Propellanes. Part LXXXI. Why are tetrakis[organoboranediylbis(oxy)]cyclobutanes formed without a trace of the isomeric tetrakis-dioxabora[3.3.2]propellanes?

MNDO and STO-3G calculations rationalize the relative instability of the title propellanes vis-à-vis the title products that are formed exclusively.     

Zement

Ohne Zusammenfassung     

Flat corannulene: when a transition state becomes a stable molecule

Flat corannulene has been considered so far only as a transition state of the bowl-to-bowl inversion process. This study was driven by the prediction that substituents with strong steric repulsion could destabilize the bowl-shaped conformation of this molecule to such an extent that the highly unstable planar geometry would become an isolable molecule. To examine the substituents'' effect on the corannulene bowl depth, optimized structures for the highly-congested decakis(t-butylsulfido)corannulene were calculated. The computations, performed with both the M06-2X/def2-TZVP and the B3LYP/def2-TZVP methods (the latter with and without Grimme''s D3 dispersion correction), predict that this molecule can achieve two minimum structures: a flat carbon framework and a bowl-shaped structure, which are very close in energy. This rather unusual compound was easily synthesized from decachlorocorannulene under mild reaction conditions, and X-ray crystallographic studies gave similar results to the theoretical predictions. This compound crystallized in two different polymorphs, one exhibiting a completely flat corannulene core and the other having a bowl-shaped conformation.

The first flat metal-free corannulene derivative was predicted by computations and achieved by synthesis.     

Nucleus Independent Chemical Shift (NICS) at Small Distances from the Molecular Plane: The Effect of Electron Density

When close to the molecular plane, the behavior of nucleus independent chemical shift (NICS) as a function of the distance from the molecular plane deviates from its behavior at larger distances. By using a dense grid of NICS-probes (BQs) it is shown that, when close to the molecular plane, maximal (absolute) NICS values are obtained above the atoms. These maxima move towards the center as the grid is elevated until the (absolute) maximum NICS is obtained at the center and stay there when the grid is further elevated. It is shown that this behavior is a result of the current density, which is influenced by the electron density, according to the Biot-Savart law, which, in turn, causes the induced magnetic field measured by the NICS. It is thus concluded that if magnetic aromaticity is studied, the NICS calculations should be carried out at a large enough distance so that only the π-ring current affects the NICS. At distances ≥2 Å, NICS(r)_π,zz=A+B*C^r. Using non-linear correlation for obtaining A, B and C and extrapolate to NICS(1)_π,zz and NICS(1.7)_π,zz is recommended as measures for aromaticity.     

Dramatic acceleration of the Menschutkin reaction and distortion of halide leaving-group order

Compared to structurally related linear trialkylamines, a simple macrocyclic amine with an anion-binding cavity exhibits very large rate enhancements (>10(5)) for stoichiometric N-alkylation with primary alkyl, allyl, and benzyl halides in the weakly polar solvent CDCl3. There is also a major distortion of the halide leaving-group order. For example, with benzyl halides the relative leaving-group order with a control amine is Cl (1) < Br (71) < I (160), whereas the leaving-group order with the macrocyclic amine is I (0.4) < Cl (1) < Br (8.5). Reaction with the macrocyclic amine is inhibited by the addition of DMSO, which is unusual because the Menschutkin reaction is normally enhanced by the presence of a polar aprotic solvent. Competitive inhibition studies indicate that the reaction proceeds through a prereaction complex. Effective molarities for the subsequent unimolecular N-alkylation step with 4-t-butylbenzyl halides are 4-t-BuBnCl (62,000 M) > 4-t-BuBnBr (2200 M) > 4-t-BuBnI (35 M); thus, the free energy of activation is selectively decreased for organohalides having smaller and more charge dense leaving groups. Likely reasons for this selective enhancement effect are: (a) increased transition-state stabilization due to hydrogen bonding in the macrocyclic pocket and (b) reduced entropic penalty in the transition state due to an increased fraction of prereaction complexes that are oriented in a near attack conformation. The study suggests that it should be possible to develop highly reactive macrocyclic amines that selectively sense or scavenge carcinogenic haloalkanes from the atmosphere.     

Double-stranded cycles: toward C84's belt region

The reactivity of the double-stranded hydrocarbon cycle with two ether bridges (1) toward iodotrimethylsilane (TMSI) was investigated in some detail. The carbon skeleton of cycle 1 resembles the belt region of a C84 fullerene which makes it a potential precursor to the long sought after fully aromatic derivative. Upon exposure to TMSI, cycle 1 undergoes a cascade of reactions which involve different states of iodination/reduction which ultimately lead to the hydrogenated cycle 5a, whose structure was proven by single-crystal X-ray analysis. A deeper insight into mechanistic aspects of this sequence of conversions was gained by performing the reaction under dry and wet conditions, whereby the latter involved both normal and deuterated water. With the help of detailed NMR correlation studies and DFT computations, all important aspects were clarified including an unexpected selective H/D exchange at the naphthalenic moieties.     

Can Substituted Cyclopentadiene Become Aromatic or Antiaromatic?

Cyclopentadiene derivatives with electronegative (F, Cl) or electropositive (H(3)Si, Me(3)Si) bis-5,5-substituents were studied at the B3LYP/6-311G* level of theory. It was found that there is no special stabilization or destabilization for any of the derivatives; the energetic effects that were previously attributed to aromatic stabilization or antiaromatic destabilization are the result of interactions in the reference systems. A nucleus-independent chemical shift (NICS) scan study at the HF-GIAO/6-311+G* theoretical level of these and similar derivatives suggest that they all show different magnitudes of diamagnetic ring current. None of the derivatives shows a paramagnetic ring current. Thus, cyclopentadienes are neither aromatic nor antiaromatic. It is also concluded that a diamagnetic ring current is perhaps necessary but certainly not a sufficient condition for aromaticity. The NICS scan procedure describes the type of ring current in the system, whereas a single isotropic NICS value (i.e., NICS(1)) may wrongly assign the type of ring current. It is shown that neither NICS(1) nor the NICS scan procedure can be used as a single aromaticity criterion.     

Crystal structure and rotational barrier of octakis(bromomethyl)naphthalene

Octakis(bromomethyl)naphthalene (4) adopts in the crystal a chiral conformation with a helical central naphthalene core and the bromomethyl groups disposed in an alternate up-down "in" arrangement. According to MM3 calculations, this conformation is less stable than the corresponding all alternated "out" form, while B3LYP/LANL2DZ calculations suggest the opposite stability order. The topomerization barrier (16.0 kcal mol(-1)) is ascribed to an enantiomerization process requiring 180 degrees rotation of all the bromomethyl groups and reversal of the helical sense of the naphthalene core.