Magnesium dianion

Isopropylmagnesium chloride is used to deprotonate the N-H and a-silyl ring proton of Me2Si(Me4HC5) (HN-r-Bu) [36]. This magnesium dianion is then reacted with TiCl3 [Eq. (29)]. The product complex is used as a Ziegler-Natta catalyst. 

The Ivanov reaction is the preparation of a 3-hydroxy acid by reaction of the magnesium dianion of a carboxylic acid with an aldehyde or ketone." In a seminal paper, Zimmerman and Traxler investigated the Ivanov reaction of phenylacetic acid and benzaldehyde and obtained anti and syn 3-hydroxy acids (127) and (128) in 69% and 22% yields, respectively (equation 85). The observed stereochemistry was rationalized with a cyclic, chair-like transition state in which a magnesium cation is chelated by one oxygen each of the carboxylate enolate and the aldehyde (the Zimmerman-Traxler transition state ). 

Thioamides of primary amines react with two equivalents of n-butyllithium or isopropylmagnesium bromide to give dianions that have been shown to have the (Z)-configuration. These species react with aldehydes to afford predominantly anti aldois (equation 102 Table 14). Again, the magnesium dianions generally show superior stereoselectivity. In certain cases, the degree of stereoselectivity is excellent. This procedure provides a convenient complement to the syn selectivity obtained with thioamides of secondary amines. 

Very high diastereoselectivities are observed in the aldol condensations of secondary thioamide dianions (Z-configuration) with aldehydes. In this reaction the magnesium dianions afford threo aldol products, whereas the monoanion generated from the A -silyl derivatives of the same secondary thioamides affords the cry/Aro-alcohol. This therefore presents a versatile and selective synthesis of both isomers of P-hydroxy-thioamides. 

In some cases, the Grignard reaction can be performed intramolecularly. For example, treatment of 5-bromo-2-pentanone with magnesium and a small amount of mercuric chloride in THE produced 1-methyl-1-cyclobutanol in 60% yield. Other four- and five-membered ring compounds were also prepared by this procedure. Similar closing of five- and six-membered rings was achieved by treatment of a 6- or s-halocarbonyl compound, not with a metal, but with a dianion derived from nickel... 

N,N -Chelation is also exhibited by the dianionic P(III)/P(V) ligands (25) in the MejSn complex (31) [39] and in the magnesium complex (32) [40], which is prepared by oxidation of [Mg(thf)2[ BuNP(p-N Bu)2PN Bu] by elemental tellurium [40]. One of the endocychc N Bu groups in (32) is also weakly coordinated to magnesium, thus providing an intramolecular base-stabihzation similar to that observed for complexes of type (8). 

The dianion 16 derived from this structure has such an intensity of negative charge compressed into a small space that an ideal microenvironment is created for divalent metals ions 17 [Eq. (3)]. The molecule enjoys a vise-like shape and a grip that holds calcium or magnesium ions tightly 15). 

Cyclo-coupling between arylalkenes and an aliphatic ester function is achieved by electrolysis in tetrahydrofuran using cathode and anode both of magnesium in an undivided cell. The first electron addition is to the arylalkene. The bond forming steps involves nucleophilic attack by radical-anions or dianions derived from the alkene. Magnesium ions generated at the anode are essential to the process. The... 

A variety of other olefins were studied 1,6-diphenylhexatriene, anthracene and cyclooc-tatetraene also displace butadiene from its polymeric magnesium complex. Now should these olefin-magnesium species be viewed as magnesacycles Or as contact ion pairs with olefin dianions In any case, no enthalpies of hydrolysis are available, nor quantitation of stabilities by even equilibrium constants. We welcome this information. 

The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean ESR spectra of the radical anion, with minimum interactions with the counterion, can be obtained (116). Further reduction to dianions is very slow, and exhaustive reduction to anion radicals minimizes problems associated with exchange between anion radicals and unreduced substrate (115). It now appears that the solvent HMPA greatly facilitates the one-electron reduction, not only for trimethylsilylsodium, but also for organolithium and magnesium reagents (110). It was found that 0.1F solutions of methyl-, n-butyl-, or f-butyllithium or benzylmagnesium chloride will quantitatively reduce biphenyl to its radical anion in less than 10 minutes (110). 

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