Pair-selectivity

There are four main factors that affect the enantioselectivity of sulfur ylide-mediated reactions i) the lone-pair selectivity of the sulfonium salt formation, ii) the conformation of the resulting ylide, iii) the face selectivity of the ylide, and iv) betaine reversibility. 

For oxathiane 1, lone pair selectivity is controlled by steric interactions of the gem-dimethyl group and an anomeric effect, which renders the equatorial lone pair less nucleophilic than the axial lone pair. Of the resulting ylide conformations, 25a will be strongly preferred and will react on the more open Re face, since the Si face is blocked by the gem-dimethyl group (Scheme 1.9) [3, 15]. 

In each of the following pairs, select the one that has the highest first ionization potential (a) Li or Be (b) AlorF ... 

Formation of a titanacyclopentane via cyclic dimerization of ethylene was reported as early as 197644 (Scheme 17). In marked contrast with the corresponding Zr-promoted reactions discussed later in this chapter, the intramolecular version of the Ti-promoted alkene-alkene coupling reactions does not appear to have been well developed. Consequently, detailed aspects of pair -selectivity and regioselectivity still remain largely unknown. 

Although a fair number of examples of the Ti-promoted intermolecular alkyne-alkene coupling reactions are known, those that display high pair selectivity and regioselectivity are still relatively limited. Some representative examples are shown in Scheme 18 51 53>53a... 

It is much more difficult to control the pair-selectivity and regioselectivity of the intermolecular cyclic carbozirconation. The fundamentally dynamic and reversible nature of most of the microsteps in these reactions is primarily responsible for the often capricious nature of these reactions. Nevertheless, considerable progress has been made recently. In particular, the use of (ethylene)zirconocene in place of (l-butene)zirconocene has been shown to provide convenient and selective procedures, as shown in Scheme 39 13,13c,212,226,227 q [lcsc procedures, however, are still not fully satisfactory, especially in terms of regioselectivity, and additional developmental works are desirable. 

Scheme 39 Pair-selective preparation of five-membered zirconacycles via (1 -butene)ZrCp2 and (ethylene)ZrCp2.

SCHEME 54. Pd-catalyzed strictly pair-selective two-step synthesis of unsymmetrically substituted conjugated diynes... 

The experimental work can be divided into two general classes (1) exploratory experiments to find ion exchange resins suitable for the proposed application, and (2) obtaining quantitative data on one resin pair selected. 

In summary, in operation of adsorption systems involving a localized and non-localized pair, selection of best separability is dependent both on the Henry constant and on the type of site interaction. Higher Henry constant combined with a localized site favours the employment of high pressure systems conversely, higher Henry constant combined with a non-localized site favours low pressure systems. The choice of appropriate temperatures depends on whether the Henry constants diverge or converge as the temperature is altered. 

Solvent pairs selected from the extremes of the list Table 2.8 are not usually sufficiently miscible to be satisfactory, e.g. methanol and light petroleum. 

Regulation of coiled-coil assembly in tropomyosins./. Struct. Biol. 137, 176-183. Arndt, K. M., Pelletier, J. N., Muller, K. M., Alber, T., Michnick, S. W., and Pliickthun, A. (2000). A heterodimeric coiled-coil peptide pair selected in vivo from a designed library-versus-library ensemble./. Mol. Biol. 295, 627-639. 

B-9. From each of the following pairs select the compound that will react faster with sodium iodide in acetone. 

Hasegawa, E., Xu, W., Mariano, P.S., Yoon, U.-C., and Kim, J.-U. (1988) Electron-transfer-induced photoadditions of the silyl amine EtjNCHjTMS to a,p-unsaturated cyclohexenones. Dual reaction pathways based on ion-pair-selective cation-radical chemistry. Journal of the American Chemical Society, 110, 8099-8111. 

For each of the following three pairs, select the salt whose solubility will be affected most by a change in pH. 

As discussed in conjunction with the intermolecular cascade carbopalladation reaction shown in Scheme 4, it has been very difficult to satisfactorily control both queuing or pair -selectivity and regioselectivity of intermolecular cascade carbopalladation processes. Consequently, essentially all of the cascade carbopalladation reactions discussed here are at least partially intramolecular. The currently known cyclic cascade carbopalladation processes can be classified into a few to several types shown in Scheme 6. 

The monomers chosen for these investigations are shown in Table I. They were selected so that a range of properties changed depending on the pair selected for the synthesis of these semi-1-IPNs (34). Factors of particular interest were the water solubility of the monomers, which could influence the locus of polymerisation and the relative hydrophilicities of the homopolymers as indexed by critical surface tension. In a semi-l-IPN the first-formed polymer is a network, but the second-formed polymer is linear. 

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