Assembling contiguous quaternary carbon atoms: Regio- and stereoselective rearrangements in cobalt-directed radical reactions of 1,4-enynes

Article abstract of DOI:10.1021/om501096x

Radical coupling reactions of Co₂(CO)₆-complexed 1,4-enynes occur in a regio- and stereoselective fashion, providing access to 3E,7E-decadiene-1,9-diynes in excellent yields (84-99%). The formation of contiguous quaternary carbon ato

Full text of DOI:10.1021/om501096x

Article
pubs.acs.org/Organometallics
Assembling Contiguous Quaternary Carbon Atoms: Regio- and
Stereoselective Rearrangements in Cobalt-Directed Radical
Reactions of 1,4-Enynes
Gagik G. Melikyan,* Rhoda Hughes, Bianca Rivas, Kellyanne Duncan, and Nare Sahakyan
Department of Chemistry and Biochemistry, California State University Northridge, Northridge, California 91330 United States
S
* Supporting Information
ABSTRACT: Radical coupling reactions of Co2(CO)6-com-
plexed 1,4-enynes occur in a regio- and stereoselective fashion,
providing access to 3E,7E-decadiene-1,9-diynes in excellent
yields (84−99%). The formation of contiguous quaternary
carbon atoms follows a tandem allylic rearrangement that
projects an original reaction site gamma to the metal core.
Propargyl alcohols with an α-alkenyl group as a substituent are
treated with HBF4, followed by the reduction of the highly
conjugated propargyl cations with zinc. The scope of the
reaction is expanded to include 1,4-enyne complexes with cyclic
and acyclic substituents gamma to the metal core, as well as aliphatic and aromatic substituents attached to the acetylenic termini.
The alternative design includes relocation of the cation generation siteα-to-γprior to the reduction step, employing either
the cation isolation technique with HBF4 or an in situ generation of ionic propargyl triflates with Tf2O. Retention of the reaction
site in 1,3-enynes is observed in both γ-alcohols and γ-Me ethers, affording respective γ,γ-radical dimers in excellent yields (98−
99%).
in solid-phase reactions of acyclic3a and cyclic3b ketones.
INTRODUCTION
■
Although being used in the total synthesis of the sesquiterpene
Assembling of organic molecules with contiguous quaternary
herbertenolide,3c radical reactions remain underrepresented in
carbon atoms has long represented a formidable synthetic
challenge.1,2 Besides a purely academic interest of developing
targeted synthesis and construction of molecular assemblies
with adjacent quaternary carbon atoms.1,2 The main reason is a
access to topologically complex molecular assemblies, signifi-
cance of this research area was determined by the presence of
lack of suitable methods for generation of tertiary, sterically
hindered carbon-centered radicals, in particular those that could
said molecular units in natural steroids and their construction
be carried out in a site-selective manner, under relatively mild
via biomimetic polyene cyclization reactions. Select natural
conditions, and also feature an enhanced degree of structural
compounds with vicinal quaternary carbons were reported in
versatility and functional tolerance. In a purely organic
folk medicine to treat certain diseases, thus providing a new
setting,3d−h tertiary radicals containing β-hydrogen atoms suffer
impetus to the synthetic studies that could potentially mimic, or
even outperform, “natural” pathways and provide access to lead
compounds for drug development. Among the alternative
approaches for assembling adjacent quaternary carbon atoms
from a low chemoselectivity with disproportionation being a
major side reaction.3d The disproportionation−combination
ratios are found to be dependent upon reaction temperature,
radical generation mode, and the physical state of the process.3d
With the steric hindrance rising, the stability of radical dimers
substantially decreases, making the central (4°−4°) carbon−
carbon bond more susceptible to homolysis and formation of
persistent tertiary radicals.3e In allylic radicals, given their
inherent ability to delocalize a spin density, regioselectivity of
dimerization is quite low, with the head-to-head dimer,
containing vicinal quaternary carbon atoms, being only a
minor product.3f
are intramolecular Heck cyclizations,2a−c biomimetic cascade
polyene cyclizations,2d,e trans-annular Diels−Alder reactions,2f
Ireland−Claisen2g and thio-Claisen2h rearrangements, and
intermolecular Michael addition reactions.2i While providing
access to topologically diverse molecular targets, most of the
existing methods are characterized by an excessive number of
steps, low overall yields, structural and functional limitations,
and a relatively narrow category of organic molecules that can
be assembled with a given methodology. Radical reactions were
used in the photochemical decarbonylation of heavily
substituted ketones with the methodology being constrained
to a set of substrates with functional groups that could lower
the C−C bond dissociation energies.3a−c Diastereoselectivity of
the radical bond formation varied in the liquid phase (dr 1.1−
2.1)3a with the major, d,l-diastereomer being exclusively formed
In the course of the systematic studies on the chemistry of
the metal-mediated radical and ionic reactions,4 we developed
alternative methods for the generation of the cobalt-complexed
Received: October 30, 2014
© XXXX American Chemical Society
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Figure 1. Topology of cobalt-complexed propargyl cations and radicals.
Figure 2. Alternative strategies for generation of the cobalt-stabilized propargyl cations.
secondary propargyl radicals via tetrafluoroborates,4a,d,e,h−k,m
triflates,4b,f,g,l,n and trifluoroacetates.4g Their subsequent inter-
and intramolecular coupling reactions provided facile access to
1,5-alkadiynes4a−d,f,g,k−n and 1,5-cycloalkadiynes4e,h−j,n contain-
ing, as a unifying structural motif, the contiguous tertiary
carbon stereocenters arranged either in d,l- or meso-fashion.
With respective tertiary radicals, dimerization failed to occur,
attesting to the fact that the steric limits for converging cobalt-
complexed propargyl radicals can be exceeded even with the
smallest alkyl groups (Me, Et) being introduced alpha to a
metal core.4n Propargyl substrates studied so far4 contained
alpha substituents [Ph (1), 1-Naph (1), alkyl (2), alkoxy (3)]
that intrinsically lack an ability to rearrange the reaction site, at
either the cation or radical generation step (Figure 1).
Synthetically, acquiring an ability to project an original reaction
site over a select number of atoms represents an attractive
prospect, on one side, benefiting from the cation-stabilizing
effect of the Co2(CO)6 moiety7 and, on the other side, creating
a spatial separation between the rearranged reaction site and
conformationally “restrictive” metal core. Herein we report on
metal-assisted radical reactions of novel propargyl systems 4
with an alkenyl group being introduced alpha to the metal core.
The presence of an allylic moiety susceptible to rearrangement5
creates a new, regioselectivity dimension in cobalt-assisted radical
reactions4 wherein the radical C−C bond formation could
potentially occur at alpha (4) and/or gamma (5) positions. If
the aforementioned rearrangements were to take place, then
the targeted introduction of two alkyl groups, both gamma to
the metal cluster, would place quaternary carbon atoms vicinal
to each other, following the tail-to-tail dimerization of the
respective tertiary radicals. Thus, the regioselectivity of radical
coupling reactions (head-to-head, head-to-tail, tail-to-tail) as
well as the stereoselectivity of allylic rearrangements became
the focal points of this investigation. The highly selective allylic
rearrangement could provide access to a topologically diverse
set of polyunsaturated compounds with vicinal quaternary
carbons, thus making this method a valuable asset for organic
chemistry, natural product synthesis, and medicinal chemistry.
RESULTS AND DISCUSSION
■
Three alternative methods for the generation of requisite
propargyl cations were developed, employing isomeric α-
alcohol 6 and γ-alcohol 7, as well as γ-Me ether 8 (Figure 2).
