Constitutional and Configurational Isomerism

Isomerizations are defined as reactions that occur with change in the chemical structure but without change in the number-average molar mass or composition. Exchange equilibria and constitutional and configurational transformations belong in this group. 

In addition to constitution and configuration, there is a third significant level of structure, that of conformation. Conformational isomerism is generally taken to refer to discrete molecular arrangements generated by rotation about formal single bonds. This aspect of stereochemistry will be dealt with more fully in Chapter 3. 

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. 

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.

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. 

Isomerism in diene polymers can be measured by infrared and nuclear magnetic resonance spectroscopy. Some of the polymerization methods described in Chapter 9 allow the production of polydienes with known controlled constitutions and geometrical configurations. 

A further isomer of senkirkine (43), crotaverrine, has been isolated from Crotalaria verrucosa Linn. The spectroscopic properties and analytical data were consistent with a constitution for crotaverrine isomeric with that of senkirkine (43). The configuration about the C-15=C-16 double bond could not be determined unambiguously from the n.m.r. spectrum since the chemical shift of the C-16 vinyl... 

The photochemical configurational isomerization of previtamin D (5) into tachysterol (6) (and back) differs from the photochemical constitutional isomerization of provitamin D (1) or lumisterol (2) into previtamin D (5) (and back see Scheme 3) not only in the reaction type, but also in which of the jr jT-spin isomers is involved. Unlike the constitutional isomerization, the configurational isomerization can proceed not only under direct light excitation (singlet mechanism), but also by triplet-triplet energy transfer (triplet mechanism), with the aid of photosensitizers. 

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... 

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-... 

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