Sp3 Centers Research Articles

Theoretical Study of Structure and Raman Spectra for Models of Carbon Nanotubes in Their Pristine and Oxidized Forms

Changes upon oxidization of carbon nanotubes (CNTs) in structure and Raman spectra are investigated using geometry optimizations and frequency calculations with density functional theory and ONIOM(MO:MO) methods. Macrocycles consisting of six and 12 fused C6 hexagon units, C24H12 and C48H24 or [6]- and [12]-cyclacene, respectively, were chosen as model systems for zigzag single wall carbon nanotubes. A vertically extended belt model with three fused [12]-cyclacenes, C96H24, was also studied as well. The oxidation was modeled by 1,2- or 1,4-cycloaddition poducts of two oxygen atoms at the furthest ends of each macrocycle; 1,4-adducts are found to be much more stable than 1,2-adducts and are possibly the actual oxidation products. It is shown for all these cases that, when oxygen atoms react, the macrocycle undergoes large deformation from the totally symmetric cyclic structure toward an oval shape. This is due to the introduction of sp3 centers on the ring and subsequent relaxation of the adjacent C6 hexagon units. Upon oxidation calculated Raman spectra show large reduction in peak intensities, which can be attributed to the loss of cylindrical symmetry due to structural deformation. The study provides a novel explanation for the experimental observation that Raman spectra of individual CNTs are highly sensitive to oxidization.

Structures and C–H bond energies of hydrogenated polycyclic aromatic hydrocarbons

Density functional calculations are employed to calculate C–H bond dissociation energies of sp3 centers at different locations of a variety of polycyclic aromatic hydrocarbons (PAHs). The computed bond dissociation energies De range from less than 80 kJ mol−1 for a hydrogen atom bound to the basal plane of a PAH layer up to 300 kJ mol−1 for a CH–H bond on a zigzag border position. Besides this not unexpected dependence on the specific process, we observe a pronounced scatter of dissociation energies with the size and shape of PAHs, which reflects the complex chemistry for graphitic carbon reactions. On the basis of the present treatment, we determine reasonable estimates for the hydrogenation energies of extended graphite sheets, which have been considered as important in the modeling of carbon deposition by pyrolysis of light hydrocarbons.

Exploratory Studies on Reactions of Cobaloxime π-Cations with C-Nucleophiles: Irreversible Alkene Decomplexation versus Nucleophilic Capture

(β-Hydroxyalkyl)- and (β-acetoxyalkyl)cobaloximes can undergo facile acid-catalyzed β-heteroatom exchange with oxygen and nitrogen nucleophiles in an SN1-like mechanism via cationic metal−alkene π-complex intermediates (cobaloxime π-cations). The reaction of cobaloxime π-cations with carbon nucleophiles has not been previously reported. The results reported in this paper demonstrate that cobaloxime π-cations are reasonably good electrophiles. They are sufficiently reactive to add to the electron-rich sp2 centers in allyltrimethylsilane and pyrrole. A major side reaction for intermolecular reactions is irreversible alkene decomplexation. An intramolecular pyrrole cyclization (22 to 23, 83% yield) significantly raised the yield for nucleophilic addition compared to that of the analogous intermolecular couplings (12b to 13b, 32% yield and 17 to 18, 29% yield). The results of these studies provide a foundation for the design and evaluation of modified cobalt ligands with the goal of suppressing alkene decomplexation and enhancing reaction of cobaloxime π-cations with C-nucleophiles.

Regio‐ and Enantioselective Silane‐Terminated Intramolecular Heck Reactions

The selective formation of tertiary sp3 centers is possible when allylsilanes 1 are employed as the alkene component in the title reaction. Application of a chiral catalyst system makes the reaction enantioselective. Primarily hetero- and carbo-cycles 2 arise with the catalyst system [Pd2(dba)3/PPh3/Ag2O]; 3 is the main product with Pd(OAc2)/PPh3/NPr4Br/KOAc.

ChemInform Abstract: The Nickel Promoted 1,3‐Migration of an sp2 Center: Ring Expansion of a Vinylcyclobutene.

AbstractDas aus (I) und (II) in literaturbekannter Weise zugängliche Vinylcyclobutan‐Derivat (III) wird durch Nickeltetracarbonyl (IV) über die isolierbare Zwischenstufe (V) in das Cyclohexadien (VI) übergeführt.

The nickel-promoted 1,3-migration of an sp2 center; ring expansion of a vinylcyclobutene

The nickel-promoted 1,3-migration of an sp2 center; ring expansion of a vinylcyclobutene

The stereochemistry of spiropiperidine cyclizations (histrionicotoxin, part I)

The spirocyclization of straingt-chain triketo-intermediates, similar to possible biogenetic precursors of histrionicotoxin, is shown to be stereoselective, generating the non-natural configuration at C-2. Aiming at an sp 2-center in this position, triketo-brassylic ester was treated with ammonia at room temperature and was shown to cleanly and efficiently yield a spiroketone.

5‐Methylenebicyclo[2.2.0]hex‐2‐ene, a Bicyclic Isomer of Toluene: Synthesis and Thermal Behavior

A compound having three sp2 centers in the bicyclo[2.2.0]hexane skeleton, viz. hydrocarbon (1), has been synthesized as shown (R = CO2Me → CH2OH → CH2OTs). Thermolysis transforms (1) into 5-methylene-1,3-cyclohexadiene, a model system for polymerization studies.