Radical reactions of the cobalt-complexed propargyl acetals: Inter- and intramolecular variants

Article abstract of DOI:10.1021/om400718w

Propargyl acetals are used as a new type of substrate in cobaltocene-induced radical reactions stereodirected by a π-bonded Co ₂(CO)₆ metal core. The experimental protocol involves treatment of the cobalt-complexed acetals with triflic anhydride, an in situ generation of ionic propargyl triflates, and a single-electron, low-temperature reduction of α-alkoxy-stabilized, Co₂(CO)₆-complexed propargyl cations. Inter- and intramolecular reactions provide a facile access to polyfunctional, topologically diverse 3,4-dialkoxy-1,5-alkadiynes and 3,4-dialkoxy-1,5-cyclodecadiynes with a predominant formation of the respective d,l-diastereomers (73-100% d,l). The stereo- and chemoselectivities are found to be dependent upon the (1) bulkiness of the gamma substituent (1,3-steric induction; d,l- 73-100%), (2) presence of a phosphorus ligand altering the π-bonded metal core (axial ligand induction; d,l- 91-97%), and (3) steric hindrance created by an alpha alkoxy group (1,1-steric induction; d,l- 83-92%).

Full text of DOI:10.1021/om400718w

Article
pubs.acs.org/Organometallics
Radical Reactions of the Cobalt-Complexed Propargyl Acetals: Inter-
and Intramolecular Variants
Gagik G. Melikyan,* Erin Voorhees, and Ruth Sepanian
Department of Chemistry and Biochemistry, California State University Northridge, Northridge, California 91330-8262, United
States
S
* Supporting Information
ABSTRACT: Propargyl acetals are used as a new type of substrate in cobaltocene-induced radical reactions stereodirected by a
π-bonded Co2(CO)6 metal core. The experimental protocol involves treatment of the cobalt-complexed acetals with triflic
anhydride, an in situ generation of ionic propargyl triflates, and a single-electron, low-temperature reduction of α-alkoxy-
stabilized, Co2(CO)6-complexed propargyl cations. Inter- and intramolecular reactions provide a facile access to polyfunctional,
topologically diverse 3,4-dialkoxy-1,5-alkadiynes and 3,4-dialkoxy-1,5-cyclodecadiynes with a predominant formation of the
respective d,l-diastereomers (73−100% d,l). The stereo- and chemoselectivities are found to be dependent upon the (1)
bulkiness of the gamma substituent (1,3-steric induction; d,l- 73−100%), (2) presence of a phosphorus ligand altering the π-
bonded metal core (axial ligand induction; d,l- 91−97%), and (3) steric hindrance created by an alpha alkoxy group (1,1-steric
induction; d,l- 83−92%).
under standard experimental conditions (Cp2Co, −50 °C).2 α-
INTRODUCTION
■
Alkoxy propargyl triflates 5 require use of propargyl acetals as
Recently we developed a novel method for an acid-free
generation of Co2(CO)6-complexed carbocations1 that includes
precursors, which, in turn, represents a substrate base expansion
for the parent Tf2O−Cp2Co reaction.2
a low-temperature treatment of methyl propargyl ethers with
triflic anhydride.2 Experimental protocol involved reduction of
RESULTS AND DISCUSSION
intermediate propargyl triflates with cobaltocene, a powerful
19e− reducing agent.3 Its compatibility with acid-sensitive
functionalities was showcased by introducing benzyloxy and
methylenedioxy groups on the periphery of the aromatic
nuclei.2 The electronic nature of the substituent, alpha to a
cobalt core, is found to be critical to the reducibility of the
propargyl triflates with cobaltocene (Figure 1). With cation-
stabilizing moieties, such as phenyl and naphthyl groups,
propargyl triflates 1−3 are supposedly of ionic nature, allowing
for a rapid electron transfer from the reducing agent onto the
recipient, an alpha cationic center. The level of stabilization will
be significantly lower in substrate 4, effectively precluding
reduction with cobaltocene due to an alleged covalent character
of the carbon−oxygen bond. Thus, reducibility with Cp2Co may
become a mechanistic tool for distinguishing between ionic and
covalent bonds located alpha to a metal core. Herein we report on
the novel type of propargyl triflates containing, as a stabilizing
substituent, an alkoxy group (5, 6). Reduction with cobaltocene
will allow us to determine if the π-donating potential of an
alkoxy group is on par with aromatic substituents, thus making
the alpha C−O bond ionic (5), or inferior to the latter,
rendering triflates covalent (6) and potentially nonreducible
■
Diastereoselectivity of Intermolecular Coupling Re-
actions of Propargyl Acetals: 1,3-Steric Induction.
Commercially available propargyl acetals 7−9 (R = H, Ph,
Me3Si) were converted to cobalt complexes 10−12 by
interaction with dicobaltoctacarbonyl (Scheme 1).1,4
A
subsequent low-temperature treatment with Tf2O (2 equiv,
−50 to −10 °C, 30 min) allows for removal of an ethoxy group
and an in situ formation of propargyl cations 13−15. From the
experimental point of view, the replacement of the “traditional”
1
−
tetrafluoroborate counterion, BF4 , with a triflate anion, TfO,−
is highly advantageous, since it allows for avoiding a laborious
cation isolation step. In addition, the presence of the metal core
immobilizes a π-bonded triple bond, thus excluding an
unwanted acetylene−allene reorganization,5 as well as ionic
transformations, such as Rupe and Meyer-Schuster rearrange-
ments.6 Propargyl triflates 13−15, represented by two
resonance contributors, are doubly stabilized species with a
Received: July 21, 2013
Published: December 17, 2013
© 2013 American Chemical Society
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Figure 1. Cobalt-complexed propargyl triflates: Ionic vs covalent forms.
Scheme 1. Propargyl Acetals as Substrates in Intermolecular Radical Coupling Reactions
dicobalthexacarbonyl group, and the oxygen atom, contributing
to the delocalization of the positive charge. The degree of
stabilization is critical for the separation between the α-cationic
center and a triflate counterion, with the ionic nature of triflate
intermediates being best verified by their reducibility with
cobaltocene, Cp2Co.3 In the case of propargyl triflates 13−15,
reduction with a 3-fold excess of the reducing agent readily
occurred at −50 °C, generating propargyl radicals 16−18,
which in turn dimerized intermolecularly to d,l- and meso-3,4-
diethoxy-1,5-alkadiynes 19−21. In comparison with α-phenyl
and α-naphthyl derivatives,2 reduction of α-ethoxy propargyl
cations 13−15 occurred at a much lower rate (2.5 h vs 10
min2) despite an excess of cobaltocene used (3 equiv vs 1.04
equiv2), indicating a higher degree of delocalization and a lower
positive charge at the cationic center located alpha to a metal
core. Besides dimerization products 19−21, a H atom
abstraction by transient radicals 16−18 and β-scission reactions
gave rise to the respective hydrocarbon, (RCCCH2OEt)-
Co2(CO)6, and propargyl aldehydes, (RCCCHO)-
Co2(CO)6, as side products. An oxidative decomplexation of
major d,l-diastereomers with ceric ammonium nitrate1a,g allows
for removal of the metal bondage, forming isomerically pure d,l-
22 and d,l-23 (Scheme 1).
Reducibility of propargyl cations 13−15 with cobaltocene
constitutes experimental proof that respective propargyl triflates
are in fact of ionic nature. These experimental data place an
alkoxy group into the same category of the cation-stabilizing
substituents as α-phenyl and α-naphthyl groups (Figure 1).2 A
relative degree of stabilization provided by α-substituents can
be assessed by using heterolytic BDE [D(R+−H−)] values as
quantitative criteria.7 Thus, an ethyl cation has a much higher
BDE than its benzylic counterpart (276.7 vs 238 kcal/mol),
+
whereas a hydroxymethyl cation, HOCH2 , exhibits an
intermediate value of 252 kcal/mol.7a,b For substrate base
expansion, this correlation has a predictive value: α-substituents
with BDE figures close to 252 kcal/mol, or less, can be
introduced to the metal complexes, since respective propargyl
triflates are expected to be ionic and thus reducible by
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Figure 2. Impact of a γ-substituent upon diastereoselectivity: Conformational analysis of 1,3-steric induction.
cobaltocene. To the contrary, propargyl substrates cannot
include α-substituents for which the BDE values are higher than
276.7 kcal/mol: due to the lack of sufficient stabilization of the
adjacent cationic center, the respective propargyl triflates will
supposedly remain covalent and thus nonreducible by
cobaltocene.
energetically more favorable, pre-d,l disposition G. The latter
dimerizes to the respective d,l-diastereomers H, in which the
cobalt−alkyne units as well as bulky γ-substituents are
positioned anti to each other (Figure 2). For comparison, α-
phenyl-substituted radical dimers, containing γ-substituents at
the acetylenic termini (γ-C6H4OMe,12a γ-Et,12b γ-t-Bu12c), were
reported by us to crystallize in the H-like configurations with
vicinal H atoms and aromatic nuclei being positioned gauche to
each other. To independently confirm the structures of the
major radical dimers derived from the metal-complexed
propargyl acetals, the relative configuration for d,l-21 (R =
Me3Si) was determined by means of X-ray crystallography
(Figure 3).13 Hydrogen atoms and vicinal ethoxy groups are
arranged gauche to each other with the respective dihedral
anglesθH3a−C3−C4‑H4a 86.0° and θO13−C3−C4−O14 43.3°
showing a significant distortion. Even a higher degree of
deviation is observed for the bulky cobalt−alkyne cores, which
relieve the steric strain by positioning anti to each other
(θC2−C3−C4−C5 152.2°). Most importantly, the relative config-
uration, as well as spatial arrangement established by X-ray
crystallography (Figure 3), is in full agreement with the d,l-
diastereomer H predicted by conformational analysis (Figure
2). Other noteworthy structural features of d,l-21 include (a) a
significant deviation from linearity in alkyne moieties caused by
the bulkiness of TMS groups (θSi1−C1−C2−C3 6.0°, θC4−C5−C6−Si2
8.0°); (b) a skew geometry of tetrahedral Co2C2 metal cores
where the angles between Co−Co and C−C triple bonds are
significantly deviated from the perpendicular arrangement1
(θC1−C2−Co1−Co2 74.6°, θC5−C6−Co3−Co4 71.1°); (c) a bent
geometry for coordinated alkyne units (C1−C2−C3 142.9°,
C6−C5−C4 140.7°, C2−C1−Si1 149.3°, C5−C6−Si2 146.5°)
reflecting a substantial rehybridization of acetylenic carbons and
strong back-bonding from the cobalt carbonyl moiety;1 and (d)
a lengthened coordinated C−C triple bond (C1−C2 1.31 Å,
C5−C6 1.32 Å vs ∼1.21 Å for free ligand) caused by the π-
complexation to the transition metal.1,8
Diastereoselectivity of dimerization reactions is found to be
dependent upon the bulkiness of a γ-substituent located at the
acetylenic terminus. Thus, parent acetal 10 (R = H) undergoes
dimerization with moderate stereoselectivity, forming predom-
inantly a d,l-diastereomer (d,l-19:meso-19, 73:27, dr 2.7).
Introducing a larger substituent, a phenyl ring, substantially
increased stereoselectivity, yielding the respective dimeric
products, d,l-20 and meso-20, in the ratio of 91:9 (dr 10.1).
