Phenols rotational barriers

A last example concerns the rotational barrier in phenoxyl radicals (Gilbert et ah, 1988). Compared to the parent phenols [37] and [39] the rotational barrier in [38] is increased by a factor of seven, whereas, with a captor substituent [40], the barrier increases only by a factor of 1.2. This could be interpreted in terms of a captodative stabilization in [38]. The captodative character of the radical [38] is represented by a resonance structure [41]. 

The lowering of the rotation barrier about the C(4)-C(4a) bond when two electron-acceptor nitro groups are introduced into the phenolate moiety is evidently due to the increased contribution of form E to the structure of this... 

As a consequence, phenol itself is planar, and the energy difference between the planar and orthogonal forms represents the barrier to internal rotation about the C—O bond. Substituents that increase conjugation between the OH group and the ring might be expected to increase the rotational barrier and vice versa (8). 

Theoretical rotational barriers for a number of para-substituted phenols have been previously reported (8). We present here results for additional substituents and consider meta substitution as well. To examine both meta and para... 

Effect of Substituents on Rotational Barriers (AE2) and Overlap Populations in Substituted Phenols... 

The effect of para substituents on nitrogen inversion barriers in anilines has been reported by Hehre et al. (9). We have extended those data to include additional substituents, as well as substituents at the meta position. Although the theoretical inversion barrier in aniline (2.72 kcal mole" ) is somewhat higher than the experimental value (1.6 kcal mole" ), the effect of the substituent on the inversion barrier might be more reliable as in the case of the rotational barriers in substituted phenols. The nitrogen inversion barriers, as well as optimized N bond angles, are summarized in Table 11. 

The results are similar to those obtained for rotational barriers in phenols although the barriers do, of course, move in opposite directions. Substituents that enhance conjugation between the NH2 group and the ring stabilize the planar transition state (4) more than the pyramidal ground state (5) leading to a reduced barrier. 

Atroposelective cycloaddition reactions of A-2-(r-butylphenyl)- and A-2,5-(di-r-butylphenyl)-maleimide show good to excellent stereoselectivities and the high rotation barriers prevent cycloadducts from interconverting. The stereospeciflc hetero-Diels-Alder reaction of o-quinone methides (80) with o-quinones (79) in MeOH at room temperature produce the 4a,8-di(hydroxymethyl)chromane derivatives (81) and (82) in high yields (Scheme 29). The intramolecular inverse-electron-demand Diels-Alder reaction of o-quinone methides (84) derived from 2-(l-hydroxy-5-alkenyl)phenol derivatives (83) produces l,2,3,3a,4,9b-hexahydrocyclopenta[c][l]benzopyrans (85) under mild acidic conditions (Scheme 30). The Diels-Alder reactions between dimethyl-cyclohexadiene derivatives and di-(-)-menthyl acetylenedicarboxylate exhibit modest diastereoselectivity. ... 

Rotation about sp2 carbon-heteroatom single bonds is usually a higher energy process compared to rotation about sp3 carbon-heteroatom single bonds due to conjugation and to resulting partial double bond character of the pivot bond. Compare the barrier of methanol (1.07 kcal/mol, Table 7) and ethanol (1.34 kcal/mol) with that of phenol (3.1 kcal/mol) [83]. 

J. Chem. Phys. 103, 584-594 (1995). (b) M. Schmitt, J. Kupper, D. Spangenberg, and A.West-phal, Determination of the structures and barriers to hindered internal rotation of the phenol-methanol cluster in the SO and SI states, Chem. Phys. 254, 349-361 (2000). (c) K. Muller-Dethlefs, Applications of ZEKE spectroscopy, J. Electron Spectrosc. Relat. Phenom. 75, 35-46 (1995). (d) J. Kupper, A. Westphal, and M. Schmitt, The structure of the binary phenol-methanol cluster a comparison of experiment and ab initio theory, Chem. Phys. 263, 41-53... 

Other problems dealt with by the Hehre-Pople method are internal rotation in vinylcyclopropane and vinylcyclobutane 153>, the structure of homoallyl cation 154> and ethylenebenzenium cation 155>, torsional barriers in -substituted phenols 156), inversion barriers in -substituted anilines 157>, the effects of a-substitution in keto-enol tautomerism 158> and the circumambulatory rearrangement in bicyclo [3.1.0] hex-3-en-2-yl cation 159> ... 

The last line in Table I presents decay parameters for a dimer in which all 10 hydroxyl groups have been acetylated. Acetylation reduces Ti and T2, but there is not much change in and a 2. More importantly, acetylation increases the size of the energy barrier that must be surmounted when the dimer undergoes the conformational transition from one rotational isomer to the other. For this reason, the rate of intramolecular conformational change is much slower in the peracetylated dimer than in the free phenol forms it is slow enough so that 400-MHz proton NMR can resolve distinct signals from the two rotational isomers in dioxane-ds solution 10). The... 

The bis(tetiafluoroethyl) ethers obtained by reacting polyethyleneglycol with tetrafluoroethylene have useful surface activity. 1 The preparation of aminoacetals of the type (CF3)2C C(NMes)OAr, by the successive reaction of the sodium salt of a phenol and then of dimethylamine with perfluoroisobutene has been described i in the compound (CFs)2C C(NMea)OMe, the CF3 groups are equivalent on the n.m.r. timescale at low temperatures, indicating a low barrier to rotation about the C=C bond. A number of fluorocarbon epoxides have been prepared... 

An experimentally accessible probe for studying x-electron interactions in substituted phenols is the barrier to rotation about the C—O bond (8). Interaction of the p-type lone pair on oxygen with the x orbitals of the ring in the planar conformation (1) is more effective than interaction of the sp -typc lone pair on oxygen in the orthogonal conformation (2) because of poorer overlap and lower orbital energy in the latter situation. 

One attractive feature of the calixarenes is their ability to host small molecules in their molecular cavities [1]. The systems may in principle exist in a large number of conformations. The conformational preferences of a calix[4]arene are usually discussed in terms of four ideal conformations cone , partial cone , 1,2-altemate and 1,3-altemate . These conformations are interconvertible by rotation of the phenolic rings [1]. The parent /7-r rf-butylcalix[4]arene (1) exists in solution in a cone conformation which undergoes a cone to cone inversion process with a barrier (in CDCI3) of 15.9 kcal mol [2]. For larger calixarenes, the number of possible ideal conformations increases [1,3]. 

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