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 πbonded Co2(CO)6 core as a triple-bond immobilizing, cationstabilizing, 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.
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adical reactions of 1,4-alkadiynes represent a scarcely 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 rearrangement 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 metalcomplexed 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 © 2015 American Chemical Society
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 precipitation 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 CH2CCH−H 88.7 kcal/ mol)12a,c with the product distributionhead-to-head, head-totail, tail-to-tailfavoring 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 4194
DOI: 10.1021/acs.organomet.5b00480 Organometallics 2015, 34, 4194−4197
Communication
Organometallics Scheme 1. Regioselective Head-to-Head Dimerization of αAlkynyl 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 gauchearranged 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 terminiC17 and C22is 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 CC) arranged in a gauche fashion, while the bulkiest substituents (complexed CC) 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-positionswas substantially minimized; that is, γHAA 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 chemoselectivity 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,αdimers 16 and 17 are comparable to each other, since the converging radicals 12 and 13 are different only in substituents relatively distantγ- 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,αdimer 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 uncomplexed 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 4195
DOI: 10.1021/acs.organomet.5b00480 Organometallics 2015, 34, 4194−4197
Communication
Organometallics
Scheme 2. π-Coordination-Induced Chirality: Regio- and Stereoselective Bis-complexation of Tetraynes
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 formationalpha to the metal coreis 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 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 complexation”. 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 complexation upon pending triple bonds. The Newman projection 31 represents the monocomplex with three additional complexation 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, biscomplexation 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 moleculethose that would potentially benefit from a newly acquired chiralityand, 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).
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00480. Full experimental and spectroscopic data (PDF) Crystallographic (CCDC 1048617) data (CIF) 4196
DOI: 10.1021/acs.organomet.5b00480 Organometallics 2015, 34, 4194−4197
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AUTHOR INFORMATION
Corresponding Author
*Phone: 818-677-2565. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under CHE-1112129. REFERENCES
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