Andrey A. Fokin

We prepared the first doubly bridgehead anti ‐Bredt alkene 1,2‐diamantene from 1,2‐diiododiamantane and trapped it as its Diels–Alder adduct with 11,12‐dimethylene‐9,10‐dihydro‐9,10‐ethanoanthracene; the structure of the adduct was confirmed through single crystal X‐ray diffraction. The structure of 1,2‐diamantene resembles a hypothetical trans ‐cyclohexene moiety at the central belt that we analyzed computationally and compared it with isomeric 2,3‐ and 3,4‐diamantenes as well as parent 1,2‐adamantene and several known bicyclic anti ‐Bredt olefins. The olefin strain energies (OSE) were computed through homodesmotic equations at the ab initio MP2/cc‐pVTZ level of theory. The choice of the computational method was based on results of CCSD(T)/cc‐pVTZ optimizations of bicyclo[2.2.1]hept‐1‐ene and norbornane as the hydrogenated reference hydrocarbon. Although isomeric diamantenes have different degrees of twisting and bending and altered substitution patterns, their OSEs are similar. Comparative orbital analyses using cis ‐cyclohexene, trans ‐cyclohexene, and 1,2‐adamantene reveal the critical role of CC hyperconjugation in stabilizing twisted alkenes. High‐yielding formation of 1,2‐diamantene is possible due to steric hindrance preventing dimerization in contrast to parent 1,2‐adamantene that readily forms its C ₂₀ ‐dimer.

Here we report our investigation of 1‐bromo‐3,5,7‐triphenyladamantane (1) and the elucidation of polymorphic crystals (1 A and 1 B) using single crystal X‐ray diffraction. In the monoclinic crystal system of 1 A (P2₁/n), we observed CH–π interactions, while Br⋅⋅⋅Br interactions are absent. Conversely, the Br⋅⋅⋅Br interactions are an apparent factor in the formation of the monoclinic crystal system of 1 B (R ). We compare our findings with 1,3,5,7‐tetraphenyladamantane (2), characterized by numerous CH–π interactions in the solid. Computational analyses were employed to investigate the interactions within the characteristic dimers present in the unit cells of 1 A and 1 B, including visualization of noncovalent interactions and the use of the atoms‐in‐molecules approach as well as MO analyses. These support the notion of London dispersion (LD) dimer‐dimer interactions in 1 A between the phenyl moieties, whereas 1 B exhibits additional dimer‐dimer Br⋅⋅⋅Br contacts. In contrast, the crystals of 2 are exclusively held together by CH–π stacking LD interactions, a feature absent in the polymorphs of 1. Both polymorphic forms of 1 emit white light when subjected to 900 nm continuous wave laser irradiation, displaying a subtle blue shift compared to 2. The absence of CH–π stacking interactions between the dimers of 1 causes a small red‐shift in the emission spectrum.

anti- and syn-sesquihomodiamantenes (SDs) were prepared and structurally characterized. anti-SD and parent sesquihomoadamantene were CH-bond functionalized by utilizing a phase-transfer protocol. The density functional theory-computed ionization potentials of unsaturated diamondoid dimers correlate well with the experimental oxidation potentials obtained from cyclic voltammetry. Similar geometries ensue for both the reduced and ionized SD states, whose persistence is supported by the β-hydrogen's spatial sheltering. This makes SDs promising building blocks for the construction of diamond materials with high stability and carrier mobility.

Herein we present previously unavailable C(sp3)-rich polycyclic hydrocarbon scaffolds which have a potential to become valuable tools in medicinal chemistry and crop science as saturated bioisosteres of benzenoids. We have developed a scalable protocol (up to 50 g from a single synthetic run) for the synthesis of tricyclo[3.3.0.03,7]octane (bisnoradamantane or stellane) 1,5-dicarbonic acid derivatives. X-ray crystallographic analysis of the stellane 1,5-dicarbonic acid dimethyl ester has revealed that this scaffold is an optimal saturated isostere for ortho-disubstituted benzene where substituents exhibit the in-plane topology. The synthetic protocol is based on the oxidative cyclization of dimethyl octahydropentalene-2,5-dicarboxylate (DMOD) through lithiation followed by I2 oxidation. The reaction outcome is determined by the stereochemistry of the substrate. While the endo,endo cis-DMOD, exclusively gives the "unwanted" Claisen cyclization product the exo,endo cis- and exo,exo cis- stereoisomers afford the desired stellane 1,5-dicarbonic acid dimethyl ester quantitatively. DFT computations have revealed that the reaction proceeds via the dianion of dimethyl octahydropentalene-2,5-dicarboxylate, which undergoes SET oxidation by I2 to form a radical anion. The subsequent cyclization followed by a second SET oxidation gives desired stellane derivative.

Theoretical challenges in describing molecules with anomalously long single C−C bonds are analyzed in terms of the relative contributions of stabilizing and destabilizing intramolecular interactions. Diamondoid dimers that are stable despite the presence of C−C bonds up to 1.7 Å long, as well as other bulky molecules stabilized due to intramolecular noncovalent interactions (London dispersions) are discussed. The unexpected stability of highly crowded molecules, such as diamondoid dimers and tert‐butyl‐substituted hexaphenylethanes, calls for reconsideration of the “steric effect” traditionally thought to destabilize the molecule. Alternatively, “steric attraction” helps to understand bonding in sterically overloaded molecules, whose structural and energetic analysis requires a proper theoretical description of noncovalent interactions.