An exclusive formation of the d,l-diastereomer 21 was observed
in the case of a bulky Me3Si group introduced at the acetylenic
terminus (d,l-21:meso-21, 100:0). A positive impact of the
substituentlocated gamma to the C−C bond formation
siteupon diastereoselectivity is achievable due to the bent
geometry (θR‑CC‑R 140−145°)1 of the alkyne unit (Figure 2,
A). The trend thus observed highlights another useful function
of the metal cluster: π-coordination to the acetylenic bond, and,
attendant with it, the back-bonding from the triple bond toward
the cobalt atoms,8 effectively lowers the bond order and
provides an effective tool for stereodirecting radical reactions.
In a purely organic setting, in the absence of these effective
levers for controlling the spatial orientation of the converging
radicals, dimerization reactions occur with low, or no,
stereoselectivity.9,10 As depicted in Figure 2, converging
staggered radicals can be arranged both in pre-d,l and pre-
meso fashions. In particular, in pre-d,l orientation B, cobalt−
alkyne units are positioned gauche to each other, along with the
vicinal alkoxy groups. Upon formation of the carbon−carbon
bond, the bulky metal cores remain gauche in C, thus bringing
the γ-H atoms into close proximity to each other. This
arrangement was confirmed by X-ray crystallographic analyses
of the radical dimers with terminal triple bonds.2,11 Pre-meso
orientation D, with a higher number of gauche interactions (3
vs 2), is energetically less favored, thus giving rise to minor
diastereomers E with meso configurations (R = H: d,l-19:meso-
19, 73:27). Introducing the larger substituents gamma to the
reaction siteγ-Ph [ΔV(Ph−H) 108.6 Å3], γ-Me3Si
[ΔV(Me3Si−H) 125.1 Å3]destabilizes a pre-d,l orientation
F, thus prompting the converging radicals to adopt an
An observed 1,3-steric induction exemplifies the concept of
“remote stereocontrol” that has effectively been used in organic
chemistry in order to impact the stereoselectivity in ionic and
radical reactions.14 In the cobalt−alkyne series, the 1,4-
asymmetric induction was previously employed in condensa-
tion of γ-alkoxy-substituted propargyl aldehydes with a methyl
anion and subsequent reduction of γ-alkoxy ketones.15 The
diastereoselectivity of nucleophilic addition reactions was found
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(Figure 4). An increased bulkiness at the acetylenic termini was
already shown by us12 to place the cobalt−alkyne groups anti to
each other (γ-C6H4OMe,12a γ-Et,12b γ-t-Bu12c). Thus, converg-
ing propargyl radicals I, while relieving the strain caused by the
bulky γ-phenyl groups, can put the alkoxy groups gauche to
each other in the d,l-diastereomer J (Figure 4). A close spatial
proximity of alkoxy groups, also corroborated by X-ray
crystallography data (Figure 3), could play a critical role in
determining the stereochemical outcome of the dimerization
reaction. In particular, an increased bulkiness of alkoxy groups
could destabilize pre-d,l orientation I, thus decreasing the
amount of d,l-diastereomer J formed. An alternative spatial
arrangement of converging radicalspre-meso orientation K
would place the alkoxy groups gauche to γ-phenyl-substituted
cobalt−alkyne moieties (Figure 4). If the dual repulsion
between alkoxy groups and cobalt−alkyne moieties in K
exceeds that of gauche-oriented alkoxy groups in I, then
increasing the volume of alkoxy groups would enhance the
stereoselectivity of dimerization in favor of d,l-diastereomers.
Acetal 24 was synthesized in two steps from diethyl
derivative 11: treatment with acidic Amberlyst-15 allowed for
retrieval of aldehyde 26, followed by its acetalization with
trimethyl orthoester16 (Scheme 2). A bulkier isopropyl group in
acetal 25 was best introduced by the direct interaction of
diethyl acetal 11 with 2-propanol mediated by boron
trifluoride.16 A subsequent low-temperature treatment with
Tf2O (2 equiv, −50 to −10 °C, 30 min), followed by low-
temperature (−50 °C) reduction with a 3-fold excess of
cobaltocene, Cp2Co,3 produced a diastereomeric mixture of d,l-
27 and meso-27 in the ratio of 83:17. The observed level of d,l-
diastereoselectivity (dr 4.9) represents a significant decline
from the reference value detected for diethyl acetal 11 (d,l-20
and meso-20, 91:9; dr 10.1) and should be attributed to a
decreased volume of alpha alkoxy groups [VOEt 75.1 Å3 vs VOMe
51.6 Å3]. With a large, isopropoxy group (VOiPr 99.5 Å3), an
increase in diastereoselectivity was only incremental, affording,
under standard conditions d,l-28 and meso-28 in a ratio of 92:8
(dr 11.5, Scheme 2). The d,l-diastereoselectivity range for 1,1-
steric induction (d,l- 83−92%) is more narrow than that for 1,3-
steric induction (d,l- 73−100%). The latter allows for achieving
an exclusive formation of d,l-diastereomers, which makes it a
more efficient mechanistic tool for controlling the stereo-
selectivity of radical dimerization reactions. It is notable that
introducing alpha alkoxy groups larger than isopropoxy is not
Figure 3. ORTEP diagram of d,l-μ-η2-[3,4-diethoxy-1,6-di-
(trimethylsilyl)-1,5-hexadiyne]bis(dicobalthexacarbonyl) (21). Se-
lected bond distances (Å), bond lengths (Å), and angles (deg):
C1−C2 1.31, C3−C4 1.49, C5−C6 1.32, Co1−Co2 2.48, Co3−Co4
2.47, C2−C1−Si1 149.3, C1−C2−C3 142.9, C6−C5−C4 140.7, C5−
C6−Si2 146.5, Si1−C1−C2−C3 6.0, C4−C5−C6−Si2 8.0, O13−
C3−C4−O14 43.3, C2−C3−C4−C5 152.2, H3a−C3−C4−H4a 86.0,
C1−C2−Co1−Co2 74.6, C5−C6−Co3−Co4 71.1.
to undergo a complete reversal, ranging from anti:syn 96:4 to
6:94.15 In our laboratory, a 1,3-steric induction concept was
previously applied to cobalt−alkyne complexes with aromatic
substituents alpha to a metal core, achieving levels of d,l-
diastereoselection as high as 100%.12c
Diastereoselectivity of Intermolecular Coupling Re-
actions of Propargyl Acetals: 1,1-Steric Induction.
Besides introducing gamma substituents to effect a 1,3-steric
induction, another mechanistic tool for controlling the
stereoselectivity of the radical reactions could be the variation
in bulkiness of alkoxy groups directly attached to the radical
center (1,1-steric induction). As model substrates, cobalt-
complexed acetals 24 (R = Me), 11 (R = Et), and 25 (R = i-Pr)
were chosen containing phenyl groups gamma to a metal core
Figure 4. Impact of an α-substituent upon diastereoselectivity: Conformational analysis of 1,1-steric induction.
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Scheme 2. γ-Phenyl-Substituted Propargyl Acetals: Impact of an α-Alkoxy Group on Diastereoselectivity
Scheme 3. γ-Phenyl- vs γ-Alkyl-Substituted Propargyl Acetals: 1,3-Steric Induction in Dimethyl Acetals
feasible because of the steric constraints induced by a bulky
dicobalthexacarbonyl core. Besides dimerization products 27
and 28, a H atom abstraction by transient radicals and β-
scission reactions gave rise to the respective hydrocarbons
(PhCCCH2OR)Co2(CO)6 (R = Me, i-Pr; 5.0−6.8%) and
propargyl aldehyde (PhCCCHO)Co2(CO)6 (26, 10.8%) as
side products.
Diastereoselectivity of Intermolecular Coupling Re-
actions: γ-Alkyl vs γ-Aryl Groups or a 1,3-Steric
Induction in the Dimethyl Acetal Series. Having
established 1,3-steric induction as an efficient tool for
controlling the stereoselectivity of dimerization reactions in
the diethyl acetal series, it was of synthetic interest to probe if
the same mechanistic phenomenon could also be exploited in
the dimethyl acetal series. Thus, n-pentyl propargyl aldehyde 29
was converted to dimethyl acetal 30 by interaction with
trimethyl orthoester under acidic conditions (Scheme 3).16 The
latter was treated with Tf2O (2 equiv, −50 to −10 °C, 30 min)
and Cp2Co (3 equiv, −50 °C) to generate first the respective
ionic propargyl triflate and then to effect a single-electron
reduction with subsequent dimerization of α-OMe propargyl
radicals. The diastereomeric mixture of d,l-31 and meso-31,
89:11, was formed with the major diastereomer being easily
separable by chromatographic means and producing, after
oxidative decomplexation with ceric ammonium nitrate,1 an
isomerically pure organic product, d,l-32. Besides the
dimerization product 31, a H atom abstraction and β-scission
reactions gave rise to the respective hydrocarbons, (n-
C5H11CCCH2OMe)Co2(CO)6 (2.9%), and propargyl alde-
hyde 29 (13.5%) as side products. The level of stereocontrol
provided by a gamma-positioned n-pentyl group (d,l-31:meso-
31, 89:11; dr 8.1) is higher than that observed for the γ-Ph-
substituted counterpart (d,l-27:meso-27, 83:17, dr 4.9).
Computational data provide a higher steric value for the n-
pentyl group (Vn‑C5H11 137.2 Å3) compared to the phenyl
moiety (VPh 114.2 Å3), thus attesting to the fact that the
stereoselectivity in the dimethyl acetal series can also be
controlled by varying the volume of the substituents, both γ-
alkyl and γ-aryl. It is worthy to note that not only can the
tertiary atom (TMS group) or an sp2-hybridized carbon atom
(phenyl group) be used but also a straight chain alkyl moiety
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Scheme 4. Axial Ligand Induction in Intermolecular Radical Coupling Reactions
(n-pentyl group) with an sp3-hybridized carbon attached to the
acetylenic terminus.
diastereoselection is substantially higher than that observed for
unsubstituted Co2(CO)6 cores, i.e., d,l-20:meso-20, 91:9
(Scheme 1). Quantitatively, the phosphorus ligand introduced
into an axial position is much larger than carbon monoxide
(VPMe3 124.1 Å3 vs VCO 39.8 Å3), making the substituted metal
core, Co2(CO)5PMe3, much bulkier than its unsubstituted
counterpart, Co2(CO)6.
Propargyl Ketals as Substrates for Radical Dimeriza-
tion Reactions. To further expand the substrate base and also
to test if the Tf2O/Cp2Co tandem could provide access to
sterically crowded molecular assemblies, propargyl ketal 35 was
probed as a substrate. It is an α-methylated analogue of acetal
24 (Scheme 2), which should produce the respective ionic
triflates due to the additional cation stabilization provided by an
alkyl group, although, on the other hand, radical 36 could be
sterically too hindered to dimerize at alpha positions (Scheme
5). Its alleged coupling to dimer 37 could allow for placing
tertiary carbon atoms vicinal to each other, while carrying the
contiguous alpha methoxy groups. Dimethyl ketal 35 was
synthesized in two steps from commercially available ketone 38
and then consecutively treated with Tf2O and Cp2O under
standardized conditions (2 equiv Tf2O, −50 to −10 °C, 30
min; 3 equiv Cp2Co, −50 °C). The careful NMR and
Phosphorus Ligand-Modified Metal Clusters: Impact
upon Diastereoselectivity (Axial Ligand Induction). The
presence of a π-bonded metal core in alkyne complexes
provides another avenue for directing the stereoselectivity of
radical reactions with C−C bonds forming alpha to a cobalt
cluster. Introducing bulky substituents to gamma positions of
the propargyl acetals (γ-Ph, γ-Me3Si) allowed us to substantially
improve the d,l-diastereoselectivity (73−100%; Scheme 1) due
to the destabilization of spatial arrangements that place triple
bonds gauche to each other (B and D, Figure 2). As a result,
metal−alkyne moieties were pushed as far apart from each
other as possible (G and H, Figure 2), forming d,l-
diastereomers with anti disposition of metal clusters (Figure
3). To further develop this concept, a volume of the metal
cluster, Co2(CO)6, can be increased by replacing an axial ligand,
or ligands, with larger substituents, such as trialkylphosphines
or phosphites.17 Diethyl acetal 11 was chosen as a model
substrate, and an axial CO was replaced with a PMe3 ligand at
ambient temperature (Scheme 4). Using an equimolar amount
of the nucleophile allows for the selective substitution of one of
two axial ligands, affording dicobaltpentacarbonyl complex 33.
