Radical Reactions of 1,4-Alkadiynes: Metal Coordination as an Effective Tool for Controlling the Regio- and Stereoselectivity of the C-C Bond Formation

Article abstract of DOI:10.1021/acs.organomet.5b00480

A novel method for selective generation of radicals in 1,4-alkadiynes is developed by employing a π-bonded Co₂(CO)₆ core as a triple-bond immobilizing, cation-stabilizing, and radical-guiding auxiliary group. The isolation of α-alkyn

Full text of DOI:10.1021/acs.organomet.5b00480

Communication
pubs.acs.org/Organometallics
Radical Reactions of 1,4-Alkadiynes: Metal Coordination as an
Effective Tool for Controlling the Regio- and Stereoselectivity of the
C−C Bond Formation
Gagik G. Melikyan* and Bryan Anker
Department of Chemistry and Biochemistry, California State University Northridge, 18111 Nordhoff Street, Northridge, California
91330, United States
S
* Supporting Information
ABSTRACT: A novel method for selective generation of
radicals in 1,4-alkadiynes is developed by employing a π-
bonded Co2(CO)6 core as a triple-bond immobilizing, cation-
stabilizing, and radical-guiding auxiliary group. The isolation of
α-alkynyl-cobalt-complexed propargyl cations and their
reduction with zinc occurred in a regio- and stereoselective
manner, giving rise to tetraethynylethanes with predominant formation of d,l-diastereomers (82−93%). The methodology
provides an easy access to tetraethynylethanes and, upon oxidation, to practically important tetraethynylethenes. We also report
on the novel phenomenon of “chiralization by metal complexation”, with achiral tetraynes being converted to chiral bis-clusters
due to highly regio- and stereoselective complexation of the acetylenic groups.
adical reactions of 1,4-alkadiynes represent a scarcely
R
studied field of organic chemistry1 due to the lack of
methods for selective introduction of an unpaired electron to
the propargyl position and an intrinsic ability of the triple
bonds to rearrange into isomeric allenes in both ionic2a,b and
radical reactions.2c−e The said reorganization compromises the
regioselectivity of the coupling reactions, giving rise to isomeric,
hard-to-separate head-to-head, head-to-tail, and tail-to-tail
dimers.2c−e Even in the transition-metal-catalyzed dimerizations
of propargyl alcohols, an unwanted acetylene-allene rearrange-
ment cannot be avoided, rendering coupling reactions
inefficient.3 In organic chemistry, to the best of our knowledge,
the only radical reaction of 1,4-alkadiynes ever reported is an
iodide-induced dimerization of 3-bromo-1,4-pentadiynes
wherein the formation of the requisite radicals occurs due to
an in situ homolysis of the C−I bond.4 Over the past decade, in
the course of the systematic studies on the chemistry of metal-
complexed propargyl radicals,5 we have demonstrated that the
complexation of the triple bond with a dicobalthexacarbonyl
group6,7 allows avoidance of an unwanted acetylene−allene
rearrangement and also provides multiple mechanistic tools for
controlling a spatial orientation of converging propargyl
radicals. The level of diastereoselectivity (up to 100%) achieved
in inter- and intramolecular radical reactions exceeds by far that
in all-carbon organic1,8 and organometallic9,10 dimerizations,
occurring with little, or no, stereoselectivity. In this account, we
report on radical reactions of 1,4-alkadiynes 1 in which one of
the triple bonds is immobilized by the metal complexation
(Figure 1), thus precluding an acetylene−allene rearrangement,
restricting the reaction sites (α, γ, γ′), and decreasing the
number of regioisomers that could potentially be formed (α,α;
α,γ; γ,γ). The regioselectivity of zinc-induced dimerization
reactions was studied to probe the relative reactivities of the
Figure 1. Topology of cobalt-complexed 1,4-alkadiynyl radicals.
propargyl radical 2 and its allenic counterpart 3, with an
unpaired electron being located alpha and gamma to the metal
core, respectively.
