Aldehydes rotational barriers

A careful analysis of the NMR line shape provides the BH2 + /BH + ratio. Because of a large difference in activation energies between the rotation barriers in the mono-and diprotonated aldehyde, the observed rates are very sensitive to the concentration of BH+. Thus ionization ratios of the order of 10-4 could be measured and approximately 4 log units of Hq could be covered with the same indicator.45... 

Thioamides have become a ubiquitous but important class of compounds - much like ordinary organic compounds such as aldehydes, ketones, and carboxylic acid derivatives - and extensive studies into their syntheses and reactions have been carried out in recent years. When an oxygen atom in an amide is replaced with a sulfur atom, the compounds become less polar and more soluble in various organic solvents. They are reactive but still stable enough to be handled in air. Therefore, a tremendous number of studies have been made into their fundamental properties, biological aspects, and broad applicabilities. Theoretical studies into the rotational barriers that exist in the thioformamides 4 and the thioacetamides 5 have also been made (for example [1]). 

A number of aldehydes have been studied by NMR and found to have analogous rotameric compositions. Only when the substituent is exceptionally stericaliy demanding, as in (CH3)3CCH2CHO, does the hydrogen-eclipsed conformation become more stable. The barrier heights are somewhat smaller than for the analogous 1-alkenes. For acetaldehyde, the rotational barrier is 1.1 kcal/mol, versus 2.0 kcal/mol for propene. ... 

Another study of an active antineoplastic agent, 2-formyl-4-morpholino-pyridine thiosemicarbazone (107), revealed that the conformation of the molecule is similar to that observed with other related thiosemicarbazone derivatives [51]. Comparison of these molecular structures indicates that the rotational barrier about the 2-carbon of the pyridine ring and the aldehydic carbon, which is influenced by the dihedral angle of this bond, may be important for the antineoplastic activity of this class of agents. 

Zhu, L., and J.W. BozzeUi (2002), Structures, rotational barriers, and thermochemical properties of chlorinated aldehydes and corresponding acetyl (CC =0) and formyl methyl radicals (C C=0) and additivity groups, J. Phys. Chem. A, 106, 345-355. 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

Comparing the barrier to rotation of the aldehyde (68, X = H) with that of the corresponding methyl compound (54), one sees that the barrier in the former is diminished to some extent. This phenomenon may be attributed to the small size of the CHO group relative to methyl, which causes diminution of the interaction in the transition state for rotation. 

Ri = R2 = Me), only the ZZ form could be observed, but with R, = H,R2 = Me, a 2 1 ratio of EE and ZZ forms was found. Signal broadenings without observable splitting in the temperature range of - 39 to - 64°C indicated equilibria with low concentrations of the ZE and EZ forms, respectively, and with barriers to aldehyde group rotation (major - minor) in the range of 10 to 12 kcal/mol. 

Electron dilfraction (64TL705) and microwave spectroscopy (65JCP647) of cyclopropanecarboxaldehyde exhibit a twofold barrier for internal rotation, and the two conformers possess almost the same energy content in the gas phase. Ab initio MO calculations (83JST(104)1 IS) in the extended b-BlG basis set predict the s-cis form to be more stable than the s-trans, with the transition state located at 0 a 100°. A twofold barrier was also found (65JCP3043) by electron diffraction for cyclopropyl methyl ketone, with the s-trans isomer destabilized by steric interactions with respect to the aldehyde. 

Substituted pyrrole aldehydes, ketones and esters have been studied in a similar fashion to the 2-substituted derivatives. The barrier to rotation about the ring to carbonyl group... 

Hindered rotation occurs on the NMR time scale for numerous other systems with partial double bonds, including carbamates, thioamides, enamines, nitrosamines, alkyl nitrites, diazoketones, aminoboranes, and aromatic aldehydes. Formal double bonds can exhibit free rotation when alternative resonance structures suggest partial single bonding. The calicene 5-5, for example, has a barrier to rotation about the central bond of only 20 kcal mol . ... 

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