1,4,6,9-Tetra-tert-butyl-2,7-dioxa-tricyclo-[6.3.0.0(3,6)]deca-3,8-diene.

The title compound, C(24)H(40)O(2), lies on an inversion center with a half-mol-ecule in the asymmetric unit. The central dioxane ring adopts a chair conformation. The four-membered ring is slightly puckered with a butterfly angle of 13.50 (14)°.

3,5-Dibromo-2,2,6,6,7,7-hexa-methyl-octane-4-one.

In the title mol-ecule, C14H26Br2O, the central carbonyl group (C3O) is essentially planar (r.m.s. deviation = 0.0021 Å). The Br atoms lie on the same side of the mol-ecule and are approximately syn, with a Br-C⋯C-Br torsion angle of -43.52 (13)°. The crystal structure is devoid of any specific inter-molecular inter-actions.

Borenium cations derived from BODIPY dyes.

Fluoride abstraction from a BODIPY dye gives a well defined borenium ion which can be converted to a borenium hydride via treatment with DIBAL-H.

A theoretical study of Favorskii reaction stereochemistry. Lessons in torquoselectivity.

The mechanisms of the chloroenolate-->cyclopropanone step of the "normal" Favorskii rearrangement have been investigated in detail using high-level ab initio calculations. A series of simple alpha-chloroenolates, based on chloroacetone (6), all monomethyl derivatives (7-9), a dimethyl analogue (10), and 1-acetyl-1-chlorocyclohexane (11) was first used to explore and define the basic features of the mechanism, which include the finding of both an "inversion" and a "retention" transition state and that in most cases these arise from separate ground-state conformations of the chloroenolate. These theoretical studies were then extended to an isomeric pair of chloroenolates 1 and 2, the cis- and trans-2-methyl derivatives of 11, which are the reactive intermediates involved in a well-known experimental study carried out by Stork and Borowitz (S-B). Finally, three alpha-chlorocyclohexanone enolate systems 12-14 were studied, since these intermediates have a more restricted enolate geometry. The "inversion" mechanism has been described as an SN2 process but the present results, while supporting a concerted process, is better described as an oxyallyl structure undergoing concerted ring closure. The "retention" mechanism has been described as SN1-like, but the calculations show that this process is also concerted, although much less so, and again involves oxyallyl-like transition-states. The model systems 6-8, 10, and 11 with a potential plane of symmetry have two enantiomeric transition states for inversion and another two for retention of configuration (at the C-Cl center). With 9 and the S-B models 1 and 2, with no symmetry plane, there are a calculated total of four diastereomeric transition states for cyclopropanone ring closure in each case, two for inversion and two for retention. While the transition-state energies calculated for simple chloroenolates favor the inversion process, the S-B models 1 and 2 have almost equal inversion-retention transition-state energies. Solvation simulation calculations of ground states and transition states suggest that the retention mechanism becomes relatively more favored in polar solvents, in agreement with some experimental results. In the chloroenolates 12-14, both inversion and retention mechanisms were also located, these arising from two different ground-state ring conformations of the enolate. In these models, one also finds similar inversion and retention transition-state energies, but again with a small preference for the inversion process.

Gas-phase diastereoselectivity of secondary 5-substituted (X)-adamant-2-yl (X = F, Si(CH(3))(3)) cations.

Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimers as well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for IX, that is, (Z)-5-F-adamant-2-yl (I(Z)F) and (E)-5-Si(CH3)3-adamant-2-yl cations (I(E)Si). The temperature dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX*CH318OH], characterized by a specific C2-H+...O18(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (II(Z)X) and (E)-5-X-adamantan-2-ols (II(E)X; X = F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.

Blue fluorescent 4a-Aza-4b-boraphenanthrenes.

[structure: see text]. Phenanthrene analogues with internalized B-N moieties were found to afford blue light emission with good quantum efficiencies, whereas the isomeric species with peripheral B-N moieties displayed only UV emission behavior, like the all-carbon framework.

Triphenylene analogues with B2N2C2 cores: synthesis, structure, redox behavior, and photophysical properties.

A series of alkyl (1-3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B(2)N(2)C(2) cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (-11.3 ppm) and outer borabenzene rings (-7.7 ppm), along with nonaromatic behavior for the pyridazine ring (-0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution (1, 3), planar structures resulted, but upon substitution, a twisted B(2)N(2)C(2) core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., 5). The increase of steric bulk from H (1) to (i)Pr (3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B...N interactions of 3.39 Angstrom. One-electron reduction by Cp(2)Co was found to afford the radical anions of 3 and 5, which were characterized by broad, featureless singlets in the EPR spectra; [3](.)(-)[Cp(2)Co](+) was characterized by X-ray crystallography. While the planar structures (1-4) were observed to possess weak fluorescence (Phi(F) = 0.02-0.08) with either yellow-orange (ca. 555 nm) or green emission (521 nm), the twisted structures (5, 6) were found to be nonfluorescent.

Preparation and crystal structure of two bicyclo[1.1.0]butan-2-ones: a hybrid oxyallyl-cyclopropanone motif.