The requisite diacetylenic alcohols (4, 5) were synthesized
by the condensation of ethyl formate11 with lithiated acetylenes
(6, R = TMS; 7, R = Ph), followed by the complexation of the
triple bond with a dicobalthexacarbonyl group (Scheme 1).6
The treatment with tetrafluoroboric acid allowed for precip-
itation of the respective cobalt-stabilized propargyl cations,7
each represented by two resonance contributors, i.e., α-alkynyl
propargyl (8, 9) and alkynyl allenyl cations (10, 11). Reduction
with zinc generated α-alkynyl propargyl radicals (12, 13), which
can project a spin density (α-to-γ), forming isomeric allenyl
radicals (14, 15). Thus, given the very nature of a propargyl
triad, the formation of head-to-head (16, 17), tail-to-tail (18,
19), and head-to-tail (20, 21) dimers can be envisioned.2c−e In
a purely organic setting,12 the stabilities of the propargyl and
allenyl radicals are comparable to each other (BDE: HC
CCH2−H 88.9 kcal/mol;12a,b CH2CCH−H 88.7 kcal/
mol)12a,c with the product distributionhead-to-head, head-to-
tail, tail-to-tailfavoring the propargyl termini in the ratio of
62:392d or 89:11.2e Introducing an alkyne-cobalt core at the
propargyl carbon could impede the formation of α,α-dimers, for
Received: June 12, 2015
Published: September 2, 2015
© 2015 American Chemical Society
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Communication
Scheme 1. Regioselective Head-to-Head Dimerization of α-
Alkynyl Propargyl Radicals
Figure 2. X-ray structure of d,l-μ-η2-[3,4-bis[(trimethylsilyl)ethynyl]-
1,6-bis(trimethylsilyl)-1,5-hexadiyne]bis(dicobalthexacarbonyl) (16).
up anti to each other, minimizing the repulsion between the
bulky cobalt-alkyne moieties (θC2−C3−C4−C5 155.8°). Despite
the size of TMS groups, linearity around the metal-coordinated
triple bonds is ideally maintained (θSi4−C1−C2−C3 0.9°,
θC4−C5−C6−Si1 2.1°). A spatial arrangement between gauche-
arranged triple bonds is such that the distance between the
acetylenic carbons (C16−C21 2.87 Å) is less than the sum of van
der Waals radii (C 1.70 Å),16 suggesting a through-space
interaction between the π-bonds. Curiously enough, the
distance between the acetylenic terminiC17 and C22is
equal to 4.04 Å, making it almost identical to that in acyclic
enediynes (4.12 Å) susceptible to a Bergman cyclization at
elevated temperatures.17 In other words, the saturated C3−C4
unit positions the triple bonds in the same way as a double
bond of cis-configuration.17 Overall, an X-ray structure (Figure
2) may adequately represent the actual spatial arrangement for
converging propargyl radicals 12 with the smallest atoms (H)
and groups (uncomplexed CC) arranged in a gauche fashion,
while the bulkiest substituents (complexed CC) are
positioned as far apart from each other as possible in order
to facilitate a C−C bond formation alpha to a metal core.
