Barrier, rotational isomers

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

Compounds of type 111 should, in principle, be resolvable into stable enantiomers if the barrier is high enough. However, these compounds do not carry a functional group convenient for resolution. Oki and associates (162) modified one of the methyls in the isopropyl group to make the rotational isomers dia-stereomeric. They prepared 9-(2-methoxy-l-methylethyl)triptycenes (114) and... 

In the first of four chapters in this volume of Topics in Stereochemistry, Michinori Oki presents a comprehensive review of atropisomerism with special reference to the literature of the past two decades. The review summarizes restricted rotation about sp2-sp2, sp2-sp, and sp3-sp3 bonds and it concludes with an analysis of reactions of isolated rotational isomers. It places particular emphasis on the magnitude of rotation barriers as a function of structure (incidentally identifying some of the largest barriers yet measured to conformer interconversion) and on the isolation of stable single-bond rotational diastereomers. 

Risk labels, lATA/ICAO, 751-3 Risk phrases, 621, 748, 749 River water, peroxide determination, 642 RNA, ozone disinfection, 616 ROS see Reactive oxygen species Rose Bengal sensitized photooxidation, 890 Rotational barriers, regioselective allylic hydroperoxide formation, 836, 847-9 Rotational isomers, peroxynitrous add, 8-9 Rotational spectra, ozonides, 721, 722-3 RP-HPLC, hydrogen peroxide determination, 627... 

N-acyl derivatives of /V-benzoylpyrrolidines is believed (68JOC3627) to be a factor in the better adaptability of the molecule to biological processes when employed as a drug. In the thiocarbonyl derivative [in which R = l-(2-OMe-naphthyl)] the barrier around the R—C(S) bond seems exceptionally high, allowing chemical separation (76TL4573) of two rotational isomers below room temperature. 

This observation is used in practice when NMR spectra give poor results because of slow rotation about bonds. Amides of many kinds, particularly carbamates, show slow rotation about the C-N bond at room temperature because of the amide delocalization. These amides have bigger barriers to rotation than the 70 kj mol-1 of the example in the table. The result is a poor spectrum with broad signals. In this example, the two sides of the five-membered ring are different in the two rotational isomers and give different spectra. 

The kinetics of the rotation about the C—N bond in hindered systems have been studied by Mannschreck and Klle. AG values evaluated from the H-NMR methyl signals (see Table 8) indicate that the rate of rotation in enamines 31-35 depends upon the electron-attracting properties of the substituents. In 36 and 37 the benzo ring reduces the rotational barriers by 9.7-12.5 kj mol as compared to 38 and 39. A formyl group raises the free energies of activation by 13.3-17.9 kJ mol , as can be seen in the table by comparing 36 to 37,38 to 39 and 39 to 40. If one of the N-methyl groups is replaced by another substituent, rotational isomers are possible. It will be difficult to separate such isomers in systems like 40 or 41, since the AG values for the isomerization are expected to be only 84.4 or 89.9 kJ mol ... 

S This barrier is between rotational isomers which are mirror-image isomers and cannot be measured by the nmr method. 

The hindered rotation about the C—C bond in ethane derivatives is a periodic function of the vicinal angle specified to describe the rotation. The symmetry of the periodic potential function is dependent upon the symmetry of the substitution in the ethane derivative. In most ethane derivatives the potential energy barriers to rotation are low (2-6 kcal mole-1) and rates of inter conversion between rotational isomers are too... 

This energy difference is not, however, the rotational energy barrier between the rotational isomers. The latter can be estimated from the rate of interconversion between the isomers as a function of temperature. That rate of interconversion is roughly 4 x I02 s l at —30°C. At —60°C, as estimated from the line width at that temperature [13.19], it is roughly 1/3 of that value, or 1.3 x 102s-1, Assuming that the rate of interconversion satisfies an Arrhenius type of behavior, lc oc e F-3 RT, where E is the rotational energy barrier,... 

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... 

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