Rearrangement processes

In a reaction, bonds are broken and made. In some cases free electrons are shifted also. The rcaciion center contains all the bond.s being broken or made during the reaction as well as all the electron rearrangement processes. The reaction uhstme-ture is the structural subunit of atoms and bonds around the reaction center that has to be present in a compound in order for the reaction to proceed in the foi"ward (synthesis) direction (Figure 10,3-32). Both characteristics of a reaction can be used to. search for reactions with an identical reaction center and reaction substructure but with different structural units beyond the reaction substructure. For example, this can be achieved by searching in a reaction database. 

When fragmentation is accompanied by the formation of a new bond as well as by the breaking of an existing bond, a rearrangement process is involved. These will be even mass peaks when only C, H, and O are involved. The migrating atom is almost exclusively hydrogen six-membered cyclic transition states are most important. 

Although manufacture of benzidine itself has virtually ceased in Europe and the United States, similar rearrangement processes are operated for 3,3 -dichlotobenzidine [91-94-1J, o-dianisidine (3,3 -dimethoxybenzidine), and benzidine-2,2 -disulfonic acid. 

Some fouling occurs simply by contact, almost certainly due to adsorption. Some occurs slowly as material is processed, some of that due to trace components in the feed and some due to slow accumulation and rearrangement processes. 

Since alkaline decomposition of p-toluenesulfonylhydrazones of steroid 12-ketones leads directly to 14(13 I2) jr e >-13(17)-enes whereas solvolysis of 12j5-mesylates gives mixtures of 13(18)- and 13(17)-enes, the former approach is preferred for the preparation of C-nor-D-homosteroids. If the particular system under consideration is not stable to base at elevated temperatures, the milder solvolysis method could be used. Either approach starts with a 12-oxygenated steroid. Additional base-stable functional groups should be compatible under the conditions for the rearrangement process. 

Dithiadiazolyl radicals are typically prepared by reduction of the corresponding cations with SbPh3. They are unstable with respect to isomerization to the 1,2,3,5-isomers both in solution " and in the solid state. The isomerization is a photochemically symmetry-allowed process, which is thermally symmetry forbidden. A bimolecular head-to-tail rearrangement has been proposed to account for this isomerization (Scheme 11.1). This rearrangement process is conveniently monitored... 

Reaction of 8-aminoquinoline 567 with 3,4-dichlorodithiazolium chloride gave the quinolyl iminodithiazole 568 whose thermal rearrangement gave 569 via a molecular rearrangement process (96MI2775) (Scheme 95). 

Intramolecular dipolar azide-olefin cycloaddition of 723 took place upon heating in benzene to afford 724 (83JA3273). An alternative rearrangement process can take place upon photolysis of 724 to give 725. Mesylation of 4-(3-hydroxypropyl)-2,4,6-trimethyl-2,5-cyclohexadiene-l-one (78JA4618) and subsequent treatment with sodium azide in DMF afforded the respective azide 726 which underwent intramolecular cycloaddition to afford the triazoline 727 (83JOC2432). Irradiation of 727 gave the triazole derivative 728 (Scheme 126). 

Metastable ions are almost always the result of fragmentation after a rearrangement process. The reason for this is that direct fragmentation is prompt, occurring before the ions leave the ion source. Metastable ions... 

Duffield and coworkers65 studied the El- induced mass spectra of five arene- (215-219) and four alkane sulfonylthioureas (220-223) and observed two rearrangement processes, namely loss of S02 from 215-219 and the elimination of ArS02 and RS02 with the thione sulfur atom from 215-223. The other fragmentations involved simple bond cleavages with and without hydrogen transfer (equation 48). The loss of H2S was evident for all the compounds studied except 221 and 222. It was, however, found to be a thermal and not an ionization process. 

By contrast in compounds 14, 15, and 16, from which MW — 43 also cannot be formed by single bond fission, the intensities of MW — 43 are negligibly small. For these we find that no feasible rearrangement process such as Equation 16 can be written. Thus for compound 14, for example,... 

Neuschuetz K., Velker J., Neier R. Tandem Reactions Combining Diels-Alder Reactions With Sigmatropic Rearrangement Processes and Their Use in Synthesis Synthesis 1998 227-255... 

C4-insertions by means of a sigmatropic rearrangement process have been described using either a thermal Cope reaction, anionic Claisen amide enolate and zwitterionic aza-Claisen rearrangements. 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

Scheme 1.3. Domino Prins/pinacol rearrangement process.

An elimination/double Wagner-Meerwein rearrangement process has recently been developed by Langer and coworkers [39]. Treatment of compound 1-136, obtained by reaction of 1-134 and 1-135, with trifluoroacetic acid (TFA) led to the cationic species 1-137, which then underwent a twofold Wagner-Meerwein rearrangement to give the bicydic compound 1-139 via 1-138 (Scheme 1.34). 

Scheme 2.193. Domino [2,3]-Wittig/anionic-oxy-Cope rearrangement process...

In the case of n-hexane, the three positional isomers can, in principle be determined from the three equations in (5). However, the results in this case are very sensitive to ion source conditions, presumably because of ion rearrangement processes, so the evaluated proportions are subject to greater errors than with 2- or 3-methylpentane. 

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