Marilyn Monroe knew that "diamonds are a girl's best friend" but, in the meantime, many chemists have realized that they are also extremely attractive objects in contemporary chemistry. The chemist's ...diamonds are usually quite small (herein: nanometer-sized "diamondoids") and as a result of their unique structure are unusual chemical building blocks. Since lower diamondoids (up to triamantane) have recently become available in large amounts from petroleum and higher diamondoids (starting from tetramantane) are now also accessible from crude oil new research involving them has begun to emerge. Having well-defined structures makes these cage compounds so special compared to other nanometer-scale diamonds. Selective and high-yielding synthetic approaches to the functionalization of diamondoids gives derivatives that can find applications in, for example, polymers, coating materials, drugs, and molecular electronics.
Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from ...overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes and the possibility of constructing hydrocarbons with extraordinarily long C-C single bonds through steric crowding. Although empirical bond distance/bond strength relationships have been established for C-C bonds (longer C-C bonds have smaller bond dissociation energies), these have no present theoretical basis. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry, as summarized by Pauling as early as 1960 and confirmed more recently. Here we report the preparation of hydrocarbons with extremely long C-C bonds (up to 1.704 Å), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable--noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with and without inclusion of dispersion corrections.
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.
Saturated diamondoid dimers with ultralong central C−C bonds are stabilized by noncovalent intramolecular interactions between bulky groups.
We present the Pd‐catalyzed arylation of (N−H)‐indoles with functionalized haloarenes “on water” using hitherto untested primary diamantyl phosphine oxides (PPO) as ligands. Remarkable C2−H arylation ...selectivity was achieved by employing functionalized iodoarenes and N‐unprotected indoles. We provide evidence that the in situ generated oxide of (9‐hydroxydiamant‐4‐yl)phosphine L1 is key for the reaction efficiency by comparing a set of diamantane‐based compounds structurally related to L1. Our results demonstrate the power of the new PPO ligands for the C−H functionalization of unprotected (N−H)‐heterocycles.
Phosphine possibilities: (9‐Hydroxydiamant‐4‐yl)phosphine is key for the aerobic Pd‐catalyzed C2‐arylation of unprotected (N−H)‐indoles with functionalized haloarenes “on water”. Our results demonstrate the power of the new primary phosphine oxide ligands for the C−H functionalization of unprotected (N−H)‐heterocycles.
The structures, strain energies, and enthalpies of formation of diamantane 1, triamantane 2, isomeric tetramantanes 3–5, Td‐pentamantane 6, and D3d‐hexamantane 7, and the structures of their ...respective radicals, cations, as well as radical cations, were computed at the B3LYP/6‐31G* level of theory. For the most symmetrical hydrocarbons, the relative strain (per carbon atom) decreases from the lower to the higher diamondoids. The relative stabilities of isomeric diamondoidyl radicals vary only within small limits, while the stabilities of the diamondoidyl cations increase with cage size and depend strongly on the geometric position of the charge. Positive charge located close to the geometrical center of the molecule is stabilized by 2–5 kcal mol−1. In contrast, diamondoid radical cations preferentially form highly delocalized structures with elongated peripheral CH bonds. The effective spin/charge delocalization lowers the ionization potentials of diamondoids significantly (down to 176.9 kcal mol−1 for 7). The reactivity of 1 was extensively studied experimentally. Whereas reactions with carbon‐centered radicals (Hal)3C. (Hal=halogen) lead to mixtures of all possible tertiary and secondary halodiamantanes, uncharged electrophiles (dimethyldioxirane, m‐chloroperbenzoic acid, and CrO2Cl2) give much higher tertiary versus secondary selectivities. Medial bridgehead substitution dominates in the reactions with strong electrophiles (Br2, 100 % HNO3), whereas with strong single‐electron transfer (SET) acceptors (photoexcited 1,2,4,5‐tetracyanobenzene) apical C4H bridgehead substitution is preferred. For diamondoids that form well‐defined radical cations (such as 1 and 4–7), exceptionally high selectivities are expected upon oxidation with outer‐sphere SET reagents.
Crude oil provides diamondoids that have the potential to be spectacular nanomolecular organic building blocks. Yet, methods for selective functionalizations of such (chemical) gems are scarce. We present a wide array of methods to probe CH substitution selectivities involving radicals, cations, and radical cations (schematized above).
The present overview of alkane functionalization reactions presents comparisons between radical and metal‐initiated (sometimes metal‐catalyzed) methodologies. While metal‐catalyzed processes are ...excellent approaches to this problem, metal‐free alternatives are equally if not, at least from an environmental and cost perspective, more useful. This conclusion is supported by the fact that many so‐called metal‐catalyzed reactions also work without the metal present, and the large variety of metals showing the same product distributions emphasizes that the metal often just aids in the generation of the active species, i.e., the metal itself is not participating in the crucial CH activation step. Highly selective alkane functionalization reactions such as those derived from nitroxyl and related radicals as well as through radical reactions conducted in phase‐transfer catalyzed systems are available but generally underutilized. These systems, in contrast to typical metal‐catalyzed approaches, are also applicable to highly strained alkanes and offer the highest 3°/2° CH selectivities reported to date in a radical reaction. The article closes with representative experimental protocols for the PTC bromination of cubane as an example of the applicability of this method to strained hydrocarbons and the direct iodination of cyclohexane as well as adamantane as typical alkanes bearing secondary and tertiary CH bonds.
Recent photoemission experiments have discovered a highly monochromatized secondary electron peak emitted from diamondoid self-assembled monolayers on metal substrates. New experimental data and ...simulation results are presented to show that a combination of negative electron affinity and strong electron-phonon scattering is responsible for this behavior. The simulation results are generated using a simple Monte Carlo transport algorithm. The simulated spectra recreate the main spectral features of the measured ones.
Abstract 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 ( P 2 1 / 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.
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