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-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 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 siteα-to-γprior 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%).
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in solid-phase reactions of acyclic3a and cyclic3b ketones. Although being used in the total synthesis of the sesquiterpene herbertenolide,3c radical reactions remain underrepresented in targeted synthesis and construction of molecular assemblies with adjacent quaternary carbon atoms.1,2 The main reason is a lack of suitable methods for generation of tertiary, sterically hindered carbon-centered radicals, in particular those that could be carried out in a site-selective manner, under relatively mild conditions, and also feature an enhanced degree of structural versatility and functional tolerance. In a purely organic setting,3d−h tertiary radicals containing β-hydrogen atoms suffer 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 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
INTRODUCTION Assembling of organic molecules with contiguous quaternary carbon atoms has long represented a formidable synthetic challenge.1,2 Besides a purely academic interest of developing access to topologically complex molecular assemblies, significance of this research area was determined by the presence of said molecular units in natural steroids and their construction via biomimetic polyene cyclization reactions. Select natural compounds with vicinal quaternary carbons were reported in folk medicine to treat certain diseases, thus providing a new 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 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 © XXXX American Chemical Society
Received: October 30, 2014
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DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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 interand intramolecular coupling reactions provided facile access to 1,5-alkadiynes4a−d,f,g,k−n and 1,5-cycloalkadiynes4e,h−j,n containing, 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 cobaltcomplexed 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.
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RESULTS AND DISCUSSION Three alternative methods for the generation of requisite propargyl cations were developed, employing isomeric αalcohol 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 B
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics 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.
γ,γ-Dimethyl-substituted propargyl alcohol 13 was synthesized by the condensation of the lithiated trimethylsilylacetylene 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
intimate details still remain to be comprehended,9 including the 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 headto-tail dimer. C
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Regioselective Radical Dimerization of Isomeric γ-Alcohols
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 scis 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,γdimer 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,βdouble bond could act as an efficient conduit between a πbonded 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
triad could undergo a coupling reaction, forming isomeric headto-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 transstereoselective 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 stereoselectivities, 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 tailto-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; αC2° V ( H C C ) C o 2( C O ) 6+ ( C H C H2 ) + H 256.98 Å 3 ; γ-C3° V2Me+(CHCH2) 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-cyclopropyl propargyl alcohols also undergo a stereoselective allylic shift to the respective E-homoallylic bromides.13d 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°). D
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics 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%). 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 componentsπ-bonded metal core, a double bond, and a p-orbital of the tertiary cationic centerallowed 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
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 2naphthyl 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 dicobalthexacarbonyl 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 stereoselective manner. γ,γ-Dimers 31 and 32, as single products, were isolated in excellent yields99% 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,γdimer 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 E
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Organometallics Table 1. Regioselective α-to-γ Rearrangement in Intermolecular Coupling Reactions of α-Alkenyl Propargyl Radicals
Yield over two steps. bStandard dimerization protocol included treatment of the cobalt-complexed 1,4-enynes with a 6-fold excess of HBF4 at −20 °C and reduction with a 10-fold excess of zinc at +20 °C. cStandard decomplexation protocol included treatment of dimeric bis-complexes with ceric ammonium nitrate (8−12 equiv) at −78 to 0 °C. dYield over three steps.
a
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 F
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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.