The former represents the most conventional approach,
wherein the cobalt complex 6 is treated with a strong acid,
HBF4, and the respective cation 9 is isolated at low
temperatures (−20 °C) as a tetrafluoroborate salt.6,7
Repositioning a hydroxyl group, α-to-γ, creates a new type of
propargyl alcohol, 7, in which a leaving group, OH, and a
metal-alkyne core are separated by a vinyl group (vinylogy
principle).8 In other words, an original cation generation site is
moved further away from a stabilizing π-bonded metal core,
with a vinyl group acting as a conduit for conjugation. It should
be mentioned that tetrafluoroborates 9 and 10 may not
necessarily be related as resonance contributors if the said
counterions are positioned, or coordinated, differently with
respect to the allylic triad, in particular its electrophilic termini.9
γ-Methyl ether 8, a vinylog of previously studied α-Me ethers,4l
could potentially interact with Tf2O under similar conditions
(−50 to −10 °C, 30 min), forming an in situ propargyl triflate
11. Its reducibility with cobaltocene will depend upon the
nature of the γ-C−OTf bond, since a degree of separation in an
ionic pair and, dependent upon it, the partial charge of the
cationic center were shown to be critical for the rate of
reduction step. The resonance hybrid 12 represents a general
structure for transient cations and radicals for which the
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Scheme 1. Regioselective Tail-to-Tail Dimerization of α-Alkenyl Propargyl Radicals
Figure 3. ORTEP diagram of μ-η2-[5,5,6,6-tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-diene-1,9-diyne]bis(dicobalthexacarbonyl) (20). Select
bond lengths (Å), bond angles (deg), and torsional angles (deg): C1−C2 1.33, C1−Si1 1.84, C1−Co1 1.99, C1−Co2 1.99, C2−C3 1.47, C2−Co1
1.95, C2−Co2 1.95, Co1−Co2 2.48, C3−C4 1.34, C5−C5′ 1.59, C2−C1−Si1 147.4, C1−C2−C3 143.7, C4−C3−C2 121.9, C3−C4−C5 126.6,
Si1−C1−C2−C3 2.3, C1−C2−C3-C4 10.2, C4−C5−C5′-C4′ 180.0, C6−C5−C5′−C7′ 58.8, C7−C5−C5′−C6′ 58.8, C1−C2−Co1−Co2 74.7,
Co1−C2−C1−Co2 83.9.
intimate details still remain to be comprehended,9 including the
γ,γ-Dimethyl-substituted propargyl alcohol 13 was synthe-
sized by the condensation of the lithiated trimethylsilylacety-
lene with dimethylacrolein,10a followed by the protection of a
triple bond10b with a dicobalthexacarbonyl group (Scheme 1).
Treatment with HBF4 allowed for isolation of the propargyl
cation 14, which could then delocalize over the double bond,
forming an isomeric cation 15. The reduction step was carried
out with a 10-fold excess of zinc at ambient temperatures,
producing the transient radicals that can be best represented by
two resonance structures, 16 and 17. Both termini of an allylic
impact of alternative counterions, BF4 and OTf,− on the
−
structure of the respective ionic pairs, as well as the locale and
geometry of their coordination to an allylic triad. Hybrid 12
also underlines the presence of the alternative competing
reaction sites (head and tail) and, attendant with it, the
regioselectivity aspect of the radical coupling reactions that
could give rise to symmetrical head-to-head and tail-to-tail
dimers, as well as to their unsymmetrical counterpart, a head-
to-tail dimer.
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Scheme 2. Regioselective Radical Dimerization of Isomeric γ-Alcohols
triad could undergo a coupling reaction, forming isomeric head-
to-head (18), head-to-tail (19), and/or tail-to-tail (20) dimers.
A carbon−carbon bond formation occurred exclusively gamma
to a metal core with the double bond isomerizing in the trans-
stereoselective manner and producing γ,γ-dimer 20 in a nearly
quantitative yield (99%). Despite a multitude of regio- and
stereoisomers that could potentially be formed (seven), the
radical reaction features excellent chemo-, regio-, and stereo-
selectivities, to the extent that the crude product represents an
analytically pure sample that does not need any additional
purification. It is worth mentioning that the presence of a
Co2(CO)6 core is critical since it provides for the stabilization
of the intermediate propargyl cations,7 precludes an unwanted
acetylene-allene reorganization,11 and also prevents the Rupe
and Meyer−Schuster rearrangements.12 An exclusive formation
of γ,γ-dimer 20 can be explained in kinetic terms with the tail-
to-tail dimerization occurring at much higher rates than the
competing head-to-head coupling (kγ,γ > kα,α). Although
seemingly counterintuitive, the tertiary γ-atom can be better
suited for the coupling reaction than the secondary α-carbon
because of the close proximity to a bulky Co2(CO)6 core. This
assumption is corroborated by the computational data: a total
volume of substituents attached to the secondary, α position is
higher than that for the tertiary, γ position (Spartan 2010; α-
C2° V(HCC)Co2(CO)6+(CHCH2)+H 256.98 Å3; γ-C3°
V2Me+(CHCH2) 91.69 Å3). A high sensitivity of organometallic
coupling reactions toward steric hindrance was recently
demonstrated by us for propargyl acetals, propargyl ketals,
and tertiary propargyl ethers: even a single extra carbon atom
introduced alpha to the metal core can be critical to the success
of the α,α-coupling reaction.4n For comparison, ionic reactions
of the terminal cobalt-complexed 1,4-enynes are largely
accompanied by a stereoselective allylic rearrangement, placing
incoming C-nucleophiles gamma to a metal core (68−82%).13a
A lower regioselectivity was observed with ethanol (α:γ, 7:1),13a
isopropenyl acetate (α:γ, 1:13),13a and furan (α:γ, 1:3)13b as
nucleophiles and cyclic seven-membered 1,4-enynes (α:γ, from
1:24 to 1.3:1)13c as substrates. Structurally related α-cyclo-
propyl propargyl alcohols also undergo a stereoselective allylic
shift to the respective E-homoallylic bromides.13d
Both double bonds have trans-configurations, attesting to their
stereoselective repositioning in the course of an allylic
rearrangement. A steric constraint introduced by the bulky
Me3Si groups does not impact the linearity of the coordinated
triple bonds (θSi1−C1−C2−C3 2.3°, θSi1′−C1′−C2′−C3′ 2.3°). Due to
rehybridization of the acetylenic carbons (sp3-to-sp2) and a
back-bonding from the cobalt atoms,7,15 triple bonds are
noticeably lengthened (C1−C2 1.33 Å vs ∼1.21 Å for free
ligand) and alkyne units are unevenly bent (C2−C1−Si1
147.4°, C1−C2−C3 143.7°). Both enyne moieties adopt an s-
cis conformation with minimal distortion in coplanarity of
multiple bonds (θC1−C2−C3−C4 10.2°, θC1′−C2′−C3′−C4′ 10.2°).
Amazingly, the double bonds which are spatially separated by a
heavily populated two-carbon spacer (4.8 Å) remain coplanar,
as attested by the torsional angles between nonconnecting C−
H bonds (θH3−C3/C3′−H3′ 180.0°, θH4−C4/C4′−H4′ 180.0°). Overall,
two identical parts of the dimer 20 are located in parallel planes
with a spatial separation of some 1.5 Å.
Decomplexation of γ,γ-dimer 20 with ceric ammonium
nitrate16 proceeded smoothly at −78 °C (10 equiv), providing
symmetrical γ,γ-dimer 21 in an excellent yield (81%) (Scheme
1). A TMS group at the sp-hybridized carbons is known to be
removable under basic conditions, thus allowing for retrieval of
terminal alkyne moieties.4k,17 The treatment of dimer 21 with
sodium hydroxide allowed for tandem desilylation, forming γ,γ-
dimer 22 in a high yield (97%) (Scheme 1). It is worthy to
mention that the presence of a TMS group in propargyl alcohol
13 is critical since our attempts to involve parent alcohol 23 in
the dimerization reaction were not successful due to the rapid
polymerization of the respective cationic species. Thus, an
auxiliary TMS substituent plays multiple roles in enhancing the
efficiency of the overall process: it provides for stabilization of
reactive intermediates, enhances the crystallinity of radical
dimers, and also increases yields of the dimeric products (50−
70%4), both metal-free and metal-complexed.
To expand the substrate base and also to test if the site of the
initial cation generation could have a higher degree of
separation from the stabilizing metal core, γ-alcohol 24, a
vinylog8 of the propargyl alcohol 13, was probed as a new class
of precursors to cobalt-stabilized conjugated cations (Scheme
2).7 In other words, the main goal was to establish if an α,β-
double bond could act as an efficient conduit between a π-
bonded electron reservoir and a positive charge developing on
the periphery of the organic ligand. α-Alcohol 13 was treated
with water in the presence of BF3·Et2O18 with the former
attacking the cationic allylic triad exclusively at the γ-carbon
The structure of γ,γ-dimer 20 was independently confirmed
by means of X-ray crystallography14 (Figure 3). The molecule
adopts an extended conformation in which the enyne moieties
are positioned anti to each other (θC4−C5−C5′‑C4′ 180.0°), and
geminal methyl groups are arranged gauche around the central
C5−C5′ bond (θC6−C5−C5′−C7′ 58.8°, θC7−C5−C5′−C6′ 58.8°).
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Scheme 3. Tf2O-Cp2Co-Induced Radical Dimerization of γ Methyl Ethers
atom and yielding γ-alcohol 24 in a high yield (96%). The
experimental data showed that HBF4 is still capable of
precipitating the cationic salt 15 under conditions similar to
those applied for α-alcohol 13 (6 equiv, −20 °C, ≤30 min).
The subsequent reduction with Zn (10 equiv) generated the
highly delocalized radical 25, which underwent dimerization
exclusively at the γ-position, forming γ,γ-dimer 20 in an
excellent yield (99%).
can also be converted to the respective tetrafluoroborate
cationic salt by treatment with HBF4 (Scheme 3); its reduction
with zinc also afforded γ,γ-dimer 20 in excellent yield (99%).
