Pseudo-double bond

Mulliken draws on the equivalence between two descriptions of a localized double bond, as (a) a tt and a bond superposed and (b) two bent a bonds, as applied, for example, to the case of bonding in ethylene (Hall and Lennard-Jones, 1951). In the present case a pseudo-double bond is visualized involving the tt orbital 4>r oii fhe atom r under attack and a pseudo-77 orbital formed by combining the valence orbitals of the... 

An excellent synopsis of the influence of double-bond (and pseudo double bonds, such as the amide bond) geometry on biological activity has been published [8], The central role of the amide pseudo double bond on the architecture of proteins is illustrated when the trans preference of amide bonds is recognized. The enthalpic difference between the two conformers has been shown to lie between 0.5 and 2.5 kcal/mol with a barrier to cis-trans interconversion of approximately 61-22 kcal/mol [8] (Fig. 8). 

Figure 5.13 Hyperconjugation can be viewed as formation of a pseudo-double-bond.

When the alignment of a carbon-hydrogen bond with a vacant p orbital takes place allowing for hyperconjugation, a pseudo-double-bond develops. As illustrated in Figure 5.13, this can be envisioned as a double bond with a closely associated hydrogen ion. 

Z/E isomerism is not limited to true double bonds and may be used when sp2 electrons of a heteroatom are conjugated with a 7r-system to form a planar pseudo double bond. In particular, in the case of amides, the cis isomer is called E. Although the general tendency now is to use the E/Z nomenclature in chemistry, despite their inaccuracy cis and trans are still utilized by biochemists because they give a more readily understandable description of molecular shape, in particular for amides in peptides and proteins. When the chains are connected through a motif containing more than three dihedral angles (i.e. carbamates), the syn-anti... 

Fig. 2.5 Possible mechanisms for CTI via the heterolytic disruption of the conjugation of double bonds and pseudo double bonds (radical cleavage/recombination pathway was omitted there since it is not a true CTI process).

The pseudo-double bond character of amides is much more pronounced than for esters due to the conjugation of the H-N-C=0 moiety and is correlated to the ability of distorted amides to be hydrolyzed to bases [19]. For this reason, the barrier to interconversion is significantly higher that for the ester series, with AGl typically ranging from 16 to 22 kcal mol-1 [17]. However, the rotational barrier is not solely due to conjugation and also partly arises from the orientation of the nitrogen lone pair which is perpendicular to the amide plane [20]. Therefore, the rates of isomerization are considerably slower than for esters. This means that both isomers can be observed by simple techniques, for example at room temperature by H and 13C NMR spectrometry and UV spectrophotometry [21]. 

In particular, for the secondary amides of nonproline dipeptides, A St may vary depending on the protonation state [21] and hence influence the temperature-de-pendent energy barrier in a way described by the equation AGt = A Hi - TASt, though amide and carbamate CTI are usually essentially enthalpy-driven. The solvent dielectric constant e also varies depending on temperature (i.e. 47.6 at -50 °C and 32.1 at 50 °C for acetonitrile) and strongly influences the charge distribution on the pseudo double bond. 

The concept of cis-trans (Z-E) isomerism, originally used for the description of the relative geometry of olefins, has been extended to many other functions which feature a double bond character (pseudo double bonds), such as amides, as well as single bonds with a partial or complete limited rotation due to steric or stereoelec-tronic effects. Cis-trans isomerization (CTI) therefore exists in non-re-bonded or overcrowded molecules that switch from a given stable conformational state to another. This is the case of biaryl compounds which have been utilized in organic chemistry as the basis of molecular switches and rotors [1,2]. Nature has also exploited CTI of single bonds to increasing molecular diversity, in particular with the bulky thyroxin, a thyroid hormone, and the well-known disulfide bond which plays a critical role in the structure of peptides and in the conformation of proteins. 

Semmler deduces from these results that natural selinene is composed of a mixture containing principally the hemicyclic pseudo-(yS)-selinene, together with a small quantity of ortho-(a)-selinene. By passing through the dihydrochloride it is possible to convert the pseudo-(y8)-selinene into the ortho-(a)-selinene or regenerated selinene, which contains only a. small proportion of the (3 form. Both yield the same solid dihydrochloride. Selinene affords a typical example of the possibility of the displacement of the double bond from the side chain into the nucleus. 

Pseudo-/ -DL-gi Zopyranose triacetate (36) was prepared by hydroxyla-tion of the enetriol triacetate (32) and converted to the corresponding pentol and pentaacetate. The intermediate 32 was obtained by Diels-Alder reaction (200°C., two days) of rans/ rans-l,4-diacetoxy-l,3-buta-diene with allyl acetate. The double bond was surprisingly inert to the usual additive reagents and not detectable by infrared spectroscopy because of near-symmetry, but it did react with tert-butyl hydroxperoxide to give 36 in about 30% yield (27). 

Supplement A The chemistry of double-bonded functional groups (2 parts) Supplement B The chemistry of acid derivatives (2 parts) Supplement C The chemistry of triple-bonded functional groups (2 parts) Supplement D The chemistry of halides, pseudo-halides and azides (2 parts) Supplement E The chemistry of ethers, crown ethers, hydroxyl groups and their sulphur analogues (2 parts)... 

Stimulated by extensive research activities on donor/acceptor substituted stilbenes, Mullen and Klarner have reported a donor/acceptor substituted poly(4,4 -biphenyl-diylvinylene) derivative (85) in which the NR2 donor and CN acceptor substituents are located on the vinylene unit [111]. The synthesis is based on a C-C-coupling reaction of in situ generated carbanion functions with a (pseudo)cation function, followed by a subsequent elimination of MeSH with formation of the olefinic double bond. 

Frouin (44), and more recently good evidence has been published (45) that nitric oxide reacts with palmitodiolein to form a pseudo-nitrosite across the double bonds of the triglyceride. 

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

Rapid rotation of the end groups and/or bridging hydrides is required to account for apparent magnetic equivalencies. The molecule does, however, have built in pseudo-cylindrical symmetry, i.e., one set of metal TT-type orbitals binds the bridging hydrides and the other set forms the TT-component of the metal-metal double bond. 

FIGURE 10. Correlation diagram showing the influence of non-bonded through-space (n.bd.th.sp) interaction between the pseudo w-orbitals. t(( IU ) (circles) and the double-bond tr-orbitals tra and (ovals) on the 7r-orbital energies of the butadiene w-system. Aa is the basis energy of tra and 7Tb and SA the inductive and hyperconjugative destabilization (see equation 34)... 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C),... 

Figure 19. Interactions between the jt MOs of the two double bonds and the pseudo-jt orbitals of the methano group in 20 and the methane molecule in 21.

However, there are also many systems in which the evidence indicates that the propagating species cannot be a carbenium ion. Such reactions have been termed pseudo-cationic and in these polymerisations the propagating species is believed to be an ester. The most thoroughly investigated systems comprise aromatic monomers (styrene, acenaphthylene [11]) and protonic acids (HC104) or iodine [11] as initiators. The simplest representation of the propagation is as the addition of the ester (stabilised by four styrene molecules) across the double-bond of the monomer [12] ... 

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