Ring-switching process

Tandem Passerini/Knoevenagel reactions were also performed by employing 2-nitrophenylacetic acid as the acid component to give the butenolides 65 that were reduced to the intermediate amines 66, which immediately cyclized to give indoles 67 in very high yields via a ring-switching process (Scheme 2.24) [52],... 

It has been argued that this rearrangement may occur via a bond-switch process in 167. In this process the sulfur acts as a nucleophilic center, which is opposite to the electrophilic behavior of the pivotal sulfur in, for example, the conversion of 149 into 150. The alternative intermediacy of a bipolar sulfur tetra-azapentalene structure or a ring-opened intermediate in... 

Figure 13.36 Switching processes of catenane 41H5+ in solution. Starting from the deprotonated catenane 414+, the position of the ring switches under acid-base and redox inputs according to AND logic.

Cyclooctatetraene offers another example of fluxional behavior. In an operation distinct from boat-boat ring reversal depicted in 5-14, the locations of the single and double bonds are switched. Anet and co-workers used the same molecule to examine the bond-switching process, whose antiaromatic transition state is 5-25b. (The transition state to ring reversal... 

The switching properties are determined by the electrochemical conditions employed during growth, the counterion incorporated, and the substituents attached to the thiophene ring.146 For example, poly(3-heptoxyl-4-methylthiophene) 29 switches at 0.76 V, whereas under the same conditions, poly(3-octylthiophene) switches at 1.01 V. The electrolyte used for switching also has a marked effect on the potential, rate, and reversibility of the switching process.147... 

After the photoinduced switching process, back switching should be easily effected by allowing O2 to enter the irradiated solution. The reduction of n-electron acceptors (bipyridinium-type systems) is easy and reversible, whereas oxidation of Ji-electron donors (aryloxy-type systems) is difficult and often irreversible. Thus, it is more convenient to use systems which contain the n-electron acceptor in the dumbbell component and the jt-electron donor in the ring component. 

A number of other carbocations similar to CsHs have been studied, the most notable being CsMes. It has been demonstrated [47,48] that the structure of this cation is Csv, I 1.65, rather than a rapidly equilibrating series of classical structures, I 1.66, where a bond-switching process permutes three two-center-two-electron bonds around the five-membered ring. An orbital interaction diagram for 11.65... 

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

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