Allylation reaction metal-mediated

Barbier coupling reaction Metal-mediated addition of alkyl, allyl or benzyl halides to carbonyl compounds. 38... 

Cross-coupling Reactions. Metal-mediated C-C and C-X bond formation via various cross-coupling reactions has emerged as a powerful tool in organic synthesis. Palladium-catalyzed processes are ubiquitous and Pd(dba)2 is frequently employed as a catalyst precursor. Cross-coupling sequences involving jr-allyl palladium complexes generally proceed with overall inversion of... 

Among all the nucleophilic addition reactions of carbonyl compounds, allylation reaction has been the most successful, partly due to the relatively high reactivity of allyl halides. Various metals have been found to be effective in mediating such a reaction (Scheme 8.4). Among them, indium has emerged as the most popular metal for such a reaction. 

Allylations, allenylations, and propargylations of carbonyl compounds in aqueous media can also be carried out with preformed organic tin reagent, rather than the use of metals.86,87,88 For example, the allylation reaction of a wide variety of carbonyl compounds with tetraal-lyltin was successfully carried out in aqueous media by using scandium trifluoromethanesulfonate (scandium triflate) as a catalyst (Eq. 8.40).89 A phase-transfer catalyst (PTC) was found to help the allylation mediated by tin at room temperature without any other assistance.90... 

Mechanistic Discussion. For the mechanism of the metal-mediated allylation reaction in aqueous media, Li proposed a carbanion-allylmetal-radical triad (Figure 8.1) in which the specific mechanism of the... 

Fe. Iron metal was found to be able to mediate the allylation reactions of aryl aldehydes with allyl bromide using sodium fluoride as the promoter. The formation of an allyliron species was proposed as the reactive intermediate in the reaction (Scheme 8.20).187 In view of... 

Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in aqueous environments.208 Different metal promoters showed varied regioselec-tivities on the product formation during allenylation/propargylation reactions of the kcto-fi-lactams. The stereochemistry of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral substituent at C4. The process led to a convenient entry to densely functionalized hydroxy-ji-lactams (Eq. 8.82). 

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>. 

Transition-metal-mediated C-O bond cleavage reactions are interesting in view of environmentally benign halogen-free chemical processes [59]. Zerovalent ruthenium complexes are also active toward C-O bond-deavage reactions, and a number of catalytic processes have been developed in this respect. For example, Ru(l,5-COD)(l,3,5-COT) catalyzes allylic alkylation of carbon nucleophiles with allylic carbonates in basic solvent (Scheme 14.24) [60]. 

In addition to the design of the solubility properties, the reactivity of organome-tallic species toward CO2 [13] (and many other potential supercritical reaction media) must be considered as important criteria for the choice of the catalyst. For example, the bisallyl ruthenium complex shown in Table 1 cannot be utilized as a precursor for ring-opening metathesis polymerization (ROMP) in SCCO2, because the insertion of CO2 into the Ru-allyl bond prevents the initiation mechanism [14]. Metal-mediated oxygen transfer to form CO and phosphine oxide was found to lead to deactivation of the [Ni(cod)2]/PMe3 (cod = 1,5-m-cycloocta-diene) catalyst system [15]. On the other hand, the reactivity of CO2 with metal... 

Several years ago transition metal mediated reactions in the area of C-glycoside synthesis were primarily limited to palladium and to a lesser extent nickel and manganese. Over the last few years several other metals, including chromium, molybdenum, tungsten, cobalt, and rhodium, have been utilized in C-glycoside synthesis. This section discusses the chemistry of palladium, which is divided into Stille-type couplings and 7T-allyl complexes. This is followed by considerations of the chemistry of chromium and the above-listed metals. A review by Frappa and Sinou entitled Transition Metal Catalysed Fimctionalization at the Anomeric Center of Carbohydrates appeared in early 1997 [55]. 

Since the observation that allylation of carbonyl compounds could be mediated by tin in aqueous medium [77], there has been an intensive development of the Barbier-type allylation reaction in water. Three metals were particularly investigated zinc, tin, and indium. In the aqueous zinc-promoted allylation, allylzinc species are considered unlikely. The initiation of the reaction could be attributed to the formation of an allylic radical anion on the metal surface this radical surface could then react with the carbonyl compound to give an alkoxide radical, which could add an electron and form the alcohols [82]. Allyl bromide or even chloride reacts with aldehydes and ketones in the presence of commercial zinc powder in a mixture of tetrahydrofuran and saturated ammonium chloride aqueous solution (Eq. 7) [83]. 

Development of a highly regioselective metal-mediated allylation reaction IN AQUEOUS MEDIA... 

In conclusion, a general regioselective metal-mediated allylation reaction has been developed. It has been demonstrated that indium, tin and zinc can be used to carry out this reaction, giving moderate to good yields and selectivities. Table 8.1 gives a brief summary of the different aldehydes and allyl bromides which are used to synthesize homoallylic alcohols in this procedure. 

Meldrum s acid derivatives have been shown to react with allylpalladium complexes derived from a variety of palladium sources. Several types of allylic acetates have been used as substrates, and the major product in these reactions is the diallyl Meldrum s acid derivative (e.g. (29)), often formed exclusively (Equation (12)). This and other metal-mediated reactions of Meldrum s acid and its derivatives are thoroughly discussed in a review <91H(32)529>. 

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