Functionalization of homodiamantane: oxygen insertion reactions without rearrangement with dimethyldioxirane

Homodiamantane bromination and nitroxylation are accompanied by contraction of the seven-membered ring to give the corresponding substituted 1-diamantylmethyl derivatives. In contrast, CH-bond hydroxylations with dimethyldioxirane retain the cage and give both apically and medially substituted homodiamantanes. The product ratios are in accord with the barriers for the oxygen insertion computed with density functional theory methods only if solvation is included through a polarizable continuum model. B3LYP-D3 and M06-2X computations with a 6-31G(d,p) basis set on the oligomeric van der Waals complexes predict the potential of homodiamantane derivatives for surface modifications with conformationally slightly flexible diamondoid homologues.

Stable alkanes containing very long carbon-carbon bonds

The metal-induced coupling of tertiary diamondoid bromides gave highly sterically congested hydrocarbon (hetero)dimers with exceptionally long central C-C bonds of up to 1.71 Å in 2-(1-diamantyl)[121]tetramantane. Yet, these dimers are thermally very stable even at temperatures above 200 °C, which is not in line with common C-C bond length versus bond strengths correlations. We suggest that the extraordinary stabilization arises from numerous intramolecular van der Waals attractions between the neighboring H-terminated diamond-like surfaces. The C-C bond rotational dynamics of 1-(1-adamantyl)diamantane, 1-(1-diamantyl)diamantane, 2-(1-adamantyl)triamantane, 2-(1-diamantyl)triamantane, and 2-(1-diamantyl)[121]tetramantane were studied through variable-temperature (1)H- and (13)C NMR spectroscopies. The shapes of the inward (endo) CH surfaces determine the dynamic behavior, changing the central C-C bond rotation barriers from 7 to 33 kcal mol(-1). We probe the ability of popular density functional theory (DFT) approaches (including BLYP, B3LYP, B98, B3LYP-Dn, B97D, B3PW91, BHandHLYP, B3P86, PBE1PBE, wB97XD, and M06-2X) with 6-31G(d,p) and cc-pVDZ basis sets to describe such an unusual bonding situation. Only functionals accounting for dispersion are able to reproduce the experimental geometries, while most DFT functionals are able to reproduce the experimental rotational barriers due to error cancellations. Computations on larger diamondoids reveal that the interplay between the shapes and the sizes of the CH surfaces may even allow the preparation of open-shell alkyl radical dimers (and possibly polymers) that are strongly held together exclusively by dispersion forces.

Overcoming lability of extremely long alkane carbon-carbon bonds through dispersion forces

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.

Synthesis of the antimalarial drug FR900098 utilizing the nitroso-ene reaction

The antimalarial drug FR900098 was prepared from diethyl allylphosphonate involving the nitroso-ene reaction with nitrosocarbonyl methane as the key step followed by hydrogenation and dealkylation. The utilization of dibenzyl allylphosphonate as the starting compound allows one-step hydrogenation with dealkylation, which simplifies the preparative scheme further.

Functionalized Nanodiamonds Part I. An Experimental Assessment of Diamantane and Computational Predictions for Higher Diamondoids

The structures, strain energies, and enthalpies of formation of diamantane 1, triamantane 2, isomeric tetramantanes 3-5, T(d)-pentamantane 6, and D(3d)-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 C-H 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)(3)C(*) (Hal=halogen) lead to mixtures of all possible tertiary and secondary halodiamantanes, uncharged electrophiles (dimethyldioxirane, m-chloroperbenzoic acid, and CrO(2)Cl(2)) give much higher tertiary versus secondary selectivities. Medial bridgehead substitution dominates in the reactions with strong electrophiles (Br(2), 100 % HNO(3)), whereas with strong single-electron transfer (SET) acceptors (photoexcited 1,2,4,5-tetracyanobenzene) apical C(4)-H 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.

