Isomerization, configurational constitutional

Just like triazenes (Sections II,B,2-II,B,4), tetrazenes can isomerize with change of conformation (rotational isomerization), configuration (cis-trans isomerization), and constitution (1,3- and 1,4-rearrangements). 

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

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.

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

This isomerization can lead to a molecule with different constitution, configuration or conformation, or to a valence isomer. 

Isomerism in all these compounds is due to the different spatial arrangement of the atoms within the molecule. The difference in properties between the two forms is sometimes so marked, that it constitutes a method for the determination of configuration of the cis-trans isomeric forms. 

Figure 18. The preparation of constitutionally isomeric aa-DD-bisglycosido-18-crown-6 derivatives with gluco, galacto, and manno configurations.

Each of the scaffolds reported in Scheme 24 can be used for the production of a stereo-isomeric sublibrary based on the appropriate peptide sequence. For example, with the sequence A-B-C-D-E and scaffold (1) two types of stereoisomeric sublibraries can be prepared. One type includes the sublibraries A and B of Scheme 26 in which within a given sequence the configuration of each residue is successively inverted thus, retaining the identical connectivity as in the parent linear peptide. In the second type 265 of sublibraries C and D (Scheme 26) the direction of the amide bond is inverted and hence the connectivity is not maintained. In most members of these sublibraries the overall conformation of the scaffold is maintained and therefore these components constitute stereoisomeric sublibraries of the parent library. Conversely, by introducing amide bond surrogates such as reduced amide bonds1465 or thioamide bonds 260,466 the conformation of the scaffolds are changed and their conformational flexibility enhanced. 

It is obvious that this constitution/ configuration dichotomy does not readily accommodate isomerism of the special types (e.g. spin isomers)13 mentioned above but it does form a very convenient basis for the consideration of the most frequently encountered and most important... 

The isomeric optically active substituted ammonium salts which have been reported by organic chemists are not readily explained by an ammonium theory such as has been wrongly assumed to have been established by the experiments of V. Meyer and M. T. Lecco—vide supra. J. C. Cain argued from the above hypothesis of the constitution of the ammonium compounds that the optical activity is the result of a similar spatial configuration as that assumed for the carbon compounds where the quadrivalent nucleus, say NCI", instead of quadrivalent carbon, is at the centre of a tetrahedron so that... 

Insertion of imsymmetrical heterocumulenes such as 0=C=X (X = S, NR), into an N-Si bond can produce two different constitutional isomers [8]. In the case of compound 1 and X = NR, R = alkyl or aryl, there is rapid insertion into both N-Si functions and the resulting situation is extremely complex with respect to the configurational (0,N-silylation) and conformational isomerism [5, 8, 10]. In contrast, 0=C=S inserts rapidly into only one N-Si bond of 1, there is no evidence for any second insertion even after several days. The formation of small amounts of pyrazine as oxidation product even under strictly anaerobic conditions suggests some electron transfer reactivity. Spectroscopic [10] and especially the structural evidence given below show that of the two conceivable constitutional isomers [11] of the insertion reaction only the 0-Si bonded species 2 with intact thiocarbonyl function is formed. 

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