and concentrated. Fractionation by preparative
TLC (SiO2, pentane) afforded 8 and 9 (17 mg, 20% [15])
JH(A)–H(X)=4.5, JH(B)–H(X)=8.6), 4.62 (s, 2H, CH),
6.70–7.10 (m, 10H, 2Ph). 13C-NMR (50 MHz, C6D6):
28.5, 33.0, 55.3, 97.5, 107.9, 127.0, 128.2, 128.3, 129.9,
130.1, 200.1. FAB: 842 (M+, 3.5%), 814 (MꢀCO,
5.5%), 786 (Mꢀ2CO, 6.7%), 758 (Mꢀ3CO, 16.9%), 730
(Mꢀ4CO, 8.9%), 702 (Mꢀ5CO, 4.0%), 674 (Mꢀ6CO,
6.9%), 646 (Mꢀ7CO, 5.1%), 615 (Mꢀ6COꢀCo, 20.4%),
562 (Mꢀ10CO, 3.1%), 528 (Mꢀ7COꢀ2Co, 15.9%). HR-
FAB: calc. for C33H18O12Co4 M+ 841.812618, found

74
G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
841.814800. Single crystals suitable for X-ray structure
analysis (Fig. 3) were obtained by methanol vapor
diffusion into a solution of 13 in benzene.
for complex 9, and No. 102653 for complex 13. Copies
of this information may be obtained free of charge
from the Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK [fax (int code) +44 (1223) 336-033, or
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk].
3.2. Crystallography
Suitable crystals, each, of D, L-8, meso-9 and D, L-13,
were mounted and placed in a goniometer head on the
Enraf-Nonius CAD4 diffractometer, and centered opti-
cally. The crystal of D, L-8 was glued to a glass fiber
held in a brass pin, but the crystals of meso-9 and
D, L-13 were coated with Paratone oil and placed on the
ends of 3 mm silica rods which had been drawn out to
a 0.10 mm fiber and cooled to 173 and 198 K, respec-
tively. Cell parameters and an orientation matrix for
data collection were obtained by using the centering
program in the CAD 4 system. Details of the crystal
data for each compound are given in Table 1. For each
crystal, the actual scan range was calculated by scan
width=scan range+0.35 tan q and backgrounds were
measured by using the moving-crystal-moving-counter
technique at the beginning and end of each scan. Two
representative reflections were monitored every 2 h as a
check on instrument and crystal stability. Lorentz, po-
larization, and decay corrections were applied to all
data sets, and an empirical absorption correction, based
on a series of c scans, was applied to the data for D, L-8
and meso-9. The weighting scheme used during refine-
ment was 1/|2, based on counting statistics.
The structures were solved by the direct methods
using SHELXS-86 [21], which revealed the positions of
most of the atoms. All other non-hydrogen atoms were
found by successive difference Fourier syntheses. Hy-
drogen atoms were placed in their expected chemical
positions using the HFIX command in SHELXL-93
[22] and all hydrogens were included in the final cycles
of least squares with isotropic Uij values related to the
atom’s ridden upon. All other non-hydrogen atoms
were refined anisotropically, except the noncarbonyl
carbon atoms in complex 13.
Acknowledgements
Financial support from the Office of Graduate Stud-
ies, Research and International Programs and Univer-
sity Corporation, California State University
Northridge is greatly appreciated. We thank Ms. Lisa
Barron and Ms. Linda Hayrapetyan for technical
assistance.
References
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Scattering factors were taken from the International
Tables for X-ray Crystallography [23]. All data process-
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tables for publication were carried out on PCs using
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4. Supplementary material
[7] For SET-induced organic and organometallic transformations
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Supplementary materials consist of coordinates, ther-
mal parameters, complete bond distances and angles for
complexes 8, 9 and 13. Crystallographic data has been
deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 102651 for complex 8, No. 102652

G.G. Melikyan et al. / Journal of Organometallic Chemistry 578 (1999) 68–75
75
Gelling, F. Mohmand, J.C. Jeffery, M.J. Went, J. Chem. Soc.
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[14] A detailed mechanistic study, including measurements of kinetic
isotope effect, are currently in progress; the results will be
reported in forthcoming full account.
[15] The actual yield of coupling products 3, 4, 8, 9, 12 and 13 might
be twice that shown in the experimental part if the structurally
and electronically altered metal cluster acts as an electron donor.
[16] Bis-clusters 10 and 11 have been synthesized by condensation of
benzaldehyde with 1,5-hexadiyne and 1,6-heptadiyne (L.
Brandsma, Preparative Acetylenic Chemistry, 2nd ed., Elsevier,
Amsterdam, 1988, 82), respectively, and subsequent complexa-
tion with Co2(CO)6 (H.W. Sternberg, H. Greenfield, R.H.
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Paquette, I. Itoh, K.B. Lipkowitz, J. Org. Chem. 41 (1976) 3524.
[19] Partial decomplexation takes place producing variable quantities
of (3,4-diphenyl-1,5-hexadiyne)dicobalt hexacarbonyl (14).
[20] (a) Methoxy-group transfer product [HCꢁCCH(OMe)Ph]Co2-
(CO)6] was isolated by column chromatography (SiO2,
petroleum ether:benzene, 2:1) in a yield of 11%. TLC (PE):
Rf=0.11. 1H-NMR (CDCl3): 3.46 (s, 3H, OMe), 5.29 (d, 1H,
CH, J=0.83 Hz), 6.03 (d, 1H, HCꢁ, J=0.92 Hz), 7.28–7.42 (m,
5H, Ph). FAB: 432 (M+, 3%), 348 (−3CO, 100%); (b) Yield of
[HCꢁCCH(OMe)Ph]Co2(CO)6 was 60%.
[21] G.M. Sheldrick, Acta Crystallogr. A46 (1990) 467.
[22] G.M. Sheldrick, Program for Structure Refinement, University
of Goettingen, Germany, 1993.
[23] A.J.C. Wilson (Ed.), International Tables for X-ray Crystallog-
raphy, vol. C, Kluwer Academic Publishers, Dordrecht, 1992,
Tables 6.1.1.4 (pp. 500–502) and 4.2.6.8 (pp. 219–222).
[24] C.K. Fair, ‘MolEN’ Structure Determination System, Enraf-No-
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[25] ‘XP/PC’ Molecular Graphics Software, Bruker Analytical X-Ray
Instruments, Inc., Madison, WI, USA.
.