Alkenes catalyst hindrance

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

Cyclic alkyl nitronates may be used in tandem [4+2]/[3+2] cycloadditions of nitroalkanes, and this reaction has been extensively studied by Denmark et al. (64,333-335). In recent work, they developed the silicon-tethered heterodiene-alkene 219 (Scheme 12.63). Steric hindrance and the fact that both the nitroalkene and the a,p-unsaturated ester in 219 are electron deficient renders the possibility of self-condensation. Instead, 219 reacts with the electron-rich chiral vinyl ether 220 in the presence of the catalyst 224 to form the intermediate chiral nitronate 221. The tandem reaction proceeds from 221 with an intramolecular 1,3-dipolar cycloaddition to form 222 with 93% de. Further synthetic steps led to the formation of ( )-detoxinine 223 (333). A similar type of tandem reaction has also been applied by Chattopadhyaya and co-workers (336), using 2, 3 -dideoxy-3 -nitro-2, 3 -didehydrothymidine as the starting material (336). 

Aldehydes are easily hydrogenated to alcohols but ketones are more difficult to reduce because of steric hindrance. Hydrogenolysis is a problem with the catalytic reduction of carbonyls, particularly when linked to aromatic systems. Pd and H2 reduce alkenes faster than carbonyls. Metal catalyst Pt is commonly used for the reduction of carbonyls. For example, the Adams catalyst (Pt02) reduces 2-naphthaldehyde (6.31) to 6.32 in 80% when used with FeCls as a promoter. When excess of the promoter is used the product is 2-methylnaphthalene (6.33), which is also obtained by the reduction of 6.31 with Pd on BaS04 and H2. 

It is sometimes possible to estimate the extent of hindrance by an examination of a molecular model of the substrate. While the sites that promote hydrogenation reactions are the comer atoms or adatoms on the catalyst siuface, models of such surface species are not commonly available. The classic procedure for determining the relative steric hindrance to adsorption has been to place a model of the substrate on a flat surface with the It cloud of the alkene perpendicular to the surface as depicted in Fig. 14.3. In this case adsorption from side B is clearly favored. A similar conclusion can be drawn on examining the adsorption of the n cloud on a surface comer atom as depicted in Fig. 14.4. Here adsorption from the B direction is favored because of the interference of the 3,5 diaxial hydrogens to adsorption from side A, but the difference between the two modes of adsorption does not appear to be as great as that assumed from consideration of Fig. 14.3. 

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