It should be mentioned that given the powerful donating ability
of the P ligand,18 the respective propargyl cation, in comparison
to its counterparts 13−15 (Scheme 1), will be much better
stabilized, thus making the propargyl triflate ionic and therefore
reducible by cobaltocene. Indeed, the treatment of acetal 33
with Tf2O, and then with Cp2Co, under standard conditions (2
equiv Tf2O, −50 °C to −10 °C, 30 min; 3 equiv Cp2Co, −50
°C), afforded dimers 34 as a complex isomeric mixture. The
complexity is caused by the chiral nature of monosubstituted
metal cores,18 which does not allow for a reliable determination
of the diastereomeric composition by means of NMR
spectroscopy. Decomplexation with ceric ammonium nitrate,
followed by the reverse complexation of organic dimers 22 with
dicobaltoctacarbonyl,4 allowed us to determine the composition
of the diastereomeric mixture as d,l-20:meso-20, 97:3. This
value adequately represents the original stereochemistryd,l-
34:meso-34with respect to the internal stereocenters formed
in the course of the radical reaction. The observed level of d,l-
Scheme 5. Propargyl Ketals as Substrates
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Scheme 6. All-Carbon Analogue of Sterically Hindered Propargyl Ketals
Scheme 7. Cyclic Propargyl Acetals as Substrates
chromatographic examinations of the crude mixtures did not
allow for finding of any detectable amounts of dimeric product
37. Instead, methyl ether 39 and ketone 40 were isolated as
main products. The former can derive from a H atom transfer
to radical 36, while a β-scission in either radical 36 or the
respective carbocation is responsible for the formation of
oxidized product 40. To clarify that a lack of dimerization is not
immune only to α-alkoxy secondary radical 36, its all-carbon
analogue, radical 41, was assembled and its dimerization
reaction tested (Scheme 6). Thus, phenylacetylene was
metalated with n-BuLi and condensed with 2-butanone,19
followed by the protection of the triple bond with Co2(CO)8.4
Propargyl alcohol 42 was treated with HBF4 to isolate the
propargyl cation 43, which was then reduced with Cp2Co to
generate a tertiary radical 41. Again, the presence of dimer 44
was not spectroscopically established, while a H atom transfer
product 45 was isolated as the main derivative. Methyl ether 46
was derived from quenching of the crude mixture with
methanol in order to retrieve an unreacted propargyl cation,
a standard protocol in kinetic studies involving the cobalt-
complexed propargyl cations.20 It becomes apparent that the
lack of dimerization in radicals 36 and 41 is not caused by the
presence of the O atom or an excessive stabilization of the
intermediate cations by a tandem action of α-methoxy and α-
methyl groups. The most plausible explanation is the steric
factor that is supposedly “exceeded” in transient species 36 and
41 due to the presence of a bulky Co2(CO)6 core. For
comparison, in a purely organic setting, tertiary radicals with
relatively small alkyl groups do undergo coupling, affording
radical dimers with vicinal quaternary carbon atoms.9b,10d It
should be mentioned that in metal-complexed propargyl
radicals the feasibility of radical C−C bond formation reactions
is determined not by the total volume of substituents attached
to the propargylic center but by its degree of substitution (2° vs
3°), or branching at the reaction site, alpha to a metal core. As
recently reported by us, in cobalt−alkyne series, secondary
propargyl radicals with α-substituents as bulky as 1- and 2-
naphthyls,2 or trimethoxyphenyl,12a,b undergo successful
dimerization with their total volume exceeding by far that of
alkyl groups introduced to radicals 36 and 41.
Cyclic Acetals as Substrates for Radical Dimerization
Reactions. Propargyl cyclic acetals 47 and 48 represent
another avenue for substrate base expansion wherein the cation
generation constitutes not an elimination step releasing the
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Scheme 8. Intramolecular Cyclizations of Bis-propargyl Acetals
respective alkyl triflates but a ring-opening reaction with
hydroxyl groups remaining tethered to the carbon framework.
Thus, successful dimerization reactions, triggered by a tandem
action of Tf2O and Cp2Co, could provide access to branched,
highly functionalized 1,5-alkadiynes, such as 49 and 50
(Scheme 7). Interaction of diethyl acetal 11 with ethylene
glycol and propane-1,3-diol,16 mediated by boron trifluoride,
afforded 1,3-dioxolane 47 and 1,3-dioxane 48 in good yields.
Their behavioral profile in dimerization reactions, contrary to
our expectations, was strikingly different. When treated with
Tf2O under standardized conditions (2 equiv, −50 to −10 °C,
30 min), followed by the Cp2Co-induced reduction (3 equiv,
−50 °C), acetal 47 was mostly recovered following an aqueous
workup. Further adaptations involving the usage of larger
quantities of reagents, higher temperatures, or prolonged
reaction times did not allow us to detect the expected dimeric
product 49. It is conceivable that the coordination with Tf2O
would proceed as expected with an in situ formation of an ionic
pair 51. For reasons that are not apparent at the present time,
intermediate species 51 do not undergo a C−O bond
heterolysis that would release acyclic ionic triflate 52. Were
the latter formed in the course of the reaction, a large excess of
cobaltocene present could have been able to reduce it to the
respective propargyl radical, which in turn would undergo a
dimerization reaction. Surprisingly, an exposure of alleged cyclic
species 51 to an excess of cobaltocene at room temperature, or
even at 40 °C, as well as using zinc as an alternative reducing
agent, did not provide any indication that the radical
dimerization reaction is in fact taking place. The six-membered
acetal 48 exhibited different behavior: when treated with Tf2O
and Cp2Co at low temperatures, as the standardized protocol
stipulates, it formed aldehyde 26, an apparent result of β-
dealkylation reaction occurring in acyclic propargyl triflate 53.
An inability of cobaltocene to reduce the cation 53 at the rates
comparable to an unwanted intramolecular elimination reaction
might be caused by the conformational and topological
differences between ionic triflates derived from acyclic and
cyclic acetals.
that is also of immediate relevance to medicinal chemistry.21,22
Bis-propargyl acetal 54 was synthesized in four steps from 1,7-
octadiyne (55) (Scheme 8). The metalation of acetylenic
termini with n-BuLi generated the respective dilithio derivative,
which was then condensed with formaldehyde and oxidized
with PCC to yield unstable dialdehyde 56. The complexation of
the triple bond with Co2(CO)8,4 followed by the acetalization
with trimethyl orthoester,16 afforded requisite substrate 54. Its
treatment with a 4-fold excess of Tf2O generated the ionic bis-
cation 57, which was then reduced with a 6-fold excess of
Cp2Co to diradical 58. The latter manifested its formation by
undergoing an intramolecular cyclization to d,l- and meso-1,5-
cyclodecadiyne 59 in a ratio of 88:12. Decomplexation with
ceric ammonium nitrate1 followed by the chromatographic
separation allowed for isolation of an isomerically pure d,l-60.
Previously, we reported on the intramolecular cyclization of bis-
propargyl alcohols containing aromatic rings alpha to the metal
core,23 but the methodology did not allow for incorporation of
alkoxy groups to an ethylene tether that connects the acetylenic
units, a structural feature that can prove advantageous for
development of drugs with controllable cytotoxicity.
Cobalt-complexed propargyl acetals have previously been
studied in cobalt-mediated ionic reactions24−27 that involve in
situ generation of α-alkoxy propargyl cations with boron
trifluoride, as a Lewis acid, and trapping of the transient
electrophilic species with nucleophiles, such as silyl enol ethers
or allyl stannanes. Both inter-24 and intramolecular25 versions
were reported with mono-24 and bis-acetals26 acting as cation
precursors. Along with achiral acetals,24a−d their chiral counter-
parts24e were also probed to examine the transfer of chirality
upon newly formed stereocenters in electrophilic propargyla-
tion reactions. α,α-Bis-alkylation at the propargylic carbon was
also described,27 further expanding the substrate base of ionic
reactions of propargyl acetals and enhancing their synthetic
potential. Radical transformations of propargyl acetals mediated
by a cobalt core represent a new type of reaction that provides
entry to functionalized 3,4-dialkoxy-1,5-(cyclo)alkadiynes in
regio- and stereoselective manners. It should be emphasized
that although 1,5-alkadiynes have remarkable synthetic
versatility that allows for their conversion to a variety of classes
of organic compounds (furans, tetrahydrofurans, pyrroles, 1,5-
alkadienes, 1,4-/1,6-diketones, cyclopentenes, cyclopentenones,
cycloalkane-1,2-diols, enediynes, fused carbocycles),28 they are
not easily accessible by alternative means.5
Intramolecular Radical Cyclizations of Propargyl
Acetals. Intramolecular cyclizations represent the “natural”
progression of any novel synthetic method; this is why an
attempt was made by us to use the cobaltocene-induced
reduction of propargyl acetals for the synthesis of 1,5-
cyclodecadiynes, an important class of organic compounds
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(19), μ-η2-(3-Ethoxy-1-propyne)dicobalthexacarbonyl, and μ-
η2-(2-Propyn-1-al)dicobalthexacarbonyl. Under an atmosphere of
nitrogen, Tf2O (141 mg, 0.50 mmol) was added dropwise (17 min) to
a solution of acetal 10 (104 mg, 0.25 mmol) in dry CH2Cl2 (4 mL) at
−50 °C. The reaction mixture was warmed to −10 °C in 30 min, then
cooled to −50 °C, and a solution of Cp2Co (142 mg, 0.75 mmol) in
dry CH2Cl2 (3 mL) was added dropwise (3 min). Upon stirring at
−50 °C for 2.5 h, the reaction mixture was diluted with brine (10 mL)
and gradually warmed to 0 °C (40 min). An aqueous layer was
extracted with CH2Cl2 (2 × 10 mL), and combined organic fractions
were washed with water (2 × 25 mL), dried over Na2SO4, and
concentrated under reduced pressure. The crude mixture consists of
d,l-19:meso-19:(HCCCH2OEt)Co2(CO)6:(HCCCHO)-
Co2(CO)6:10 in the ratio of 53:20:6:12:9, respectively (d,l-19:meso-19,
73:27). The fractionation on a preparative TLC plate (PE:E, 20:1)
yielded a mixture of d,l-19 and meso-19, which were further purified on
a SiO2 column (75 g, PE; d,l-19 19.3 mg, 20.9%; 23.0% with a 91%
conversion) and preparative TLC plate (PE:E, 20:1, meso-19 2.0 mg,
2.2%; 2.4% with a 91% conversion). d,l-19: dark red solid. Mp: 69−73
°C (sealed capillary; dried by coevaporation with benzene, 3 × 1 mL).
