Twisting process

Fig. 7. Spin-drawing processes for nylon yam (a) draw-twisting process, (b) conventional spinning process, and (c) coupled process.

Dunham and Abbott have studied ligand exchange in cis- and trans-[Pt(ox)2(H20)2] and shown that it occurs too slowly to be involved in the formation of metal-metal bonded polymers.507 They also observed a trans to cis isomerization that occurred without ligand exchange and proposed a Bailar or Ray-Dutt twist process.507... 

The mechanism of its action is not completely understood, although it is presumed, that mitoxantrone acts by binding with DNA, thus disturbing the twisting process of the chains. It is used intravenously for treating severe nonlymphatic leukemia, breast cancer, and so on. A synonym of this drug is novantrone. 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

Fig. 14. A clockwise (CW) Ray-Dutt twist followed by a CW Bailar twist, applied schematically to a model complex like 30-32. Each of the two processes exchanges diastereomers, but the combined action is a topomerization equivalent to a (X, Cl)-1,2 shift. The eye drawn near one of the structures shows which triangular face was selected for the second twist process.45 Reproduced with permission from the American Chemical Society.

There are two pathways without bond rupture that have been widely discussed. One is the trigonal, or Bailar, twist and the other is the rhombic, or Ray-Dutt, twist, shown in Fig. 1-12 (a) and (b), respectively. Twist processes are, of course, not confined to chelate complexes. 

In another study,a chiral templating procedure was employed to direct the helicity of an oligo-bipyridine, double-stranded copper(I) helicate. The strategy employed is illustrated in Figure 6.29 in which spiro-bisindanol 71 and dimethyl-biphenic acid 72 were employed as the chiral template starters for the twisting process inherent in helicate formation. The above approach serves as a model for a general route towards the synthesis of enantio-pure helicates and is also of considerable intrinsic interest since it illustrates the manner by which stereochemical information may be transmitted over nanometer distances. 

This type of isomerization cannot take place by a twisting process and requires a metal-ligand bond rupture mechanism. 

Figure 10. Additional twisting processes for the racemization and isomerization of M AB)t...

This is particularly true now with the recent discovery of trigonal prismatic structures for some 6-coordinated metal complexes (21). Surely the proper metal-ligand combination should give a system in which the difference in energy between an octahedral and a trigonal prismatic structure is small. As a result, racemization may take place by a trigonal twist mechanism. Twisting processes of the square-planar-tetrahedral type are known to occur readily in certain nickel(II) complexes (20). 

The thermal racemization of (+)5g5-Cr(phen)3(C104)3 has been examined under a variety of medium conditions. The racemization is considered to occur via an intramolecular twist process. The effect of acid, hydroxide, and various other added ions on the rate constant has been investigated. The four diastereoisomers of Cr[( +)-acp]3 (acp = 3-acetylcamphorate) can be interconverted by photolysis or thermolysis at temperatures exceeding 373 K. ... 

A review has highlighted the photoisomerism of dienes in confined conditions. The results obtained are apparently consistent with the so-called hula-twist mechanism. Liu has reviewed the recent literature dealing with cis,transisomers involving the hula-twist mechanism. The H-vinyl conical intersection, the hula-twist process, of buta-1,3-diene have been explored. ... 

In the case of the much more labile tris-diketonate complexes of aluminum and gallium, the techniques of study are more complex since it is impossible to isolate even partially resolved samples. The most probable mechanisms for these labile complexes appear to be certain twist processes along with bond rupture to give spy transition states. 

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