Extreme Oxatriquinanes: Structural Characterization of α-Oxyoxonium Species with Extraordinarily Long Carbon–Oxygen Bonds

Günbaş, Görkem; Sheppard, William; Fettinger, James C.; Olmstead, Marilyn M.; Mascal, Mark

doi:10.1021/ja4032715

Cited by 15 publications

(23 citation statements)

References 10 publications

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“…61 The considerably longer C−O bond length observed in the oxocarbenium ions stemming from the cyclohexane-derived acetal trans-6, which ionized eight times faster than an acetal without an alkoxy group, however, cannot be readily interpreted as deriving from neighboringgroup participation. 60 These distances are, however, consistent with electrostatic stabilization of an oxocarbenium ion, which is even observed in cases where the C−O distance is around 3.3 Å. 24 If the accelerated ionization of trans-substituted acetals trans-7 and trans-8 involved neighboring-group participation and the accelerated ionization of trans-6 did not, then the difference between the efficiency of these mechanisms is small (leading to rate differences less than three-fold), much smaller than normally observed in cases of neighboring-group participation (rate accelerations of several orders of magnitude are more common).…”

Section: The Journal Of Organic Chemistrymentioning

confidence: 99%

Influence of Alkoxy Groups on Rates of Acetal Hydrolysis and Tosylate Solvolysis: Electrostatic Stabilization of Developing Oxocarbenium Ion Intermediates and Neighboring-Group Participation To Form Oxonium Ions

et al. 2015

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The hydrolysis of 4-alkoxy-substituted acetals was accelerated by about 20-fold compared to that of sterically comparable substrates that do not have an alkoxy group. Rate accelerations are largest when the two functional groups are linked by a flexible cyclic tether. When controlled for the inductive destabilization, an alkoxy group can accelerate acetal hydrolysis by up to 200-fold. The difference in rates of acetal hydrolysis between a substrate where the alkoxy group was tethered to the acetal group by a five-membered ring compared to one where it was tethered by an eight-membered ring was less than 100-fold, suggesting that fused-ring intermediates were not formed. By comparison, the difference in rates of solvolysis of structurally related tosylates were nearly 10(6)-fold between the five- and eight-membered ring series. This observation implicates neighboring-group participation in the solvolysis of tosylates but not in the hydrolysis of acetals. The acceleration of acetal hydrolysis by an alkoxy group is better explained by electrostatic stabilization of intermediates that accumulate positive charge at the acetal carbon atom.

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Section: The Journal Of Organic Chemistrymentioning

confidence: 99%

Influence of Alkoxy Groups on Rates of Acetal Hydrolysis and Tosylate Solvolysis: Electrostatic Stabilization of Developing Oxocarbenium Ion Intermediates and Neighboring-Group Participation To Form Oxonium Ions

et al. 2015

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“…The solvent was evaporated and the residue was chromatographed on silica gel (20% to 40% AcOEt/n-hexane as the eluent) to give 13 (215. 8 To a solution of 14 (275.9 mg, 0.632 mmol) in acetone (6 mL) was added 20% K 2 CO 3 aq. (3 mL).…”

Section: Methodsmentioning

confidence: 99%

“…7) Most recently, oxatriquinane having a hydroxy group at the α-position to oxygen (4) was found to have the longest C-O bond length so far reported among organic compounds. 8) We became interested in these stable oxonium cation species, and set out to synthesize further examples for examination of their potential utility in materials science or medicinal chemistry.…”

