Double bonds, restricted rotation about

Amino acids are combined (linked together) through peptide bonds (-C-N-) (Figure 8.1) the peptide bond formed is planar (flat), due to the delocalisation of electrons that form the partial double bond, restricting rotation about the bond. The rigid peptide dihedral angle, co (the bond between C and N), is always close to 180°. The dihedral angles phi (the bond between N and Ca) and psi (the bond between Ca and C) can only have a number of possible values, and so effectively control the protein s three-dimensional structure. 

In sulfenamides (R S—NR R ), the cation-radicals keep an nnpaired electron occupying a n orbital. This orbital is localized between the sulfur and nitrogen atoms. As a matter of fact, a slight S=N double bond character exists in the neutral sulfenamides. A consequence of this double bond character is an increase in the energy barrier, which restricts rotation around the S—N bond. Restricted rotation about the S—N bond is known in neutral sulfenamides (Kost and Raban 1990). Notably, the energy barrier to this rotation is greater for the derived cation-radicals when compared to the parent compounds (Bassindale and Iley 1990). 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

Some simple azo compounds, because of restricted rotation about the ( N=N ) double bond, are capable of exhibiting geometrical isomerism. The geometrical isomerism of azobenzene, the simplest aromatic azo compound which may be considered as the parent system on which the structures of most azo colorants are based, is illustrated in Figure 3.1. The compound is only weakly coloured because it absorbs mainly in the UV region giving a 2m.lx value of 320 nm in solution in ethanol, a feature which may be attributed to the absence of auxochromes (see Chapter 2). 

On account of this overlap there is considerable resistance to rotation about a double bond and it produces a rigid molecule, hi other words the disposition of groups attached to the carbon atom can be shown in different ways in space, giving rise to isomers. Therefore geometrical isomerism is a consequence of restricted rotation about double bonds. 

Alkylamino crotonic esters (22) all show in solution the presence of the isomers of both E and Z configurations, and there is also some evidence for rotamers involving restricted rotation about the (C=C)-N and C-C02R bonds (64). In all cases studied, when only one crystal form was found the molecular configuration was Z about the double bond. For R = CH2Ph, R = CJI, two crystal forms were obtained, one with E and one with Z molecular configuration. 

The restricted rotation about exocyclic partial double bonds in some 3-azapyrylium salts was investigated by temperature-dependent C- and H-NMR spectroscopy (87MRC688). 

Restricted rotation about double bonds or due to the presence of ring systems leads to configurational isomers termed geometric isomers. Thus, we recognize two isomers of but-2-ene, as shown below, and we term these cis and trans isomers. We have met these terms earlier (see Section 3.3.2). 

Using the double bonds, we conclude that the twisted configuration shown in Figure 6-17 should not be very stable. Here the p orbitals are not in position to overlap effectively in the tt manner. The favored configuration is expected to have the axes of the p-tt orbitals parallel. Because considerable energy would have to be expended to break the p-tr double bond and to permit rotation about the remaining sp2-cr bond, restricted rotation and stable cis-trans isomers are expected. Similar conclusions can be reached on the basis of the r model of the double bond. 

The chirality observed in this kind of substituted allene is a consequence of dissymmetry resulting from restricted rotation about the double bonds, not because of a tetrahedral atom carrying four different groups. Restricted rotation occurs in many other kinds of compounds and a few examples are shown in Table 13-3, which includes trans-cycloalkenes (Section 12-7), cycloalkyli-denes, spiranes, and ort/zo-substituted biphenyl compounds. To have enantiomers, the structure must not have a plane or center of symmetry (Section 5-5). 

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

The synthesis of a series of l//-pyrazolo[3,4-3]quinoxalines (flavazoles) 55 by acylation, alkylation, halogenation, and aminomethylation of the parent compound was reported and their structures were investigated by H, and N NMR spectroscopy <2005T2373>. Restricted rotation about the partial C-N double bond of the A -acyl derivatives was studied by dynamic NMR spectroscopy and the barriers to rotation were determined. N NMR data of a series of 3-alkyl [aryl]-substituted 5-trichloromethyl-l,2-dimethyl-l//-pyrazolium chlorides 56 (where the 3-substituents are H, Me, Et, -Pr, -Bu, -Pent, -Hex, (CH2)sC02Et, CH2Br, Ph, and 4-Br-C6H4) were reported <2002MRC182>. The N substituent chemical shift (SCS) parameters were determined and these data were compared with the SCS values and data obtained by molecular orbital (MO) calculations. 

In a few cases, single-bond rotation is so slowed that cis and trans isomers can be isolated even where no double bond exists (see also p. 230). One example is N-methyl-A -benzylthiomesitylide (69 and 70), the isomers of which are stable in the crystalline state but interconvert with a half-life of 25 h in CDCI3 at 50°C. This type of isomerism is rare it is found chiefly in certain amides and thioamides, because resonance gives the single bond some double-bond character and slows rota-tion. (For other examples of restricted rotation about single bonds, see pp. 230-233). 

Geometric isomerism resulting from restricted rotation about double bonds... 

The C-N bond length to the carbonyl group is closer to that of a standard C-N double bond (127 pm) than to that of a single bond (149 pm). This partial double bond character is responsible for the restricted rotation about this C-N bond. We must supply 88 k) moH if we want to rotate the C-N bond in DMF (remember a full C-C double bond takes about 260 k) moC ). This amount of energy is not available at room temperature and so, for all intents and purposes, the amide C-N bond is locked at room tern- o... 

Molecules chiral due to restricted rotation about single or double bonds (atropisomers) may act as the only chiromers in stereochemical reaction cycles. Each individual reaction is presumed to occur with retention, although conceivably a reaction could be induced to occur with inversion. Such a possibility is outlined in Chart XIII, but is contrived and specialized. 

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