Azatriquinanes. 2.(1) synthesis of azatriquinadiene and azatriquinacene

Chemistry of the Heterotriquinanes and Heterotriquinacenes

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 Introduction

2 Azatriquinane

3 Azatriquinacene

4 Aromatic Azatriquinacene-Based Systems

5 Oxatriquinane

6 Tetravalent Oxygen

7 Oxatriquinacene

8 The Future

Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres

To probe the limits of the internal charge compensation approach for the generation of 10π aromatic hemispheres derived from hetero-acepentalenide zwitterions, the phospha- and arsa-bridged cyclononatetraenides were studied computationally by DFT at the B3LYP/6-311++G(d,p) level, and compared with the known aza-analog.

A Zwitterionic, 10 π Aromatic Hemisphere

A new concept in anionic 10 π aromaticity is described by the embedding of a compensating charge within an aromatic cyclononatetraenide ring by the symmetric superposition of an alkyl ammonium bridge. This is accomplished by the methylation of azatriquinacene to give a quaternary ammonium salt, followed by oxidation to the tetraene and final deprotonation. The resulting zwitterion is a stable [9]annulene with strong aromaticity as shown by its degree of C-C bond equalization and a nucleus-independent chemical shift value lower than that of benzene. The solid-state structure shows an eclipsed stacking motif with the electron-poor ammonium methyl groups occupying the electron-rich cavity of the aromatic bowl.

Semibullvalene and diazasemibullvalene: recent advances in the synthesis, reaction chemistry, and synthetic applications

Semibullvalene (SBV) and its aza analogue 2,6-diazasemibullvalene (NSBV) are theoretically interesting and experimentally challenging organic molecules because of four unique features: highly strained ring systems, intramolecular skeletal rearrangement, extremely rapid degenerate (aza-)Cope rearrangement, and the predicted existence of neutral homoaromatic delocalized structures. SBV has received much attention in the past 50 years. In contrast, after NSBV was predicted in 1971 and the first in situ synthesis was realized in 1982, no progress on NSBV chemistry was made until our results in 2012. We have been interested in the reaction chemistry of 1,4-dilithio-1,3-butadienes (dilithio reagents for short), especially for their applications in the synthesis of SBV and NSBV, because (i) the cyclodimerization of dilithio reagents could provide the potential eight-carbon skeleton of SBV from four-carbon butadiene units and (ii) the insertion reaction of dilithio reagents with C≡N bonds of two nitriles could provide a 6C + 2N skeleton that might be a good precursor for the synthesis of NSBV. Therefore, we initiated a journey into the synthesis and reaction chemistry of SBV and NSBV starting from dilithio reagents that has been ongoing since 2006. In this Account, we outline mainly our recent achievements in the synthesis, structural characterization, reaction chemistry, synthetic application, and theoretical/computational analysis of NSBV. Two efficient strategies for the synthesis of NSBV from dilithio reagents and nitriles via oxidant-induced C-N bond formation are described. Structural investigations of NSBV, including X-ray crystal structure analysis, determination of the activation barrier for the aza-Cope rearrangement, and theoretical analysis, show that the localized structure of NSBV is the predominant form and that the homoaromatic delocalized structure exists as a minor component in the equilibrium. We also discuss the reaction chemistry and synthetic applications of NSBV. Several novel reaction patterns have been explored, including thermolysis, C-N bond insertion, rearrangement-cycloaddition, oxidation, and nucleophilic ring-opening reactions. Diverse and interesting N-containing polycyclic skeletons can be constructed, such as nickelaazetidine, 1,5-diazatriquinacenes, and triazabrexadienes, which are not available by other means. Our results show that NSBV not only features a rapid aza-Cope rearrangement with a low activation barrier but also acts as unique synthetic reagent that is significantly different from aziridine. The strained rigid ring systems as a whole can be involved in the reactions. Our achievements highlight two significant advances: (i) the well-established efficient synthesis and isolation of NSBV has greatly accelerated the development of NSBV chemistry, and (ii) the previously unattainable molecules have become "normal" and routine starting materials for the synthesis of otherwise unavailable but interesting structures. We expect that our pursuits will inspire and help direct future chemical and physical research on NSBV.

