Extended transition structure

There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

In contrast, the tricyclic ketone 2.77 and the azepine dimer 2.79 from pp. 15 and 16 are the products of attack with extended transition structures 2.106 and 2.107 in neither case is the alternative product detected. Clearly,... 

Extended transition structures are usually preferred in [6+4] cycloadditions... 

A secondary orbital interaction has been used to explain other puzzling features of selectivity, but, like frontier orbital theory itself, it has not stood the test of higher levels of theoretical investigation. Although still much cited, it does not appear to be the whole story, yet it remains the only simple explanation. It works for several other cycloadditions too, with the cyclopentadiene+tropone reaction favouring the extended transition structure 2.106 because the frontier orbitals have a repulsive interaction (wavy lines) between C-3, C-4, C-5 and C-6 on the tropone and C-2 and C-3 on the diene in the compressed transition structure 3.55. Similarly, the allyl anion+alkene interaction 3.56 is a model for a 1,3-dipolar cycloaddition, which has no secondary orbital interaction between the HOMO of the anion, with a node on C-2, and the LUMO of the dipolarophile, and only has a favourable interaction between the LUMO of the anion and the HOMO of the dipolarophile 3.57, which might explain the low level or absence of endo selectivity that dipolar cycloadditions show. 

On the other hand, phosphorane intermediates are not expected to be involved in the hydrolysis of phosphate monoesters, so the effective observed catalysis by the carboxyl group of salicyl phosphate 3.21 [51] (Scheme 2.26) is presumed to be concerted vith nucleophilic attack. (The hydrolysis reaction involves the less abundant tautomer 3.22 of the dianion 3.21, and the acceleration is >10 -fold relative to the expected rate for the pH-independent hydrolysis of the phosphate monoester dianion of a phenol of pK 8.52.) However, this system differs from the methoxy-methyl acetals discussed above, in that there is a clear distinction between neutral nucleophiles, which react through an extended transition structure similar to 3.16 in Scheme 2.23, and anions, which do not react at a significant rate, presumably because of electrostatic repulsion. This distinction is well-established for the dianions of phosphate monoesters with good leaving groups (p-nitrophenyl [52] and... 

The rotationally active absorption band of the peptide group produces an ORD peak at about 199 nm for the helix conformation and at about 205 nm for the extended ft structure. The negative trough near 230 nm is associated with a weaker transition of the peptide link. The Cotton effects observed in the 270-290-nm region are a result of the side chain phenolic groups of the tyrosine residues. 

As a consequence, it has been proposed [35, 36] that the torquoelectronic model developed by Houk [35-37] for this latter reaction can be extended to the Staudinger reaction between ketenes and imines. This model relies on the nonequivalent positions of the substituents in the conrotatory transition structures, as shown in Fig. 3. In these transition structures the outward and inward positions are not equivalent, which has profound consequences on the stereochemical outcome of the reaction (vide infra). 

The classical reaction with chlorosulfonyl isocyanate has been extended to it-vinyl sulfide 449 to give a 2.5 1 diastereomeric mixture of 4-(phenylthio)azetidin-2-ones 343 and 450 (Equation 180) <2000MI935>. The facial selectivity in the cycloaddition has been explained by the conformational preference of the allylic groups in the transition structure. A similar reaction with styrene resulted into synthesis of the racemic 4-aryl-azetidin-2-one (Equation 181) <2000TA2351>. The divinyl ether 451 reacted with acid-free chlorosulfonylisocyanate to form 4-vinyloxyazetidin-2-one 452 (Equation 182) <1996SL895, 1997TA2553, 1998TL8349>. Most of the results in the reactions of isocyanate with vinyl ethers could be rationalized by a -conformational preference of the ether in... 

Thus, we feel that the a-helical and the extended" helical structure are well established in VCD, and that there exists a simple method for the interpretation of the data. The VCD features of the B-pleated sheet structure appear reasonably well established, too, although its interpretation is much more difficult. Since the data are mono-signate, the DECO model is not appropriate (it always predicts conservative couplets). Nafie and coworkers explained such monosignate VCD in terms of a model similar to one described earlier by Schellman [24], with nearly co-linear (and antiparallel) electric and magnetic dipole transition moments [30]. 

We now extend the structural basis set for cobalt(III) hexaamines with one additional structure with relatively long Coin-N bonds. The [Co(tmen)3]3+ cation (tmen = 2,3-dimethylpropane-2,3-diamine) is a highly strained species with long Com-N bonds because of the four methyl substituents (see Fig. 17.12.1). The structure of the cation has been determined by an X-ray diffraction study, and the conformation in the crystal has been defined as 065 (see Section 17.3 for the nomenclature of the conformers). Due to the elongation of the Com-N bonds to 1.997 A, there is a remarkable shift in the ligand field spectra (the first d-d transition ( Ai- ) is at 515 nm vs 470 nm for [Co(en)3p ) and the redox potential (-0.18 V vs +0.28 V)[56>231]. 

In Figure 3.7, a selection of metal clusters containing interstitial atoms is shown. Examples with interstitial H atoms as well as transition-metal atoms are also known. Addition of an interstitial metal atom is the first step towards extended metal structures. The term interstitial derives from its use in solid-state chemistry where atoms are found in the interstices of metal lattices, e.g., the tetrahedral or octahedral... 

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