Barbier coupling, aldehyde

Indium mediates a highly enantioselective Barbier-type allylation of both aromatic and aliphatic aldehydes, using a chiral ethanolamine auxiliary, readily recoverable by acid extraction.193 Barbier coupling of aldehydes can be carried out in water using tin(II) chloride, with cobalt(II) acetylacetonate as catalyst.194... 

In a thorough study it was determined that the Barbier coupling of allylic halides 69a to aldehydes, ketones, or a,p-unsaturated carbonyl compounds 68 catalyzed by Cp2TiCl2 47a in the presence of manganese as the terminal reducing agent proceeds most likely via a radical mechanism (Fig. 21) [149,150], The active titanocene(III)... 

Although organosamariums can react with esters and lactones, they preferentially undergo addition to aldehydes or ketones. This allows organosamariums bearing an ester functionality to be generated and used in intermolecular Barbier couplings (Scheme 5.74).98,99... 

Williams used Sml2 to mediate the Barbier coupling of functionalised a-halo-2-methylazoles with aldehydes.127 For example, coupling of iodide 106 with aldehyde 107 proceeded in good yield to give 108, thus illustrating the value of the method for a future approach to phorboxazole A (Scheme 5.75). 

Aryl radical cyclization.10 Sml2 in HMPA/THF at 25° can effect cyclization of l-allyloxy-2-iodobenzene to a Sm(III) intermediate (a) that can be trapped by electrophiles, including aldehydes or ketones. The report suggests that a similar mechanism operates in the Barbier-type coupling generation of an alkyl radical followed by formation of RSml2, which adds to a carbonyl compound to form an adduct that is hydrolyzed to an alcohol. 

In diethyl ether, the yields of the zinc-mediated Barbier condensation are medium or poor,1 and THF should be preferred for most of the synthetic uses. Sonication is frequently used, e.g. in the reaction of methallyl bromide and salicylaldehyde which provide the expected alcohol27 The condensation of 4-bromo-2-sulfolene with a series of aldehydes and ketones was effected with the zinc-silver couple and ultrasound.28 The reaction occurs without transposition, which is not the case in the presence of magnesium. In refluxing... 

Sml2, which can be prepared conveniently from samarium powder and 1,2-diiodoethane in THF, finds application as a versatile one-electron reducing agent in organic synthesis. Two typical synthetic procedures mediated by Sml2 are the pinacol coupling of aldehydes and the Barbier reaction, as shown in the following schemes ... 

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... 

Indium trichloride promotes catalytically the addition of alkynylstannanes to aldehydes (Table 25).42 Metallic indium also mediates the same Barbier-type coupling between alkynyl halides and aldehydes or ketones to give secondary or tertiary propargyl alcohols (Table 26). Secondary alcohols can be oxidized in situ according an Oppenauer process.395 Thus, alkynyl ketones have been prepared from aldehydes via an indium-mediated alkynylation reaction followed by an indium-mediated Oppenauer oxidation. They are also obtained via an indium-mediated alkynylation of the relevant acyl chlorides (Table 27).396... 

The samarium(ll)-mediated Barbier reaction has been used as an alternative to the deprotonation chemistry to generate 2-alkyl-metallated oxazoles (Scheme 43). This reaction is very useful for the coupling of 2-iodomethyl oxazoles and aliphatic aldehydes. However, reactions with aromatic aldehydes gave mainly pinacol coupling products <20050L4099>. 

Barbier-Wieland degradation Boudroux-Chichibabin synthesis Bouveault aldehyde synthesis Darzens synthesis Friedel-Crafts Reaction Gatterman-Koch reaction Grignard reaction Reformatsky reaction Saytzeff reaction Ullmann coupling reaction Wiirtz coupling reaction Wiirtz-Fittig reaction... 

The general reaction of ketones with halides under Barbier conditions is analogous to that of aldehydes. Usually, the reaction products are alcohols. The major by-products are diols resulting from the coupling of ketyl radical anions [Eq. (5)]. 

This indium-mediated palladium-catalyzed Barbier-type allylation of aldehydes is expanded to cascade reactions with allenes, which give three-component coupling products (Tab. 8.12 and 8.13) [89]. 

Of interest also is the aqueous Reformatsky cross-coupling reaction between a-halo ketones or aldehydes and carbonyl compounds in the presence of zinc, tin and indium. Like the Barbier-type reactions, indium was found to be the most efficient metal and it strongly reduced undesirable side reactions such as the reduction of the halide [134]. As usual in aqueous reactions implicating carbonyl compounds, lanthanum(III) triflate promoted the reaction [146]. 

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