Allylic acetates reactions with carbonyl compounds

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

The Pd(0)-catalyzed displacement of allylic acetates (297) with various nucleophiles via the allylic Pd(II) complex (298) is a well-established procedure (Scheme 114). Through attack of electrons (+2e ) in place of nucleophiles, (298) is expected to undergo a reductive cleavage providing allylic carbanions (299) and the acetate anion along with Pd(0) complexes. The latter can then be captured by various electrophiles (polarity inversion. Scheme 114) leading to (300) [434]. This procedure is useful for the deprotection of allyl esters under neutral conditions. Recently, a mechanistic study of the Pd-catalyzed reaction of allylic acetate (297), using carbonyl compounds as an electrophile, has been reported [435]. 

Although 2-[(trimethylsilyl)methyl]allyl esters and halides react with imines to give pyrrolidines (Scheme 22),the cycloaddition reaction with carbonyl compounds requires much more drastic conditions. On the other hand, 2-[(trialkyIstannyl)methyl]allyl acetate can react with aldehydes to give furanyl compound, where the presence of trialkyltin acetate, as a by-product, is crucial to compel the reaction successfully (Scheme 23). In fact, in the presence of the hialkyltin acetate, even the 2-[(trimethyl silyl)methyl]allyl ester undergoes [3 + 2] cycloaddition reactions with aldehydes (Scheme 24) and ketones successfully.Similarly, InCls is also an efficient additive for the cycloaddition reaction and both aldehydes and ketones are suitable as sub-strates. ... 

Propargylic lithium alanates or lithium borates react with allylic halides or with carbonyl compounds in a regioselective manner to furnish 1,1 -disubstituted allenes (Scheme The reaction between alkyl halides and aluminum metal is the basis of the oldest method for the synthesis of organoaluminum compounds. For example, propargylic bromides react with aluminum in ether giving organoaluminum compounds that on treatment with acetals yield solely a-allenic ethers (equation 1)." However, the reaction of simple alkyl halides with aluminum metal requires a long reaction time. 

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water (Eq. 8.49).108 When the allylation was mediated by indium in water, the reaction went smoothly at room temperature without any promoter, whereas the use of zinc and tin usually requires acid catalysis, heat, or sonication. The mildness of the reaction conditions makes it possible to use the indium method to allylate a methyl ketone in the presence of an acid-sensitive acetal functional group (Eq. 8.50). Furthermore, the coupling of ethyl 2-(bromomethyl)acrylate with carbonyl compounds proceeds equally well under the same reaction conditions, giving ready access to various hydroxyl acids including, for example, sialic acids. 

Yet another development which is worth mentioning in this context is the 5n substitution of acetals of unsaturated carbonyl compounds. The phenomenon that the reaction of an allylic acetal with a Grignard reagent in the presence of CuBr may occur as a vinylogous substitution with double bond shift has been long known.The reaction can be utilized in an efficient synthesis of 3-substituted propionaldehydes using the acrolein acetal as a homoenolate cation equivalent (Scheme 38). ... 

Hollis, T. K., Robinson, N. p., Whelan, J., Bosnich, B. Homogeneous catalysis. Use of the TiCp2(CF3S03)2] catalyst for the Sakurai reaction of allylic silanes with orthoesters, acetals, ketals and carbonyl compounds. Tetrahedron Lett. 1993, 34,4309-4312. 

Allylations. The reaction of tetraallylstannane with carbonyl compounds or acetals ... 

The cycHc urea moiety provides structural rigidity as well as hydrogen-bonding possibihties similar to those of the imidazoles described above. The corresponding 2-imidazolones have been prepared on a soHd phase by tandem aminoacylation of a resin-bound allylic amine with an isocyanate followed by intramolecular Michael addition [73]. However, due to the paucity of data presented on the characterized compounds and the brief experimental procedure, this synthesis is not discussed in detail. Access to cyclic ureas or thioureas has also been obtained by reaction with carbonyl- or thiocarbonyldiimidazole through a cyclo-release mechanism [74—76]. 1,3-Dihydroimidazolones have been obtained by treatment of ureido acetals with TFA and subsequent conversion in an intramolecular cyclization via an N-acyliminium ion [77]. 

Allylation. The reaction of allylsilanes with carbonyl compounds, acetals, and ortho esters is catalyzed by Cp2Ti(OTf)2. Rapid reaction rates and good conversion... 

