Allyl bromide Benzaldehyde

In that same period systematic studies [70] of the Barbier-type reaction of allylic bromides, benzaldehyde and zinc in ethanol as the solvent (see Sect. 3.4.4, p. Ill) also had led to the conclusion that the organometallic reagent was actually involved as an intermediate in these reactions. 

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,... 

A novel method of generating finely divided zinc metal is by the use of pulsed sonoelectrochemistry using an ultrasonic horn as the cathode [85], Normal electrolysis of ZnCl2 in aqueous NH4CI affords a zinc deposit on the cathode. When the electrolysis is pulsed at 300 ms on/off and the cathode is pulsed ultrasonically at a 100 ms 200 ms on off ratio the zinc is produced as a fine powder. This powder is considerably more active than commercial zinc powder e. g. in the addition of allyl bromide to benzaldehyde (Eq. 3.9). 

Li, et al. reported ethyl-bridged PMOs with Pd(ll) complexed to 3-aminopropyl-Itrimethoxysilane grafted onto the mesoporous walls to be an efficient catalysts for Barbier reaction of benzaldehyde and allyl bromide (Figure 16) [74]. Use of water as the reaction medium combined with the presence of ethyl moiety in the framework (which increased hydrophobicity of the pores) enhanced diffusion of the organic substrates. As can be seen in Table 3 the PMO material showed superior catalytic efficiency compared to grafted SBA-15 and MCM-41 materials with values comparable to homogeneous trials. 

Table 3. Catalysis results for the Barbier reaction of benzaldehyde and allyl bromide in presence of homogeneous palladium catalyst (entry 1) and those prepared by complexing palladium to amine functionalized mesoporous silica (entry 2-4) [74]...

A conjugated dienyl zinc reagent is easily obtained from propargyl bromide and 3 as shown in Scheme 24. The species reacted with allyl bromide as a dienylzinc, and with benzaldehyde as an allenylzinc species48. 

A copper(O) complex, electro-generated from Cu(acac)2, is able to undergo an oxidative addition with benzyl and allyl bromides. Further reduction leads to the coupling products bibenzyl and 1,5-hexadienes Methyl-3-hexene-l,6-dicarb-oxylate can be prepared from butadiene and CO by electroreduction if di-Fe dicyclopentadienyl tetracarbonyl is used as redox catalyst Electro-generated low-valent tungsten species are able to reductively dimerize benzaldehyde to stilbene according to Eq. 83. The reduction potential was controlled at the third wave of the WClg catalyst (V = -1900 mV/SCE)... 

Elemental antimony is known to mediate the Barbier-type allylation of aldehydes by allylic halides.35 The active intermediates are believed to be allylic antimony(m) species, which are generated from the antimony(O) and the halides. In fact, allylic dichlorostibanes, produced by metathesis of SbCl3 with the corresponding allylic stannanes, react with benzaldehyde to give homoallyl alcohols, where the C-C bond is constructed with -selectivity (Equation (l)).36 Fluoride salts such as KF, NaF, RbF, and CsF accelerate the Sb-mediated Barbier-type allylation with allyl bromide in aqueous media (Equation (2)).37 In the absence of the fluoride ion, no allylation occurs. Although aromatic and aliphatic aldehydes are allylated in good yields by a combined use of Sb-KF, acetophenone, cyclohexanone, and methyl pyruvate remain untouched. 

Aldimines react with allyl bromide in the presence of metallic bismuth and tetrabutylammonium bromide in acetonitrile to give homoallyl amines (Equation (42)).77 When a chiral imine derived from benzaldehyde and (S)-valine methyl ester is used as the substrate, the allylation with allylic bismuth(m) species takes place smoothly by the assistance of a Lewis acid such as BF3-OEt2 and A1C13 to afford a chiral homoallylic amine with high diastereoselec-tivity (Equation (43)).78... 

The reaction of benzaldehyde with unsymmetrical allylic bromides in the ionic liquid bmim also proceeds regioselectively to realize a carbon-carbon bond formation at the more substituted allylic carbon (Scheme 5). In the coupling of crotyl bromide with benzaldehyde, the product is a nearly 50 50 mixture of antilsyn diastereomers. Cinnamyl bromide predominantly gives the //-diastereomer. The regio- and diastereoselectivity is similar to that observed for these in aqueous media.104... 