Topologically, substrate base expansion was carried out in a
tridimensional fashion in which parent α-alcohol 13 was
modified at select locations, i.e., α, γ, and γ′ to the cobalt core
(Table 1). The latter involves the replacement of an auxiliary
TMS group at the acetylenic termini with phenyl and 2-
naphthyl groups (Table 1, entries 2, 3). Requisite alcohols 29
and 30 were synthesized in two steps by the condensation of
the lithiated acetylides with dimethylacrolein,10a followed by
the protection of the triple bonds10b with a dicobalthex-
acarbonyl group. The cation-generation step was carried out
with HBF4 under standard conditions (6 equiv, −20 °C, 30/15
min), followed by the reduction with zinc (10 equiv, 20 °C, 30
min). In both substrates, formation of the C−C bond occurred
exclusively at the gamma positions with double bonds
undergoing the trans-allylic rearrangement in a highly stereo-
selective manner. γ,γ-Dimers 31 and 32, as single products,
were isolated in excellent yields99% and 86%attesting to
the fact that the regio- and stereoselectivities of the tail-to-tail
radical coupling are not compromised by the bulkier
substituents attached to the acetylenic termini. Incorporation
of an additional methyl group, alpha to the metal cluster, could
facilitate the generation of the intermediate tertiary cation from
α-alcohol 33, although the regioselectivity, as well as the
stereoselectivity of the allylic rearrangement, was hard to
predict (Table 1, entry 4). The synthesis of tertiary alcohol 33
was carried out by conventional means, although its isolation
proved challenging (7% over two steps) due to its powerful
tendency to rearrange to the respective γ-alcohol in the course
of chromatographic separation. Its consecutive treatment with
HBF4, and then Zn, under standardized conditions, formed γ,γ-
dimer 34 (86%), as a single product, although the configuration
of the rearranged trisubstituted double bonds could not be
reliably determined by NMR spectroscopy.20 The intimate
structural details of the radical dimer 34 were established by
means of X-ray crystallography (Figure 4) with the basic
structural characteristics being similar to those in its lower
homologue 20 (Figure 3). In particular, bis-complex 34 adopts
an extended conformation (θC4−C2−C2′−C4′ 180.0°) with geminal
methyl groups being arranged in an ideally gauche fashion
around the central C2−C2′ bond (θC1−C2−C2′−C1′ 62.8°,
θC3−C2−C2′−C3′ 62.8°). Both pairs of α-C-Me and β-C−H
bonds are parallel to each other, although remain pointed in the
Having established that a gamma position of the 1,3-enyne
cobalt complexes is a viable locale for the generation of the
cations from respective alcohols, we then switched to a new
class of gamma derivatives, γ-Me ethers, such as 26 (Scheme 3).
Methyl propargyl ethers, containing a MeO group at the
secondary carbon atoms alpha to the metal core, were reported
by us to undergo a low-temperature conversion to ionic
triflates, which then can be reduced with cobaltocene to the
respective radicals.4l It was of significant synthetic interest to
examine if the parent process could be expanded toward the
novel molecular assemblies in which a tertiary γ-carbon atom
and a cobalt-alkyne core are separated by a double bond, in
other words, if the treatment of γ-Me ether 26 with triflic
anhydride could produce the desired substitution product 27,
or the steric hindrance at the tertiary carbon atom would
prevent a nucleophilic attack. γ-Me ether 26 was synthesized
from alcohol 13 in one step (BF3·OMe2, MeOH,18 65%) and
then treated with Tf2O under the standardized conditions (−50
to −10 °C, 30 min), followed by the reduction with
cobaltocene19 at −50 °C (10 min). γ,γ-Dimer 20 was isolated
in a 98% yield, attesting to the following: (a) the substitution
reaction did occur at the tertiary carbon atom, forming ionic
pair 27, (b) the latter undergoes a C−O bond heterolysis to
form ionic triflate 28, thus meeting the key structural
requirement for being reducible with cobaltocene,4l (c) spatial
alignment of all three conjugating componentsπ-bonded
metal core, a double bond, and a p-orbital of the tertiary
cationic centerallowed for an efficient delocalization of the
positive charge, thus enabling the double bond to act as an
electronic conduit, and (d) despite its bulkiness, cobaltocene is
capable of reducing not only secondary4l but also the tertiary
cationic centers. Overall, from the substrate expansion
perspective, γ-methyl ether 26 represents the third class of
substrates, along with α-Me ethers4l and propargyl acetals,4n for
which a tandem action of Tf2O-Cp2Co can apply, allowing for a
rapid, low-temperature, high-yield, and regioselective coupling
reaction occurring gamma to a metal core. γ-Methyl ether 26
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Table 1. Regioselective α-to-γ Rearrangement in Intermolecular Coupling Reactions of α-Alkenyl Propargyl Radicals
a
b
Yield over two steps. Standard dimerization protocol included treatment of the cobalt-complexed 1,4-enynes with a 6-fold excess of HBF4 at −20
c
°C and reduction with a 10-fold excess of zinc at +20 °C. Standard decomplexation protocol included treatment of dimeric bis-complexes with ceric
ammonium nitrate (8−12 equiv) at −78 to 0 °C. Yield over three steps.
d
opposite directions (θC6−C5/C5′−C6′ 180.0°, θC4−H4/C4′−H4′
180.0°). Most importantly, the newly formed trisubstituted
double bonds have an E-configuration, attesting to the fact that
even with an α-methyl group present, allylic rearrangement
occurs stereoselectively by placing the bulky cobalt-alkyne cores
and geminal methyl groups trans to each other. Unexpectedly, a
lesser distortion from planarity is observed for double and triple
bonds, adopting an s-cis conformation in both enyne moieties
(34 θC8−C7−C5−C4 2.3°; 20 θC1−C2−C3−C4 10.2°). Analogously, a
better linearity was found around the coordinated triple bonds
(34 θC5−C7−C8−Si1 0.5°; 20 θSi1−C1−C2−C3 2.4°). A methyl group
located alpha to the metal core most noticeably presents itself
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Figure 4. ORTEP diagram of μ-η2-[3,5,5,6,6,8-hexamethyl-1,10-bis(trimethylsilyl)deca-3E,7E-diene-1,9-diyne]bis(dicobalthexacarbonyl) (34).
Selected bond lengths (Å), bond angles (deg), and torsional angles (deg): Co1−Co2 2.48, Co1−C7 1.97, Co2−C7 1.97, Co1−C8 1.98, C7−C8
1.34, C7−C5 1.47, C5−C4 1.34, C7−Co1−Co2 51.0, C7−Co1−Co8 39.8, C7−C8−Si1 147.7, C8−C7−C5 143.1, C7−C5−C6 115.8, C4−C5−C7
116.1, C4−C5−C6 128.1, C5−C4−C2 132.8, C3−C2−C1 105.6, C8−C7−C5−C4 2.3, C5−C7−C8−Si1 0.5, C6−C5−C4−C2 2.6, C4−C2−C2′−
C4′ 180.0, C1−C2−C2′−C1′ 62.8, C3−C2−C2′−C3′ 62.8, C6−C5/C5′−C6′ 180, C4−H4/C4′−H4′ 180.
Figure 5. ORTEP diagram of {1-[(E)-4″-(trimethylsilyl)but-1″-en-3″-ynyl]-1′-[(E)-4‴-(trimethylsilyl)but-1‴-en-3‴-ynyl]-1,1′-bi(cyclohexane)}-
bis(dicobalthexacarbonyl) (36). Selected bond lengths (Å), bond angles (deg), and torsional angles (deg): Co1−Co2 2.48, Co1−C10 2.00, Co1−C9
1.97, Si1−C10 1.85, C10−C9 1.34, C9−C8 1.45, C1−C1′ 1.61, C7−C8 1.34, Co2−C10−Co1 76.8, C9−C10−Si1 149.2, C10−C9−C8 144.8, C8−
C7−C1 126.9, C7−C8−C9 123.1, Si1−C10−C9−C8 3.5, C10−C9−C8−C7 1.4, C1−C7−C8−C9 177.3, C7−C1−C1′−C7′ 180.0, C2−C1−C1′−
C6′ 60.6, C2′−C1′−C1−C6 60.6, H8−C8/C8′−H8′ 180.0, H7−C7/C7′−H7′ 180.0.
by expanding the bond angles around the vinylicC4 and
C5carbon atoms (θC4−C5−C6 128.1°; θC5−C4−C2 132.8°).