With R = Ph (5), an exclusive formation of α,α-dimers was
observed (d,l-17:meso-17, 82:18) with the chemoselectivity of
the process being superior by far to that of substrate 4 (R =
TMS). The respective monodecomplexation product was not
observed in the crude mixture, thus eliminating the necessity of
an in situ partial recomplexation, preceding the isolation of
individual products. Besides this, hydrogen atom abstraction
in α- and γ-positionswas substantially minimized; that is, γ-
HAA 25 was not observed in the crude mixture, while α-HAA
23 was formed only in trace amounts (d,l-17:meso-17:23,
81.3:18.0:0.7). For identification purposes, the latter was
independently synthesized from alcohol 5 by treatment with
NaBH4−CF3COOH.13,14 Overall, the enhanced chemoselec-
tivity also manifested itself in a higher isolated yield of
diastereomeric products (79.8%; d,l-17:meso-17, 77:23). It is
conceivable that the rate constants for the formation of α,α-
dimers 16 and 17 are comparable to each other, since the
converging radicals 12 and 13 are different only in substituents
relatively distantγ- and γ′from the site of the C−C bond
formation. This assumption allows explaining the observed
difference in chemoselectivities (24, 35.7%; 25, 0%) by
postulating that compared to the γ-radical 14, γ-radical 15 is
steric reasons, while providing an additional stabilization to the
radical center located alpha to the electron-donating metal
core.7 A careful examination of the crude mixture by NMR
revealed that the dimerization reactions occur in a highly
regioselective manner with only head-to-head dimeric products
(16, 17) being formed. When R = TMS, the crude mixture
consisted of d,l-16 and meso-16 in the ratio of 89:11, along with
the H atom abstraction (HAA) products 22 and 24 (1:34) and
monodecomplexation product derived from the partial
decomplexation of the radical dimer (14%). Upon the
recomplexation with Co2(CO)8, the product distribution, by
NMR, was found to be 59:4.8:0.5:35.7 d,l-16:meso-16:α-HAA
22:γ-HAA 24 with the diastereomeric composition of 92.5:7.5
d,l-16:meso-16. The major diastereomer, d,l-16, was isolated in
good yield (57.8%), with the preparative TLC separation being
carried out at low temperatures (+6 °C) in order to avoid a
partial decomplexation reaction. α-HAA 22 was independently
synthesized from alcohol 4 by treatment with NaBH4−
CF3COOH,13,14 while γ-HAA 24, a highly unstable compound,
was isolated at low temperatures (+6 °C) by means of
preparative TLC (29.4%). The relative configuration of α,α-
dimer 16 was established by means of X-ray crystallography15
since in the absence of terminal acetylenic hydrogens5i the
NMR spectroscopy does not allow for an unambiguous
stereochemical assignment. As Figure 2 shows, the uncom-
plexed triple bonds capped with TMS groups are positioned
gauche to each other (θC21−C3−C4−C16 46.3°), as are the vicinal
hydrogen atoms (θH3−C3−C4−H4 79.3°), both exhibiting a
significant level of distortion from an ideal staggered
conformation. In contrast, the complexed triple bonds sprung
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Communication
much less reactive in an HAA reaction due to the conjugation
of an unpaired electron with a phenyl group. An alleged
stabilization of the allenyl radical by a gamma π-donor provides
a useful mechanistic tool for controlling the chemoselectivity of
the radical dimerization reactions. It is also noteworthy that the
stereoselectivity of the radical C−C bond formationalpha to
the metal coreis noticeably higher with a TMS group being
present at the acetylenic terminus (R = TMS: d,l-16:meso-16,
92.5:7.5; R = Ph: d,l-17:meso-17, 82:18), thus providing
another example of a 1,3-steric induction by a remote, and
removable, trimethylsilyl group.5g
Scheme 2. π-Coordination-Induced Chirality: Regio- and
Stereoselective Bis-complexation of Tetraynes
Oxidative decomplexation of d,l-16 with ceric ammonium
nitrate18 allowed for release of tetrayne 26 (80.5%) from a
metal bondage, thus concluding a four-step synthetic scheme in
an overall yield of 15.7%. Analogously, organic product 27 can
be liberated from the diastereomeric mixture (d,l-17:meso-17,
77:23) with an overall yield of 37.1% over the four steps. For
comparison, competing literature protocols for 26 and 27
afforded the former, in three steps, in a 10.7% overall yield,4,11a
while the latter was synthesized also in three steps, but in a
lower overall yield (5.4%).4,11b,19 It is noteworthy that the
decomplexation step also accomplishes dechiralization of the
radical dimers due to the elimination of C3 and C4