Prenyl bromide affords better chemical yields and better enantioselectivities than allyl bromide (Table 6). The best result (99% yield, 90% (R) ee) is obtained when benzaldehyde is treated with prenyl bromide in the presence of (—)-cinchonidine. A similar enantioselective propargylation reaction of aldehydes with enantioselectivity up to 85% has been achieved in organic solvents by using stoichiometric amounts of (—)-cinchonidine as the chiral source (Table 7).190... 

Z75. (a) Dieckmann of dimethyl adipate, alkylation by allyl bromide, hydrolysis and decarboxylation (c) Robinson with CH3CH = CHCOCH3 then reduction (d) form enamine or enolate, acylate with CICOOEt, methylate with CH3I, do aldol with benzaldehyde. 

Electron-rich phenols react with isoprene in ionic liquids and with added Sc(OTf)3 to give 2,2-dimethylchromans <07SL3050>. Ionic liquids also promote the one-pot synthesis of 4-arylchromans from benzaldehydes, phenols and allyl bromide which involves sequential Barbier allylation, Friedel-Crafts alkylation and an intramolecular hydroalkoxylation <07SL1357>. 

Reaction conditions A MeOH (1 equiv.) B CuCN (1 equiv.) C ethyl 2-(bromomethyl)acrylate D nitrostyrene E MeaSnCI (1 equiv.) F allyl bromide G i-BuOH (1 equiv) (E)-1-iodo-l-butene, Pd(PPh3) cat I CITi(OiPr)3 (2 equiv.), benzaldehyde (1.5 equiv.). 

Allyl bromide and benzaldehyde af ar to be equally reactive towards a-selenoalkyllithiums (Scheme 12, entries a and c), whereas a-selenoalkylcuprates exhibit a different behavior with lylation occurring almost exclusively, even when the reaction is performed in the presence of benzaldehyde (Scheme 12, entries b and d). "... 

Chiral allylic indium reagents and achiral aldehydes. Chiral indium reagents have been generated by the oxidative metallation of allylic bromides, which bear remote stereogenic subunits. For example, the 1,4-asymmetric induction in the indium-mediated coupling of allyl bromides 298 with benzaldehyde affords the syn-adducts as the major products (Table 10-35) [199]. The stereoselectivity of the reaction improves with more sterically encumbered allyl bromides. 

The achiral allylation of aldehydes has also been achieved in recyclable ionic liquids" and in water." Greener still is the corresponding Barbier allylation" of aldehydes and ketones with allylic bromides in water mediated by tin metal. The atom efficiency of this reaction is actually less than the corresponding tetraallyltin allylation of (say) benzaldehyde (65% and 83%, respectively) because of the loss of the heavy bromine atom, but this neglects the synthesis of tetraallyltin, which is prepared from allyl bromide or chloride. A particularly intriguing recent advance with this thoroughly studied reaction is the use of nano-tin " (Scheme 5.8.17). 

Elemental antimony [20] and SbCls-M (M = Fe, Al) binary systems [21] mediate the Barbier-type allylation of aldehydes by allylic halides or phosphates to afford homoallyl alcohols (Scheme 14.1). In the presence of fluoride salts a similar allylation with allyl bromide proceeds in H2O [22]. Allylic dichlorostibanes, generated from allylic stannanes and SbCls, react with benzaldehyde to give homoallyl alcohols (Scheme 14.2) [23]. 

Note that there are several modern versions of the Barbier reaction that use transition metals or their derivatives.Examples include indium metal, samarium iodide, and lead. An example using lead reacted benzaldehyde with allyl bromide in the presence of lead metal to give a 99% yield of 45.56 A retro-Barbier fragmentation has also been reported in which an alcohol was treated with 10 equivalents of bromine and 15 equivalents of potassium carbonate in chloroform to give a bromo ketone.5 ... 

Homoallylic alcohols were directly prepared from aromatic acetals and dioxolanes (> 80%) using a Barbier-type allylation, Zn/allyl bromide/NH4Cl, in the presence of j3-CD in water at 50 °C (Figure 4.8). The synthesis of enantiomerically enriched homoallylic alcohols is an important goal in organic synthesis. An enantioselective metal-mediated allylation of substitnted benzaldehydes with jS-CD was achieved. The reaction proceeds in short reaction times (Ih) at room temperatnre. The homoallylic alcohols are obtained in moderate yields with up to 93% enantioexcess. 

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