TLC (PE:E, 20:1): Rf 0.60. 1H NMR (400 MHz, CDCl3): δ 1.29 (6H,
t, 2CH3, J = 7.0), 3.79 (2H, dq, 2CH, Jgem = 8.8, JCH−CH3 = 7.2), 4.05
(2H, dq, 2CH, Jgem = 8.6, JCH−CH3 = 7.0), 4.16 (2H, s, 2CH), 5.98 (2H,
s, HC). 13C NMR (100 MHz, CDCl3): δ 15.3 (CH3), 68.7 (OCH2),
73.1 (OCH), 84.8 (HCC), 93.1 (HCC), 199.7 (CO). MS
TOF: m/z calcd for C22H14O14NaCo4 MNa+ 760.7604, found
760.7616. Anal. Found: C, 35.95; H, 2.30. C22H14O14Co4 requires:
C, 35.80; H, 1.91. meso-19: dark red solid (crystallizes at −20 °C in 30
days). Mp: 87−89 °C (sealed capillary; dried by coevaporation with
benzene, 3 × 1 mL). TLC (PE:E, 20:1): Rf 0.56. 1H NMR (400 MHz,
CDCl3): δ 1.28 (6H, t, 2CH3, J = 7.0), 3.76 (2H, dq, 2CH, Jgem = 8.6,
JCH−CH3 = 7.0), 3.90 (2H, dq, 2CH, Jgem = 8.6, JCH−CH3 = 7.2), 4.46
(2H, s, 2CH), 6.11 (2H, s, HC). MS TOF: m/z calcd for
C22H14O14NaCo4 MNa+ 760.7604, found 760.7570 (low intensity).
μ - η 2 - ( 3 , 3 - D i e t h o x y - 1 - p h e n y l - 1 - p r o p y n e ) -
dicobalthexacarbonyl (11). According to protocol A, PhC
CCH(OEt)2 (8, 1.02 g, 5.00 mmol) and dicobaltoctacarbonyl (1.88
g, 5.50 mmol) afforded 11 (2.25 g, 91.8%) as a dark red solid.
Physicochemical and spectral parameters were consistent with
literature data.24
CONCLUSION
■
Cobalt-complexed propargyl acetals can be used as precursors
to α-alkoxy propargyl cations and radicals with the latter
dimerizing to 3,4-dialkoxy-1,5-alkadiynes and 3,4-dialkoxy-1,5-
cyclodecadiynes in good to excellent d,l-diastereoselectivities
(d,l- 73−100%). Being reducible with cobaltocene at −50 °C,
intermediate propargyl triflates are deemed to be ionic in nature
due to the stabilization by a powerful π-donor, an oxygen atom.
It is established that the stereo- and chemoselectivities can be
controlled by three alternative mechanistic tools, including (1)
gamma substituents introduced to an acetylenic terminus (1,3-
steric induction; d,l- 73−100%), (2) alpha substituents directly
attached to the radical reaction site (1,1-steric induction; d,l-
83−92%), and (3) axial ligands incorporated into a π-bonded
metal core (axial ligand induction; d,l- 91−97%). The said
leverages can be usedalone or in any combinationto
further enhance the stereoselectivity of the radical dimerizations
and also to fine-tune the radical C−C bond-forming reactions
in order to access much larger, topologically and functionally
diverse molecular assemblies. The presence of vicinal alkoxy
groups is essential for the secondary functionalizationsα,α′
by using radical and ionic reactions, either in a purely organic
setting or in the transition-metal-mediated reactions.
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)4 were purchased from Strem. NMR
1
solvents were supplied by Cambridge Isotope Laboratories. H and
13C NMR spectra were recorded on a Bruker DRX-400 (1H, 400
MHz) spectrometer. Chemical shifts were referenced to internal
solvent resonances and are reported relative to tetramethylsilane.
Spin−spin coupling constants (J) are given in hertz. Elemental
analyses were performed by Atlantic Microlab (Norcross, GA, USA).
Melting temperatures (uncorrected) were measured on a Mel-Temp II
(Laboratory Devices) apparatus and Optimelt Automated Meltemp.
Silica gel S735-1 (60−100 mesh; Fisher) was used for flash column
chromatography. Analytical and preparative TLC analyses were
conducted on silica gel 60 F254 (EM Science; aluminum sheets) and
silica gel 60 PF254 (EM Science; w/gypsum; 20 × 20 cm), respectively.
Column chromatography was conducted on silica gel (Sorbent
Technologies; standard grade, 63−200 μm) and basic alumina
(Sorbent Technologies, pH = 10, activity II/III, 50−200 μm). Eluents
are ether (E), petroleum ether (PE), pentane (P), and benzene (B).
Mass spectra were run at the Regional Center on Mass-Spectroscopy,
UC Riverside, Riverside, CA, USA (FAB, ZAB-SE; CI-NH3, 7070EHF;
Micromass; TOF Agilent 6210 LCTOF instrument with a Multimode
source).
d,l- and meso-μ-η2-(3,4-Diethoxy-1,6-diphenyl-1,5-
hexadiyne)bis(dicobalthexacarbonyl) (20), μ-η2-(3-Ethoxy-1-
phenyl-1-propyne)dicobalthexacarbonyl, and μ-η2-(3-Phenyl-
2-propyn-1-al)dicobalthexacarbonyl (26). According to protocol
B, Tf2O (141 mg, 0.50 mmol), acetal 11 (123 mg, 0.25 mmol), and
Cp2Co (142 mg, 0.75 mmol) afforded the crude mixture, containing,
by NMR, d,l-20, meso-20, (PhCCCH2OEt)Co2(CO)6, 26, and 11 in
the ratio of 72:7:4:14:3, respectively (d,l-20:meso-20, 91:9). The
fractionation on a silica gel column (25 g) afforded meso-20 (PE),
(PhCCCH2OEt)Co2(CO)6 (PE), and d,l-20 (E), which were
further repurified on preparative TLC plates (PE:E, 20:1): d,l-20
35.3 mg (31.7%; 32.7% with a 97% conversion), meso-20 2.4 mg
(2.16%; 2.22% with a 97% conversion); (PhCCCH2OEt)Co2(CO)6
1.1 mg (0.99%; 1.02% with a 97% conversion). d,l-20: dark brown oil.
TLC (PE:E, 20:1): Rf 0.37. 1H NMR (400 MHz, CDCl3): δ 1.04 (6H,
t, 2CH3, J = 7.0), 3.34 (2H, quintet, 2CH, J = 7.3), 3.74 (2H, quintet,
2CH, J = 7.3), 5.19 (2H, s, 2CH), 7.26−7.38 (6H, m, aromatic H),
7.56 (4H, d, aromatic H, J = 6.8). 13C NMR (100 MHz, CDCl3): δ
15.0 (CH3), 66.2, 83.5 (OCH2, OCH), 93.8, 95.7 (CC), 128.0,
128.9, 129.8, 138.2 (aromatic C), 199.7 (CO). MS TOF: m/z calcd for
C35H25O15Co4 [M + MeO]− 920.8527, found 920.8507. meso-20: dark
brown solid. Tdec 88−110 °C (sealed capillary; dried by coevaporation
with benzene, 3 × 1 mL; partial decomposition). TLC (PE:E, 20:1): Rf
0.44. 1H NMR (400 MHz, CDCl3): δ 0.57 (6H, t, 2CH3, J = 7.0), 3.19
μ-η2-(3,3-Diethoxy-1-propyne)dicobalthexacarbonyl (10)
(Protocol A). Under an atmosphere of nitrogen, a solution of
HCCCH(OEt)2 (7, 717 mg, 5.60 mmol) in ether (20 mL) was
added dropwise (15 min) to a solution of dicobaltoctacarbonyl (2.02 g,
5.90 mmol) in ether (30 mL) at 0 °C. The reaction was stirred for 3.5
h (TLC control) at 20 °C, and ether was evaporated under reduced
pressure. The crude mixture was fractionated on an alumina column
(220 g, PE) under anaerobic conditions to give 10 (1.48 g, 63.8%) as a
red oil. Physicochemical and spectral parameters were consistent with
literature data.24
(2H, dq, 2CH, Jgem = 8.4, JCH−CH3 = 7.0), 3.85 (2H, dq, 2CH, Jgem
=
8.4, JCH−CH3 = 6.8), 4.71 (2H, s, 2CH), 7.22−7.36 (6H, m, aromatic
H), 7.45−7.50 (4H, m, aromatic H). MS TOF: m/z calcd for
C35H25O15Co4 [M + MeO]− 920.8527, found 920.8510. (PhC
In Situ Generation and Cp2Co Reduction of Co2(CO)6-
Complexed Propargyl Triflates (Protocol B): d,l- and meso-μ-
η2-(3,4-Diethoxy-1,5-hexadiyne)bis(dicobalthexacarbonyl)
1
CCH2OEt)Co2(CO)6: tar-like solid. TLC (PE:E, 20:1): Rf 0.38. H
NMR (400 MHz, CDCl3): δ 1.29 (3H, t, CH3, J = 7.0), 3.74 (2H, q,
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OCH2, J = 7.1), 4.85 (2H, s, OCH2), 7.30−7.38 (3H, m, aromatic H),
7.52−7.57 (2H, m, aromatic H). MS TOF: m/z calcd for
C18H15O8Co2 [M + MeO]+ 476.9436, found 476.9437.
solution, refinement, graphics, and generation of publication materials
were performed by using SHELXTL, V6.12 software.
d,l-3,4-Diethoxy-1,6-di(trimethylsilyl)-1,5-hexadiyne (23).
According to protocol C, d,l-21 (35 mg, 0.04 mmol) was treated
with Ce(NH4)2(NO3)6 (263 mg, 0.48 mmol; 30 min at −50 °C; 30
min at −30 °C; 30 min at −10 °C), affording, after preparatory TLC
isolation (PE:E, 20:1), d,l-23 (9.8 mg, 79.0%) as a yellow oil. TLC
Decomplexation Procedure (Protocol C): d,l-3,4-Diethoxy-
1,6-diphenyl-1,5-hexadiyne (22). Under an atmosphere of nitro-
gen, a solution of Ce(NH4)2(NO3)6 (362 mg, 0.66 mmol) in degassed
acetone (11 mL) was added dropwise (8 min) to a solution of d,l-20
(57 mg, 0.06 mmol) in degassed acetone (8 mL) at −78 °C. The
reaction mixture was stirred for 1 h at −78 °C, stirred an additional
hour at −30 °C, and then treated with brine (20 mL, degassed) at −50
°C. The temperature was raised to 20 °C, and an aqueous layer was
extracted with ether (2 × 10 mL). Combined ethereal fractions were
dried over activated molecular sieves (4 Å) and concentrated under
reduced pressure. The crude product was fractionated on a preparatory
TLC plate (PE:E, 20:1) to give d,l-22 (11.4 mg, 59.7%) as pale yellow
1
(PE:E, 20:1): Rf 0.50. H NMR (400 MHz, CDCl3): δ 0.19 (18H, s,
6CH3), 1.24 (6H, t, 2CH3, J = 7.0), 3.59 (2H, dq, 2CH, Jgem = 9.4,
JCH−CH3 = 7.0), 3.80 (2H, dq, 2CH, Jgem = 9.2, JCH−CH3 = 7.0), 4.19
(2H, s, 2CH). 13C NMR (100 MHz, CDCl3): δ 0.0 (TMS), 15.3
(CH3), 65.2 (OCH2), 72.8 (OCH), 92.1, 101.8 (CC). MS TOF EI
+: m/z calcd for C16H30O2Si2 M+ 310.1779, found 310.1774. Anal.
Found: C, 62.54; H, 9.18. Calcd for C16H30O2Si2: C, 61.88; H, 9.74.
μ - η 2 - ( 3 , 3 - D i m e t h o x y - 1 - p h e n y l - 1 - p r o p y n e ) -
dicobalthexacarbonyl (24). Under an atmosphere of nitrogen,
Amberlyst-15 (500 mg) was added to a solution of acetal 11 (0.98 g,
2.00 mmol) in dry acetone (50 mL), followed by the addition of
deionized water (1 mL) at 20 °C. Upon stirring for 2 h, the reaction
mixture was filtered through a short bed of Florisil (2 cm), dried
(Na2SO4), and evaporated to dryness under reduced pressure.