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confidence: 99%

Functionalized Oxatriquinanes and Their Structural Equilibrium in Protic Solvent

Suzuki

Muratake

2014

CHEMICAL & PHARMACEUTICAL BULLETIN

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We synthesized oxatriquinane hexafluorophosphate bearing an ethoxycarbonylmethyl group 7 or a 2-oxopropyl group 11. Both of these organic oxonium cation compounds were obtained as stable solids. However, 1 H-NMR analysis showed that oxatriquinane 7 was present as the oxonium cation in aprotic solvent CD 3 CN, but was in rapid equilibrium with ring-opened bicyclic compound 8 in protic solvent CD 3 OD. The oxatriquinane 11 also showed similar behavior in protic solvent. Phenyl-substituted oxatriquinanes 12 and 14 were also obtained as stable solids, and showed similar properties to 7 and 11. Key words oxatriquinane; organic oxonium cation; equilibrium; protic solventThere have been few reports on stable organic oxonium salts, 1) though alkyl oxonium salts such as triethyl oxonium tetrafluoroborate 1 (known as Meerwein's reagent) have been utilized in organic synthesis as powerful alkylation reagents for alcohol or amine 2,3) ( Fig. 1). However, they are chemically unstable and sensitive to moisture, so they are difficult to handle. Recently, Mascal et al. reported the synthesis of oxatriquinane 2 and oxatriquinacene 3, which have a tricyclic condensed ring structure with an oxygen cationic center. 4)Compound 2 was isolated as a crystalline solid suitable for X-ray crystal structure analysis. Interestingly, 2 could be purified by column chromatography and was stable under reflux in alcohol or water. Subsequently, they synthesized several oxatriquinanes with simple alkyl groups and carried out X-ray crystal structure analysis.5,6) A unique S N 2 reaction at tertiary carbon in the α-position to the oxygen center was also reported.7) Most recently, oxatriquinane having a hydroxy group at the α-position to oxygen (4) was found to have the longest C-O bond length so far reported among organic compounds. 8)We became interested in these stable oxonium cation species, and set out to synthesize further examples for examination of their potential utility in materials science or medicinal chemistry.First, we focused on oxatriquinane having a simple ester group, to see whether such a group is compatible with the oxonium cation. As shown in Chart 1, the starting ketone 5, 7) which was synthesized from 1,5-cyclooctadiene, was subjected to Reformatsky reaction to afford β-hydroxyester 6 in good yield. Compound 6 was obtained as nearly a single stereoisomer, and its stereochemistry was determined by examination of the nuclear Overhauser effect (NOE) between methine proton at C1 and methylene protons at the α-position to the carbonyl group. Next, 6 was treated with trifluoromethanesulfonic acid (TfOH) in acetonitrile to give oxatriquinane trifluoromethanesulfonate, but we could not obtain this in crystalline form, so the salt was purified by simple anion exchange using sat. aqueous KPF 6 to afford oxatriquinane hexafluorophosphate 7 in an excellent yield as a stable off-white solid after usual work-up. Recrystallization from CH 2 Cl 2 /Et 2 O afforded colorless fine needles. Compound 7 showed good solubility in acetonitrile, dichlo...

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“…Although the approach in Scheme 1 is not amenable to starting with furan, related annulation strategies presented themselves. Ultimately, we settled on an approach that began with closing a THF ring across 1,4,7-cyclononatriene (41). Oxygen could be introduced into 41 via epoxidation using the approach of Winstein et al, 33 giving 42 in good yield (Scheme 11).…”

Section: Oxatriquinanementioning

confidence: 99%

“…Fortunately, the methyl ether of 61 could be smoothly displaced upon protonation with triflic acid to generate 62. 41 Cations 62 and 63 proved remarkably stable and X-ray quality crystals could be grown by dilution of their acetonitrile solutions with diethyl ether. Crystallographic analysis revealed substantially elongated C-O bond distances, effectively superseding 60 as the C-O bond record holder (Figure 17).…”

Section: Scheme 15 Higher Trialkyloxatriquinane Synthesismentioning

confidence: 99%

Chemistry of the Heterotriquinanes and Heterotriquinacenes

Hafezi¹,

Mascal²

2021

Synlett

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Triquinanes are tricyclic hydrocarbons that have fused cyclopentane rings. Although there are linear and angular triquinanes that are doubly fused, this Account focuses exclusively on the ‘triquinacane’, or triply fused structure with a heteroatom (nitrogen or oxygen) on the C 3v symmetry axis. Azatriquinane- and oxatriquinane-based species tend to show remarkable and often unexpected chemistry, and have variously comprised the most basic trialkyl amine, a superbasic proton chelate, trigonal pyramidal ligand platforms, novel calixiform hosts, aromatic hemispheres of hetero-C20 fullerenes, cocrystallizing agents for eliminating rotational disorder in fullerene crystals, the first water-stable, chromatographable trialkyloxonium species, the first isolable allylic oxonium species, world-record C–O bond lengths, rapid SN2 reaction at a tertiary center, and R4O2+ (oxadionium) species.1 Introduction2 Azatriquinane3 Azatriquinacene4 Aromatic Azatriquinacene-Based Systems5 Oxatriquinane6 Tetravalent Oxygen7 Oxatriquinacene8 The Future

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Extreme Oxatriquinanes: Structural Characterization of α-Oxyoxonium Species with Extraordinarily Long Carbon–Oxygen Bonds

Cited by 15 publications

References 10 publications

Influence of Alkoxy Groups on Rates of Acetal Hydrolysis and Tosylate Solvolysis: Electrostatic Stabilization of Developing Oxocarbenium Ion Intermediates and Neighboring-Group Participation To Form Oxonium Ions

Influence of Alkoxy Groups on Rates of Acetal Hydrolysis and Tosylate Solvolysis: Electrostatic Stabilization of Developing Oxocarbenium Ion Intermediates and Neighboring-Group Participation To Form Oxonium Ions

Functionalized Oxatriquinanes and Their Structural Equilibrium in Protic Solvent

Chemistry of the Heterotriquinanes and Heterotriquinacenes

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