Allylation of carbonyl compounds mediated

Regio- and Stereoselectivity. For the allylation of carbonyl compounds mediated by indium and other compounds in aqueous media, usually the carbon-carbon bond forms at the more substituted carbon of the allyl halide, irrespective of the position of halogen in the starting material. However, the carbon-carbon bond forms at the less-substituted carbon when the y-substituents of allyl halides are large enough (e.g., trimethylsilyl or tert-butyl) as shown by Chan et al.139 (Scheme 8.10). The following conclusions can be drawn ... 

Allylation of Carbonyl Compounds Mediated by Nanometer-Sized Bismuth... 

Mediated by Tin. In 1983, Nokami et al. observed an acceleration of the reaction rate during the allylation of carbonyl compounds with diallyltin dibromide in ether through the addition of water to the reaction mixture.74 In one case, by the use of a 1 1 mixture of ether/water as solvent, benzaldehyde was allylated in 75% yield in 1.5 h, while the same reaction gave only less than 50% yield in a variety of other organic solvents such as ether, benzene, or ethyl acetate, even after a reaction time of 10 h. The reaction was equally successful with a combination of allyl bromide, tin metal, and a catalytic amount of hydrobromic acid. In the latter case, the addition of metallic aluminum powder or foil to the reaction mixture dramatically improved the yield of the product. The use of allyl chloride for such a reaction,... 

Ga. Gallium was used to mediate the allylation of carbonyl compounds in water.170 The reaction can also be carried out by using preformed allylgallium reagents.171 The corresponding homoallyl alcohols were obtained in high yields without the assistance of either acidic media or sonication (Eq. 8.72). 

Tin compounds can be produced by the Nalco-type reactions39 as well as by Cd and Zn mediated methods42,43. The use of organotin compounds in electrodically induced transmetallations has also been described in a study in which Grignard-type allylation of carbonyl compounds has been carried out by electrochemically recycled allyltin reagents44. 

Scheme 149 Tin-mediated reductive allylation of carbonyl compounds.

The Barhier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Sml2, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barhier-type allylation of carbonyl compounds in an SmH-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amount of SmCfi [570]. 

Indium mediates the coupling of a,a-difluoroallyl carbanion with aldehydes, to give geuz-difluorohomoallyl alcohols.89 hi contrast to many comparable allylations of carbonyl compounds, ketones do not react. 

Stereo specific generation and reactions of allylic alkali and alkaline earth metals have been reviewed121 and solvent-mediated allylation of carbonyl compounds with allylstannanes has been explored.122 Chiral phosphoramides derived from (5 )-proliiie have been used to catalyse asymmetric allylation of aromatic aldehydes by allylic trichlorosilanes.123... 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Indium-mediated Barbier-type coupling between carbonyl compounds and allyl halides has been revealed to proceed effectively in diverse reaction media. Even under solvent-free conditions, allylation works well, although no reaction is observed with benzyl bromide and a-halo carbonyl compounds.59 Various aldehydes react with allyl bromide mediated by indium in liquid carbon dioxide to give homoallylic alcohols (Scheme 1). In contrast to the corresponding neat allylation, the liquid C02-mediated reaction can allylate solid aldehydes successfully.60 Indium-mediated allylations of carbonyl compounds with allyl bromide proceed in room temperature ionic liquids. In [bmim][BF4] and [bmim][PF6] (bmin l-butyl-3-methylimidazolium), the desired homoallylic alcohols are formed with good levels of conversion.61 Homoallyllic alcohols are also prepared by the reaction of resin-bound aldehydes (Equation (l)).62... 

Table 10-32. Indium-mediated allylation of carbonyl compounds in water. B2 289...

Bu3Bi mediates the allylation of carbonyl compounds with allyl bromide to give a mixture of homoallyl alcohols and fheir allyl ethers (Scheme 14.133) [278], Tris(4-methoxyphenyl)bismuthane catalyzes the cyanation of carbonyl compounds with MesSiCN [175], In the presence of fluoride ions (CeFsjsBi undergoes nucleophilic substitution with an activated C-F bond of perfluorinated compounds to transfer three CgFj groups [279]. (CFsjsBi can also act as a trifluoromethyl anion equivalent [280]. 

The SnCl2-mediated allylation of carbonyl compounds is even possible with allylic alcohol or its corresponding methylcarbonate in the presence of palladium catalyst (Scheme 26) [123]. In the reaction of (E)-but-2-enoI with benzaldehyde in the presence of SnCl2, the rate and the anti selectivity were shown to increase with the amount of water in the reaction mixture [124]. 

