Lewis tetraallyltin

During our investigations of the reactions mediated by LASCs, we have found that addition of a small amount of a Bronsted acid dramatically increased the rate of the aldol reaction (Eq. 5).[191 This cooperative effect of a LASC and an added Bronsted acid was also observed in the allylation ofbenzalde-hyde with tetraallyltin in water.1201 Although, from a mechanistic point of view, little is known about the real catalytic function of scandium and proton, this cooperative effect of a Lewis acid and a Bronsted acid provides a new methodology for efficient catalytic systems in synthetic chemistry. 

Stereoselective addition of allyl metal reagents to various functionalities is an important reaction in organic synthesis [32, 33]. The allylation of epoxides and aziridines with allyltin reagent is catalyzed by Lewis acids. Even though many Lewis acids have been reported to catalyze this reaction, Bi(OTf)3 is distinct because it avoids the formation of byproducts and is also environmentally more compatible. It catalyzes the reaction of aryl epoxides with tetraallyltin to afford the corresponding homoallyllic alcohol [34]. 

Maruoka has reported that chiral bimetallic Lewis acid catalysts 9-11, prepared from (S)-BINOL, M(0-i -Pr)4 (M=Ti, Zr, Hf), and the corresponding spacer, strongly enhance the reactivity of aldehydes or ketones toward allyl transfer from allylstannanes [18-20]. For example, treatment of acetophenone (42) with tetraallyltin (41) in the presence of 30 mol% of the chiral bidentate Ti(IV) catalyst 10 provided the (S)-enriched homoallylic alcohol 43 in 95% yield with 90% ee (Scheme 2) [19]. A suggested reaction mechanism involves double activation of carbonyls owing to the simultaneous coordination of two Ti atoms to a carbonyl oxygen atom. 

Besides Lewis acids, tetraallyltin is activated by methanol to promote the reaction with aldehydes or activated... 

Certain Lewis acids are known to induce an epoxide-aldehyde rearrangement <01TL8129>, and this chemistry has recently been combined in tandem with metal-mediated allylations. For example, epoxides react with tetraallyltin in the presence of bismuth(III) triflate to give homoallylic alcohols 116. The reaction involves an initial 1,2-shift to form an aldehyde 115, which is then attacked by the allyl tin species <03TL6501>. A similar but operationally more straightforward protocol is available by combining allyl bromide with indium metal, followed by the addition of epoxide <03TL2911>. 

Both Sc(OTf)3 and Yb(OTf)3 have been employed as Lewis acid catalysts for additions of allylic stannanes to aldehydes. Reactions with the former catalyst can be conducted in a variety of solvents and are not sensitive to water [24]. All four allyl groups of tetraallyltin are consumed in the addition (Table 13). The latter reaction is performed with allyl tributyltin in CH2CI2 (Eq. 16) [25]. 

Nafion is another choice of polymer support for Sc-based Lewis acids. Nafion-Sc catalyst is readily prepared by treatment of Nafion with ScCb 6H2O in acetonitrile under reflux [116]. Nafion-Sc catalyst has been found to be effective in several synthetic reactions including allylation of carbonyl compounds with tetraallyltin, Diels-Alder reaction, Friedel-Crafts acylation, and imino Diels-Alder reactions. The use of Nafion-Sc in flow systems has also been tested. 

Denmark has spectroscopically examined the reaction of both allyl- and 2-bute-nylstannanes with aldehydes using the Lewis acids SnCU and BF3-OEt2 [73, 82]. First, the metathesis of both allyltributylstannane and tetraallyltin with SnCl4 was determined (by C NMR spectroscopy) to be instantaneous at -80 °C. The reaction of allyltributylstannane with a complexed aldehyde was detemiined to be significantly more complicated. When a molar equivalent of SnCU per aldehyde was employed, metathesis was determined to be the preferred pathway for aldehydes. When one half a molar equivalent of SnC per aldehyde is used, the reaction pathways and product distribution become very sensitive to both the aldehyde structure and addition order. A spectrum of mechanistic pathways was documented ranging from direct addition (acetaldehyde) to complete metathesis (pivalalde-hyde) to a competitive addition and metathesis (4-t-butylbenzaldehyde). The results obtained with a molar equivalent of SnCl4 are most relevant, as this reagent stoichiometry is most commonly used in the addition reactions. 

Most scandium Lewis acids are water-compatible and are often used as Lewis acid catalysts in aqueous media. It was demonstrated that in the presence of 5mol% Sc(OTf)3 acyl hydrazones and imines reacted with tetraallyltin in water-tetrahydorfuran (THF) to give the desired allylated adducts in good yields (Scheme 12.59) [160]. 

In addition to rare earth triflates, copper triflate was also found to be a stable Lewis acid in aqueous media. In a mixed aqueous solvent system (H20-EtOH-toIuene = 1 7 4), allylation of various aldehydes with tetraallyltin and aldol reactions with silyl enol ethers proceeded smoothly in the presence of Cu(OTf)2 (20 mol%) to give homoallylic alcohols and aldol adducts, respectively, in high yields (Schemes 3.9 and 3.10). 

Hachiya, 1. and Kobayashi, S., Aqueous reactions with a Lewis acid and an organometalhc reagent. The scandium trifluoromethanesulfonate-catalyzed allylation reaction of carbonyl compounds with tetraallyltin, /. Org. Chem., 1993, 58, 6958-6960. 

Polymer-supported scandium-based Lewis acid also shows high activity in this reaction in water and can be easily recovered and reused [70]. Experiments carried out with 4-phenyl-2-butanone and tetraallyltin found that the reaction proceeded smoothly at room temperature to afford the... 

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