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When a solution of a carbonyl compound in alcohol (primary or secondary) is heated to ca. 300 degrees C, a disproportionation reaction, in which a carbonyl compound is reduced to the corresponding alcohol and the alcohol is oxidized to the corresponding ketone, takes place. This uncatalyzed variation of the Meerwein-Ponndorf-Oppenauer-Verley reaction gives, in certain cases, e.g., reduction of acetophenone or benzaldehyde by i-PrOH, almost quantitative yields. Yields are higher with secondary alcohols such as i-PrOH than with a primary alcohol such as EtOH. The reactions were also performed in a flow system by passing at a slow rate the same solutions through a glass or a metal coil heated to elevated temperatures. Ab initio calculations performed at the B3LYP/6-31G* level show that thermodynamically i-PrOH is a more potent reducing agent than EtOH by ca. 4 kcal/mol. The computations also show that in cases of aromatic carbonyl compounds, part of the deriving force is obtained from the entropy change of the reaction. The major contributor to the high yield, however, is the excess alcohol used, which shifts the equilibrium to the right. Calculated entropy of activation as well as isotopic H/D labeling suggest a cyclic transition state.

This paper reports computational data for the energetics of internal attacks, both in ring-opening reactions (eq 3) where strain energy is released and in model, strain-free systems (eq 4). A comparison is drawn with the corresponding bimolecular processes. The exothermicity of three-membered ring-opening reactions is significantly larger than that of the four-membered ring systems. However, using the Marcus equation, it is shown that the higher reactivity of the three-membered rings is intrinsic to the system and does not stem only from a higher thermodynamic driving force. The intrinsic barriers for the strain-free reactions are shown to be dominated by the position of the nucleophilic and nucleofugic atoms in the periodic table, as in the bimolecular SN2 reactions, although a pi rather than a sigma bond is formed in these reactions.

Carbanion 1, obtained by a nucleophilic attack of PhSe- on 3-chlorobicyclobutane-carbonitrile in DME undergoes both protonation and elimination as shown in eq 1. Alcohols of increasing acidity in the following order: t-BuOH, i-PrOH, MeOH, trifluoroethanol (TFE), and hexafluoro2-propanol (HFIP) were used as proton donors. An Eigen-type plot of the log of the product ratio (protonation/elimination) vs the pK(a) of the alcohols, levels off for the two most acidic alcohols, TFE and HFIP which react at a diffusion-controlled rate. The partitioning of the products between protonation and elimination enables, therefore, the determination of the rate constant for the internal elimination as approximately 3 x 10(10) s(-1). Ab initio calculations at the B3LYP/6-31G level show that the elimination from a model carbanion (4, eq 4) occurs in a barrierless process. Simulation of the experimental reaction by including solvation effects using the Onsager model, shows that using the dielectric constant of DME (7.2) stabilizes, as expected, the carbanion and prevents a spontaneous elimination. In the absence of solvation effects, using Me- as a base, a complete elimination of HCl (proton removal and leaving-group expulsion) took place from 3-chlorocyclobutanecarbonitrile in a barrierless process without the formation of any discrete intermediate.

In small rings, strain energy is believed to be dominated by bond angles (Baeyer strain). However, it was found (Dill et al.) that, in bicyclobutane, bridgehead substitution effects on strain energy range from +10 to -30 kcal. Ab initio computations at the 6-31G and 6-31+G* levels have shown that the deformation caused by the bridgehead substitution is minimal and so are its energetic consequences.Key words: bicyclobutane, strain energy, geometry distortion.

Ab initio methods were used to calculate the geometry and the charge distribution (natural bond orbital) in end-protonated polyynes. The geometry obtained is practically identical to that of the corresponding anion and the neutral radical. Thus, the geometry is not much dependent on charge dispersal. Moreover, it is shown that regardless of whether the imposed geometry is that of a cumulenic structure which localizes the charge at one end or that of the neutral molecule which localizes the charge at the other end, the same amount of charge is delocalized to the remote end of the protonated molecule regardless of the imposed structure. The same phenomenon is observed also for polyenes. It is interesting to note that regardless of the charge or its absence, as in the case of the radical, the optimal geometry is obtained as the arithmetic sum of the main resonance structures. Thus, it is concluded that, in these cases, the wave function is only weakly coupled to the geometry of the molecule.

When a homogenous electric field is applied to polyynes (C10 and C20) perpendicular to their long axis, they bend to form an arch. The height of the arch is proportional to the intensity of the electric field. The direction of the bend and its magnitude depend on the electronic nature (donor/acceptor) of the substituents at the termini of the polyyne. The driving force for the formation of the arch is the dipole moment produced in the system parallel to the electric field. This dipole moment stems from the substituents and from additional polarization by the field. The bend of the linear polyyne fits a parabolic distortion. According to mechanical engineering analysis, this results from a moment that operates at the two end zones of the polyynes, in accordance with the natural bond order (NBO) charge distribution. It is shown that solutions relevant to beam deflection due to a central load or a uniformly distributed load are not satisfactory. Various parameters, such as the dipole moment and the height of the arch, are better correlated with σ than with σ+ or σ−. Application of the electric field to more complex systems enables the sculpting of interesting nanoshapes.

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