Rotation and rings

A number of ring systems have been converted into 1,2,4-thiadiazole derivatives. The most common include 5-imino-1,2,4-dithiazolidines, isoxazoles, oxadiazoles, and 5-imino-l,2,3,4-thiatriazolines. In general, a ring-opening reaction is followed by rotation and ring closure, or the heterocyclic ring may act as a masked 1,3-dipole which reacts with a suitable dipolarophile. 

Ab initio calculations on (39) estimate activation energies of amine rotation and ring inversion (39a) and (39b) to be 17.1 and 17.3 kcal mol-1, respectively, which is in good agreement with values derived from NMR experiments (94JA5167). 

The results for the carbon-oxygen attack are summarized schematically in Figure 7.38. The excited-state branch of the reaction path terminates in a conical intersection point at a CO distance of 177 pm before the biradical is fully formed (cf. Figure 7.37a). Thus the system can evolve back to the reactants or produce a transient C,C-biradical intermediate that is isolated by small barriers (< 3 kcal/mol) to fragmentation (TS,) or to rotation and ring closure to oxetane (TS2). The singlet and triplet biradical minima are essentially coincident. 

Kleinpeter and coworkers79 have recently studied restricted rotation and ring inversion in highly substituted anilines, 23-31 (Figure 10), prepared by the reaction of cyclic ylidene malonitriles with acetylene (di)carboxylic acids. Table 8 presents the experimental and calculated activation energies for the restricted rotation of the amino group in 23-31. 

Fig. 11.3. Energy profile for rotation and ring opening of the cyclopropyl methyl radical derived from CCSD(T)/cc-pvTZ computations. Reproduced from J. Org. Chem., 68, 9441 (2003), by permission of the American Chemical Society.

Amide C—N Bond Rotation and Ring-Flip of Cyclohexanes... 

Addition of benzyne, from benzene diazonium carboxylate, to ( )-deuterio-t-butylethylene gave l-deuterio-2-t-butylbenzocyclobutene with 75% retention of the olefin configuration. The preference for retention is similar to that exhibited in the [2+2] addition of benzyne to trans-1,2-disubstituted olefins. The observation of retention with a monosubstituted olefin is consistent with a biradical mechanism, with the stereochemical result determined by the relative barrier heights to rotation and ring closure in the biradical. 

For free radicals with more than two conformations, information about their dynamical processes such as internal rotation and ring inversion can be obtained by measuring the spectrum over a range of temperature. However, analy si s of the spectrum becomes more difficult and uncertain. 

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