Metal ate” complexes

For amine protection, sulfonamides such as 9 offer several advantages over urethanes 5 or amides 7. In particular, secondary amines protected as the urethane or the amide exist as mixtures of rotational isomers, confusing NMR characterization and making crystallization more difficult. The limitation has been that sulfonamides have been difficult to remove. Masanobu Uchiyama of the University of Tokyo reports (J. Am. Chem. Soc. 2004,126, 8755) the development of transition metal ate complexes that catalyze electron-transfer reduction. While the sulfonamide 10 is inert to Mg in THF, inclusion of a catalytic amount of the ate complex 11 led to 12 in quantitative yield. 

Is there a small cluster chemistry based on lanthanide amides Is alkali metal ate complexation just an annoying fact Is there a real potential for lanthanide amides in material sciences and catalysis The synthetic chemist is challenged by these questions and more, and hopefully the previous sections can deliver some basic tools for tackling these topics. 

The first metal-ate complex, sodium triethylzincate, " was obtained by John Wanklyn who treated diethylzinc with elemental sodium in an attempt to produce ethylsodium. A systematic and reliable approach to ate complexes is the addition of a more polar organometallic reagent (eg., an organolithium) to a less polar organic derivative of an alkali-earth metal, transition element, or metalloid. In this way, a wealth of ate complexes of berrylium, magnesium, zinc, copper, boron, aluminum, silicon, tin, and mercury as well as of nonmetals like phosphorus, sulfur, selenium, tellurium, and even iodine " have been obtained. 

Although isostmctural with hexaaLkylben2ene metal TT-complexes, the bota2iae complexes ate less stable than the ben2ene analogues, and spectroscopic... 

Thiophene is also metal ated by a benzene-soluble ethyl sodium-diethyl zinc complex. Upon carbonation, 65% of 2-thiophenecar-boxylic acid was obtained. ... 

These reagents are not isolated but are used directly in reactions with aldehydes, after generation of ate complexes via the addition of an alkyllithium reagent or pyridine11. 2-(2-Propenyl)-1,3,2-dioxaborolane is also metalated upon treatment with lithium tetramethylpiperidide, but mixtures of a- and y-substitution products are obtained upon treatment of this anion with alkylating agents20. Consequently, this route to a-substituted allylboron compounds appears to be rather limited in scope. 

While Fe(- -3)- and Fe(+2)-ate complexes are formed by the coordination of four anionic ligands, more electron-rich metal centers tend to bind neutral or even... 

One of the most prominent characteristics of Fe(+2) is its ability to undergo oxidation leading to Fe(+3). This was used by Uchiyama et al. when they reported on Fe(+2)-ate complexes as potent electron transfer catalysts [7, 8]. These ferrates are accessible from FeCl2 and 3 equiv. of MeLi. The Fe(+2/+3) oxidation potential of [Me3Fe(+2)]Li 19 in THF is —2.50 V, thus being in between those of Sml2 (—2.33 V) and Mg (—3.05 V). With these alkyliron-ate complexes it was possible to realize a reductive desulfonylation of various A -sulfonylated amines 20 with different basicity. By using Mg metal to restore the active Fe(+2) species 19 a catalytic reductive desulfonylation process was achieved (Scheme 4). 

This section surveys the rapid development which has occurred in the synthesis and structural analyses of mixed metal complexes in the 12 years since the publication of COMC (1995). The limited information which was available then on ate complexes has expanded into a more sophisticated understanding of how different metals gathered in a single complex influence the adopted structure and observed reactivity. 

Incorporation of heavier alkali metals into ate complexes is usually disfavored because of the large lattice energies of the simple salt elimination products MX. However, Stephan et al. have reported that reaction of Cp2ZrHCl with KPH(mes ) in the presence of KH gives the... 

An alternative explanation is that the regioselectivity of insertion (180 vs. 181) is determined by the rate of 1,2-metallate rearrangement, the formation of the regioisomeric ate complexes 176 and 177 being fast and reversible (Scheme 3.39). Interconversion of 176... 

Protonation of an oxo ligand in any of these four metal systems results in electron density changes on the metal centers in the proton-ated complexes. The decrease in electron density on any one oxo ligand upon protonation of the other oxo ligand trans thereof is clear from the 170 chemical shifts for all four metal systems (see Table II and III and Fig. 11). On the other hand, protonation in these systems results in an increase in the trans M = 0 bond strength as was shown crystallographically (see Table I), which in turn also results in an increase in electron density on the cyano carbon atoms as observed from the 13C chemical shifts. 

Only one example of electrophilic behavior of silicon-stabilized lithiooxiranes is reported. Intermolecular C—Li insertion followed by Li20 elimination occurs by raising the temperature, and ( ) vinylsilanes are obtained stereoselectively (Scheme 80). Reaction of lithiooxiranes with aluminum , zirconium and silicon reagents leads to the corresponding ate complexes, which undergo 1,2-metallate rearrangements. 

Metal dithiophosphate complexes are involved in a wide variety of analytical methods for metals. Diethyldithiophosphoric acid reportedly 13-1 is) fonns complexes with thirty-five elements, mainly metals, in various oxidation states which are useful for solvent extraction. No attempt is made here to detail all the applications to solvent extraction methods which have been described since these ate summarized in the texts listed. Dithiophosphoric... 

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