We present a strategy for the skeletal editing of diamondoid structures to selectively displace methylene for heteroatom moieties in the carbon framework. This constitutes a synthetic approach to doping diamond-like structures with electron donor dopants (O, N, and S). The key steps involve two subsequent retro-Barbier fragmentations followed by cage reconstruction in the presence of a dopant. Remarkably, the incorporation of n-dopants reduces the strain of the diamondoid cage as shown through homodesmotic equations.

Novel nanocarbons such as fullerenes, nanotubes, graphene, and nanodiamond reside at the cutting edge of nanoscience and technology. Along with chemical functionalization, geometric constraints (such as extreme curvature in nanotubes or defects within or at the surfaces of diamond nanoparticles) significantly alter the electronic states of the nanocarbon material. Understanding the effects of steric strain on the electronic structure is critical to developing nanoelectronic applications based on these materials. This paper presents a fundamental study of how strain affects the electronic structure in a benchmark series of some fundamental saturated carbon cage compounds. Adamantane, C10H16, the smallest diamondoid and arguably the smallest nanodiamond crystallite, has carbon atoms essentially commensurate with diamond lattice positions and possesses by far the least molecular strain of this series. Twistane also is a C10H16 isomer but the fixed cyclohexane twist conformation of the central ring introduces additional strain into the cage. Octahedrane [(CH)12] and cubane [(CH)8] are considerably more strained, culminating in cubane where carbon–carbon bonds lie either parallel or orthogonal to one another. Using gas-phase near-edge x-ray absorption fine structure spectroscopy to probe the unoccupied electronic states, we observe two major progressions across this series. First, a broad C–C σ* resonance in the absorption splits into two more narrow and intense resonances with increasing strain. Second, the first manifold of states previously associated with tertiary C–H σ* in the diamondoid series appears to broaden and shift to lower energy. This feature is more than twice as intense in cubane than in octahedrane, even though these two molecules have only tertiary carbons, with the chemical formula (CH)x. The spectral differences are entirely due to the shape of the molecules; in particular, in cubane, the features arise from a high degree of p-p interaction between parallel C–C bonds. In contrast to the conventional wisdom that near-edge x-ray absorption is primarily an atomically localized spectroscopy, molecular shape and associated strain lead to the dominant features in spectra acquired from this fundamental series of carbon cage structures.

Platinum-based antineoplastic agents play a major role in the treatment of numerous types of cancer. A new bulky, lipophilic, and chiral ligand based on 1,2-diaminodiamantane in both of its enantiomeric forms was employed for the preparation of new platinum(II) complexes with chloride and oxalate ligands. The dichloride complexes have a higher solubility and were evaluated as anti-proliferation agents for human ovarian cancer cell lines A2780 and cisplatin-resistant A2780cis. Its R,R-enantiomer showed increased efficacy compared to cisplatin for both cancer cell lines. A chromatographic approach was used to estimate the solvent partition coefficient of the dichloride complex. The binding of diamondoid-based platinum complexes to nucleotides was tested for both enantiomers with guanosine monophosphate (GMP) and deoxyguanosine monophosphate (dGMP) and occurs at a similar or faster rate for both isomers compared to cisplatin despite greatly increased steric demand. These findings highlight the potential in 1,2-diaminodiamantane as a viable pharmacophore.

Aim. To study the effect of electronic (α- and β-hyperconjugations) and steric (noncovalent interactions) factors on the structures of olefinic radical cations. Results and discussion. The effect of intramolecular dispersion interactions on the structures of crowded alkenes in the neutral and ionized forms has been studied at the density functional theory (DFT) level with and without dispersion corrections included, as well as at the MP2 theory level with medium size basis sets. The results obtained are compared to the available experimental data. An excellent agreement has been found between the experimental and MP2/DFT-computed geometries of sesquihomoadamantene, adamantylidene adamantane, bis-2,2,5,5-tetramethylcyclopentylidene, bis-D₃-homocub-4-ylidene, and bis-C_S-homocub-8-ylidene in the neutral and ionized forms. The experimental ionization potentials are better reproduced with the DFT-methods. Experimental part. The structure and composition of compounds were proved by the methods of ¹H and ¹³C NMR-spectroscopy, and GC-MS-analysis. Elemental analysis was performed for the compounds obtained. Conclusions. The twisting of the olefinic moieties in the sesquihomoadamantene and adamantylidene adamantane radical cations is determined by the balance between the σ-π-hyperconjugation and residual one-electron π-bonding and is close to that of the prototypical ethylene radical cation (29°). The twisting reaches 55° for the bis-2,2,5,5-tetramethylcyclopentylidene radical cation due to substantial steric repulsions between methyl groups. At the same time, the ionized states of bis-D₃-homocub-4-ylidene and bis-C_S-homocub-8-ylidene retain their planarity due to β-CC-hyperconjugation and intramolecular dispersion attractions. Received: 26.12.2019 Revised: 17.01.2020 Accepted: 27.02.2020