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,γdimers. 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
by expanding the bond angles around the vinylicC4 and C5carbon 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,γdimers 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 G
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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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 (θ Si1 − C1 0− C 9− C 8 3.5°, θSi1′‑C10′‑C9′−C8′ 3.5°) effectively place the five-atom sequencesSi1−C10−C9−C8−C7 and Si1′−C10′−C9′−C8′− C7′into two parallel planes. 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 bisdesilylation 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 regioselectivity, 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
Article
EXPERIMENTAL SECTION
All manipulations of air-sensitive materials were carried out in flamedried 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, ZABSE; CI-NH3, 7070EHF; Micromass; TOF Agilent 6210 LC-TOF instrument with a Multimode source). Synthesis of Co2(CO)6-Complexed Propargyl Alcohols (Protocol A): μ-η2-[5-Methyl-1-(trimethylsilyl)hex-4-en-1-yn-3ol]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). TLC (PE:E, 5:1): Rf 0.47. 1H 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 (CC), 127.9, 135.5 (C4, C5), 200.2 (CO). 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-η 2 -[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca3E,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
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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 coreat the radical dimerization step. 1,3Enynes 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-projection” 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. H
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics 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 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, 3H, 4-H). 13C NMR (100 MHz, δ, CDCl3): 0.7 (6CH3), 23.1 (4CH3), 42.2 (C5, C6), 80.1, 105.0 (CC), 124.0, 145.4 (CC), 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). μ-η 2 -(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 (CC), 123.3, 145.6 (C3, C4), 200.9 (CO). 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,7Ediene-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 mg, 65%) as dark red oil. TLC (PE:E, 20:1): Rf 0.49. 1H 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 (CC), 126.2, 142.2 (CC), 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. μ-η2-[5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7Ediene-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,7Ediene-1,9-diyne]bis(dicobalthexacarbonyl) (20). Cp2Co-Mediated Dimerization Reaction. Under an atmosphere of nitrogen, Tf2O (73.3 mg, 0.26 mmol) was added dropwise (15 min) to a
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%). Decomplexation with Ceric Ammonium Nitrate (Protocol C): 5,5,6,6-Tetramethyl-1,10-bis(trimethylsilyl)deca-3E,7E-diene1,9-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 (CCH), 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-(5-Methyl-1-phenylhex-4-en-1-yn-3-ol)dicobalt Hexacarbonyl (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 dicobaltoctacarbonyl (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 with benzene, 3 × 1 mL). TLC (PE:E, 2:1): Rf 0.55. 1H 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 (CC), 127.4, 127.99, 128.95, 129.90 (aromatic C, HCCMe2), 136.4, 138.0 (aromatic C, HCCMe2), 199.6 (CO). 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-diphenyldeca-3E,7E-diene1,9-diyne)bis(dicobalthexacarbonyl) (31). According to protocol B, HBF4·OMe2 (201 mg, 1.50 mmol), alcohol 29 (118 mg, 0.25 I
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
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): Rf 0.63. 1H 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 (CC), 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, HCCH). MS TOF FD+: m/z M+ 498. MS HR TOF FD+: m/z calcd for C36H34O2 M+ 498.2553, found 498.2568. μ-η 2 -[3,5-Dimethyl-1-(trimethylsilyl)hex-4-en-1-yn-3-ol]dicobalt Hexacarbonyl (33). According to protocol A, nbutyllithium (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-penten2-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 (PE:E, 10:1): Rf 0.59. 1H 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 (CC), 131.5, 134.2 (C4, C5), 200.6 (CO). 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)deca3E,7E-diene-1,9-diyne)bis(dicobalthexacarbonyl) (34). According 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 coevaporation with benzene, 3 × 1 mL). TLC (P): Rf 0.69. 1H 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 (CC), 131.5, 141.5 (CC), 200.7 (CO). 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,7Ediene-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). 13 C NMR (100 MHz, δ, CDCl3): 0.4 (6CH3), 19.1, 24.4 (4CH3, 3CH3, 8-CH3), 45.0 (C-5, C-6), 88.3, 110.9 (CC), 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-η 2 -[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-