Trimethyl orthoformate (255 mg, 2.40 mmol) was added to a
solution of 26 (832 mg, 2.00 mmol; assuming 100% yield) in dry
methanol (30 mL), followed by the addition of p-TsOH (190 mg, 1.00
mmol) at 20 °C. The reaction mixture was stirred for 2 h, then
quenched with NaHCO3 (84 mg, 1.00 mmol) dissolved in deionized
water (30 mL). An organic layer was extracted with ether (3 × 60
mL), and combined ethereal fractions were dried (Na2SO4). Upon
concentration under reduced pressure, the crude mixture was
fractionated on an alumina column (120 g, anaerobic conditions,
PE) to afford 24 (800 mg, 86.5%) as a dark red solid. TLC (PE:E,
20:1): Rf 0.29. Mp: 78−80 °C (sealed capillary; dried by coevaporation
with benzene, 3 × 1 mL). 1H NMR (400 MHz, CDCl3): δ 3.55 (6H, s,
2OCH3), 5.63 (1H, s, CH), 7.30−7.39 (3H, m, aromatic H), 7.61
(2H, d, aromatic H, J = 6.4). 13C NMR (100 MHz, CDCl3): δ 55.1
(CH), 90.3, 91.9 (CC), 104.7 (OCO), 128.0, 129.0, 130.1, 137.8
(aromatic C), 199.5 (CO). MS TOF: m/z calcd for C18H15O9Co2
[M + OCH3]− 492.9386, found 492.9395. Anal. Found: C, 44.27; H,
2.34. C17H12O8Co2 requires: C, 44.18; H, 2.62.
1
oil. TLC (PE:E, 20:1): Rf 0.17. H NMR (400 MHz, CDCl3): δ 1.31
(6H, t, 2CH3, J = 7.0), 3.71 (2H, dq, 2CH, Jgem = 9.4, JCH−CH3 = 7.0),
3.94 (2H, dq, 2CH, Jgem = 9.2, JCH−CH3 = 7.0), 4.55 (2H, s, 2CH),
7.27−7.33 (6H, m, aromatic H), 7.44−7.49 (4H, m, aromatic H). 13C
NMR (100 MHz, CDCl3): δ 15.4 (CH3), 65.4, 73.0 (OCH, OCH2),
85.8, 87.2 (CC), 122.9, 128.4, 128.6, 132.1 (aromatic C). MS TOF:
m/z calcd for C22H22O2Na MNa+ 341.1512, found 341.1520.
μ-η2-(3,3-Diethoxy-1-trimethylsilyl)dicobalthexacarbonyl
(12). According to protocol A, Me3SiCCCH(OEt)2 (9, 1.00 g, 5.00
mmol) and dicobaltoctacarbonyl (1.88 g, 5.50 mmol) afforded 12
1
(2.26 g, 93.0%) as dark red oil. TLC (PE:E, 10:1): Rf 0.62. H NMR
(400 MHz, CDCl3): δ 0.31 (9H, s, 3CH3), 1.26 (6H, t, 2CH3, J = 7.2),
3.75 (4H, ABX3, 2CHAHB, JH(A)‑H(B) = 9.1, JH(A)‑CH3 = 6.8, JH(B)‑CH3
=
7.0), 5.49 (1H, s, CH). MS TOF: m/z calcd for C17H23O9SiCo2
516.9781, found 516.9775. Anal. Found: C, 39.44; H, 3.71. Calcd for
C16H20Co2O8Si: C, 39.52; H, 4.15.
d,l-μ-η2-[3,4-Diethoxy-1,6-di(trimethylsilyl)-1,5-hexadiyne]-
bis(dicobalthexacarbonyl) (21), μ-η2-(3-Ethoxy-1-trimethylsil-
yl-1-propyne)dicobalthexacarbonyl, and μ-η2-(3-Trimethylsil-
yl-2-propyn-1-al)dicobalthexacarbonyl. According to protocol B,
Tf2O (141 mg, 0.50 mmol), acetal 12 (122 mg, 0.25 mmol), and
Cp2Co (142 mg, 0.75 mmol) afforded the crude mixture, containing,
by NMR, d,l-21, (TMSCCCH2OEt)Co2(CO)6, (TMSCCCHO)-
Co2(CO)6, and 12 in the ratio of 76.7:13.9:2.8:6.6, respectively (d,l-21
≥ 99%). The crude mixture was fractionated on a silica gel column (50
g, PE) to give d,l-21 (52.5 mg, 47.6%; 51.2% with a 93% conversion)
d,l- and meso-μ-η2-(3,4-Dimethoxy-1,6-diphenyl-1,5-
hexadiyne)bis(dicobalthexacarbonyl) (27), μ-η2-(3-Methoxy-1-
phenyl-1-propyne)dicobalthexacarbonyl, and μ-η2-(3-Phenyl-
2-propyn-1-al)dicobalthexacarbonyl (26). According to protocol
B, Tf2O (141 mg, 0.50 mmol), acetal 24 (116 mg, 0.25 mmol), and
Cp2Co (142 mg, 0.75 mmol) afforded the crude mixture, containing,
by NMR, d,l-27, meso-27, (PhCCCH2OMe)Co2(CO)6, and
(PhCCCHO)Co2(CO)6 in the ratio of 57:12:17:14, respectively
(d,l-27:meso-27, 83:17). The crude product was fractioned on a
preparative TLC plate (PE:E, 20:1) to afford meso-27, (PhC
CCH2OMe)Co2(CO)6, and d,l-27, which were further purified on
preparative TLC plates (meso-27, PE; (PhCCCH2OMe)Co2(CO)6,
PE; d,l-27, PE:E, 20:1/twice), as well as minute quantities of reduction
product (PhCCCH3)Co2(CO)6 (3.3 mg, 3.3%). Obtained were
meso-27 (6.4 mg, 5.9%), (PhCCCH2OMe)Co2(CO)6 (7.3 mg,
6.8%), and d,l-27 (30.0 mg, 28.0%). d,l-27: dark red-brown oil. TLC
1
as a brown tar-like solid. TLC (PE:E, 20:1): Rf 0.59. H NMR (400
MHz, CDCl3): δ 0.36 (18H, s, 6CH3), 1.22 (6H, t, 2CH3, J = 7.0),
3.62 (2H, dq, 2CH, Jgem = 8.6, JCH−CH3 = 6.8), 3.99 (2H, dq, 2CH, Jgem
= 8.6, JCH−CH3 = 7.0), 5.03 (2H, s, 2CH). 13C NMR (100 MHz,
CDCl3): δ 1.4 (TMS), 15.2 (CH3), 64.8 (CH2), 81.2, 107.9 (CC),
83.9 (OCH), 200.5 (CO). MS TOF: m/z calcd for
C28H30O14NaSi2Co4 [MNa]+ 904.8394, found 904.8401. Anal.
Found: C, 39.12; H, 3.77. Calcd for C28H30Co4O14Si2: C, 38.11; H,
3.43. Single crystals suitable for X-ray structure analysis (Figure 3)
were obtained by methanol vapor diffusion into a solution of d,l-21 in
ether at −20 °C (3 days).
X-ray Crystallography of d,l-21. A suitable crystal of d,l-21 was
coated with Paratone N oil, suspended in a small fiber loop, and placed
in a cooled nitrogen gas stream at 173 K on a Bruker D8 APEX II
CCD sealed tube diffractometer with graphite-monochromated Cu Kα
(1.54178 Å) radiation. Data were measured using a series of
combinations of phi and omega scans with 10 s frame exposures
and 0.5° frame widths. Data collection, indexing, and initial cell
refinements were all carried out using APEX 213a software. Frame
integration and final cell refinements were done using SAINT13b
software. The structure was solved using direct methods and difference
Fourier techniques (SHELXTL, V6.12).13c Hydrogen atoms were
placed in their expected chemical positions using the HFIX command
and were included in the final cycles of least-squares with isotropic
Uij’s related to the atoms’ ridden upon. All non-hydrogen atoms were
refined anisotropically except for the oxygen atoms and C26.
Scattering factors and anomalous dispersion corrections are taken
from the International Tables for X-ray Crystallography.13d Structure
1
(PE:E, 20:1): Rf 0.21. H NMR (400 MHz, CDCl3): δ 3.26 (6H, s,
2OCH3), 5.13 (2H, s, 2CH), 7.29−7.39 (6H, m, aromatic H), 7.48
(4H, d, aromatic H, J = 7.2). 13C NMR (100 MHz, CDCl3): δ 58.6
(OCH3), 84.5 (CH), 93.8, 94.8 (CC), 127.9, 129.1, 129.5, 138.4
(aromatic C), 199.6 (CO). MS TOF: m/z calcd for C31H18O13Co4 [M
− CO]+ 833.8070, found 833.8039. meso-27: dark brown solid. TLC
1
(PE:E, 20:1): Rf 0.48. H NMR (400 MHz, CDCl3): δ 3.11 (6H, s,
2OCH3), 4.62 (2H, s, 2CH), 7.28−7.36 (6H, m, aromatic H), 7.50−
7.54 (4H, m, aromatic H). 13C NMR (100 MHz, CDCl3): δ 58.6
(OCH3), 85.6 (CH), 94.0, 95.3 (CC), 127.7, 128.7, 129.9, 138.6
(aromatic C), 199.8 (CO). MS TOF: m/z calcd for C31H18O13Co4 [M
− CO]+ 833.8070, found 833.8099. (PhCCCH2OMe)Co2(CO)6:
dark red oil. TLC (PE:E, 20:1): Rf 0.34. 1H NMR (400 MHz, CDCl3):
δ 3.58 (3H, s, OCH3), 4.81 (2H, s, CH2), 7.30−7.38 (3H, m, aromatic
H), 7.52−7.57 (2H, m, aromatic H). MS TOF: m/z calcd for
78
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C16H10O7Co2 M+ 431.9085, found 431.9101. (PhCCCH3)-
d,l- and meso-μ-η2-(8,9-Dimethoxy-6,10-hexadecadiyne)bis-
(dicobalthexacarbonyl) (31), μ-η2-(1-Methoxy-2-octyne)-
dicobalthexacarbonyl, and μ-η2 -(2-Octyn-1-al)-
dicobalthexacarbonyl (29). According to protocol B, Tf2O (141
mg, 0.50 mmol), acetal 30 (114 mg, 0.25 mmol), and Cp2Co (142 mg,
0.75 mmol) afforded the crude mixture, containing, by NMR, d,l-31,
meso-31, (n-C5H11CCCH2OMe)Co2(CO)6, and 29 in the ratio of
59:7:15:19, respectively (d,l-31:meso-31, 89:11). The crude product
was fractionated on a preparatory TLC plate (PE:E, 20:1) to afford d,l-
31, meso-31, (C5H11CCCH2OMe)Co2(CO)6, and 29 (14 mg,
13.5%). d,l-31, meso-31, and (C5H11CCCH2OMe)Co2(CO)6 were
repurified on separate preparative TLC plates (d,l-31:PE, 2 runs; meso-
31:PE, 1 run; (C5H11CCCH2OMe)Co2(CO)6:PE, 1 run) to give
d,l-31 (27.0 mg, 25.2%), meso-31 (2.4 mg, 2.3%), and (C5H11C
CCH2OMe)Co2(CO)6 (3.2 mg, 2.9%). d,l-31: dark red-brown oil.