Tethering γ,γ-Me,Me substituents of the allylic triad was a
significant structural change in parent alcohol 13 that could
potentially compromise an exclusive tail-to-tail dimerization
observed so far (Table 1, entries 1−4). The formation of γ,γ-
dimers would involve tertiary cycloalkyl radicals that could
kinetically differ, for steric and conformational reasons, from
tertiary alkyl radicals 17 derived from parent alcohol 13. In
particular, an increased steric hindrance could decrease the
dimerization rate at a gamma site (kγ,γ), thus making the C−C
bond formation alpha to a metal core kinetically more
competitive (kα,α). As a result, the regioselectivity could decline
with dimerization reactions occurring at both termini of the
allylic triad and producing α,α-dimers, along with isomeric γ,γ-
dimers. A cyclohexyl ring, gamma to a metal core, was
introduced in alcohol 35 in three steps (13%) by using
commercially available α-vinylcyclohexanol in a ReVII-induced
rearrangement-oxidation reaction,21 followed by condensation
with lithium TMS-acetylide10a and subsequent protection of the
triple bond with a dicobalthexacarbonyl core.10b The treatment
with a 6-fold excess of HBF4 rapidly converted alcohol 35 to
the respective cation (4 min, −20 °C), which then smoothly
converted to γ,γ-dimer 36, in a high yield (84%), when reduced
G
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with a 10-fold excess of zinc (20 °C, 30 min) (Table 1, entry 5).
Its formation stands as an experimental proof that tertiary
cyclohexyl radicals can dimerize as efficiently as less bulkier,
acyclic tertiary alkyl radicals. The X-ray structure reveals
(Figure 5) that γ,γ-dimer 36 mimics, conformationally and
configurationally, its acyclic counterparts 20 and 34 (Figures 3,
4). Both double bonds have a trans-configuration, with the
cyclohexyl rings adopting a chair conformation. A newly
formed C−C bond, extended to 1.61 Å due to the increased
steric pressure, holds the metal-clustered enyne moieties anti to
each other (θC7−C1−C1′−C7′ 180°), while the cyclohexane rings
position their internal C−C bonds ideally gauche to each other
(θC2−C1−C‑1′−C6′ 60.6°, θC2′‑C1′−C1−C6 60.6°). With respect to the
six-membered ring, γ,γ-dimer 36 represents a 1,1-disubstituted
bis-cyclohexane derivative with cyclohexane rings being placed
into the respective equatorial positions, while the less bulkier
double bonds occupy the respective axial positions. The
disposition of the vinylic C−H bonds (H8−C8/C8′−H8′
180°, H7−C7/C7′−H7′ 180°) is indicative of an ideal
coplanarity of the double bonds. An s-cis conformation of
enyne moieties (θC10−C9−C8−C7 1.4°, θC10′‑C9′‑C8′−C7′ 1.4°) and a
minimal deviation from planarity for the substituents around
the coordinated triple bonds (θSi1−C10−C9−C8 3.5°,
θSi1′‑C10′‑C9′−C8′ 3.5°) effectively place the five-atom sequen-
cesSi1−C10−C9−C8−C7 and Si1′−C10′−C9′−C8′−
C7′into two parallel planes.
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).
Methylene chloride was stored over CaCl2 and distilled under dry
nitrogen from CaH2; ether was stored over sodium and filtered
through an alumina−silica gel column. All reagents and solvents
(pentane, THF, acetone, benzene, methanol) were purchased from
Acros, TCI, Alpha Aesar, and Sigma-Aldrich and used as received. Zinc
was acquired from Aldrich (dust, 10 μm). Co2(CO)8 and Ce-
(NH4)2(NO3)6 were purchased from Strem. NMR solvents were
supplied by Cambridge Isotope Laboratories. 1H and 13C NMR
spectra were recorded on Bruker Avance III-400 (1H, 400 MHz) and
Varian 400-MR (1H, 400 MHz) spectrometers. Chemical shifts were
referenced to internal solvent resonances and are reported relative to
tetramethylsilane. Spin−spin coupling constants (JHH) are given in
hertz. Elemental analyses were performed by Columbia Analytical
Services (Kelso, WA, USA). Melting temperatures (uncorrected) were
measured on a Mel-Temp II (Laboratory Devices) apparatus and an
Optimelt Automated Meltemp. Silica gel S735-1 (60−100 mesh;
Fisher) was used for flash column chromatography. Analytical and
preparative TLC analyses (PTLC) were conducted on silica gel 60 F254
(EM Science; aluminum sheets) and silica gel 60 PF254 (EM Science;
w/gypsum; 20 × 20 cm), respectively. Visualization was carried out
with potassium permanganate aqueous solution unless indicated
otherwise. 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 LC-TOF
instrument with a Multimode source).
Decomplexation reactions for γ,γ-dimers 31, 32, 34, and 36
were carried out with ceric ammonium nitrate16 (−78 °C, 10
equiv), providing symmetrical γ,γ-dimers 37−40 in good to
high yields (55−86%; Table 1). Analogous to the bis-
desilylation process demonstrated for the parent reaction (21
→ 22, Scheme 1), γ,γ-dimers 34 and 36, as well as their
topologically and functionally diverse structural analogues, can
be treated with base4k,17 to retrieve the acetylenic termini,
which in turn are ripe for a multitude of secondary synthetic
transformations. For comparison, in a purely organic setting,
unsymmetrical allylic radicals dimerize with a low regioselec-
tivity, affording a mixture of head-to-head (4°−4°), head-to-tail
(4°−2°), and tail-to-tail (2°−2°) dimers in the ratio 16:30:54.3f
Synthesis of Co2(CO)6-Complexed Propargyl Alcohols
(Protocol A): μ-η2-[5-Methyl-1-(trimethylsilyl)hex-4-en-1-yn-3-
ol]dicobalt Hexacarbonyl (13). Under an atmosphere of nitrogen,
n-butyllithium (352 mg, 5.50 mmol; 3.44 mL, 1.6 M in hexane) was
added dropwise (15 min) to a solution of trimethylsilylacetylene (539
mg, 5.50 mmol) in dry THF (20 mL) at −10 °C. Upon addition, the
reaction mixture was stirred for 5 h at −10 °C, a solution of
dimethylacrolein (420 mg, 5.00 mmol) in dry THF (5 mL) was added
dropwise (15 min) at −10 °C, and the reaction mixture was stirred for
2.5 h at 20 °C (TLC monitoring). The crude mixture was quenched
with saturated NaClaq (25 mL) at 0 °C and diluted with ether (25
mL), and an aqueous layer was separated and extracted with ether (3
× 25 mL). Combined ethereal layers were dried (Na2SO4), and
solvents were evaporated under reduced pressure. Under an
atmosphere of nitrogen, the crude alcohol 3 (910 mg, 5.00 mmol,
assuming 100% yield) was redissolved in dry ether (10 mL) and added
dropwise (20 min) to a solution of dicobaltoctacarbonyl (1.88 g, 5.50
mmol) in dry ether (20 mL) at 20 °C. The reaction mixture was
stirred for 5 h at 20 °C (TLC control), concentrated under reduced
pressure, and fractionated on Florisil (200 g, PE:E, 5:1) to afford 13
(1.31 g, 56% over two steps) as a brick-red solid. Mp: 38−39 °C
(sealed capillary, dried by coevaporation with benzene, 3 × 1 mL).
CONCLUSIONS
■
A double bond introduced alpha to a Co2(CO)6-alkyne cluster
was used in radical coupling reactions of 1,4-enynes as a
molecular lever for projecting an original reaction site over an
allylic triad. The strategy was proven effective with a set of
substrates derived from alterations of α-, γ-, and γ′-carbon
centers in cyclic and acyclic Co2(CO)6-complexed propargyl
alcohols. Selective formation of γ,γ-dimers (84−99%) was
enabled by the proximity of a stabilizing metal core at the
cation generation step and an ability of an α-alkenyl group to
project the reaction site away from a highly congested area
alpha to a metal coreat the radical dimerization step. 1,3-
Enynes with a hydroxyl group gamma to a metal core behave as
vinylogs of propargyl alcohols by preserving the most essential
characteristic of the latter, i.e., an ability to undergo a C−O
bond heterolysis under acidic conditions and to form isolable,
metal-stabilized propargyl cations. The “reaction-site-projec-
tion” strategy can be applied for the synthesis of organic
molecules containing contiguous quaternary carbon atoms and
also for probing an upper limit of projection span in ionic and
radical reactions.
1
TLC (PE:E, 5:1): Rf 0.47. H NMR (400 MHz, δ, CDCl3): 0.29 (s,
9H, 3CH3), 1.75 (s, 3H, CH3), 1.80 (s, 3H, CH3), 1.84 (d, J = 4.4, 1H,
OH), 5.28 (d, J = 8.4, 1H, 4-H), 5.54 (dd, 1H, 3-H). 13C NMR (100
MHz, δ, CDCl3): 0.7 (3CH3), 18.7 (CH3), 25.6 (CH3), 69.7 (C3),
78.1, 115.0 (CC), 127.9, 135.5 (C4, C5), 200.2 (CO). MS HR
TOF: m/z calcd for C16H17O7SiCo2 (M − H)− 466.9413, found
466.9400. Anal. Found: C, 40.80; H, 3.79. C16H17O7SiCo2 requires: C,
41.04; H, 3.87.