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. 1 H 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 (CC), 124.1, 128.1, 129.10, 129.38, 138.6, 146.8 (aromatic C, HCCH), 199.7 (CO). 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-1,9diyne (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 (CC), 108.2, 123.8, 128.1, 128.5, 131.6, 151.7 (aromatic C, HCCH). MS TOF FD+: m/z M+ 338. MS HR TOF FD+: m/z calcd for C26H26 M+ 338.2029, found 338.2021. μ-η2-[5-Methyl-1-(6′-methoxy-2′-naphthyl)hex-4-en-1-yn-3ol]dicobalt Hexacarbonyl (30). According to protocol A, nbutyllithium (70 mg, 1.10 mmol; 0.69 mL, 1.6 M in hexane), 2ethynyl-6-methoxynaphthalene (200 mg, 1.10 mmol), dimethylacrolein (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 (CC), 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 (CO). 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. 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 0.50. 1H 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 (CC), 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 (CO). 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)deca3E,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) J
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
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Organometallics 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)5CCHCHO (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)5CCHCHO 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 brickred oil. TLC (PE:E, 10:1): Rf 0.44. 1H 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, 2H). 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 (CC), 124.7, 143.5 (CC), 200.5 (CO). 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. 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 benzene, 3 × 1 mL). TLC (PE): Rf 0.74. 1H 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). 13 C NMR (100 MHz, δ, C6D6): 0.9 (6CH3), 23.5, 27.2, 30.8, 48.4 (cyclohexyl C), 80.6, 105.5 (CC), 129.4, 143.7 (CC), 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). 1-[(E)-4″-(Trimethylsilyl)but-1″-en-3″-ynyl]-1′-[(E)-4‴(trimethylsilyl)but-1‴-en-3‴-ynyl]-1,1′-bi(cyclohexane) (40). According to protocol C, Ce(NH4)2(NO3)6 [241 mg, 0.44 mmol/ acetone (11 mL); added in four portions (8 + 1 + 1 + 1) mL] and 36 (39 mg, 0.04 mmol) afforded, upon fractionation by preparative TLC (PE) and repurification by preparative TLC (P:E, 100:1), 40 (11 mg, 67%) as a white solid. Mp: 133−134 °C (sealed capillary, dried by coevaporation with benzene, 3 × 1 mL). TLC (PE): Rf 0.35. 1H NMR (400 MHz, δ, C6D6): 0.22 (s, 18H, 6CH3), 0.76 (m, 2H, CH2), 1.00− 1.40 (m, ∼18H, 9CH2), 5.36 (d, J = 16.8, 2H, 2CH), 5.97 (d, 2H, 2CH). 13C NMR (100 MHz, δ, C6D6): 0.5 (6CH3), 23.0, 26.8, 29.9, 48.2 (cyclohexyl C), 94.4, 105.4 (CC), 113.3, 150.8 (CC). MS TOF FD+: m/z M+ 410. MS HR TOF FD+: m/z calcd for C26H42Si2 M+ 410.2820, found 410.2828. X-ray Crystallographic Analysis of μ-η2-[5,5,6,6-Tetramethyl1,10-bis(trimethylsilyl)deca-3E,7E-diene-1,9-diyne]bis(dicobalthexacarbonyl) (20), μ-η2-(3,5,5,6,6,8-Hexamethyl-
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)but-1‴-en-3‴-ynyl]-1,1′-bi(cyclohexane)}bis(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 TWINABS2008/3 (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
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
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. Corresponding Author
*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. 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.
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REFERENCES
(1) (a) Peterson, E. A.; Overman, L. E. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 11943. (b) Trost, B. M.; Jiang, C. Synthesis 2006, 369. (c) Nicolaou, K. C.; Sorensen, E. J. Classics in Total Synthesis; VCH: Weinheim, 1996; Chapters 14, 25, 26. (d) Nicolaou, K. C.; Snyder, S. A. Classics in Total Synthesis II; Wiley VCH: Weinheim, 2003; Chapters 11, 14, 19. (2) Nonradical approaches to vicinal quaternary carbon atoms: (a) Overman, L. E.; Paone, D. V.; Stearns, B. A. J. Am. Chem. Soc. 1999, 121, 7702. (b) Fox, M. E.; Li, C.; Marino, J. P.; Overman, L. E. J. Am. Chem. Soc. 1999, 121, 5467. (c) Overman, L. E.; Watson, D. A. J. Org. Chem. 2006, 71, 2600. (d) Johnson, W. S.; Lindell, S. D.; Steele, J. J. Am. Chem. Soc. 1987, 109, 5852. (e) Corey, E. J.; Lin, S. J. Am. Chem. Soc. 1996, 118, 8765. (f) Toro, A.; Nowak, P.; Deslongchamps, P. J. Am. Chem. Soc. 2000, 122, 4526. (g) Gilbert, J. C.; Selliah, R. D. J. Org. Chem. 1993, 58, 6255. (h) Lemieux, R. M.; Meyers, A. I. J. Am. Chem. Soc. 1998, 120, 5453. (i) Holton, R. A.; Kennedy, R. M.; Kim, H.; Krafft, M. E. J. Am. Chem. Soc. 1987, 109, 1597. (3) Radical approaches to vicinal quaternary carbon atoms: (a) Shiraki, S.; Natarajan, A.; Garcia-Garibay, M. A. Photochem. Photobiol. Sci. 2011, 10, 1480. (b) Mortko, C. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2005, 127, 7994. (c) Ng, D.; Yang, Z.; GarciaK
DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Garibay, M. A. Org. Lett. 2004, 6, 645. (d) Gibian, M. J.; Corley, R. C. Chem. Rev. 1973, 73, 441. (e) Tidwell, T. T. In Encyclopedia of Radicals in Chemistry, Biology, and Materials; Chatgilialoglu, C., Armido Studer, A., Eds.; Wiley: New York, 2012; Vol. 1, pp 2−35. (f) Engel, P. S.; Bishop, D. J. J. Am. Chem. Soc. 1972, 94, 2148. For related vitamin B12-catalyzed dimerization of alkenes see: (g) Shey, J.; McGinley, C. M.; McCauley, K. M.; Dearth, A. S.; Young, B. T.; van der Donk, W. A. J. Org. Chem. 2002, 67, 837. (h) For cobalt-mediated coupling of tertiary benzylic radicals see: Movassaghi, M.; Schmidt, M. A. Angew. Chem., Int. Ed. 2007, 46, 3725. For steric effects in radical chemistry see: (i) Ruchardt, C. In Organic Chemistry: Syntheses and Reactivity; Springer-Verlag: Berlin, 1980; pp 2−32. (4) (a) Melikyan, G. G.; Sepanian, S.; Riahi, B.; Villena, F.; Jerome, J.; Ahrens, B.; McClain, R.; Matchett, J.; Scanlon, S.; Abrenica, E.; Paulsen, K.; Hardcastle, K. I. J. Organomet. Chem. 2003, 683, 324. (b) Melikyan, G. G.; Villena, F.; Sepanian, S.; Pulido, M.; Sarkissian, H.; Florut, A. Org. Lett. 2003, 5, 3395. (c) Melikyan, G. G.; Villena, F.; Florut, A.; Sepanian, S.; Sarkissian, H.; Rowe, A.; Toure, P.; Mehta, D.; Christian, N.; Myer, S.; Miller, D.; Scanlon, S.; Porazik, M. Organometallics 2006, 25, 4680. (d) Melikyan, G. G.; Floruti, A.; Devletyan, L.; Toure, P.; Dean, N.; Carlson, L. Organometallics 2007, 26, 3173. (e) Melikyan, G. G.; Wild, C.; Toure, P. Organometallics 2008, 27, 1569. (f) Melikyan, G. G.; Mikailian, B.; Sepanian, R.; Toure, P. J. Organomet. Chem. 2009, 694, 785. (g) Melikyan, G. G.; Sepanian, R.; Spencer, R.; Rowe, A.; Toure, P. Organometallics 2009, 28, 5541. (h) Melikyan, G. G.; Spencer, R.; Abedi, E. J. Org. Chem. 2009, 74, 8541. (i) Melikyan, G. G.; Voorhees, E.; Wild, C.; Spencer, R.; Molnar, J. Tetrahedron Lett. 2010, 51, 2287. (j) Melikyan, G. G.; Spencer, R. Tetrahedron 2010, 66, 5321. (k) Melikyan, G. G.; Spencer, R.; Rowe, A. Organometallics 2010, 29, 3556. (l) Melikyan, G. G.; Rivas, B.; Harutyunyan, S.; Carlson, L.; Sepanian, R. Organometallics 2012, 31, 1653. (m) Melikyan, G. G.; Carlson, L.; Sahakyan, N.; Floruti, A.; Garrison, M. Dalton Trans. 2013, 42, 14801. (n) Melikyan, G. G.; Voorhees, E.; Sepanian, R. Organometallics 2014, 33, 69. (5) (a) Smith, M. B. March’s Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, 7th ed.; Wiley: Hoboken, NJ, 2013; pp 208−249, 409−412, 657−658, 812−817 and references therein. (b) DeWolfe, R. H.; Young, W. G. Chem. Rev. 1956, 56, 753. (c) Davies, H. M. L.; Lian, Y. Acc. Chem. Res. 2012, 45, 923. For electron diffraction-derived allylic radical structure, see: (d) Vajda, E.; Tremmel, J.; Rozsondai, B.; Hargittai, I.; Maltsev, A. K.; Kagramanov, N. D.; Nefedov, O. M. J. Am. Chem. Soc. 1986, 108, 4352. (6) Varghese, V.; Saha, M.; Nicholas, K. M. Org. Synth. 1989, 67, 141. (7) (a) Melikyan, G. G.; Nicholas, K. M. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Ed.; VCH Publishers: Weinheim, 1995; Chapter 4, p 99. (b) McGlinchey, M. J.; Girard, L.; Ruffolo, R. Coord. Chem. Rev. 1995, 143, 331. (c) Went, M. Adv. Organomet. Chem. 1997, 41, 69. (d) Green, J. R. Curr. Org. Chem. 2001, 5, 809. (e) Muller, T. J. J. Eur. J. Org. Chem. 2001, 2021. (f) Teobald, B. J. Tetrahedron 2002, 58, 4133. (g) Omae, I. Appl. Organomet. Chem. 2007, 21, 318. (8) (a) Smith, M. B. March’s Advanced Organic Chemistry. Reactions, Mechanisms, and Structure, 7th ed.; Wiley: Hoboken, NJ, 2013; pp 264, 549, 1143, 1148. (b) Besseau, F.; Laurence, C.; Berthelot, M. J. Chem. Soc., Perkin Trans. 2 1994, 485. (c) Bruneau, P.; Taylor, P. J.; Wilkinson, A. J. J. Chem. Soc., Perkin Trans. 2 1996, 2263. For vinylogous aldol reactions see: (d) Casiraghi, G.; Zanardi, F.; Appendino, G.; Rassu, G. Chem. Rev. 2000, 100, 1929. (e) Denmark, S. E.; Heemstra, J. R.; Beutner, G. L. Angew. Chem., Int. Ed. 2005, 44, 4782. (f) Takikawa, H.; Ishihara, K.; Saito, S.; Yamamoto, H. Synlett 2004, 732. For vinylogous acylvinyl systems see: (g) Chincilla, R.; Najera, C. Chem. Rev. 2000, 100, 1891. For conceptually related methylenology principle see: (h) Zipse, H. Acc. Chem. Res. 1999, 32, 571. (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.