H-coupled electron transfer in alkane C-h activations with halogen electrophiles

The mechanisms for the reactions of isobutane and adamantane with polyhalogen electrophiles (HHal(2)(+), Hal(3)(+), Hal(5)(+), and Hal(7)(+), Hal = Cl, Br, or I) were studied computationally at the MP2 and B3LYP levels of theory with the 6-31G (C, H, Cl, Br) and 3-21G (I) basis sets, as well as experimentally for adamantane halogenations in Br(2), Br(2)/HBr, and I(+)Cl(-)/CCl(4). The transition structures for the activation step display almost linear C...H...Hal interactions and are characterized by significant charge transfer to the electrophile; the hydrocarbon moieties resemble the respective radical cation structures. The regiospecificities for polar halogenations of the 3-degree C-H bonds of adamantane, the high experimental kinetic isotope effects (k(H)/k(D) = 3-4), the rate accelerations in the presence of Lewis and proton (HBr) acids, and the high kinetic orders for halogen (7.5 for Br(2)) can only be understood in terms of an H-coupled electron-transfer mechanism. The three centered-two electron (3c-2e) electrophilic mechanistic concept based on the attack of the electrophile on a C-H bond does not apply; electrophilic 3c-2e interactions dominate the C-H activations only with nonoxidizing electrophiles such as carbocations. This was shown by a comparative computational analysis of the electrophilic and H-coupled electron-transfer activation mechanisms for the isobutane reaction with an ambident electrophile, the allyl cation, at the above levels of theory.

Selective radical reactions in multiphase systems: phase-transfer halogenations of alkanes

The present paper shows that selective radical reactions can be initiated and carried out in multiphase systems. This concept is applied to the selective functionalization of unactivated aliphatic hydrocarbons, which may be linear, branched, and (poly)cyclic, strained as well as unstrained. The phase-transfer system avoids overfunctionalization of the products and simplifies the workup; the selectivities are excellent and the yields are good. This is the only method for direct preparative iodination of alkanes applicable to large scale as well. We demonstrate that the reaction systems are indeed phase-transfer catalyzed through a systematic study of variations of the reactants, solvents, catalysts, and by measuring as well as computing the H/D kinetic isotope effects for the rate-limiting C-H abstraction step by *CHal3 radicals which are held responsible for the observed radical reactions. In the case of *CBr3, this key intermediate could also be trapped under otherwise very similar reaction conditions. To stimulate further work, the tolerance of some functional groups was tested as well.

Molecule-induced alkane homolysis with dioxiranes

The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2.

The rearrangement of the cubane radical cation in solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.

Halogenation of cubane under phase-transfer conditions: single and double C-H-bond substitution with conservation of the cage structure

The first highly selective C-H chlorination, bromination, and iodination of cubane (1) utilizing polyhalomethanes as halogen sources under phase-transfer (PT) conditions is described. Isomeric dihalocubanes with all possible combinations of chlorine, bromine, and iodine in ortho, meta, and para positions were also prepared by this method; m-dihalo products form preferentially. Ab initio and density functional theory (DFT) computations were used to rationalize the pronounced differences in the reactions of 1 with halogen (Hal(*)) vs carbon-centered trihalomethyl (Hal(3)C(*)) radicals (Hal = Cl, Br). For Hal(3)C radicals the C-H abstraction pathway is less unfavorable (DeltaG(double dagger)(298) = 21.6 kcal/mol for Cl(3)C(*) and 19.4 kcal/mol for Br(3)C(*) at B3LYP/6-311+G//B3LYP/6-31G) than the fragmentation of the cubane skeleton via S(H)2-attack on one of the carbon atoms of 1 (DeltaG(double dagger)(298) = 33.8 and 35.1 kcal/mol, respectively). In stark contrast, the reaction of 1 with halogen atoms preferentially follows the fragmentation pathway (DeltaG(double dagger)(298) = 2.1 and 7.5 kcal/mol) and C-H abstraction is more unfavorable (DeltaG(double dagger)(298) = 4.6 and 12.0 kcal/mol). Our computational results nicely agree with the behavior of 1 under PT halogenation conditions (where Hal(3)C(*) is involved in the activation step) and under free-radical photohalogenation with Hal(2) (Della, E. W., et al. J. Am. Chem. Soc. 1992, 114, 10730). The incorporation of a second halogen atom preferentially in the meta position of halocubanes demonstrates the control of the regioselectivity by molecular orbital symmetry.