TLC (PE:E, 20:1): Rf 0.35. 1H NMR (400 MHz, CDCl3): δ 0.94 (6H,
t, 2CH3, J = 7.2), 1.43 (8H, m, 4CH2), 1.70 (4H, quintet, 2CH2, J =
7.6), 2.92 (4H, three lines, 2CH2, J = 8.0), 3.56 (6H, s, 2CH3), 4.81
(2H, s, 2CH). 13C NMR (100 MHz, CDCl3): δ 14.1 (CH3), 22.6,
31.96, 32.01, 34.4 (CH2), 58.3 (OCH3), 84.9 (OCH), 93.8, 101.2
(CC), 200.1 (CO). MS TOF: m/z calcd for C30H30O14Co4 M+
849.8958, found 849.8978. meso-31: dark brown solid. Mp: 68−75 °C
(sealed capillary; dried by coevaporation with benzene, 3 × 1 mL).
TLC (PE:E, 20:1): Rf 0.68. 1H NMR (400 MHz, CDCl3): δ 0.93 (6H,
t, 2CH3, J = 7.0), 1.33−1.63 (10H, m, 5CH2), 1.72 (2H, m, CH2), 2.91
(4H, m, 2CH2), 3.58 (6H, s, 2CH3), 4.30 (2H, s, 2CH). MS TOF: m/
z calcd for C30H30O14Co4 [M]+ 849.8958, found 849.8941. (n-
C5H11CCCH2OMe)Co2(CO)6: red oil. TLC (PE): Rf 0.17. 1H
NMR (400 MHz, CDCl3): δ 0.94 (3H, t, CH3, J = 7.2), 1.41 (4H, m,
2CH2), 1.65 (2H, quintet, CH2, J = 7.6), 2.85 (2H, three lines, CH2, J
= 8.0), 3.51 (3H, s, CH3), 4.60 (2H, s, CH2). MS TOF: m/z calcd for
C15H16O7Co2 M+ 425.9555, found 425.9539. 29: dark red-brown oil.
TLC (PE:E, 20:1): Rf 0.28. 1H NMR (400 MHz, CDCl3): δ 0.94 (3H,
t, CH3, J = 7.0), 1.42 (4H, m, 2CH2), 1.69 (2H, quintet, CH2, J = 7.6),
2.92 (2H, three lines, CH2, J = 7.8), 10.32 (1H, s, CHO). MS TOF:
m/z calcd for C14H12O7Co2 M+ 409.9242, found 409.9223.
1
Co2(CO)6: dark red oil. TLC (PE:E, 20:1): Rf 0.63. H NMR (400
MHz, CDCl3): δ 2.89 (3H, s, CH3), 7.29−7.39 (3H, m, aromatic H),
7.53 (2H, m, aromatic H). MS TOF: m/z calcd for C15H8O6Co2 M+
401.8979, found 401.8976.
μ - η 2 - ( 3, 3 - D i i s o p r op o x y - 1 - p h e n yl - 1- p r o p yn e ) -
dicobalthexacarbonyl (25). Under an atmosphere of nitrogen, BF3·
Et2O (114 mg, 1.00 mmol) was added to a solution of 11 (490 mg,
1.00 mmol) in 2-propanol (20 mL) at 0 °C. The reaction mixture was
stirred for 1 h, another equivalent of BF3·Et2O (114 mg, 1.00 mmol)
was added, and stirring was continued for another hour. The reaction
was quenched with NaHCO3 aqueous solution (20 mL), extracted
with CH2Cl2, dried (Na2SO4), and then fractionated on an alumina
column (120 g, anaerobic conditions, PE) to afford 25 (501 mg,
1
96.6%; 89% purity) as a dark red oil. TLC (PE:E, 20:1): Rf 0.38. H
NMR (400 MHz, CDCl3): δ 1.22 (6H, d, 2CH3, J = 6.4), 1.26 (6H, d,
2CH3, J = 6.4), 4.07 (2H, septet, 2CH, J = 6.0), 5.86 (1H, s, CH),
7.29−7.37 (3H, m, aromatic H), 7.61 (2H, m, aromatic H). MS TOF:
m/z calcd for C21H20O8Co2 M+ 517.9817, found 517.9826.
d,l-μ-η2-(3,4-Diisopropoxy-1,6-diphenyl-1,5-hexadiyne)bis-
(dicobalthexacarbonyl) (28), μ-η2-(3-Isopropoxy-1-phenyl-1-
propyne)dicobalthexacarbonyl, and μ-η2-(3-Phenyl-2-propyn-
1-al)dicobalthexacarbonyl (26). According to protocol B, Tf2O
(141 mg, 0.50 mmol), acetal 25 (130 mg, 0.25 mmol), and Cp2Co
(142 mg, 0.75 mmol) afforded the crude mixture, containing, by
NMR, d,l-28, meso-28, [PhCCCH2OCH(CH3)2]Co2(CO)6, and
(PhCCCHO)Co2(CO)6 in the ratio of 70:6:11:13, respectively (d,l-
28:meso-28, 92:8). The crude product was fractionated on a
preparatory TLC plate (PE:E, 20:1) to afford d,l-28, [PhC
CCH2OCH(CH3)2]Co2(CO)6, (PhCCCHO)Co2(CO)6 (11.2 mg,
10.8%), and minute quantities of the reduction product (PhC
CCH3)Co2(CO)6 (3.2 mg, 3.2%). d,l-28 was repurified twice on
preparative TLC plates (PE, 1 run; PE, 2 runs) to give pure d,l-28 (31
mg, 27.0%). [PhCCCH2OCH(CH3)2]Co2(CO)6 was further
purified on a preparative TLC plate (PE, 2 runs; 5.7 mg, 5.0%).
meso-28 could not be isolated in a homogeneous state because of the
spontaneous decomplexation on inorganic sorbents. Recomplexation
of the partially decomplexed meso-28 allowed us to reliably identify its
signals in the crude mixture [δ 3.72 (2H, septet, 2CH, J = 6.4), 4.12
(1H, s, CH), 5.21 (2H, s, CH)]. d,l-28: dark red-brown oil. TLC
d,l-(8,9-Dimethoxy-6,10-hexadecadiyne) (32). According to
protocol C, d,l-31 (51 mg, 0.06 mmol) was treated with Ce-
(NH4)2(NO3)6 (395 mg, 0.72 mmol; −50 °C, 1 h), affording, after
chromatographic fractionation (preparatory TLC plate, PE:E, 10:1),
d,l-32 (14.0 mg, 86.2% yield) as a yellow oil. TLC (PE:E, 10:1): Rf
1
(PE:E, 20:1): Rf 0.54. H NMR (400 MHz, CDCl3): δ 1.04 (6H, d,
1
2CH3, J = 6.0), 1.22 (6H, d, 2CH3, J = 6.4), 4.09 (2H, septet, 2CH, J =
5.8), 5.05 (2H, s, 2CH), 7.16−7.22 (6H, m, aromatic H), 7.39−7.45
(4H, m, aromatic H). 13C NMR (100 MHz, CDCl3): δ 20.4 (CH3),
22.7 (CH3), 71.0 (OCH), 80.3 (OCH), 93.4, 100.7 (CC), 127.7,
128.7, 129.8, 138.0 (aromatic C), 199.8 (CO). MS TOF: m/z calcd for
C36H26O14Co4 M+ 917.8645, found 917.8682. Anal. Found: C, 47.09;
H, 2.92. C36H26O14Co4 requires: C, 47.08; H, 2.85. [PhC
CCH2OCH(CH3)2]Co2(CO)6: red oil. TLC (PE:E, 20:1): Rf 0.36.
1H NMR (400 MHz, CDCl3): δ 1.27 (6H, d, 2CH3, J = 6.4), 3.87 (1H,
0.23. H NMR (400 MHz, CDCl3): δ 0.91 (6H, t, 2CH3, J = 7.2),
1.28−1.44 (8H, m, 4CH2), 1.55 (4H, quintet spl, 2CH2, J = 7.2), 2.25
(4H, t spl, 2CH2, J = 7.0), 3.46 (6H, s, 2OMe), 4.10 (2H, quintet,
2CH, J = 1.0). 13C NMR (100 MHz, CDCl3): δ 14.2 (CH3), 19.0,
22.4, 28.5, 31.2 (CH2), 56.9 (OMe), 74.2 (OCH), 75.8, 88.5 (CC).
MS TOF: m/z calcd for C18H31O2 MH+ 279.2319, found 279.2323.
Anal. Found: C, 77.77; H, 10.92. C18H30O2 requires: C, 77.65; H,
10.86.
μ-η2 -(3,3-Diethoxy-1-phenyl-1-propyne)dicobalt-
(trimethylphosphine)pentacarbonyl (33). Under an atmosphere
of nitrogen, trimethylphosphine (4.0 mL; 4.00 mmol; 1 M solution in
THF) was added dropwise (4 min) to a solution of 11 (1.96 g, 4.00
mmol) in THF (80 mL) at 20 °C. The reaction mixture was stirred 21
h, then concentrated under reduced pressure and fractionated on an
alumina column (120 g, anaerobic conditions, PE) to give 33 (1.42 g,
66.0%) as a red solid. Mp: 73−75 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). TLC (PE:E, 10:1): Rf 0.17.
1H NMR (400 MHz, CDCl3): δ 1.20 (9H, d, P(CH3)3, J = 10.4), 1.28
septet, CH, J = 6.0), 4.82 (2H, s, CH2), 7.29−7.38 (3H, m, aromatic
H), 7.56 (2H, d, aromatic H, J = 7.2). MS TOF: m/z calcd for
C18H14O7Co2 M+ 459.9398, found 459.9407.
μ-η2-(1,1-Dimethoxy-2-octyne)dicobalthexacarbonyl (30)
(Protocol F). Under an atmosphere of nitrogen, trimethylorthofor-
mate (184 mg, 1.73 mmol) was added to a solution of 29 (590 mg,
1.44 mmol) in dry methanol (20 mL) at 20 °C. p-TsOH (137 mg,
0.72 mmol) was added, the reaction mixture was stirred for 2 h and
quenched with NaHCO3 (61 mg, 0.72 mmol) in deionized water (20
mL), and an organic layer was extracted with ether (3 × 40 mL).
Combined ethereal fractions were dried (Na2SO4), concentrated
under reduced pressure, and fractionated on an alumina column (120
g, anaerobic conditions, PE) to afford 30 (628 mg, 95.6%) as a dark
(3H, t, CH3, J = 7.2), 1.29 (3H, t, CH3, J = 7.6), 3.77 (2H, quintet,
CH2, J = 8.0), 3.94 (2H, quintet br, CH2), 5.71 (1H, s, CH), 7.18−
7.30 (3H, m, aromatic H), 7.61 (2H, d, aromatic H, J = 7.2). 13C NMR
(100 MHz, CDCl3): δ 15.4 (CH3), 19.3, 19.6 (CH3), 63.6, 63.9
(CH2), 85.2, 86.8 (CC), 104.0 (OCO), 126.8, 128.6, 130.4, 140.3
(aromatic C), 202.2, 205.2 (CO). MS TOF: m/z calcd for
C21H25O7PCo2 M+ 537.9996, found 537.9984. Anal. Found: C, 46.95;
H, 4.81. C21H25O7PCo2 requires: C, 46.86; H, 4.68.
