Lewis acid catalysts transition metal promoters

The use of Bu2BOTf or AICI3 led to increased yields of the cyclobutane product 2a, but production of the desilylated alcohol 2b was also observed with these catalysts (entries 2 and 3). Broad screening of a number of Lewis acids showed that EtAlCL and TiCL were the optimal catalysts, both serving to promote formation of cycloadduct 2a in 76 and 61% yields, respectively (entries 4 and 5). Diastereoselective formation of 2a occurred in these reactions, as was seen in the stoichiometric reactions reported earlier [10b]. Several Lewis acids, including Et2AlCl, Sn(OTf)2, SnCLt, TMSI, and InCL, were found to catalyze the formation of 2a and/or 2b, but in low yields (not shown in Table 4.1). On the contrary, no cyclobutane product is produced in reactions catalyzed by lanthanide Lewis acids and transition metal Lewis acids starting enol ether 1 or desilylated ketone 3 were recovered in these cases. Thus, these results indicated that EtAlCla is an efficient catalyst for catalytic [2+2] cycloaddition reaction. 

In the Bu3SnH-promoted radical reactions to aliphatic alkynes, using initiators such as AIBN, Et3B, and ultrasound104 furnishes /3-adducts as a mixture of (E)- and (Z)-isomers. Lewis acid catalysts give /3-(Z) isomers,96 whereas transition metal catalysts furnish the predominant formation of (3-(E) isomers.105 The a-stannylation of simple aliphatic alkynes, however, is particularly difficult because of the absence of anchor substituents such as ethers. In the general hydrostannations of aliphatic alkynes, a-adducts are obtained only as minor adducts in the Pd-catalyzed reaction (Equation (35)). 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

Nb monomer catalyst (1) is remarkably suppressed to 1/300 and the dehydration is promoted 4 times on the dimer catalyst (2). It is to be noted that the change of the number of Nb atoms at the active sites from one to two metal atoms gives rise to a complete reverse of basicity/acidity in the catalytic properties. In the dimer catalyst (2) the preferable conformation at the transition state observed with the monomer catalyst (1) is difficult. Furthermore, the Lewis acidity of the Nb atoms in the dimer (2) is increased by the oxygen-bridge. 

Late-transition metal salts have been utilized as catalysts to promote Friedel-Crafts acylation of arenes and heteroarenes with anhydrides. A mismatch between their soft metal center and the hard carbonyl oxygen atoms of the products avoids the formation of a kinetically inert complex and results in catalytic turnovers. Although late-transition metal salts exhibit, a priori, rather poor Lewis acidity, sufficient reactivity can be gained by rendering them cationic. The acylation of variously substituted... 

Often the rates and selectivities of catalytic reactions are enhanced by additional reaction components that are added in small amoimts. These added materials are often called promoters or co-catalysts. Protic acids or Lewis acids are common promoters. As is presented in Chapter 16, triarylboranes are Lewis acid promoters in hydrocyanations catalyzed by transition metal complexes. 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

As an example, Yoon [34] has studied the inter- and intramolecular 2+2 cycloaddition of unsaturated ketones (an example in Scheme 8.19). A chiral version of the reaction cannot be obtained by simply adding a chiral catalyst, because electronic excitation promotes the addition of the aromatic ketone also when not complexed, as indeed it is often the case with this kind of reactions. However, he was successful when he used two catalysts, viz., a visible-absorbing transition metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. Under these conditions, the excited molecule could react only with a molecule complexed with a chiral ligand (Scheme 8.20). 

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