stereocenters, a structural feature that was not previously
observed in conventional releases of π-bonded organic ligands
(dechiralization by decomplexation).7 Tetrayne 27 was
dehydrogenated, at the C3−C4 position, by treatment with a
2-fold excess of DDQ20a (benzene, anaerobic conditions). The
light-sensitive, neon yellow enetetrayne 2819,20b,c was isolated
in good yield (46.8%), providing a novel, synthetically versatile
access to the tetraethynylethene (TEE)21 carbon framework
that can be accomplished in five steps with a total yield of
17.4%. The latter represents a practically important class of
organic compounds with a cross-conjugated π-electron system
that has been used, as a building block, for assembling complex,
topologically diverse, and highly delocalized molecular
assemblies with unique optical and redox properties.21
Introducing Chirality by the Selective Complexation of the Triple
Bonds in Tetraethynylethanes. In the course of recomplexation
studies on tetraynes 26 and 27, we observed a chiralization of
the achiral substrate due to the selective coordination of the
competing triple bonds, i.e., “chiralization by metal complex-
ation”. When tetrayne 26, an achiral compound, was treated
with Co2(CO)8, the complexation of acetylenic moieties
occurred in a highly regio- and stereoselective manner (Scheme
2). The substrate topology is such that the treatment with two
equivalents of metal carbonyl could theoretically form three
isomeric bis-complexes: achiral 29 with geminal complexation
sites, achiral/meso-16, and chiral/d,l-16. The reaction yielded
only the latter, as a single product, in good yield (71.1%) and
with an excellent stereoselectivity (d,l-16:meso-16, 98:2).
Remarkably, even when tetrayne 26 is treated with four
equivalents of the complexation agent in order to form
tetracomplex 30, still only bis-complex 16 is formed, indicating
that despite the abundance of the metal carbonyl, the triple
bonds are not subject to complexation. Furthermore, the
treatment of d,l-16 with two additional equivalents of
Co2(CO)8 does not show any signs of subsequent complex-
ation upon pending triple bonds. The Newman projection 31
represents the monocomplex with three additional complex-
ation sites available to the metal coordination. It is conceivable
that the spatial proximity of gauche triple bonds to the bulky
metal cluster impedes an incorporation of another π-bonded
metal core, with an anti-alkynyl moiety being the only one
susceptible to repeat coordination. An X-ray structure for d,l-16
(Figure 2) validates the said interpretation, with uncomplexed
triple bonds being positioned between two bulky cobalt-alkyne
cores, in a sterically congested area. Analogous results were
obtained for tetrayne 27, which, when treated with four
equivalents of Co2(CO)8, afforded d,l-17 in high yield (80.0%)
as a single diastereomer. Conceptually, these reactions are
preceded by the selective complexation of the sterically
hindered triple bonds with transition metals (Co,22a,b Pt21c
)
albeit in cyclic molecules. In an all-carbon environment,22a as
well as in silicon22b and germanium22b heterocyclynes, bis-
complexation with dicobalt octacarbonyl occurred selectively at
the least hindered sites, in a highly symmetrical fashion, without
introducing chirality to the parent molecule.
A highly regio- and stereoselective bis-complexation of
acyclic tetraynes has two synthetically important ramifications.
First, the concept of “chiralization by metal complexation”
would allow for chirality to be introduced into an achiral
molecule without compromising the carbon framework of the
substrate, or forming any new σ-bonds, then for targeted
modifications in select parts of the moleculethose that would
potentially benefit from a newly acquired chiralityand, at the
final step, for an easy removal of π-bonded metal cores and
retrieval of the original carbon framework. Second, introducing
a triple bond, by design, into a constrained area of the molecule
that would make it unreactive toward metal coordination, or
possibly other reagents, represents a new method for protecting
acetylenic moieties wherein the structural integrity of the
functional group is fully preserved, while access of reagents to
the said functionality is restricted for steric reasons (“caged”
triple bond).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00480.
Full experimental and spectroscopic data (PDF)
Crystallographic (CCDC 1048617) data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 818-677-2565. 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.
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