Other Metals in Barbier-Type Reactions

References quoted are mainly from the recent literature and there is little doubt that in the time between the finishing of this manuscript and its publishing a number of important new contributions will have been made. 

Although in the main, the Saytzefif reactions of allylic halides have been investigated it is to be expected that new developments will be aimed at the use of other, less reactive halides. 

The inclusion in this group of 1-bromoadamantane (p. 117, ref. [100b]), with which successful Barbier-type reactions have been reported mainly with lithium in Sect. 3.3.6, indicates that other less reactive halides will soon follow. 

The reported use of water and alcohols as solvents for Zn-Barbier reactions is an astounding development and will stimulate more research in the use of water as solvent in organic synthesis, also in view of increasingly rigid environmental laws. 

Further researches with these solvents will also lead to a better understanding of the mechanism of the (Zn-)Barbier reaction. 

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Chapter 3 will deal with the application of other metals in Barbier-type reactions, quite often also referred to as Saytzeff reactions, Grignard reactions, etc. 

After having started this monograph with one-step carbon-carbon bond formation reactions in which magnesium was the metallic reagent (chap. 2) and then having progressed on to the use of lithium and zinc, some attention will now be focussed on the application of numerous other metals in Barbier-type reactions. 

The indium-mediated Barbier reaction has certainly become one of the most popular reactions for creating a carbon-carbon bond under aqueous conditions and has led to spectacular developments in recent years. Compared to other metals, indium is resistant to oxidation, hydrolysis, and has a very low first ionization potential (5.79 eV, in contrast to the second one which is quite normal) which confers on it a remarkable reactivity in Barbier-type reactions. In 1991, Li and co-workers reported the first allylation of aldehydes and ketones mediated by indium in water without any additives or special activation [133]. In particular, the use of indium allowed reactions with acid-sensitive compounds [134] or the preparation of complex carbohydrates such as deaminated... 

Several other metals have been used in this Barbier-type reaction. Thus, a mixture of bismuth(III) chloride/aluminum [160] or of bismuth(III)chloride/ magnesium [161] was shown to promote allylation of aldehydes under aqueous conditions yielding homoallylic alcohols in good yields. Similarly, the same reaction can be promoted with metallic lead [162], highly reactive antimony prepared by the NaBH4 reduction of SbClj [163] or a mixture of manganese and catalytic copper [164]. In the latter case, aromatic aldehydes reacted whereas ali-... 

As for the names of these reactions with metals, other than magnesium, it has already been stated in Chapter One that preference will be given to the name Barbier-type reaction instead of simply Barbier reaction. Furthermore the name of each individual metal will be included in the indication of the processes hence there will be mention of Li-Barbier reactions, Zn-Barbier reactions and so on. 

The oxidative insertion of zinc into a-halo carbonyl compounds and the subsequent reaction of the zinc enolates formed with various electrophiles can either be carried out in a one-pot Barbier-type fashion or in two consecutive steps.1-3 Zinc enolates exhibit a reasonably high stability over a wide temperature range (from -78°C to above 80°C for short periods of time) compared to other metal enolates. Although it has been reported that solutions of BrZnCH2COOtBu can be stored for several days without loss in activity,5 it is generally advisable to use freshly prepared reagents in order to avoid... 

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water.12 The work was designed on the basis of the first ionisation potentials of different elements, in which indium has the lowest first ionisation potential relative to the other metallic elements near it in the periodic table. On the other hand, indium metal is not sensitive to boiling water or alkali and does not form oxides readily in air. Such special properties of indium indicate that it is perhaps a promising metal for aqueous Barbier-Grignard-type reactions. Indeed, it appears that indium is the most reactive and... 

As C—M bonds of most post-transition metals have a strong covalent character and because many reactions can occur also via radical and radical anion processes on metal surfaces, it is not surprising that many other metals have been foimd to mediate the Barbier-Grignard-type reactions in water. 

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