The first X-ray crystal structures of two different bicyclo[1.1.0]butanones show a very long transannular bond of 1.69 A and a large carbonyl pyramidal distortion of 0.26 A out of the plane of the three carbons. These features are those expected of molecules with a hybrid cyclopropanone-oxyallyl structure, and these structures differ markedly from those of the parent bicyclo[1.1.0]butane skeleton.

Importance of entropy in the diastereoselectivity of 5-substituted 2-methyladamant-2-yl cations.

The diastereofacial selectivity of 2-methyl-5-X-adamant-2-yl cations IX (X = CN, Cl, Br, CH3O, COOCH3, C6H5, CH3, and (CH3)3Sn) toward methanol has been investigated in the gas phase at 750 Torr and in the 40-120 degrees C temperature range and compared with that of IF (X = F) and ISi (X = (CH3)3Si) measured previously under similar conditions. Detailed analysis of the energy surface of the IMe (X = CH3) ion reveals that the activation barrier of its syn addition to methanol is significantly lower than that of the anti attack. In the 40-100 degrees C range, such a difference is strongly reduced by adverse entropic factors which are large enough to invert the IMe diastereoselectivity from syn to anti at T > 69 degrees C. The behavior of IMe diverges markedly from that of IF and ISi. Large adverse entropic factors account for the predominant syn diastereoselectivity observed in the reaction with IF (X = F), notwithstanding the anti enthalpy barrier is lower than the syn one. Adverse entropy plays a minor role in the reaction with ISi (X = (CH3)3Si) which instead exhibits a preferred anti diastereoselectivity governed by the activation enthalpies. Depending on the electronic properties of X, the kinetic behavior of the other IX ions obeys one of the above models. The gas-phase diastereoselectivity of IX ions responds to a subtle interplay between the sigma-hyperconjugative/electrostatic effects of the X substituent and the activation entropy terms. sigma-Hyperconjugation/field effects determine the pyramidal structure and the relative stability of the syn and anti conformers of IX as well as the relative stability of their addition transition structures and their position along the reaction coordinate. The diastereoselectivity of IX in the gas phase is compared with that measured in solution and with theoretical predictions.

Solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate: mechanistic implications of a record-breaking secondary alpha-deuterium kinetic isotope effect for an SN1 substrate.

The secondary alpha-deuterium kinetic isotope effect (alpha-kie) for the solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate in 97% w/w aqueous 2,2,2-trifluoroethanol (97T) at 25 degrees C has been measured (k(H)/k(D) = 1.33). The alpha-kie is abnormally high compared to the value of 1.23 for the corresponding limiting S(N)1 solvolysis of 2-adamantyl p-bromobenzenesulfonate, which proceeds via an extended ion-pair mechanism. A novel mechanism for the solvolysis of the tin compound is proposed that accommodates not only the high alpha-kie but also the absence of internal return.

Chiroptical Properties of the Ketene and Diazo Chromophores. 1. Conformation and Optical Activity of 1-Alken-1-ones and 1-Diazoalkanes vs Aldehydes.

Ab initio methods have been employed to study the conformational behavior and chiroptical properties of acyclic, structurally related aldehydes, ketenes, and diazoalkanes of the type MeCHRCH=XY. The study involved aldehydes 1, 4, and 7 (XY = O, R = H, Me, Et, correspondingly), ketenes (1-alken-1-ones) 2, 5, and 8 (XY = CO, R = H, Me, Et), and 1-diazoalkanes 3, 6, and 9 (XY = NN, R = H, Me, Et). Geometries were optimized at the B3LYP/6-31G level, stationary points were characterized by vibrational frequency analysis, and final energies of 7-9 were obtained at the B3LYP/6-311+G//B3LYP/6-31G level. The chiroptical properties were calculated by the CIS/6-31+G method. It was found that rotational barriers of the functional group (CHXY) about the CC bond are lowered in the following order: aldehydes (1.5-2.1 kcal mol(-1)) > ketenes (1.3-1.6) > diazoalkanes (0.5-0.7). A conformer with the C=X bond eclipsed by an alpha-hydrogen is the global minimum of the ketenes and diazoalkanes, unlike the aldehydes where eclipsing by an alpha-alkyl group is preferred over a hydrogen. In all three classes of compounds, the optical rotational strength of the n-pi transition is greatest for conformers with the dihedral angle, CCCX, in the range 80-100 degrees. Within this range, the signs of the n-pi rotational strengths for the ketenes and diazoalkanes are opposite to ones for the aldehydes. As a whole, the torsional dependencies of the n-pi rotational strength of the ketene and diazo chromophores can be explained within the framework of a model of "through-bond" perturbation. CD spectra of (S)-alpha-methyl-substituted butanal (7), 1-penten-1-one (8), and 1-diazobutane (9) were obtained experimentally. The calculated chiroptical properties of equilibrium mixtures of conformers of compounds 7-9 are in agreement with experimental data.

New Developments in Superacid Chemistry: Characterization of HC+ =O and FC+ =O Cations.

A high gas pressure and use of the superacid HF/SbF5 (1/1) were required for direct spectroscopic characterization of the HC+ =O cation, which was until recently discussed as the short-lived intermediate of carbonylation reactions. The fluorocarbonyl cation FC+ =O, which was equally difficult to characterize, was also directly observed with NMR spectroscopy as the product of protolytic cleavage of tert-butyl fluoroformate [Eq. (a)].