Syntheses of an isoannelated annulene,a bisdehydro[14]annuleno[c]furan and bisdehydro[14]annulene derivatives

A bisdehydro[14]annuleno[c]furan, an isoannelated diatropic annulene, has been synthesized. Cyclic glycol, a precursor of the annuleno[c]furan, could be converted into bisdehydro[14]annulene derivatives under mild acidic conditions.     

Tunneling and sterically induced ring puckering in a substituted [8]annulene anion radical

Electron paramagnetic resonance (EPR) studies have revealed that the steric interaction between the methyl hydrogens on a tert-butoxy substituent and the cyclooctatetraene (COT) ring system sterically induces a puckering of the eight-membered ring in the anion radical of tert-butoxy-COT. The induced nonplanarity of the COT ring system causes a large attenuation of the EPR coupling constants. Since the C-D bond length is slightly shorter than is the C-H bond length, replacement of the tert-butyl group with a tert-butyl-d(9) group results in less steric interaction and measurably larger electron proton coupling constants. The oscillation between the two close to planar alternating bond length (ABL) D(2d) conformers of the COT moiety was found to be extremely rapid (k > 10(12) s(-1)) and quantum mechanical tunneling is proposed to be involved.     

Spin densities in dialkoxy-[16]annulene anion radicals: dimerization of alkoxy-[8]annulenes

The anion radicals of alkoxy-substituted cyclooctatetraenes in hexamethylphosphoramide spontaneously dimerize to form the dianions of dialkoxy-[16]annulenes. The dianions reveal the expected high-field NMR resonance for the internal protons. After electron transfer, the EPR spectra of the corresponding anion radicals reveal that only the 1,5-dialkoxy systems are formed. Further, the measured proton and (13)C spin densities show that the odd electron resides in a molecular orbital with six hydrogens in "deep" nodal positions that completely hide them from EPR detection. This MO corresponds to the nonbonding (singly occupied) MO of higher energy after splitting of the degenerate nonbonding MOs by the two-electron-withdrawing substituents. The surprising electron-withdrawing nature of the alkoxy substituents is attributed to a rather strong mixing of the sigma and pi systems in [16]annulene.     

C-C bond insertion of a complexed phosphinidene into 1,6-methano[10]annulene

Reaction of electrophilic phosphinidene complex [MePW(CO)5] with 1,6-methano-[10]annulene results in the sole formation of the isomeric C-C insertion products 6 c (main) and 6 d (minor). The single-crystal X-ray structure of the complexed 1,7-methano-3-phospha[11]annulene (6 c) shows a syn-W(CO)5 group at the exo bent phosphorus. The structure displays C-C bond alternation without bonding between the bridgehead carbon atoms. Density functional theory calculations indicate 6 c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate. The endo-anti phosphirane cannot undergo ring expansion, due to the high barrier that is associated with an intramolecular antara-antara retro Diels-Alder reaction. The stabilizing effect of transition-metal coordination is discussed.     

A synthesis of a doubly-bridged [24] annulene

Reductive coupling of 1, 6-bis(2-formylvinyl)cyclohepta-1, 3, 5-triene with a low-valent titanium reagent afforded a paratropic bismethano [24] annulene.     

The Radical Anions of Heptalene and 1, 7-Methano-[12]annulene

The radical anions of heptalene and of 1, 7-methano-[12]annulene are generated by metal reduction and characterized by means of their ESR-spectra. Whereas the neutral hydrocarbons are π-bond localized their corresponding radical anions turn out to be π-bond delocalized. This could be deduced from an interpretation of the different hyperfine splittings using a simple MO-model.     

A simple synthesis of a doubly-bridged [16]annulene

Reductive coupling of cychoheptatriene-1,6-dialdehyde using a low-valent titanium reagent provides a one-step route to a paratropic bismethano [16] annulene.     

Grid inhomogeneous solvation theory: Hydration structure and thermodynamics of the miniature receptor cucurbit[7]uril

The displacement of perturbed water upon binding is believed to play a critical role in the thermodynamics of biomolecular recognition, but it is nontrivial to unambiguously define and answer questions about this process. We address this issue by introducing grid inhomogeneous solvation theory (GIST), which discretizes the equations of inhomogeneous solvation theory (IST) onto a three-dimensional grid situated in the region of interest around a solute molecule or complex. Snapshots from explicit solvent simulations are used to estimate localized solvation entropies, energies, and free energies associated with the grid boxes, or voxels, and properly summing these thermodynamic quantities over voxels yields information about hydration thermodynamics. GIST thus provides a smoothly varying representation of water properties as a function of position, rather than focusing on hydration sites where solvent is present at high density. It therefore accounts for full or partial displacement of water from sites that are highly occupied by water, as well as for partly occupied and water-depleted regions around the solute. GIST can also provide a well-defined estimate of the solvation free energy and therefore enables a rigorous end-states analysis of binding. For example, one may not only use a first GIST calculation to project the thermodynamic consequences of displacing water from the surface of a receptor by a ligand, but also account, in a second GIST calculation, for the thermodynamics of subsequent solvent reorganization around the bound complex. In the present study, a first GIST analysis of the molecular host cucurbit[7]uril is found to yield a rich picture of hydration structure and thermodynamics in and around this miniature receptor. One of the most striking results is the observation of a toroidal region of high water density at the center of the host's nonpolar cavity. Despite its high density, the water in this toroidal region is disfavored energetically and entropically, and hence may contribute to the known ability of this small receptor to bind guest molecules with unusually high affinities. Interestingly, the toroidal region of high water density persists even when all partial charges of the receptor are set to zero. Thus, localized regions of high solvent density can be generated in a binding site without strong, attractive solute-solvent interactions.     

Molecular structure and conformational transitions of dichloroacetylchloride

RHF and MP2 techniques in 6–31G(d) basis set have been used to determine the structure of the isolated molecule CHCl₂COCl in two stable conformations (cis-and gosh-), as well as in transition states arising due to the rotary motion of CHCl₂ group around the C—C bond. The energy gap between the conformers and the relevant potential barriers has been calculated using the obtained potential dependence of the internal rotation. Plausible conformation of dichloroacetylchloride is discussed on the basis of ³⁵Cl NQR.