A-homoallylic alcohols were prepared in good yields by allylation of dialdehydes or their acetals with allyl bromide, tin(II) chloride, and potassium iodide in water or water/THF (Eq. 8.46). ° Under ultrason-ication, it was found that SnCU could efficiently mediate the aqueous Barbier reactions between carbonyl compounds and allyl bromide to give the corresponding homoallylic alcohols in high yields without using any Lewis-acid catalyst. 

Non-fluxional vinylic boranes do not react with carbonyl compounds. Nevertheless, the vinylic borane 29 reacts with acetone (2 h under reflux) yielding the Z-isomer of the homoallylic borinic ester 30b. The cw-configuration of the reaction product 30b corresponds to the following sequence of transformations (Scheme 2.11). The [1,7]-H shift in the vinylic borane 29 gives the allylic Z, Z-isomer 28d which immediately reacts with acetone before the equilibrium among the allylic isomers is established. On the other hand, cyclopentanone reacts directly with 29 under mild conditions yielding Z, Z-1,3,5-heptatriene 31 and borinic ester 32 (Scheme 2.12). Apparently 29 reacts with the enol form of cyclopentanone, and a direct splitting of the B-Q ,2 bond takes place. Similar reaction of 29 with acetic acid was used for the preparative synthesis of previously unknown hydrocarbon 31 (Scheme 2.12) [35]. 

Synthesis of homoallylic alcohols through the reaction of organometallic allyl compounds with carbonyl compounds is one of the most important processes in organic synthesis. Sc(OTf)3-catalyzed allylation of aldehydes with tetraallyltin to prepare homoallylic alcohols has been reported. Recently, it was revealed that the addition of acetic anhydride to the reaction mixture dramatically increases the reaction s yields to more than 90% [10]. On the other hand, in the presence of a catalytic amount of Sc(OTf)3 (l-5mol%), benzoylhydrazines reacted with tetraallyltin to give the corresponding homoallylic hydrazines, which were readily converted to homoallylic amines (Scheme 12.6) [11]. Three-component reactions of aldehydes, benzoylhydrazine, and tetraallyltin also proceeded smoothly in the presence of a catalytic amount of Sc(OTf)3. 

Some organosilicon compounds undergo transmetallation. The allylic cyanide 461 was prepared by the reaction of an allylic carbonate with trimethylsi-lyl cyanide[298]. The oriho esters and acetals of the o. d-unsaturated carbonyl compounds 462 undergo cyanation with trimefhylsilyl cyanide[95]. 

In the preparation of the thiazides containing more highly functionalized side chains (183-185), an acetal of the aldehyde is usually used rather than the free carbonyl compound. Thus, trichlomethiazide (183) is prepared by reaction of 160 with the dimethyl acetal from dichloroacetaldehyde. In a similar vein, alkylation of the acetalthiol, 190, with allyl bromide affords 191. This yields altizide (184) on condensation with 160. Alkylation of 190 with 2,2,2-trifluoroethyl iodide gives 192. This leads to epithiazide (185) on condensation with 160. 

The reactions are accelerated by bromide salts, which are thought to exchange for acetate in the ir-allylic complex. The reactions of acyclic compounds occur with minimal E Z isomerization. This result implies that the TT-allyl intermediate is captured by carbonylation faster than E Z isomerization occurs. 

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,... 

Durandetti et al. have described iron-catalyzed electrochemical allylation of carbonyl compounds with allylic acetates (Equation (27)).333 In the case of aldehydes, slow addition of the corresponding aldehyde is required in order to avoid pinacol formation. With crotyl acetate (R3 = Me), the reaction proved to be highly regioselective, providing almost exclusively branched homoallylic alcohols 150. 

Preparation of o,/3-Unsaturated Carbonyl Compounds by the Reactions of Silyl End Ethers and End Acetates with Allyl Carbonates... 

Heterogeneous palladium catalysts proved to be active in the conversion of simple alkenes to the corresponding allylic acetates, carbonyl compounds, and carboxylic acids.694 704 Allyl acetate or acrylic acid from propylene was selectively produced on a palladium on charcoal catalyst depending on catalyst pretreatment and reaction conditions.694 Allylic oxidation with singlet oxygen to yield allylic hydroperoxides is discussed in Section 9.2.2. 

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