Dimerization Reaction with Zinc as a Reductant (Protocol
B): μ-η2-[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-
3E,7E-diene-1,9-diyne]bis(dicobalthexacarbonyl) (20). Under
an atmosphere of nitrogen, HBF4·OMe2 (201 mg, 1.50 mmol) was
added dropwise to a solution of 13 (117 mg, 0.25 mmol) in dry
pentane (20 mL) at −20 °C. The reaction mixture was stirred for 30
min at −20 °C, and the precipitate was washed with dry pentane (2 ×
20 mL) at −20 °C. The residual solvent was stripped under reduced
pressure (40 min, −30 °C), the precipitate was dissolved in dry
H
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Article
CH2Cl2 (5 mL) at −20 °C, and degassed zinc (163 mg, 2.50 mmol)
was added in one portion. The reaction mixture was stirred at −20 °C
for 5 min and then for an additional 20 min at 20 °C (TLC control).
Zinc was filtered off on a short bed of Florisil (1 in.), yielding 20
(111.7 mg, 99%) as a chocolate powder-like solid (crude 20 is
solution of Me-ether 26 (120.5 mg, 0.25 mmol) in dry CH2Cl2 (4 mL)
at −50 °C and stirred for 5 min. At −50 °C, a solution of Cp2Co (49.1
mg, 0.26 mmol) in dry CH2Cl2 was added dropwise (6 min), and the
reaction mixture was stirred for 10 min (TLC control). Saturated brine
(10 mL) was added at −50 °C, and the temperature was raised to 0 °C
in 40 min. The crude product was extracted with ether (3 × 15 mL),
and the combined organic layers were washed with deionized water (3
× 25 mL) and dried (Na2SO4). The crude product represents
analytically pure 20 (110 mg, 98%).
analytically pure and did not require any additional purification). Tdec
:
114−128 °C (sealed capillary; dried by coevaporation with benzene, 3
× 1 mL). TLC (PE): Rf 0.78. 1H NMR (400 MHz, δ, CDCl3): 0.33 (s,
18H, 6CH3), 1.07 (s, 12H, 4CH3), 6.34 (AB spectrum, J = 15.6, 4H, 3-
H, 4-H). 13C NMR (100 MHz, δ, CDCl3): 0.7 (6CH3), 23.1 (4CH3),
42.2 (C5, C6), 80.1, 105.0 (CC), 124.0, 145.4 (CC), 200.4
(CO). MS TOF FD+: m/z M+ 902. MS HR TOF FD+: m/z calcd for
C32H34O12Si2Co4 M+ 901.8911, found 901.8936. Anal. Found: C,
42.84; H, 3.71. C32H34O12Si2Co4 requires: C, 42.59; H, 3.80. Single
crystals suitable for X-ray structure analysis (Figure 3) were obtained
by ethanol vapor diffusion into a solution of 20 in ethyl acetate at +4
°C (3 days).
Decomplexation with Ceric Ammonium Nitrate (Protocol C):
5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-diene-
1,9-diyne (21). Under an atmosphere of nitrogen, Ce(NH4)2(NO3)6
(175 mg, 0.32 mmol) in degassed acetone (8 mL) was added dropwise
to a solution of 20 (36 mg, 0.04 mmol) in degassed acetone (10 mL)
at −78 °C. The reaction mixture was stirred at −78 °C (30 min), then
gradually warmed to 0 °C (45 min) and stirred for additional 15 min
at 0 °C. The reaction was brought to completion by adding a solution
of Ce(NH4)2(NO3)6 (44 mg, 0.08 mmol) in degassed acetone (2 mL)
at −78 °C. The reaction mixture was warmed to −10 °C, stirred for
additional 15 min, and quenched with saturated NaCl(aq) (15 mL).
The aqueous layer was extracted with ether (3 × 15 mL), and the
combined organic layers were dried over molecular sieves (4 Å). The
crude product was chromatographed on a preparative TLC plate
(silica, PE:E, 100:1) to afford 21 (10.7 mg, 81%) as a white solid. Mp:
135−136 °C (sealed capillary, dried by coevaporation with benzene, 3
× 1 mL). TLC (PE:E, 100:1): Rf 0.46. 1H NMR (400 MHz, δ, C6D6):
0.21 (s, 18H, 6CH3), 0.63 (s, 12H, 4CH3), 5.38 (d, J = 16.0, 2H,
2CH), 6.29 (d, J = 16.4, 2H, 2CH). 13C NMR (100 MHz, δ,
CDCl3): 0.2 (6CH3), 22.7 (4CH3), 42.5 (C5, C6), 93.6, 104.6 (C
C), 108.3, 152.6 (C3, C4, C7, C8). MS ESI APCI: m/z calcd for
C20H38NSi2 (M + NH4)+ 348.2537, found 348.2533. Anal. Found: C,
72.54; H, 10.31. C20H34Si2 requires: C, 72.65; H, 10.36.
5,5,6,6-Tetramethyldeca-3E,7E-diene-1,9-diyne (22). Under
an atmosphere of nitrogen, BnMe3NCl (2.4 mg, 0.013 mmol) and
21 (16.5 mg, 0.05 mmol) were dissolved in dry acetonitrile (15 mL),
and a solution of NaOH (20 mg, 0.50 mmol) in DI H2O (2 mL) was
added dropwise at 0 °C (5 min). The temperature was raised to 20 °C,
and the reaction mixture was stirred for 20 h. The suspension was
diluted with ether (100 mL), and the organic layer was washed with
saturated NaClaq (3 × 50 mL), dried over molecular sieves (4 Å), and
concentrated under reduced pressure to afford 22 (9 mg, 97%) as
white crystals. Mp: 58.0−58.5 °C (sealed capillary, dried by
coevaporation with benzene, 3 × 1 mL). TLC (P): Rf 0.36. 1H
NMR (400 MHz, δ, C6D6): 0.61 (s, 12H, 4CH3), 2.57 (dd, J = 2.0, J =
0.4, 2H, 2HC), 5.26 (dd, J = 16.0, 2H, 3-H, 8-H), 6.25 (d, 2H, 4-H,
7-H). 13C NMR (100 MHz, δ, C6D6): 22.6 (4CH3), 42.4 (C5, C6),
77.4, 83.2 (CCH), 108.1, 153.1 (C3, C4, C7, C8). MS TOF FD+:
m/z M+ 186. MS HR TOF FD+: m/z calcd for C14H18 M+ 186.1403,
found 186.1408.
μ-η2-(2-Methyl-6-(trimethylsilyl)hex-3E-en-5-yn-2-ol)-
dicobalt Hexacarbonyl (24). Under an atmosphere of nitrogen, at 0
°C, degassed H2O (3 mL; DI) was added dropwise to a solution of 13
(468 mg, 1.0 mmol) in dry THF (15 mL), followed by BF3·OMe2
(570 mg, 5.0 mmol; 1 drop/10 s). The reaction mixture was stirred at
0 °C for 15 min (TLC control), then saturated NH4Cl(aq) (15 mL)
was added dropwise, and the aqueous layer was extracted with CH2Cl2
(3 × 20 mL). The combined organic layers were rinsed with deionized
water (20 mL) and dried (Na2SO4). The crude product was
fractionated on alumina (100 g, activity II−III, pH 10; cold,
degassed/1 h, PE:E, 5:1) to afford 24 (450 mg, 96%) as a black
solid. Mp: 35−36 °C (sealed capillary, dried by coevaporation with
benzene, 3 × 1 mL). TLC (PE:E, 3:1): Rf 0.48. 1H NMR (400 MHz, δ,
C6D6): 0.25 (s, 9H, 3CH3), 0.80 (s, 1H, OH), 1.05 (s, 6H, 2CH3),
6.22 (d, J = 14.8, 1H, CH), 6.84 (d, 1H, CH). 13C NMR (100
MHz, δ, CDCl3): 0.8 (3CH3), 30.1 (2CH3), 71.1 (C2), 81.0, 104.7
(CC), 123.3, 145.6 (C3, C4), 200.9 (CO). MS TOF FD+: m/z
M+ 468. MS HR TOF FD+: m/z calcd for C16H18O7SiCo2 M+
467.9480, found 467.9464. Anal. Found: C, 41.49; H, 3.89.
C16H18O7SiCo2 requires: C, 41.04; H, 3.87.
μ-η2-[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-
diene-1,9-diyne]bis(dicobalthexacarbonyl) (20). According to
protocol B, HBF4·OMe2 (201 mg, 1.50 mmol), alcohol 24 (117 mg,
0.25 mmol; −20 °C, 15 min), and zinc (163 mg, 2.50 mmol) yielded
analytically pure 20 (111 mg, 99%).