Triorganylbismuthines can mediate such C-C bond forming reactions as shown in Scheme 5.38 in the presence or absence of a transition metal salt. Tributylbismuthine promotes the allylation of carbonyl compounds with allylic bromide [91HAC297] and the olefination of diazo compounds with carbonyl compounds in the presence of catalytic amounts of a copper(I) halide [90TL5897]. A combination of triphenylbismuthine and WCle promotes the metathesis of olefins, while a similar combination with TiC facilitates the stereoselective Diels-Alder reaction of unsaturated esters [76TMC183, 93JOC4783]. 

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]. 

A major improvement was realized with the use of indium, a metal with a very low first ionization potential (5.8 eV) which works without ultrasonic radiation even at room temperature [87]. As the zero-valent indium species is regenerated by either zinc, aluminum, or tin, a catalytic amount of indium trichloride together with zinc, aluminum [88], or tin [89] could be utilized in the allylation of carbonyl compounds in aqueous medium. The regeneration of indium after its use in an allylation process could be readily carried out by electrodeposition of the metal on an aluminum cathode [90], Compared with tin-mediated allylation in ethanol-water mixtures, the indium procedure is superior in terms of reactivity and selectivity. Indium-mediated allylation of pentoses and hexoses, which were however facilitated in dilute hydrochloric acid, produced fewer by-products and were more dia-stereoselective. The reactivity and the diastereoselectivity are compatible with a chelation-controlled reaction [84, 91]. Indeed, the methodology was used to prepare 3-deoxy-D-galacto-nonulosonic acid (KDN) [92, 93], N-acetylneuraminic acid [93, 94], and analogs [95],... 

Allylation of carbonyl compounds. The reaction of allyl bromide with carbonyl compounds in the presence of zinc iodide is mediated by germanium(II) iodide. Ketones are less reactive than aldehydes and require an excess of the reagent and longer reaction times. 

Mediated by Tin. In 1983, Nokami et al. observed an acceleration of the reaction rate during the allylation of carbonyl compounds with diallyltin dibromide in ether through the addition of water to the reaction mixture. " In one case, by the use of a 1 1 mixture of ether/water as solvent, benzaldehyde was allylated in 75% yield in... 

The allylation of carbonyl compounds via allylboration, allylstannation, and Sc(CF3S03)3- " or InCl3- mediated process for the generation of a variety of homoallylic alcohols has been performed in various imidazolium ILs. The enantioselective allylation of aldehydes has been performed by In(iii) complexes associated by PYBOX ligands in [C4GiIm]PF6 (Scheme... 

This reaction was first reported by Luche in 1985. It is the modification of the initial aqueous allylation of carbonyl compounds (in 1983) via the intermediacy of tin in the presence of a catalytic amount of hydrobromic acid (HBr), to the zinc mediated allylation of carbonyl compounds using ultrasonic irradiation in saturated aqueous NH4CI/THF solution. Thus the new reaction condition is referred to as the Luche reaction. In parallel, Li" and Chan extended the Barbier-Grignard-type reaction to successfully proceed in aqueous media and contributed to the new area called green chemistry. ... 

Indium-mediated allylations of carbonyl compounds have proved to be an important application of the metal. In a typical procedure, a carbonyl compound, an allyl halide, and indium powder are all stirred together in a solvent such as THF or DMF or, remarkably enough, water ... 

Moreover, it was found that nanometer-sized bismuth mediates the allylation of carbonyl compounds in water to afford the corresponding homoallylic alcohols in high yields. Nanometer-sized bismuth is much more effective than regular bismuth powder in this reaction. The regioselectivity of the reaction depends on which kind of nanometer-sized bismuth (Bi nanotubes or in situ Bi nanoparticles) is used (Equation 2) [25b]. 

Unlike the metallic tin-mediated allylation of carbonyl compounds, which requires either acidic medium, aluminum additive, ultrasonic radiation or heating, the metallic zinc-mediated allylation proceeds at room temperature. 

The aqueous indium-mediated allylation of carbonyl compounds could also proceed intramolecularly, as described in carbocyclizations yielding five-, six- and seven-membered rings, which further cyclized to give fused a-methylene-y-lactones ... 

Recently, Sarangi et al. (1995) discovered that the Barbier-type allylation of carbonyl compounds could be mediated by zero-valent copper. In the experimental procedure copper(ll) chloride in its hydrated form was mixed with magnesium powder. This reaction did not proceed with anhydrous CuCU, but addition of water to this anhydrous salt in the presence of magnesium allowed the allylation of aldehydes and ketones with allylic bromides. In the absence of halides, the hydrated reagent (CuCl2 2H20/Mg) turned out to be efficient in the reduction of aldehydes. However, the water of crystallization could not suffice and addition of water promoted the reduction process (Sarangi eta/., 1995). 

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