Unit 6.4 Isomerism

What you need to know before you start this unit. 

You have already come across compounds which have different structures but the same molecular formula in our discussion of alkanes, alkenes and arenes. Such compounds are called isomers. Isomerism in alcohols is also discussed in Section 7.1.2 and isomerism in aldehydes and ketones is also discussed in Section 7.2.7. 

What is meant by isomerism in some common organic compounds. 

Structural-, positional-, stereo-isomerism (optical and geometric) in aliphatic hydrocarbon systems. 

Functional group isomerism, e.g. in alcohols and ethers, aldehydes and ketones. Isomerism in aromatic compounds. 

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Table V. Kinetics of Repeating Unit Isomerization in Poly[(chloromethyl)thiirane] and Poly(3-chlorothietane)...

Repeating unit isomerization is similar in several respects to isomerization polymerization (26,27). Isomerization polymerization may be defined as a process whereby a monomer of structure A is converted to a polymer of repeating unit structure B, wherein the conversion of A to B represents a structural change which is not a simple ring opening or double bond addition ... 

We define repeating unit isomerization as a process subsequent to polymerization, in which an intramolecular rearrangement of the repeating unit leads to a thermodynamically preferred structure ... 

C NMR analysis, 356,360,3611-362/ Reduction, 2,2-bis[4 -(4"-phenylsulfonyl phenoxyl)phenyl] propane, H NMR, 9 Repeating unit isomerization vs. 

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

Anionic polymerization of 67 by the activated monomer mechanism should occur with the selective cleavage of the CO—NH bond of the monomer to give a polyamide composed of kinetically controlled cis units (68c). However, the cis units isomerize to the thermodynamically more stable trans units (68t) through the proton abstraction from the methine group adjacent to the carbonyl group. This was ascertained by the isomerization experiment in which a polymer consisting of 92% cis unit and 8% trans unit was converted to one containing 40% cis unit and 60% trans unit when heated in dimethyl sulfoxide at 80 °C for 6 hours in the presence of 15 mol% potassium pyrrolidonate. 

If the processes occurring during thermal- and photoisomerization are to be analyzed and understood a kinetic analysis is very useful. This can help give answers to the following questions How fast and how effective is the isomerization reaction What is the molecular mechanism and Do all units isomerize according to the same mechanism and kinetics Some tools for this analysis are presented here. 

There have been studies of polyanion unit isomerization in the solid state but such isomerizations are more problematical to quantify and unequivocal inferences from them are harder to come by than from the analogous studies in solution. The difficulty derives in part because high-quality kinetics data are hard to obtain for reactions in the solid state. Nonetheless, some reports on solid-state POM isomerization in the time period of this review afford valuable insights. 

The transference number for Li+ in the 13a-containing lilm was decreased significantly when the spirobenzopyran unit isomerized to its merocyanine form. The results was explained by the enhanced Li+ binding in the merocyanine form. 

It was difficult to prove how many of the azobenzene units isomerized after irradiation. Thus 73 can be used only for qualitative statements. But... 

Growing macrocations can rearrange to more stable structures when structural requirements for this are met and the lifetimes of the individual macrocations are relatively long. The monomeric unit isomerizes by intramolecular reaction under these conditions. Since the monomeric units of the polymer produced can often not be produced by existing monomers, these polymerizations are called phantom or exotic polymerizations. They can proceed by ring isomerization or by material transport. ... 

In this chapter we describe a new method of copolymer synthesis which is analogous to the method of isomerization polymerization, in that a copolymer is prepared from a single monomer The method is repeating unit isomerization, which we define as a polymerization followed by intramolecular rearrangement of the polymer repeating unit to a thermod3mamically more favorable structure (Eqn 4) ... 

In this chapter, we review the repeating unit isomerization of polymeric 3-chlorosulfides, with emphasis on the kinetics and mechanism of the reaction. We then discuss some preliminary observations concerning the isomerization of poly(chlorobutylthiirane), a... 

Isomerization of Polymeric P-Chlorosulfides. The first example of a repeating unit isomerization was found when we examined the carbon-13 NMR spectrum of a sample of poly(chloromethyIthiirane) which had been stored at room temperature for three months. Instead of the expected three lines at approximately 39, 51 and 54 ppm down-field from tetramethylsilane, we foimd two major lines at 40 and 61 ppm the expected signals were present, but of low intensity. The proton NMR spectrum was also unexpected, showing in addition to the backbone and chloromethyl signals, a downfield (6 4.28) quintet which could not be rationalized on the basis of the simple CMT repeating unit structure. When a freshly-prepared sample was analyzed, both the and the NMR spectra were as predicted. 

The repeating unit isomerization of poly(chloromethylthiirane) occurs in bulk and in non-nucleophilic solvents such as chloroform, dichloromethane and nitrobenzene. Regardless of the medium, the rearrangement appears to stop after isomerization of about 60% of... 

In a first test of this expectation, we sought evidence for repeating unit isomerization in poly[(2-chloroethyl)oxirane] (PCEO, IV), according to the mechanism shown in Eqn 9. 

Di-t-butylethene, a component also used as an additive for synthetic gasoline, is obtained from isobutene and ethene with a dual isomerization-disproportionation catalyst [20]. The process begins with dimerization in a reactor for conversion of isobutene to 2,4,4-trimethyl-1-pentene this is followed by isomerization-disproportionation in a bifunctional unit (isomerization of 2,4,4-trimethyl-1-pentene to 2,4,4-tri-methyl-2-pentene and conversion of the latter into di-r-butylethene) (Fig. 6). A by-product of the process, 2,3-dimethyl-2-butene, is recirculated to the disproportionation unit to be cleaved with ethene to isobutene, which is reintroduced into the process. When neohexene (3,3-dimethyl-1-butene) is employed as the starting material in this process, the installation consists solely of the disproportionation and fractionation units. 

The importance of repeat unit isomerism in poly (1,3-dienes) is very clearly demonstrated by the naturally-occurring polyisoprenes. Gutta... 

Crystalline Substructures. Figure 4 indicates that a number of structures are conceivable based on the two alternative arrangements of the three-fold hyckogen bond pattern connecting the melamine and barbiturate (isocyanurate) units. Isomerism around these sets of bonds leads to various possible substructures in the solid state, from the straight tape at the bottom of the figure to the cyclic hexamer at the top. Unless the substituents are tailored to fill interstitial voids efficiently, we suspect that... 

Tirrell et al. ° showed that poly(epichlorohydrin) does not undergo repeating unit isomerization however substituted polyether carrying functional groups placed five bonds away from the backbone heteroatom could undergo isomerization due to anchimeric assistance. 

Figure 10.2 shows the locations of reforming and isomerization units in refinery configurations. 

Figure C2.1.1. (a) Constitutional isomerism of poly (propylene). The upper chain has a regular constitution. The lower one contains a constitutional defect, (b) Configurational isomerism of poly(propylene). Depending on tire relative configurations of tire asymmetric carbons of two successive monomer units, tire corresponding dyad is eitlier meso or racemo.

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

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