μ-η2-(5-Methoxy-5-methyl-1-trimethylsilylhex-3E-en-1-yne)-
dicobalt Hexacarbonyl (26). Under an atmosphere of nitrogen,
BF3·OMe2 (142.5 mg, 1.25 mmol) was added dropwise (5 min) to a
solution of alcohol 13 (117 mg, 0.25 mmol) in CH2Cl2 (8 mL)
followed by methanol (0.25 mL) at −78 °C. Upon addition, the
reaction mixture was stirred for 2 h at −78 °C (TLC control), then
quenched with saturated aqueous NH4Cl solution (10 mL) at 0 °C.
The aqueous layer was separated and extracted with ether (3 × 10
mL). The combined ethereal layers were washed with deionized water
(15 mL), dried (Na2SO4), and concentrated under reduced pressure.
The fractionation by preparative TLC (PE:E, 10:1) yielded 26 (78.4
μ-η2-(5-Methyl-1-phenylhex-4-en-1-yn-3-ol)dicobalt Hexa-
carbonyl (29). According to protocol A, n-butyllithium (1.41g, 22.0
mmol; 13.75 mL, 1.6 M in hexane), phenylacetylene (2.20 g, 22.0
mmol), dimethylacrolein (1.40 g, 16.6 mmol), and dicobaltoctacar-
bonyl (6.24 g, 18.26 mmol) afforded, upon fractionation on degassed
silica (200 g, PE:E, 20:1), alcohol 29 (4.33 g, 55% over two steps) as a
dark red solid. Mp: 46−47 °C (sealed capillary, dried by coevaporation
1
mg, 65%) as dark red oil. TLC (PE:E, 20:1): Rf 0.49. H NMR (400
MHz, δ, CDCl3): 0.33 (s, 9H, 3CH3), 1.35 (s, 6H, 2CH3), 3.20 (s, 3H,
OCH3), 6.05 (d, J = 15.6, 1H, CH), 6.62 (d, 1H, CH). 13C NMR
(100 MHz, δ, CDCl3): 0.7 (3CH3), 25.9 (2CH3), 50.7 (OCH3), 75.2
(OC), 80.4, 103.3 (CC), 126.2, 142.2 (CC), 200.3 (CO). MS
TOF FD+: m/z M+ 482. MS HR TOF FD+: m/z calcd for
C17H20O7SiCo2 M+ 481.9637, found 481.9615. Anal. Found: C, 42.06;
H, 4.20. C17H20O7SiCo2 requires: C, 42.34; H, 4.18.
1
with benzene, 3 × 1 mL). TLC (PE:E, 2:1): Rf 0.55. H NMR (400
MHz, δ, CDCl3): 1.78 (d, J = 1.2, 3H, CH3), 1.82 (d, J = 1.2, 3H,
CH3), 2.02 (d, J = 4.0, 1H, OH), 5.44 (d quintet, J = 9.2, 1H, 4-H),
5.76 (dd, J = 4.4, 1H, 3-H), 7.35 (m, 3H, aromatic H), 7.59 (spl d, J =
8.2, J = 1.4, 2H, aromatic H). 13C NMR (100 MHz, δ, CDCl3): 18.8
(CH3), 25.7 (CH3), 69.5 (C3), 91.0, 100.9 (CC), 127.4, 127.99,
128.95, 129.90 (aromatic C, HCCMe2), 136.4, 138.0 (aromatic C,
HCCMe2), 199.6 (CO). MS TOF FD+: m/z M+ 472. MS HR
TOF FD+: m/z calcd for C19H14O7Co2 M+ 471.9398, found 471.9379.
Anal. Found: C, 48.14; H, 2.88. C19H14O7Co2 requires: C, 48.33; H,
2.99.
μ-η2-[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-
diene-1,9-diyne]bis(dicobalthexacarbonyl) (20). According to
protocol B, HBF4·OMe2 (201 mg, 1.50 mmol), Me-ether 26 (121 mg,
0.25 mmol), and zinc (163 mg, 2.50 mmol) yielded 20 (111.8 mg,
99%) as an analytically pure sample.
μ-η2-[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-
diene-1,9-diyne]bis(dicobalthexacarbonyl) (20). Cp2Co-Medi-
ated Dimerization Reaction. Under an atmosphere of nitrogen,
Tf2O (73.3 mg, 0.26 mmol) was added dropwise (15 min) to a
μ-η2-(5,5,6,6-Tetramethyl-1,10-diphenyldeca-3E,7E-diene-
1,9-diyne)bis(dicobalthexacarbonyl) (31). According to protocol
B, HBF4·OMe2 (201 mg, 1.50 mmol), alcohol 29 (118 mg, 0.25
I
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Article
mmol), and zinc (163 mg, 2.50 mmol) yielded 31 (113.1 mg, 99%) as
a black solid with a greenish tinge. Tdec: 108−115 °C (sealed capillary;
dried by coevaporation with benzene, 3 × 1 mL). TLC (PE): Rf 0.35.
1H NMR (400 MHz, δ, CDCl3): 1.17 (s, 12H, 4CH3), 6.43 (d, J =
15.2, 2H, 2CH), 6.71 (d, J = 15.6, 2H, 2CH), 7.30−7.41 (m, 6H,
aromatic H), 7.55 (m, 4H, aromatic H). 13C NMR (100 MHz, δ,
CDCl3): 23.3 (4CH3), 42.6 (C5, C6), 92.57, 92.61 (CC), 124.1,
128.1, 129.10, 129.38, 138.6, 146.8 (aromatic C, HCCH), 199.7
(CO). MS TOF FD+: m/z M+ 910. MS HR TOF FD+: m/z calcd
for C38H26O12Co4 M+ 909.8747, found 909.8729. Anal. Found: C,
50.36; H, 2.73. C38H26O12Co4 requires: C, 50.14; H, 2.88.
5,5,6,6-Tetramethyl-1,10-diphenyldeca-3E,7E-diene-1,9-
diyne (37). According to protocol C, Ce(NH4)2(NO3)6 (208.2 mg,
0.38 mmol) in degassed acetone [9.5 mL; the addition was conducted
in three portions (8 + 1 + 0.5 = 9.5 mL) with each step followed by
warming the reaction mixture to 0 °C and stirring for 30 min] and 31
(36.4 mg, 0.04 mmol) afforded, upon fractionation on a preparative
TLC plate (P:E, 7:1), 37 (11.6 mg, 86%) as a white solid. Mp: 102−
103 °C (sealed capillary, dried by coevaporation with benzene, 3 × 1
mL). TLC (PE:E, 10:1): Rf 0.69. 1H NMR (400 MHz, δ, C6D6): 0.77
(s, 12H, 4CH3), 5.61 (d, J = 16.0, 2H, 2CH), 6.39 (d, J = 16.4, 2H,
2CH), 6.83 (m, 6H, aromatic H), 7.50 (dd, J = 7.8, J = 1.4, 4H,
aromatic H). 13C NMR (100 MHz, δ, CDCl3): 22.8 (4CH3), 42.6 (C5,
C6), 88.77, 88.84 (CC), 108.2, 123.8, 128.1, 128.5, 131.6, 151.7
(aromatic C, HCCH). MS TOF FD+: m/z M+ 338. MS HR TOF
FD+: m/z calcd for C26H26 M+ 338.2029, found 338.2021.
38 (19 mg, 55%) as a light-sensitive, white solid (contains minute
quantities of pentane even after 48h of evaporation under reduced
pressure; partially degrades when longer evaporation times are
applied). Mp: 175−177 °C (w/decomposition; sealed capillary,
dried by coevaporation with benzene, 3 × 1 mL). TLC (P:CH2Cl2,
1
1:1): Rf 0.63. H NMR (400 MHz, δ, CDCl3): 1.10 (s, 12H, 4CH3),
3.93 (s, 6H, 2OCH3), 5.73 (d, J = 16.4, 2H, 2CH), 6.44 (d, 2H,
2CH), 7.11 (d, J = 2.4, 2H, 5′-H), 7.15 (dd, J = 8.8, J = 2.4, 2H, 7′-
H), 7.48 (dd, J = 8.4, J = 1.6, 2H, 3′-H), 7.67 (d, J = 8.8, 2H, 4′-H or
8′-H), 7.70 (d, J = 8.8, 2H, 4′-H or 8′-H), 7.90 (s, 2H, 1′-H). 13C
NMR (100 MHz, δ, CDCl3): 22.7 (4CH3), 42.5 (C5, C6), 55.4
(2OCH3), 88.3, 89.2 (CC), 105.8, 108.1, 118.5, 119.3, 126.8, 128.5,
129.0, 129.3, 131.0, 133.9, 151.4, 158.2 (aromatic C, HCCH). MS
TOF FD+: m/z M+ 498. MS HR TOF FD+: m/z calcd for C36H34O2
M+ 498.2553, found 498.2568.