(10) (a) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes; Elsevier: New York, 1981; p 80. (b) Greenfield, H.; Sternberg, H. W.; Friedel, R. A.; Wotiz, J. H.; Markby, R.; Wender, I. J. Am. Chem. Soc. 1956, 78, 120. (11) (a) Onodera, G.; Nishibayashi, Y.; Uemura, S. Organometallics 2006, 25, 35. (b) Yang, F.; Zhao, G.; Ding, Y.; Zhao, Z.; Zheng, Y. Tetrahedron Lett. 2002, 43, 1289. (c) Ogoshi, S.; Nishiguchi, S.; Tsutsumi, K.; Kurosawa, H. J. Org. Chem. 1995, 60, 4650. (d) Badanyan, Sh. O.; Voskanyan, M. G.; Chobanyan, Zh. A. Russ. Chem. Rev. 1981, 50, 1074. (e) Boutillier, P.; Zard, S. Z. Chem. Commun. 2001, 1304. (12) Swaminathan, S.; Narayanan, K. V. Chem. Rev. 1971, 71, 429. (13) (a) Padmanabhan, S.; Nicholas, K. M. Tetrahedron Lett. 1982, 23, 2555. (b) Stuart, J.; Nicholas, K. M. Heterocycles 1991, 32, 949. (c) DiMartino, J.; Green, J. R. Tetrahedron 2006, 62, 1402. (d) Descoins, C.; Samain, D. Tetrahedron Lett. 1976, 10, 745. (14) (a) Bruker. SAINT V7.68a; Bruker AXS Inc.: Madison, WI, USA, 2009. (b) Bruker. SADABS V2008-1; Bruker AXS Inc.: Madison, WI, USA, 2008. (c) Bruker. TWINABS V2008-3; Bruker AXS Inc.: Madison, WI, USA, 2008. (d) Petricek, V.; Dusek, M.; Palatinus, L. Jana2006, The Crystallographic Computing System; Institute of Physics: Praha, Czech Republic, 2006. (e) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339; Supramol. Chem. 2001, 1, 189. (f) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (15) (a) Davies, S. G.; McCarthy, T. D. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: New York, 1995; Vol. 12, p 1039. (b) Semmelhack, M. F. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: New York, 1995; Vol. 12, p 979. (c) Amouri, H. E.; Gruselle, M. Chem. Rev. 1996, 96, 1077. (d) Mann, B. E. Non-Rigidity in Organometallic Compounds. In Comprehensive Organometallic Chemistry, Vol. 3, Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: New York, 1982; p 89. (16) Seyferth, D.; Nestle, M. O.; Wehman, A. T. J. Am. Chem. Soc. 1975, 97, 7417. (17) Alphonse, F.; Karim, R.; Cano-Soumillac, C.; Hebray, M.; Collison, D.; Garner, D.; Joule, J. Tetrahedron 2005, 614, 11010. (18) Olier, C.; Gastaldi, S.; Gil, G.; Bertrand, M. P. Tetrahedron Lett. 2007, 48, 7801. (19) (a) Tyler, D. R. Acc. Chem. Res. 1991, 24, 325. (b) Zavarine, I. S.; Kubiak, C. P. Coord. Chem. Rev. 1998, 171, 419. (c) Antipin, M. Yu.; Boese, R.; Augart, N.; Schmid, G. Struct. Chem. 1993, 4, 91. (20) (a) Barfield, M.; Chakrabarti, B. Chem. Rev. 1969, 69, 757. (b) Anderson, J. E. Tetrahedron Lett. 1975, 46, 4079. (21) Vatele, J. Tetrahedron 2010, 66, 904.
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DOI: 10.1021/om501096x Organometallics XXXX, XXX, XXX−XXX
