Sensitizer highly bulky

Since then, similar temperature-switching behavior has been frequently observed upon sensitization with ortfto-benzenepolycarboxylates 45, 46, and 49 with a variety of chiral auxiliaries a-n and p-u, illustrated in Scheme 6 [31,32], and the best ees of product 47E obtained at ambient and low temperatures were further raised up to 49% (at 25°C) and 64% (at - 89°C) by using a highly bulky chiral sensitizer 45c [32]. 

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

Dehydrofluorination has also been utilized for the synthesis of stable stannaphosphenes. The stannaphosphene with bis(trimethylsilyl)methyl groups attached to tin could not be obtained in pure form because of its high reactivity92 replacing these alkyl groups with the bulky aryl substituent 2,4,6-triisopropylphenyl eliminated this problem and resulted in a stannaphosphene that is still air- and moisture-sensitive but stable under an inert atmosphere for prolonged periods of time and in solution at 60°C.93-94... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

In contrast to most of the high energy-density alkali and silver (NO- and NO2-substituted) methanides, the ionic liquids of these methanides with a bulky organic cation are neither heat nor shock sensitive, and hence can be prepared and stored in large scale. Nevertheless, this type of methanide-based ionic liquids can also be considered energetic ionic liquids since the thermodynamically unstable methanide anion is only kinetically... 

Exxon s Flexsorb SE solvents achieve high hydrogen sulfide selectivity by virtue of their molecular structure. These solvents are sterically hindered secondary amines. A bulky molecule is used to shield the available hydrogen radical on the nitrogen atom and prevent the insertion of carbon dioxide. The reaction with hydrogen sulfide is not sensitive to the amine s structure, so the steric hinderance affords higher hydrogen sulfide selectivity. 

Again, enantioselectivity is sensitive to the a substituent R1 with Me or/trim-alkyl (entries 1-6 and 11) giving very high enantioselectivities under neutral conditions. Most notable, cyclization of 18.1c under neutral conditions in THF (rather than DMA) yielded oxindole 17.3a of 97% ee. Substrates with the bulky a substituents (18.1d,e) gave markedly lower enantioselectivities under both sets of conditions (entries 7-10). Enantioselectivity is less sensitive to the p substituent R2 as can be seen by comparison of cyclizations of substrates 18.1a-c (entries... 

Both are stable metallocarbene complexes, but they have very different reactivity profiles. The molybdenum catalyst is highly reactive and is effective widi sterically demanding olefins. Its drawbacks are diat it is not highly tolerant of diverse functional groups and has high sensitivity to air, moisture, and solvent impurities. The ruthenium system, on die odier hand, is catalytically active in die presence of water or air, and it exhibits a remarkable functional group tolerance. It is not a reactive as the molybdenum catalyst, particularly toward sterically bulky substrates. However, it is readily available and is die reagent of choice for all but die most difficult substrates. 

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