Methylene group, thermodynamic

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

As urea and melamine have a functionality higher than 2, a network is rapidly formed at T > 100°C. As methylene groups are thermodynamically more stable, some rearrangements are also possible at these temperatures (Eq. (2.51)). 

As an example of the group contribution concept, consider the Benson methylene groups shown below, where C represents a tetrahedral carbon, and Cb represents an aromatic carbon, Cd represents a doubly bonded carbon. Each of these values was determined from experimental thermodynamic data. Note that the contribution of the C-(H)2(X)(Y) group to the enthalpy of formation is different in each bonded environment. 

Recently, Agasti et al.158 studied the thermodynamic and kinetic stability of MPCs that differ in the substitution pattern on the first and second carbon next to the sulfur atom (Fig. 4.5). The rate constant of the decomposition of MPCs by potassium cyanide increases in the series iso-thiolate (a methyl group on C2) < nor-thiolate (two methylene groups next to S) < sec-thiolate (a methyl group on Cl). Moreover,... 

The ability of sulfur to stabilize carbanions is exemplified in the following cases. The pentadienyldi-thiocarbamate (144 Scheme 11) can be alkylated at the methylene group to afford the methylated product (145). At 110 C, the dithiocarbamate unit walks its way to the other end of the pentadienyl chain via tandem [3,3] sigmatropic rearrangements. The formation of (14d) is thermodynamically controlled as the more-substituted pentadienyl unit is formed. Dithiocarbamate (146) can be alkylated and eventually transformed into sulfur-free products the sequence giving (148) from (146) is but one of these processes. 

There are two additional features of the hydrocarbon, cyclopropene, which have not been considered and which may contribute to its reduced kinetic and thermodynamic acidity. First of all, the reversed sign (polarity) of the dipole moment of cyclopropene compared to other alkenes, including cyclobutene, and its larger than usual magnitude (0.455 D) , is indicative of significant electron shift from the double bond to the methylene group . This has been shown to be a result of electron transfer from the n c-orbital to the <7 component orbital . Related to this polarity reversal is the observation... 

These findings are in contrast to thermodynamics favoring in the case of naphthalene the formation of conjugated 1,2-isomers. Therefore, kinetic control has to be assumed. The site selectivity for the first as well as the second protonation step has been predicted by HMO theory. One approach refers to local charge densities [13,165] the other one uses localization energies as a reactivity index [166]. In any event, the inductive effect of the methylene group, formed in the first protonation step, has to be taken into account if two sites provide comparable reactivity indices. 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

Tetrahydro-4-oxo-4H-pyrido[l,2-a]pyrimidines contain an active methylene group in position 9, which permits versatile transformations. " Depending on the molar ratio, halogenation of 4-oxo-6,7,8,9-tetra-hydro-4//-pyrido[l,2-fl]pyrimidines gave 9-halo or 9,9-dihalo com-pounds. " 9-Bromo- or 9-chloro-6-methyl-4-oxo-6,7,8,9-tetra-hydro-4H-pyrido[l,2-a]pyrimidine-3-carboxylic acids were obtained as 4 1 mixtures of the thermodynamically more stable 6,9-diaxial and the 6-axial, 9-equatorial-substituted diastereoisomers. ... 

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