Methyl group, thermodynamic

While noting that early cases [33-35] are available, we feature Nagano s 4 [36] which displays a Na" "-induced FE factor of 5.3 in water. This is a virtually spaced PET system [37] aided by the steric effect of the strategically placed methyl group. Thermodynamic arguments and quantum calculations were offered to support PET in this case. 

The molecular features of covalent hydration are also present in the dihydroxy series, i.e., in pteridine-2,6-dione (30) and in pteridine-4,6-dione. The latter compound is hydrated only at the C(7)—N(8) double bond, whereas (30) forms two hydrated species, 7-hydroxy-7,8-dihydro- (29) and 4-hydroxy-3,4-dihydro-pteridin-2,6-dione (31) (equation 8). Structure (29) is thermodynamically the more stable substance (31) is formed more rapidly in solution but disappears slowly with time (63JCS5151). Insertion of a 4-methyl group greatly reduces the extent of 3,4- in favour of 7,8-hydration by a blocking effect . 

Although the thermodynamic aspects of acylotropy are well documented, there have been few kinetic studies of the process. The activation barrier is much higher than for prototropy and only Castells et al. (72CC709) have succeeded in observing a coalescence phenomenon in H NMR spectra. At 215 °C in 1-chloronaphthalene the methyl groups of N-phenyl-3,5-dimethylpyrazole-l-carboxamide coalesce. The mechanism of dissociation-combination explains the reversible evolution of the spectra (Scheme 9). 

In theory two carbanions, (189) and (190), can be formed by deprotonation of 3,5-dimethylisoxazole with a strong base. On the basis of MINDO/2 calculations for these two carbanions, the heat of formation of (189) is calculated to be about 33 kJ moF smaller than that of (190), and the carbanion (189) is thermodynamically more stable than the carbanion (190). The calculation is supported by the deuterium exchange reaction of 3,5-dimethylisoxazole with sodium methoxide in deuterated methanol. The rate of deuterium exchange of the 5-methyl protons is about 280 times faster than that of the 3-methyl protons (AAF = 13.0 kJ moF at room temperature) and its activation energy is about 121 kJ moF These results indicate that the methyl groups of 3,5-dimethylisoxazole are much less reactive than the methyl group of 2-methylpyridine and 2-methylquinoline, whose activation energies under the same reaction conditions were reported to be 105 and 88 kJ moF respectively (79H(12)1343). 

CH3I should approach the enolate from the direction that simultaneously allows its optimum overlap with the electron-donor orbital on the enolate (this is the highest-occupied molecular orbital or HOMO), and minimizes its steric repulsion with the enolate. Examine the HOMO of enolate A. Is it more heavily concentrated on the same side of the six-membered ring as the bridgehead methyl group, on the opposite side, or is it equally concentrated on the two sides A map of the HOMO on the electron density surface (a HOMO map ) provides a clearer indication, as this also provides a measure of steric requirements. Identify the direction of attack that maximizes orbital overlap and minimizes steric repulsion, and predict the major product of each reaction. Do your predictions agree with the thermodynamic preferences Repeat your analysis for enolate B, leading to product B1 nd product B2. 

The reactivity of the methyl group in 4-methylcinnoline ethiodide indicates that the structure of this compound is 5, and this evidence has also been interpreted to mean that N-1 is the basic group in cinnolines. However, evidence of this type is only indicative since the formation of quaternary salts is subject to kinetic control, whereas protonation yields predominantly the thermodynamically more stable cation. The quinazoline cation has been shown to exist in the hydrated, resonance-stabilized form 6 7 by ultraviolet spectro-... 

Dihydroxypteridine was expected to undergo hydration but, a priori, it was difficult to decide whether covalent hydration would occur across the 3,4- or the 7,8-position, or both. Kinetic and spectroscopic evidence now indicate that addition of water occurs much more rapidly across the 3,4-positions (and, hence, that the energy of activation must be less for this site), but the 7,8-water-adduct is thermodynamically the more stable. With time, the concentration of the species hydrated in the 3,4-position reaches a maximum (about 64% of the total concentration). Thereafter, it falls steadily and the concentration of the 7,8-adduct rises until, at equilibrium, the latter accounts for 92% of the total and the 3,4-adduct for only 7.6%. In 2,6-dihydroxy-4-methylpteridine, the methyl group drastically reduces the extent of water addition to the 3,4-position but does not significantly affect 7,8-addition, so that, spectroscopically, only a first-order conversion of anhydrous molecule into the 7,8-water-adduct is observed. ... 

The Diels-Alder adduct 8 formed in a mixture with 7 by treatment of porphyrin 6 with dimethyl acetylenedicarboxylate undergoes a base-induced migration of a C —C double bond yielding a single diastereomer 9 with the thermodynamically favored tram arrangement of the methyl group and the methoxycarbonyl substituent.201... 

About twenty-five molecules (all containing a methyl group) have been studied by the doublet splitting method. The barrier values obtained are given in Table I, along with values obtained by the thermodynamic method where available. In several of these molecules, independent barrier values were obtained from a number of spectral lines for each of several isotopic species, with agreement to 5 per cent or better. While some systematic error can still be present (for example, from an error in the stucture ), the... 

Table A4.1 summarizes the equations needed to calculate the contributions to the thermodynamic functions of an ideal gas arising from the various degrees of freedom, including translation, rotation, and vibration (see Section 10.7). For most monatomic gases, only the translational contribution is used. For molecules, the contributions from rotations and vibrations must be included. If unpaired electrons are present in either the atomic or molecular species, so that degenerate electronic energy levels occur, electronic contributions may also be significant see Example 10.2. In molecules where internal rotation is present, such as those containing a methyl group, the internal rotation contribution replaces a vibrational contribution. The internal rotation contributions to the thermodynamic properties are summarized in Table A4.6.

Russian workers have studied secondary cyclic phosphites (113) and (114). Unfortunately, the situation is far from clear since m-(115) iso-nierizes to trans- 16) on standing, " but (117) is suggested to be thermodynamically more stable than (118). This apparent contradiction could be explained by either an incorrect assignment of isomers or a 1,3-interaction of the 4-methyl group in (114). 

Many molecules are composed of functional groups (hat can rotate with respect to the rest of the molecule. The classical example is ethane, as the possibility of rotation of one methyl group against the other was recognized long ego. Because the torsional mode does not result in infrared activity, its frequency was estimated from thermodynamic data. 

In an effort to further support the proposed mechanisms for the Y+propene reaction, we have examined the reactions of Y with four isomeric butenes, which are essentially propene molecules with one additional methyl group (Fig. 31). Based on estimated potential energy barrier heights22 and thermodynamics (Fig. 32))22-31-34,i56,i57 q js eXpected that analogous product channels to those observed for propene should be seen for the butenes. Therefore, a comparison of reactions of butene isomers to reactions with propene should allow us to further test the validity of the proposed mechanisms. Here we briefly summarize our most notable conclusions from this work. 

This explains why 2 methyl butadiene reacts with maleic anhydride (the dienophile) at 30°C in the medium of dioxane with twice the speed as compared with butadiene. But under the same conditions butadiene reacts 10 times faster as compared with 2-chlorobutadiene which has an electron withdrawing group. Thermodynamically unstable compounds are the most reactive dienes. 

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