Bonding in Cyclic Systems

Early work on strained double bonds has been reviewed.7-9 Double bonds in strained bicylic systems and medium-sized cycloalkenes are particularly reactive and add azides quantitatively in an exothermic reaction43-45 81,82 that could be useful in derivatization83 and quantitative analysis.84 The reaction of organic azides with strained, olefinic bonds in cyclic systems, first recorded by Alder,43-45 has been the subject of numerous theoretical and... 

Terminal olefins are easily hydrogenated. Their hydrogenation is much faster than the hydrogenation of double bonds in cyclic systems or internal double bonds. c/5-Olefms are hydrogenated faster than tran -olefins. Conjugated diole-... 

Thiols will add across the C=N bond in cyclic systems, such as 3-benzyl-2-phenylthiazolinum bromide (57), ... 

In cyclic systems, the fragmentation of alkoxy radicals can be a reversible process. The 10-decaIyloxy radical can undergo fragmentation of either the C(l)—C(9) or the C(9)-C(10) bond ... 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

Steric and conformational factors are also important, especially in cyclic systems.233 There is a preference for the migration of the group that is antiperiplanar with respect to the peroxide bond. In relatively rigid systems, this effect can outweigh the normal preference for the migration of the more branched group.234... 

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (HuckeVs rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when n = 1, i.e. that with 6n electrons. IO71 electrons (n = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol-1], and I4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mol- respectively ... 

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the truns-diaxial conformation in cyclohexane derivatives (cf. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47) (48) ... 

In open chain compounds that lack any chiral centre of course, rotation about all single bonds can be assumed to be both relatively free and fast on the NMR timescale and the 7-9 Hz range quoted is the result of averaging of this angle. The same is of course not true in cyclic systems where structures are rigid and bond angles constrained. We will deal with this topic thoroughly in Section 6.6.5. 

It is worth noting that whilst we have restricted discussion in this section to conformational interconversion based on the slow rotation of bonds, the concept of the NMR timescale is equally applicable to other types of interconversion, such as can sometimes be seen in cyclic systems which may exist in two different conformational forms. 

There is also the possibility of Davydov coupling, which is likely to appear when there are double or multiple H-bond systems [7,21-23]. It is responsible for cooperative effects between neighboring hydrogen bonds in cyclic hydrogen bonded dimers, or more generally in hydrogen-bonded chains in solids [ 10,24—34]. 

Exocyclic double bonds at cyclic systems, which contain cross-conjugated double bonds, cannot be considered as a subgroup of radialenes and shall therefore be treated separately, although many of the structural features are comparable. However, in these systems the exocyclic and endocyclic double bonds are competing with each other as sites for Diels-Alder reactions, cycloadditions and electrophilic attacks. The double bond character of both, as measured by its distance, can provide some evidence for the selec-tivities. If no strain and conjugation are expected, the double bonds should be comparable... 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

Ketone p-toluenesulphonyl hydrazones can be converted to alkenes on treatment with strong bases such as alkyl lithium or lithium dialkylamides. This reaction is known as the Shapiro reaction68. When w./i-LinsaUi rated ketones are the substrates, the products are dienes. This reaction is generally applied to the generation of dienes in cyclic systems where stereochemistry of the double bond is fixed. A few examples where dienes have been generated by the Shapiro reaction have been gathered in Table 669. 

Tellurium-lithium exchange in cyclic systems, like the telluranaphthalene 120, has been demonstrated by Sashida. The possibility of a two-fold tellurium-carbon bond cleavage introduces a route to doubly hthiated compounds. When compound 120 is treated with an excess of n-butyllithium in THF, the lithiated lithium tellurate 121 is formed, which is interconverting to the doubly hthiated compound 122 (Scheme 44, verified by deuteriolysis). 

THE DETERMINATION OF DOUBLE-BOND CHARACTER IN CYCLIC SYSTEMS IV - TETRAH YDRONAPHTH ALENE STfiRlC FACILITATION OF CHELATION ... 

A totally new situation arises from the presence of defined topological bonds in polymer systems. The last documented example is given by polyrotaxanes 7 in which defined topological bonds occur between the macrocycles and the polymer chain (considered as infinite). The polyrotaxanes are composed of a polymer chain on to which a certain number of macrocycles is threaded. For short polymer chains, the end-capping by stoppers prevents the macrocycles unthreading from the chain [27, 28] (Scheme 3). Multicatenanes 8 are structurally related to polyrotaxanes 7 and can be viewed as cyclic analogs of polyrotaxanes [29]. 

Equivalent protons. All hydrogens which are in identical environments have the same chemical shift and therefore absorb at the same frequency they are said to be chemically equivalent. This can arise in two ways. Firstly, the protons are equivalent if they are bonded to the same carbon atom which is also free to rotate. For example, the three protons in a methyl group are equivalent and appear as a singlet (see the spectra of toluene, anisole or acetophenone above), and the two protons of a methylene group, provided that it can rotate freely, are identical and appear as a singlet (see the spectrum of phenylacetic acid above) frequently this is not the case with methylene groups in cyclic systems where rotation is restricted. 

Maximum stabilization only occurs of course if the vacant p orbital and the carbon-silicon bond are in the same plane. Whilst this does not present any problems in acyclic cases, it is not always possible in cyclic systems. 

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