Mesityl-activated catalysts

Figure 18 Targeted mesityl-activated catalysts 50 and 51 and adamantyl-activated catalyst 52...

Figure 20 Mesityl-activated catalysts bearing different anionic ligands and their E/Z ratio vs. conversion for CM of allylbenzene and CDAB...

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

The aromatic complex can be a neutral t °-benzene derivative or an anionic ri -cyclopentadienyl ring. Substituents on these aromatic rings can greatly influence the effectiveness of these catalysts. For example, with benzene derivatives the unsubstituted benzene rings give lower ees and the use of hexamethylbenzene results in lower catalytic activities whilst the cumenyl or mesityl rings give optimum catalyst systems. The two types of chiral bifunctional hnkers that have been most practical are anionic ones based on monosulfonated diamines and amino alcohols. 

The addition of water and methanol to terminal alkynes has also been studied by Laguna et al. by pentafluorophenyl and mesityl gold derivatives. Both acidic and non-acidic conditions led to high activity, even in the presence of as little as 0.5 mol% of catalyst. The use of pentafluorophenyl compounds allowed them to obtain additional spectroscopic information in the stoichiometric reaction of the complex [Au (C6F5)2C1]2 and phenylacetylene, which showed that gold(III) was the active species in the catalytic process. The reaction followed the Markovnikov rule, as shown in the proposed mechanism (Scheme 8.13), delivering the corresponding ketones or diacetal products [96]. 

In addition to conjugated dienes, cyanocobalt catalysts also hydrogenate the C=C bond of activated alkenes.52 Carvene, mesityl oxide, 2-cyclohexenone, benzalacetone and an androstenone derivative were reduced in this way.53... 

Acid Catalyzed. Although ketonic carbonyl groups are less reactive than aldehydic carbonyls in the presence of basic catalysts, this is not the case with acid catalysts. Thus acetone undergoes aldol addition in the presence of sulfuric acid to give mesityl oxide, which then condenses with a third molecule of acetone to give a mixture ofphorone (2,6-dimethyl-2,6-heptadien-4-one) and mesitylene (1,3,5-trimethylbenzene). Ketones also condense with activated aromatic products in the presence of sulfuric acid to give coupled aromatic products. For example, acetone and phenol condense to bisphenol A (4,4 -isopropylidenediphenol), which is used in the manufacture of epoxy resins (qv) and polycarbonates (qv). 

Complexes 53 are efficient catalysts for the homodimerization of 1-octene and styrene. Complex 53a bearing the sterically more demanding l,3-bis(2,6-diisopropylphenyl)-NHC ligand shows a higher reactivity than the mesityl-substituted 53b. These complexes also catalyze the CM of 1-octene or styrene with methyl acrylate ( 80% yield), the RCM of diethyl diallylmalonate at 40 °C ( 95% yield), and the ROMP of cyclooctene at 60 °C ( 90% yield). By GC-MS analysis the presence of free p-cymene was detected in the beginning of the reactions. From these results it may be concluded that the first step of the catalytic cycle is arene decoordination to generate a 12-electron [OsCl(= CHPh)(NHC)] + derivative as the catalytically active species [102],... 

Through the direct coupling of 2-bromo-4,4-dimethyloxazoline 189 and 1-mesityl imidazole (188) the corresponding imidazolium salt 190 was obtained and used for the preparation of a mono-carbene-palladium complex 191 active as a catalyst in Heck and Suzuki C-C coupling reactions <020M5204>. 

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