μ-η2-[3,5-Dimethyl-1-(trimethylsilyl)hex-4-en-1-yn-3-ol]-
dicobalt Hexacarbonyl (33). According to protocol A, n-
butyllithium (704 mg, 11.0 mmol; 6.9 mL, 1.6 M in hexane),
trimethylsilylacetylene (1, 1.08 g, 11.0 mmol), and 4-methyl-3-penten-
2-one (980 mg, 10.0 mmol; 95.7% purity) afforded, upon fractionation
on silica (150 g, PE:E, 1:1), the respective crude alcohol (1.09 g). The
latter was treated with dicobaltoctacarbonyl (1.92 g, 5.60 mmol) to
yield, upon fractionation on Florisil (90 g, degassed, cold, PE:E, 50:1)
and repurification by preparative TLC (2 plates; PE:E, 20:1), 33 (331
mg, 7%; 96% purity) as a dark red solid. Mp: 43−44 °C (sealed
capillary, dried by coevaporation with benzene, 3 × 1 mL). TLC
1
μ-η2-[5-Methyl-1-(6′-methoxy-2′-naphthyl)hex-4-en-1-yn-3-
ol]dicobalt Hexacarbonyl (30). According to protocol A, n-
butyllithium (70 mg, 1.10 mmol; 0.69 mL, 1.6 M in hexane), 2-
ethynyl-6-methoxynaphthalene (200 mg, 1.10 mmol), dimethylacro-
lein (84 mg, 1.00 mmol), and dicobaltoctacarbonyl (342 mg, 1.00
mmol) afforded, upon fractionation on a preparative TLC plate (PE:E,
5:1), 30 (191 mg, 35% over two steps) as a red solid. Mp: 89−91 °C
(sealed capillary, dried by coevaporation with benzene, 3 × 1 mL).
TLC (PE:E, 5:1): Rf 0.29. 1H NMR (400 MHz, δ, CDCl3): 1.80, 1.87
(s, 6H, 2CH3), 2.06 (br s, 1H, OH), 3.95 (s, 3H, OCH3), 5.49 (d, J =
8.8, 1H, 4-H), 5.83 (dd, J = 9.0, J = 4.2, 1H, 3-H), 7.10−7.21 (m, 2H,
aromatic H), 7.63 (d, J = 8.4, 1H, aromatic H), 7.73 (m, 2H, aromatic
H), 8.01 (s, 1H, aromatic H). 13C NMR (100 MHz, δ, CDCl3): 18.9,
25.8 (2CH3), 55.5, 69.6 (C3, OMe), 91.4, 101.0 (CC), 106.2, 119.7,
127.4, 127.5, 128.47, 128.53, 129.1, 129.7, 133.0, 134.3, 136.4, 158.4
(C4, C5, aromatic C), 199.7 (CO). MS TOF FD+: m/z M+ 552.
MS HR TOF FD+: m/z calcd for C24H18O8Co2 M+ 551.9660, found
551.9664. Anal. Found: C, 52.01; H, 3.35. C24H18O8Co2 requires: C,
52.20; H, 3.29.
(PE:E, 10:1): Rf 0.59. H NMR (400 MHz, δ, CDCl3): 0.34 (s, 9H,
3CH3), 1.65 (s, 3H, CH3), 1.69 (s, 3H, CH3), 1.80 (s, 1H, OH), 1.96
(s, 3H, 3-CH3), 5.35 (s, 1H, 4-H). 13C NMR (100 MHz, δ, CDCl3):
1.2 (3CH3), 18.9 (CH3), 27.6 (CH3), 33.7 (CH3), 76.1 (C3), 78.1,
123.0 (CC), 131.5, 134.2 (C4, C5), 200.6 (CO). MS TOF FD+:
m/z M+ 482. MS HR TOF FD+: m/z calcd for C17H20O7SiCo2 M+
481.9637, found 481.9627. Anal. Found: C, 42.46; H, 4.14.
C17H20O7SiCo2 requires: C, 42.34; H, 4.18.
μ-η2-(3,5,5,6,6,8-Hexamethyl-1,10-bis(trimethylsilyl)deca-
3E,7E-diene-1,9-diyne)bis(dicobalthexacarbonyl) (34). Accord-
ing to protocol B, HBF4·OMe2 (201 mg, 1.50 mmol), alcohol 33
(120.5 mg, 0.25 mmol), and zinc (163 mg, 2.50 mmol) afforded, upon
trituration with dry MeOH (3 × 3 mL), 34 (100 mg, 86%; 98.0%
purity) as a black solid. Tdec: 177−182 °C (sealed capillary; dried by
1
coevaporation with benzene, 3 × 1 mL). TLC (P): Rf 0.69. H NMR
(400 MHz, δ, C6D6): 0.31 (s, 18H, 6CH3), 1.16 (s, 12H, 4CH3), 2.06
(d, J =1.2, 6H, 3-CH3, 8-CH3), 6.28 (q, 2H, 4-H, 7-H). 13C NMR (100
MHz, δ, CDCl3): 1.0 (6CH3), 20.5, 25.2 (4CH3, 3-CH3, 8-CH3), 45.3
(C-5, C-6), 79.9, 115.8 (CC), 131.5, 141.5 (CC), 200.7 (CO).
MS TOF FD+: m/z M+ 930. MS HR TOF FD+: m/z calcd for
C34H38O12Si2Co4 M+ 929.9224, found 929.9233. Anal. Found: C,
43.47; H, 3.89. C34H38O12Si2Co4 requires: C, 43.88; H, 4.12. Single
crystals suitable for X-ray structure analysis (Figure 4) were obtained
by methanol vapor diffusion into a solution of 34 in petroleum ether at
+4 °C (1 day).
3,5,5,6,6,8-Hexamethyl-1,10-bis(trimethylsilyl)deca-3E,7E-
diene-1,9-diyne (39). According to protocol C, Ce(NH4)2(NO3)6
[287 mg, 0.48 mmol/acetone(12mL); the addition was carried out in
two portions (10 mL + 2 mL) with each step followed by stirring at
−50 °C for 30 min, then raising temperature to −20 °C, and stirring
for additional 30 min] and 34 (37.2 mg, 0.04 mmol) afforded 39 (10
mg, 70%; purity 99%) as a white solid. Mp: 100−101 °C (sealed
capillary, dried by coevaporation with benzene, 3 × 1 mL). TLC (P):
Rf 0.31. 1H NMR (400 MHz, δ, CDCl3): 0.19 (s, 18H, 6CH3), 1.13 (s,
12H, 4CH3), 1.93 (d, J = 1.6, 6H, 2CH3), 5.97 (q, J = 1.2, 2H, CH).
13C NMR (100 MHz, δ, CDCl3): 0.4 (6CH3), 19.1, 24.4 (4CH3, 3-
μ-η2-[5,5,6,6-Tetramethyl-1,10-di(6′-methoxy-2′-naphthyl)-
deca-3E,7E-diene-1,9-diyne]bis(dicobalthexacarbonyl) (32).
According to protocol B, HBF4·OEt2 (243 mg, 1.50 mmol), alcohol
30 (138 mg, 0.25 mmol), and zinc (163 mg, 2.50 mmol) afforded,
upon fractionation by preparative TLC (2 plates, PE:E, 5:1), 32 (115
mg, 86%) as a black solid. Mp: 42−60 °C (sealed capillary; dried by
coevaporation with benzene, 3 × 1 mL). TLC (P:CH2Cl2, 2:1): Rf
1
0.50. H NMR (400 MHz, δ, CDCl3): 1.22 (s, 12H, 4CH3), 3.94 (s,
6H, 2OCH3), 6.48 (d, J = 15.2, 2H, CH), 6.79 (d, 2H, CH), 7.14
(d, J = 2.4, 2H, 5′-H), 7.18 (dd, J = 8.8, J = 2.8, 2H, 7′-H), 7.59 (dd, J
= 8.6, J = 1.8, 2H, 3′-H), 7.72 (d, J = 8.4, 2H, 4′-H or 8′-H), 7.75 (d, J
= 9.2, 2H, 4′-H or 8′-H), 7.96 (s, 2H, 1′-H). 13C NMR (100 MHz, δ,
CDCl3): 23.3 (4CH3), 42.6 (C5, C6), 55.59, 55.61 (2OCH3), 92.8,
93.1 (CC), 106.2, 119.7, 124.3, 127.6, 127.9, 128.0, 129.2, 129.7,
133.6, 134.4, 146.7, 158.5 (C3, C4, aromatic C), 199.8 (CO). MS
TOF FD+: m/z M+ 1070. MS HR TOF FD+: m/z calcd for
C48H34O14Co4 M+ 1069.9271, found 1069.9255. Anal. Found: C,
54.63; H, 3.76. C48H34O14Co4 requires: C, 53.86; H, 3.20.
