Esters, rotational barriers

Linnartz, P., Bitter, S., Schalley, C.A Deslipping of ester rotaxanes a kooperative interplay of hydrogen bonding with rotational barriers, Eur. J. Org. Chem. (2003), 4819-4829. 

Tautomerism has been investigated in 3,5-dimethyl-2-(2 -pyridyl)pyrrole and 3,5-di-/< /t-butyl-2-(2 -pyridyl)pyrroles in the gas phase, in their alcohol complexes, and as dimers <2004PCP3938>. The compounds exist preferentially in the normal ry -conformation, which in the dimethyl compound is energetically favored over the /f-conformation by 4.3 kcal moP with a rotational barrier of 11.3 kcal mol , and are more stable than their tautomers in the gas phase. Tautomerism is observed in pyrrole-2,5-diacetic acid and its diethyl ester with the pyrrole form being favored for the free acid and the pyrrolidinediylidene tautomer for the diester <2003NJC1353>. 

The pseudo-double bond character of amides is much more pronounced than for esters due to the conjugation of the H-N-C=0 moiety and is correlated to the ability of distorted amides to be hydrolyzed to bases [19]. For this reason, the barrier to interconversion is significantly higher that for the ester series, with AGl typically ranging from 16 to 22 kcal mol-1 [17]. However, the rotational barrier is not solely due to conjugation and also partly arises from the orientation of the nitrogen lone pair which is perpendicular to the amide plane [20]. Therefore, the rates of isomerization are considerably slower than for esters. This means that both isomers can be observed by simple techniques, for example at room temperature by H and 13C NMR spectrometry and UV spectrophotometry [21]. 

VT NMR showed that N3-[3]polynorbomane 164 existed as an equilibrium mixture of the syn-atropisomer 164a and anti-atropisomer 164b (ratio 1 1.7). NMR spectroscopy allowed distinction between the isomers on the basis of symmetry. The syn-isomer 164a exhibited two well-separated ester methyl resonances (8 3.67, 4.05) as predicted for the isomer with Cs-symmetry, whereas the anft -isomer 164b displayed a single ester methyl resonance (8 3.85) in accord with that expected for a compound with C2-symmetry. It was not possible to isolate the separate atropisomers in this system since the energy barriers governing rotation were too low. 

The barriers to rotation of esters deserve mention here, especially in comparison to amide barriers. The H NMR spectra of some nitrites (45) were measured in 1957 (83). The temperature had to be lowered to - 58°C at 30 MHz to see the separate signals of propyl nitrite. The barriers to rotation were ca. 10 kcal/mol. This result may be rationalized by considering the lesser electron-donating ability of the alkoxy relative to the dialkylamino group. The dipolar canonical form (46) of nitrite esters is not as stable as that of nitrosamines. 

Michalik et al. (86) have studied some similar systems (79). In these, the SMe proton resonances appear as symmetrical doublets at ambient temperature, which undergo coalescence in the temperature region of 151 to 180°C, corresponding to barriers to rotation about the C3=C4 bond of 21.6 to 23.3 kcal/mol. In comparable systems (i.e., with Ar = p-ClC6H4), the dicyano compound 79a has a AG of 21.9 kcal/mol the corresponding cyano ester 79b, 22.3 kcal/mol. Thus the cyano group acts as a more efficient acceptor than the ester group, an... 

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... 

In acidic medium, the 1° form should be present in equilibrium with the T+ form because hemi-orthoesters are weak bases. Consequently, hemiortho-esters must be allowed to undergo molecular rotation prior to their breakdown in this medium. Also, there is no evidence so far that molecular rotation can compete with the breakdown of an intermediate in the T+ ionic form. What we know is that the barrier for cleavage should be definitely lower in the T+ than in the T° form (24). At pH higher than 11, hemiortho-esters will exist exclusively in the T ionic form, and it will be seen that in some cases, the energy barrier for the cleavage of T is lower than that of molecular rotation. 

At 60 °C, 50% of the ester groups, trapped in constrained environments with high activation energy barriers, do not performed 7r-flips at a frequency higher than 10 kHz and their motions are limited to restricted rotations (rocking) around the local chain axis with an average amplitude of 7°. 

Torsional potential functions for rotation in side-chain ester groups have been suggested by Yan et al. (1968). A twofold potential function of the form (I/y/2)(l — cosy), with a barrier height of 8-75 kcal mole-1, was assigned to the (C = 0)—00 bond. The value of f7(y) for rotation about the (COO)—C bond was taken as zero for all values ofy. 

To bias the direction the macro cycle takes at each of the transformations, temporary barriers would be required in order to restrict Brownian motion in one particular direction. Such temporary barriers are intrinsically present in [3]catenane 20 (Fig. 8 and Scheme 10). Irradiation at 350 nm of , -20 causes counter-clockwise rotation of the light-blue macrocycle to the succinic amide ester (orange) station to give Z,E-20. The light-blue macrocycle cannot rotate clockwise because the purple macrocycle effectively blocks that route. 

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