Quarternary carbon atoms

Rule D. When a quarternary carbon atom is the alpha position to a secondary or tertiary double-bonded carbon atom, a pinacol type of rearrangement involving migration of a methyl group may occur. Examples of this type of rearrangement include isomerization of... 

In this review the nomenclature and numbering originally given by Newman 1( will be used. As an example the numbering of hexahelicene is given. In the example the quaternary carbon atoms are not numbered, but on addition of a group at these carbon atoms they are numbered as usual in polycondensed aromatic systems. For the x-ray analyses (and also i3C-NMR etc), the quarternary carbon atoms are numbered separately. 

Primary, secondary, tertiary, and quarternary carbon atoms are symbolized by 1°, 2°, 3°, and 4 , respectively. Using this notation, the connectivity symbol 3° (2°) in Table 4.3 indicates the observed carbon k to be tertiary and attached to a secondary carbon l. 

Hydroisomerization proceeds towards thermodynamic equilibrium which is approximately reached between the normal, mono-branched and di-branched structures at high degrees of overall conversion. Hydrocracking, however, is severe under these conditions. It is evident from Table II that monomethyl isomers are primary products the same is apparently true for monoethyl isomers although due to thermodynamic reasons lower concentrations are obtained. Dimethyl isomers including those containing a quarternary carbon atom are formed as secondary products. However, trimethyl isomers are formed very slowly so that their concentrations do not reach equilibrium values. It follows from this that the number of ramifications is deciding as to whether a branched isomer is a primary, secondary or tertiary product in hydroisomerization of n-octane and n-nonane. 

Quarternary carbon atoms of the r-butyl groups at C(4) and C(5). ) a-values also given for 227 K, 246 K, 257 K. Determination of activation parameter for ring inversion by variable-temperature ESR spectroscopy. Assignment of structure confirmed by corresp. radical obtained from 7-D-7-OH cholesterol very similar a-values (9 = 2.0022) were obtained by y-irr. of cholesterol. ... 

Quarternary carbon atoms of neopentyl groups. Protons of neopentyl groups. y-irr. of phenylacetylene. - INDO calculation of coupling constants for 7 different geometries at C(7). Protons of r-butyl group at C(8). ... 

In this case, contrary to the rules of Keulemans, a compound containing a quarternary carbon atom is made in high yield. 

Bearing in mind that the formation of the new carbon-carbon bond is an intramolecular reaction, one is not surprised to notice again that even severe steric hindrance can be overcome here, leading to the highly selective generation of quarternary carbon atoms. 

The synthetic fiexibihty of this intermediate should not be underrated. The configurationally well-defined quarternary carbon atom is an important structural element and its applications are of course not restricted to cyclopentane derivatives. Since the double bond may be split in different ways, various ring-open polyfunctional molecules can easily be reached. 

The thermal oxidative stability of poly(a-methyl styrene) is substantially lower than that of PS as a result of the weak bonds at the quarternary carbon atom which readily decompose under the attack of oxygen at elevated temperatures. 

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