5,5,6,6-Tetramethyl-1,10-di(6′-methoxy-2′-naphthyl)deca-
3E,7E-diene-1,9-diyne (38). According to protocol C, Ce-
(NH4)2(NO3)6 [306.9 mg, 0.56 mmol/acetone(8mL); the addition
was carried out in four portions (2 × 0.21 mmol + 2 × 0.07 mmol)
with each step followed by warming the reaction mixture to 20 °C and
stirring for 30 min] and 32 (75 mg, 0.07 mmol) yielded, upon
fractionation by preparative TLC (2 plates, in the dark; P:CH2Cl2, 1:1)
CH3, 8-CH3), 45.0 (C-5, C-6), 88.3, 110.9 (CC), 118.3, 146.4 (C
C). MS TOF FD+: m/z M+ 358. MS HR TOF FD+: m/z calcd for
C22H38Si2 M+ 358.2507, found 358.2504. Anal. Found: C, 72.50; H,
10.57. C22H38Si2 requires: C, 73.66; H, 10.68.
μ-η2-[1-Cyclohexylidene-4-(trimethylsilyl)but-3-yn-2-ol]-
dicobaltohexacarbonyl (35). Under an atmosphere of nitrogen,
PhIO (792 mg, 3.60 mmol), TEMPO (47 mg, 0.30 mmol), and 4 Å
molecular sieves (78 mg) were added to a solution of α-vinyl-
J
DOI: 10.1021/om501096x
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
cyclohexanol (378 mg, 3.00 mmol) in dry CH2Cl2 (15 mL) at 20 °C.
The suspension was cooled to 0 °C, Re2O7 (290 mg, 0.60 mmol) was
added in one portion, and the reaction mixture stirred for 2 h at 20 °C.
An additional amount of Re2O7 (145 mg, 0.30 mmol) was added, and
the stirring was continued until PhIO was dissolved (1 h). The
reaction mixture was filtered on silica gel (20 g; PE:EtOAc, 5:1, 300
mL) and concentrated under reduced pressure to yield crude
(CH2)5CCHCHO (183 mg). In a separate flask, n-butyllithium
(49.3 mg, 0.77 mmol; 1.2 mL, 1.6 M in hexane) was added dropwise
(5 min) to a solution of trimethylsilylacetylene (76 mg, 0.77 mmol) in
dry THF (5 mL) at −10 °C. Upon addition, the reaction mixture was
stirred for 5 h at −10 °C. A solution of crude (CH2)5CCHCHO in
dry THF (3 mL) was added dropwise (10 min) at −10 °C, and the
reaction mixture was stirred for 30 min at −10 °C and then for
additional 30 min at 20 °C (TLC monitoring). The crude mixture was
quenched with saturated NH4Claq (10 mL) at 0 °C, and the aqueous
layer was separated and extracted with ether (3 × 25 mL). The
combined ethereal layers were dried (Na2SO4), and solvents were
evaporated under reduced pressure. Under an atmosphere of nitrogen,
the crude alcohol (164 mg, 0.74 mmol, assuming 100% yield) was
redissolved in dry ether (5 mL) and added dropwise (20 min) to a
solution of dicobaltoctacarbonyl (263 mg, 0.77 mmol) in dry ether (15
mL) at 20 °C. The reaction mixture was stirred overnight at 20 °C,
concentrated under reduced pressure, and fractionated on Florisil (50
g, PE:E, 10:1) to afford 35 (45 mg, 13% over three steps) as a brick-
1,10-bis(trimethylsilyl)deca-3E,7E-diene-1,9-diyne)bis-
(dicobalthexacarbonyl) (34), and μ-η2 -{1-[(E)-4″-
(Trimethylsilyl)but-1″-en-3″-ynyl]-1′-[(E)-4‴-(trimethylsilyl)-
b u t - 1 ‴ - e n - 3 ‴ - y n y l ] - 1 , 1 ′ - b i ( c y c l o h e x a n e ) } b i s -
(dicobalthexacarbonyl) (36). Data Collection, Structure Solution,
and Refinement. Polarized microscopy (between crossed polars) was
used to select suitable crystals, quickly mount them onto a nylon fiber
with paratone oil, and place them under a cold stream at 110(2) K.
Single-crystal X-ray data were collected on a Bruker APEX2
diffractometer with 1.6 kW graphite-monochromated Mo radiation.
The detector to crystal distance was 5.1 cm. The data collection was
performed using a combination of sets of ω scans, yielding data with a
reasonable 2θ range and good completeness. The frames were
integrated with SAINT v7.68a (Bruker, 2009).14a Multiscan absorption
corrections were carried out using the program SADABS V2008-1
(Bruker, 2008).14b For nonmerohedrally twinned crystals multiscan
absorption corrections were carried out using the program TWINABS-
2008/3 (Bruker, 2008).14c Further details are given in the respective
CIF files. The structures were solved with either SHELX (Sheldrick,
2008) or JANA2006 (Palatinus, 2006)14d and refined with Olex214e
and SHELX (Sheldrick, 2008).14f
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 20,
34, and 36. 1H and 13C NMR spectra for 22, 32, 37, 38, and 40.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
red oil. TLC (PE:E, 10:1): Rf 0.44. H NMR (400 MHz, δ, CDCl3):
0.30 (s, 9H, 3CH3), 1.60 (br s, 3H, 3CH2), 1.85 (d, J = 4.4, 1H, OH),
2.01−2.44 (m, 4H, 2CH2), 5.23 (d, J = 8.8, 1H, 1-H), 5.60 (dd, 1H, 2-
H). 13C NMR (100 MHz, δ, CDCl3): 0.8 (3CH3), 26.8, 27.8, 28.1,
29.8, 37.1 (cyclohexylidene C), 68.8 (C2), 78.3, 115.4 (CC), 124.7,
143.5 (CC), 200.5 (CO). MS TOF FD+: m/z M+ 508. MS HR
TOF FD+: m/z calcd for C19H22O7SiCo2 M+ 507.9793, found
507.9795. Anal. Found: C, 45.06; H, 4.51. C19H22O7SiCo2 requires: C,
44.89; H, 4.36.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gagik.melikyan@csun.edu.
■
μ-η2-{1-[(E)-4″-(Trimethylsilyl)but-1″-en-3″-ynyl]-1′-[(E)-4‴-
(trimethylsilyl)but-1‴-en-3‴-ynyl]-1,1′-bi(cyclohexane)}bis-
(dicobalthexacarbonyl) (36). According to protocol B, HBF4·OMe2
(201 mg, 1.50 mmol), alcohol 35 (127 mg, 0.25 mmol), and zinc (163
mg, 2.50 mmol) afforded, upon fractionation by preparative TLC (2
plates, +4 °C, PE), 36 (103 mg, 84%; purity 96.6%) as black crystals.
Tdec: 112−130 °C (sealed capillary; dried by coevaporation with
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, University Corporation, and
College of Science and Mathematics, California State University
Northridge, for their generous support.
1
benzene, 3 × 1 mL). TLC (PE): Rf 0.74. H NMR (400 MHz, δ,
CDCl3): 0.35 (s, 18H, 6CH3), 0.95 (m, 2H, CH2), 1.21−1.40 (m,
10H, 5CH2), 1.54 (t, J = 14.0, 4H, 2CH2), 1.86 (d, J = 11.2, 4H,
2CH2), 5.95 (d, J = 15.6, 2H, 2CH), 6.38 (d, J = 16.0, 2H, 2CH).
13C NMR (100 MHz, δ, C6D6): 0.9 (6CH3), 23.5, 27.2, 30.8, 48.4
(cyclohexyl C), 80.6, 105.5 (CC), 129.4, 143.7 (CC), 201.0 (C
O). MS TOF FD+: m/z M+ 982. MS HR TOF FD+: m/z calcd for
C38H42O12Si2Co4 M+ 981.9537, found 981.9498. Anal. Found: C,
47.08; H, 4.28. C38H42O12Si2Co4 requires: C, 46.45; H, 4.31. Single
crystals suitable for X-ray structure analysis (Figure 5) were obtained
by methanol vapor diffusion into a solution of 36 in methylene
chloride at +4 °C (1 day).
REFERENCES
■
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1,10-bis(trimethylsilyl)deca-3E,7E-diene-1,9-diyne]bis-
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K
DOI: 10.1021/om501096x
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Organometallics
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(9) The low-temperature, multinuclear NMR studies on isomeric
cations 9 and 10 probing the coordination mode between the
tetrafluoroborate anion and cationic termini of unsymmetrical allylic
species will be reported in the forthcoming account, along with
semiempirical and ab initio computation data.
L
DOI: 10.1021/om501096x
Organometallics XXXX, XXX, XXX−XXX