1
red oil. TLC (PE:E, 20:1): Rf 0.34. H NMR (400 MHz, CDCl3): δ
0.94 (3H, t, CH3, J = 7.0), 1.43 (4H, m, 2CH2), 1.67 (2H, quintet,
CH2, J = 7.6), 2.84 (2H, three lines, CH2CC, J = 8.0), 3.51 (6H, s,
2OCH3), 5.38 (1H, s, CH). 13C NMR (100 MHz, CDCl3): δ 14.1
(CH3), 22.6, 31.6, 31.9, 33.9 (CH2), 54.9 (OCH3), 92.3, 98.7 (CC),
104.7 (OCO), 200.0 (CO). MS TOF: m/z calcd for C16H18O8Co2
M+ 455.9660, found 455.9642.
d,l- and meso-3,4-Diethoxy-1,6-diphenyl-1,5-hexadiyne (22).
According to protocol B, Tf2O (141 mg, 0.50 mmol), acetal 33 (135
mg, 0.25 mmol), and Cp2Co (142 mg, 0.75 mmol) afforded the crude
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product 34 (121 mg, 0.12 mmol), which was dissolved in degassed
acetone (10 mL) and treated (dropwise, 4 min) with a solution of
Ce(NH4)2(NO3)6 (263 mg, 0.48 mmol) in degassed acetone (4 mL)
at −78 °C. The reaction mixture was stirred for 1 h, and then an
additional equivalent of Ce(NH4)2(NO3)6 (66 mg, 0.12 mmol) in
degassed acetone (1 mL) was added dropwise (1 min). The reaction
mixture was stirred for 1 h and then treated with brine (20 mL,
degassed) at −78 °C. The temperature was raised to 20 °C, and an
aqueous layer was extracted with ether (3 × 30 mL). Combined
ethereal fractions were dried over activated molecular sieves (4 Å) and
concentrated under reduced pressure. The crude mixture (by NMR:
d,l-22:meso-22, 96:4) was fractionated on a preparative TLC plate
(PE:E, 20:1) to afford d,l-22 (12.0 mg, 29.4% over two steps).
Stereochemical assignment of meso-22 was confirmed by recomplex-
ation of the mixture of d,l-22 and meso-22 to a mixture of d,l-20 and
meso-20 in the ratio 97:3.
with saturated NH4Cl (50 mL), and an aqueous layer was extracted
with ether (3 × 40 mL). The combined organic layers were dried
(Na2SO4) and then concentrated under reduced pressure. The residue
(1.02 g, 5.88 mmol; assuming 100% yield) was dissolved in ether (20
mL) and added dropwise (15 min) to a solution of dicobaltocta-
carbonyl (2.11 g, 6.17 mmol) in ether (40 mL) at 0 °C. The reaction
was stirred for 3 h at 20 °C (TLC control) and concentrated under
reduced pressure, and then the crude mixture was fractionated on a
Florisil column (120 g, anaerobic conditions, PE) to afford 42 (2.13 g,
78.7%) as dark red solid. Mp: 47−49 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). TLC (PE:E, 7:1): Rf 0.30. 1H
NMR (400 MHz, CDCl3): δ 1.08 (3H, t, CH3, J = 7.6), 1.65 (3H, s,
CH3), 1.78 (1H, s, OH), 1.92 (2H, ABX3, CH2, JH(A)‑H(B) = 14.0),
7.28−7.39 (3H, m, aromatic H), 7.58−7.63 (2H, m, aromatic H). 13C
NMR (100 MHz, CDCl3): δ 8.9 (CH3), 28.9 (CH3), 37.6 (CH2), 75.5
(OC), 92.5, 108.1 (CC), 127.9, 129.0, 130.0, 138.2 (aromatic C),
199.8 (CO). MS TOF: m/z calcd for C18H13O7Co2 [M − H]−
458.9331, found 458.9340. Anal. Found: C, 46.20; H, 2.94.
C18H14O7Co2 requires: C, 46.98; H, 3.07.
μ - η 2 - ( 3 , 3 - D i m e t h o x y - 1 - p h e n y l - 1 - b u t y n e ) -
dicobalthexacarbonyl (35). Under an atmosphere of nitrogen,
trimethyl orthoformate (177 mg, 1.67 mmol) was added to a solution
of ketone 36 (200 mg, 1.39 mmol) in methanol (4 mL) at 20 °C. The
reaction mixture was treated with p-TsOH (30 mg, 0.16 mmol),
stirred for 1.5 h, and quenched with a solution of NaHCO3 (13.4 mg,
0.16 mmol) in DI water (5 mL). An organic layer was extracted with
ether (3 × 10 mL), and combined ethereal fractions were dried
(Na2SO4). A solution of crude ketal (264 mg, 1.39 mmol; assuming
100% yield) in dry ether (15 mL) was added dropwise (10 min) to a
solution of dicobaltoctacarbonyl (499 mg, 1.46 mmol) in dry ether (20
mL) at 0 °C. The reaction mixture was stirred for 3 h at 20 °C,
concentrated under reduced pressure, and then fractionated on an
alumina column (80 g, anaerobic conditions, PE) to afford 35 (414
mg, 62.5%) as dark red solid. Mp: 55−69 °C (w/decomposition;
sealed capillary; dried by coevaporation with benzene, 3 × 1 mL).
TLC (PE:E, 20:1): Rf 0.23. 1H NMR (400 MHz, CDCl3): δ 1.79 (3H,
s, CH3), 3.32 (6H, s, 2OCH3), 7.28−7.38 (3H, m, aromatic H), 7.66
(2H, d, aromatic H, J = 7.2). 13C NMR (100 MHz, CDCl3): δ 25.5
(CH3), 50.4 (OCH3), 92.7, 97.4 (CC), 103.2 (OCO), 127.8, 128.8,
130.2, 138.2 (aromatic C), 199.6 (CO). MS TOF: m/z calcd for
C19H17O9Co2 [M + OCH3]− 506.9542, found 506.9544. Anal. Found:
C, 45.84; H, 3.17. C18H14O8Co2 requires: C, 45.40; H, 2.96.
μ-η2-(3-Methyl-1-phenyl-1-pentyne)dicobalthexacarbonyl
(45) and μ-η2-(3-Methoxy-3-methyl-1-phenyl-1-pentyne)-
dicobalthexacarbonyl (46). Under an atmosphere of nitrogen,
HBF4·Me2O (168 mg, 1.25 mmol) was added dropwise to a solution
of 42 (115 mg, 0.25 mmol) in degassed pentane (20 mL) at −40 °C.
The reaction mixture was stirred for 1 h, warmed to −30 °C, and
stirred for an additional 45 min. The cation was washed with pentane
(3 × 10 mL) at −40 °C, and residual solvent was evaporated under
reduced pressure. The precipitate was dissolved in CH2Cl2 (5 mL) at
−40 °C; then the temperature was lowered to −78 °C, and a solution
of Cp2Co (95 mg, 0.50 mmol) in dry CH2Cl2 (2 mL) was added
dropwise (2 min). Upon stirring at −78 °C for 1 h, the temperature
was raised to −50 °C, and the reaction mixture was stirred for another
30 min. Methanol (0.5 mL) was added at −50 °C, the solution was
stirred for 30 min, warmed to −30 °C, and stirred for additional 15
min. The reaction mixture was diluted with brine (10 mL), an aqueous
layer was extracted with CH2Cl2 (2 × 10 mL), and combined organic
fractions were washed with water (2 × 25 mL), dried (Na2SO4), and
concentrated under reduced pressure. The crude product (by NMR,
HAA 45:Me-ether 46, 50:50) was fractionated on a preparatory TLC
plate (PE:E, 20:1), followed by repurification (preparatory TLC plate,
PE:E, 20:1), affording HAA product 45 (37.7 mg, 34.0%) and Me-
ether 46 (9.8 mg, 8.3%). HAA 45: dark red oil. TLC (PE:E, 20:1): Rf
0.74. 1H NMR (400 MHz, CDCl3): δ 1.07 (3H, t, CH3, J = 7.4), 1.36
(3H, d, CH3, J = 6.8), 1.49 (1H, m, CHAHBXY3), 1.85 (1H, m,
CHAHBXY3), 3.07 (1H, m, CH), 7.28−7.39 (3H, m, aromatic H), 7.51
(2H, m, aromatic H). 13C NMR (100 MHz, CDCl3): δ 12.8, 21.5,
32.3, 39.4 (3-Me, C3, C4, C5), 92.1, 107.8 (CC), 127.7, 129.0,
129.4, 138.7 (aromatic C), 200.2 (CO). MS TOF: m/z calcd for
C18H14O6Co2 M+ 443.9449, found 443.9431. Me-ether 46: dark red
μ-η2-(3-Methoxy-1-phenyl-1-butyne)dicobalthexacarbonyl
(39) and μ-η2-(4-Phenyl-3-butyn-2-one)dicobalthexacarbonyl
(40). According to protocol B, Tf2O (141 mg, 0.50 mmol), ketal 35
(119 mg, 0.25 mmol), and Cp2Co (142 mg, 0.75 mmol) afforded
(−78 °C, 2 h; −50 °C, 30 min) the crude mixture, containing, by
NMR, 39 and 40 in the ratio of 54:46. Fractionation on a preparatory
TLC plate (PE:E, 20:1) yielded 39 (37.6 mg, 33.7%) and 40 (49.8 mg,
1
46.3%). 39: dark red oil. TLC (PE:E, 20:1): Rf 0.34. H NMR (400
MHz, CDCl3): δ 1.57 (3H, d, CH3, J = 6.0), 3.51 (3H, s, OCH3), 4.73
(1H, q, CH, Jav = 6.2), 7.29−7.38 (3H, m, aromatic H), 7.52−7.56
(2H, m, aromatic H). 13C NMR (100 MHz, CDCl3): δ 22.2 (CH3),
57.7 (CH), 77.7 (OCH3), 91.1, 99.9 (CC), 127.9, 129.0, 129.8,
138.2 (aromatic C), 199.8 (CO). MS TOF: m/z calcd for
C18H15O8Co2 [M + MeO]− 476.9436, found 476.9457. Anal.
Found: C, 44.75; H, 2.57. C17H12O7Co2 requires: C, 45.77; H, 2.71.
40: brown-red solid. Mp: 45−47 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). TLC (PE:E, 20:1): Rf 0.14.
1H NMR (400 MHz, CDCl3): δ 2.59 (3H, s, CH3), 7.38 (3H, m,
1
oil. TLC (PE:E, 20:1): Rf 0.39. H NMR (400 MHz, CDCl3): δ 0.95
(3H, t, CH3, J = 7.4), 1.62 (3H, s, CH3), 1.95 (2H, ABX3, CHAHB,
JH(A)‑H(B) = 14.0, JH(A)‑CH3 = 7.2, JH(B)‑CH3 = 6.8), 3.27 (3H, s, OCH3),
7.28−7.37 (3H, m, aromatic H), 7.57 (2H, m, aromatic H). 13C NMR
(100 MHz, CDCl3): δ 9.0, 25.9, 33.9 (3-Me, C4, C5), 51.2 (OMe),
79.6 (OCH), 94.1, 105.5 (CC), 127.6, 128.8, 130.0, 138.8 (aromatic
C), 200.0 (CO). MS TOF: m/z calcd for C19H16O7Co2 M+
473.9555, found 473.9562.
aromatic H), 7.59 (2H, m, aromatic H). 13C NMR (100 MHz,
CDCl3): δ 31.4 (CH3), 87.3, 92.4 (CC), 128.9, 129.3, 129.8, 136.9
(aromatic C), 198.3, 201.4 (CO). MS TOF: m/z calcd for
C16H9O7Co2 MH+ 430.9007, found 430.9016. Anal. Found: C, 44.89;
H, 2.02. C16H8O7Co2 requires: C, 44.68; H, 1.87.