Efficient synthesis of aza-triquinacene derivatives via cycloaddition of 2,6-diazasemibullvalenes with nitroso compounds

The reaction between 2,6-diazasemibullvalenes and nitroso compounds was investigated. Aza-triquinacene derivatives of interesting structural and synthetic chemistry were generated highly selectively in good to excellent isolated yields. This reaction, which was rarely found between common aziridine derivatives and nitroso compounds, could be attributed to the rigid polycyclic ring system and the substitution patterns of 2,6-diazasemibullvalenes. Δ(1) -Bipyrroline derivatives were formed in excellent yields when these aza-triquinacene derivatives were treated with SmI2 .

One-pot tandem cyclization of enantiopure asymmetric cis-2,5-disubstituted pyrrolidines: Facile access to chiral 10-heteroazatriquinanes

A series of chiral 10-heteroazatriquinanes were synthesized from enantiopure asymmetric cis-2,5-disubstituted pyrrolidines through a one-pot tandem cyclization procedure. The structures and configurations of these new chiral 10-heteroazatriquinanes are confirmed by X-ray single-crystal diffraction analysis.

Interaction of Fullerenes with the Concave Surfaces of Perchloroazatriquinacene

[reaction: see text] Crystal structures of the one-to-one co-crystals of C(60).perchloroazatriquinacene and C(70).perchloroazatriquinacene show that the rigid, chalice-like azatriquinacene packs between completely ordered fullerene molecules with multiple, close Cl...fullerene contacts.

The Azaacepentalenide Anion: A New Aromatic, Bowl-Shaped Heterocycle

The bowl-shaped aromatic azaacepentalenide anion and its perchloro analogue were derived from 10-azatriquinacene and 2,3,5,6,8,9-hexachloro-10-azatriquinacene via the N-oxides. Spectroscopic and computational studies, as well as chemical derivitization experiments, were used to confirm the structural assignments.

Resurrection of neutral tris-homoaromaticity

Neutral in-plane tris-homoaromaticity is evaluated in tris(bismethano)benzene (15) and modifications of this parent structure in which the pi-orbitals might interact in the plane established by the unsaturated carbon atoms (in-plane conjugation). On the basis of magnetic susceptibility exaltation, nucleus-independent shift (NICS), and aromatic stabilization energy (ASE, evaluated via homodesmotic and isodesmic equations using B3LYP/6-311+G + ZPVE energies, as well as by MM3 and MM4 force field computations), we identified triene 17, a triply bridged analogue of 15, as the system where homoaromaticity is most effective. The NICS(total) in the center of 17 is -30.1 ppm and the diatropic pi-contribution is -18.0 ppm. This structure possesses more than one-third of the aromatic stabilization of benzene and is the best candidate for neutral tris-homoaromaticity ever proposed. The previously described tris-(bismethano)-benzene (15) also shows homoaromaticity but to a smaller extent compared to 17. Structure 18, which is closely related to 17, also is significantly homoaromatic but, as evaluated by MM3, strain partially counteracts the stabilizing effects from homoconjugation. Such a counteracting increase in strain largely cancels or even overwhelms the stabilization from homoconjugation in all other species considered in this study.

Structures and Energies of 10-Azatriquinacene and Its Hydrogenation Products: A Computational Study

Enthalpies of formation and hydrogenation, vertical ionization potentials, and proton affinities of 10-azatriquinacene and its hydrogenation products have been computed at the B3LYP/6-311+G level of density functional theory. Like the parent triquinacene, 10-azatriquinacene is not homoaromatic on the basis of the energetic, geometric, and magnetic criteria. The hyperfine coupling constants of the nitrogen-centered cation radicals are also calculated.