μ-η2-(3-Phenyl-2-propynal ethylene glycol acetal)-
dicobalthexacarbonyl (47). Under an atmosphere of nitrogen,
ethylene glycol (314 mg, 5.05 mmol) was added to a solution of 11
(495 mg, 1.01 mmol) in CH2Cl2 (20 mL) at 20 °C. The reaction
temperature was lowered to 0 °C, and BF3·Et2O (230 mg, 2.02 mmol)
was added dropwise (1 min). The reaction mixture was stirred for 1 h,
then diluted with saturated NaHCO3 (20 mL), extracted with CH2Cl2
(3 × 10 mL), dried (Na2SO4), and concentrated under reduced
pressure. Fractionation on an alumina column (80 g, anaerobic
conditions, PE) afforded 47 (418 mg, 89.9%) as a dark red solid. TLC
(PE:E, 20:1): Rf 0.21. Mp: 63−64 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). 1H NMR (400 MHz,
CDCl3): δ 4.08−4.22 (4H, m, 2CH2), 6.27 (1H, s, CH), 7.30−7.38
μ - η 2 - ( 3 - M e t h y l - 1 - p h e n y l - 1 - p e n t y n - 3 - o l ) -
dicobalthexacarbonyl (42). Under an atmosphere of nitrogen, a
solution of n-butyllithium in hexane (3.9 mL; 1.6 M; 6.17 mmol) was
added dropwise (3 min) to a solution of phenylacetylene (600 mg,
5.88 mmol) in THF (40 mL) at −20 °C. The reaction mixture was
stirred for 5 h at −20 °C, and a solution of 2-butanone (466 mg; 6.47
mmol) in THF (5 mL) was added dropwise (5 min) at −40 °C. The
reaction was stirred overnight at ambient temperature, then quenched
80
dx.doi.org/10.1021/om400718w | Organometallics 2014, 33, 69−83

Organometallics
Article
(3H, m, aromatic H), 7.62−7.66 (2H, m, aromatic H). 13C NMR (100
MHz, CDCl3): δ 66.1 (OCH2), 90.5, 92.1 (CC), 103.6 (OCO),
128.2, 129.0, 130.1, 137.7 (aromatic C), 199.3 (CO). MS TOF: m/z
calcd for C17H10O8Co2 M+ 459.9034, found 459.9021. Anal. Found: C,
44.30; H, 2.09. C17H10O8Co2 requires: C, 44.38; H, 2.19.
dried by coevaporation with benzene, 3 × 1 mL). TLC (PE:E, 10:1):
1
Rf 0.25. H NMR (400 MHz, CDCl3): δ 1.83 (4H, m, 2CH2), 2.91
(4H, m, 2CH2), 3.52 (12H, s, 4OCH3), 5.40 (2H, s, 2CH). 13C NMR
(100 MHz, CDCl3): δ 31.9, 33.8 (CH2), 55.1 (OCH3), 92.3, 97.7
(CC), 104.8 (CH), 199.8 (CO). MS TOF FD+: M+ 826. MS
TOF: m/z calcd for C26H22O16ClCo4 [M + Cl]− 860.7930, found
860.7949.
μ-η2-(3-Phenyl-2-propynal propylene glycol acetal)-
dicobalthexacarbonyl (48). Under an atmosphere of nitrogen,
propane-1,3-diol (381 mg, 5.00 mmol) was added to a solution of 11
(490 mg, 1.00 mmol) in CH2Cl2 (20 mL) at 20 °C. The reaction
temperature was lowered to 0 °C, and BF3·Et2O (228 mg, 2.00 mmol)
was added dropwise (1 min). The reaction mixture was stirred for 1 h,
then diluted with saturated NaHCO3 (20 mL), extracted with CH2Cl2
(2 × 10 mL), dried (Na2SO4), and concentrated under reduced
pressure. Fractionation on an alumina column (120 g, anaerobic
conditions, PE) afforded 48 (326 mg, 68.7%) as a dark red solid. TLC
(PE:E, 20:1): Rf 0.28. Mp: 77−91 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). 1H NMR (400 MHz,
CDCl3): δ 1.44 (1H, d septets, CH, J = 13.2, J = 1.2), 2.22 (1H, m,
CH), 4.03 (2H, td, CH2, J = 12.0, J = 2.4), 4.27 (2H, dd, CH2, J = 10.8,
J = 5.2), 5.77 (1H, s, CH), 7.28−7.36 (3H, m, aromatic H), 7.63−7.67
(2H, m, aromatic H). 13C NMR (100 MHz, CDCl3): δ 26.0 (CH2),
67.6 (2CH2), 88.8, 92.4 (CC), 100.7 (OCO), 128.0, 128.8, 130.3,
138.0 (aromatic C), 199.5 (CO). MS TOF: m/z calcd for
C18H12O8Co2 M+ 473.9191, found 473.9170. Anal. Found: C, 45.52;
H, 2.45. C18H12O8Co2 requires: C, 45.60; H, 2.55.
d,l- and meso-μ-η2-(3,4-Dimethoxy-1,5-cyclodecadiyne)bis-
(dicobalthexacarbonyl) (59). According to protocol B, Tf2O (338
mg, 1.20 mmol), bis-acetal 54 (330 mg, 0.40 mmol; CH2Cl2, 32 mL),
and Cp2Co (302 mg, 1.60 mmol; CH2Cl2, 8 mL; −50 °C, 70 min)
afforded, upon extraction with CH2Cl2 (2 × 40 mL) and fractionation
on a silica gel column (120 g, PE), a mixture of d,l-59 and meso-59 (98
mg, 32.1%; NMR: d,l-59:meso-59, 88:12; d,l-59 4.22 ppm; meso-59
4.50, 4.77 ppm).23a Individual d,l-59 was obtained on a silica gel
column (40 g) by using PE as an eluent. d,l-59: dark red solid. Tdec
83−105 °C (sealed capillary; dried by coevaporation with benzene, 3
× 1 mL; decomposition with slight melting). TLC (PE:E, 20:1): Rf
1
0.67. H NMR (400 MHz, CDCl3): δ 1.62 (2H, m, 2CH), 2.00 (2H,
m, 2CH), 3.00 (2H, d, 2CH, J = 16.0), 3.27 (2H, m, 2CH), 3.72 (6H,
s, 2OCH3), 4.22 (2H, s, 2CH). 13C NMR (100 MHz, CDCl3): δ 30.0
(2CH2), 32.7 (2CH2), 60.9 (2OCH3), 87.0 (2OCH), 96.1, 98.7 (C
C), 200.0, 200.3 (CO). MS TOF: m/z calcd for C24H16O14Co4 M+
763.7862, found 763.7865.
d,l-3,4-Dimethoxy-1,5-cyclodecadiyne (60). According to
protocol C, a mixture of d,l-59 and meso-59 (61 mg, 0.08 mmol; d,l-
59:meso-59, 94:6) was treated with Ce(NH4)2(NO3)6 (526 mg, 0.96
mmol; −78 °C, 1 h; −10 °C, 1 h) to afford, upon fractionation on a
preparatory TLC plate (PE:E, 7:1), d,l-60 (6.0 mg, 39.0%) as a pale
yellow solid. Mp: 43−44 °C (sealed capillary; dried by coevaporation
μ-η2-(3-Phenyl-2-propynal)dicobalthexacarbonyl (26). Under
an atmosphere of nitrogen, Tf2O (564 mg, 2.00 mmol) was added
dropwise (15 min) to a solution of 48 (119 mg, 0.25 mmol) in dry
CH2Cl2 (5 mL) at −20 °C. The reaction mixture was stirred for 15
min, then zinc (325 mg, 5.00 mmol) was added, and the stirring was
continued for an additional 15 min. The temperature was raised to 20
°C, and upon stirring for 2 h, the reaction mixture was diluted with
brine (10 mL) and an aqueous layer was extracted with CH2Cl2 (2 ×
10 mL). The crude mixture was subsequently filtered through a short
bed of Florisil (2 cm), and combined organic fractions were dried
(Na2SO4) and concentrated under reduced pressure. Fractionation on
a preparative TLC plate (PE:E, 20:1) afforded 26 (23 mg, 21.9%).
μ-η2 -(1,1,10,10-Tetramethoxy-2,8-decadiyne)bis-
(dicobalthexacarbonyl) (54). Under an atmosphere of nitrogen, n-
butyllithium (25 mL, 40.0 mmol; 1.6 M hexane solution) was added
dropwise (20 min) to a solution of 1,7-octadiyne (55, 2.12 g, 20.0
mmol) in dry THF (40 mL) at −78 °C. Upon stirring for 40 min,
paraformaldehyde (1.20 g, 40.0 mmol) was added to the reaction
mixture. The temperature was increased to 20 °C (10 min), and the
reaction mixture was stirred for 14 h. The crude product was quenched
with DI water (60 mL) at 0 °C, an organic layer was extracted with
ether (3 × 40 mL), and combined ethereal fractions were dried
(Na2SO4). Upon concentration under reduced pressure, the reaction
afforded crude HOCH2CC(CH2)4CCCH2OH (2.41 g), which
was fractionally used in subsequent steps. Under an atmosphere of
nitrogen, pyridinium chlorochromate (4.83 g, 22.40 mmol) was added
to a slurry of diol (930 mg, 5.60 mmol) and Celite (6.0 g) in CH2Cl2
(40 mL) at 0 °C. The temperature was raised to 20 °C, and the
reaction mixture was stirred for 1.5 h, then filtered through cotton and
concentrated under reduced pressure. The crude bis-aldehyde 56 (907
mg, 5.60 mmol; assuming 100% yield) was dissolved in dry ether (20
mL) and added dropwise (20 min) to a solution of dicobaltocta-
carbonyl (3.83 g, 11.2 mmol) in dry ether (30 mL) at 0 °C. The
reaction mixture was stirred for 3 h at 20 °C, then concentrated under
reduced pressure. Under an atmosphere of nitrogen, trimethyl
orthoformate (1.31 g, 12.30 mmol) was added to a solution of bis-
complex (4.11 g, 5.60 mmol; assuming 100% yield) in methanol (60
mL) at 0 °C, followed by p-TsOH (1.07 g, 5.60 mmol). The reaction
was stirred for 2 h at 20 °C, then quenched with a solution of
NaHCO3 (470 mg, 5.60 mmol) in DI water (60 mL). An organic layer
was extracted with ether (3 × 100 mL), and combined ethereal
fractions were dried (Na2SO4) and concentrated under reduced
pressure. The crude mixture was fractionated on an alumina column
(120g, PE:E, 20:1) under anaerobic conditions to give 54 (865 mg,
18.8% over four steps) as a dark red solid. Mp: 77 °C (sealed capillary;
1
with benzene, 3 × 1 mL). TLC (PE:E, 7:1): Rf 0.22. H NMR (400
MHz, CDCl3): δ 1.61−1.71 (2H, m, CH2), 1.80−1.92 (2H, m, CH2),
2.13−2.31 (4H, m, 2CH2), 3.45 (6H, s, 2OCH3), 4.21 (2H, m, 2CH).
13C NMR (100 MHz, CDCl3): δ 20.9 (2CH2), 27.6 (2CH2), 57.5
(2OCH3), 77.4 (2OCH) 79.3, 91.9 (CC). MS TOF: m/z calcd for
C12H17O2 MH+ 193.1223, found 193.1219. Anal. Found: C, 74.49; H,
8.10. C12H16O2 requires: C, 74.97; H, 8.39.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files and tables giving crystallographic details, bond
distances and angles, atomic coordinates and equivalent
isotropic displacement parameters, and torsion angles for d,l-
21. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gagik.melikyan@csun.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under CHE-1112129. The authors are
greatly indebted to the Office of Graduate Studies, Research
and International Programs, and University Corporation,
California State University Northridge, for their generous
support. We are also thankful to McKinzie Garrison for
technical assistance.
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