Metal dialkylamides

Metal dialkylamides were reviewed by Bradley in 1972.61 A comprehensive list of all zirconium and hafnium amides reported prior to 1979 may be found in the book by Lappert et al.62... 

Although cyclometallation reactions have seldom been observed with d° metal compounds, intramolecular metallation involving early transition metal dialkylamides with formation of azametallacyclopropane, which occurs by /3 H abstraction, has been reported.281... 

Ionic States Resulting from Lower Energy Ionizations of Open-Shell Metal Dialkylamides... 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

By Reactions of Metal Dialkylamides with Alcohols and Phenols 339... 

This method is particularly useful for the synthesis of the alkoxide and phenoxide derivatives of the earlier transition elements. The method is extremely convenient in view of the high volatility of the generated dialkylamines, which are readily removed in vacuum. One major drawback is the synthetic availability of the corresponding metal dialkylamide complex. In some cases the method represents not only the most convenient but also the only synthetic route to an alkoxide derivative. Hence, zirconium tetra-t-butoxide is formed in excellent yield from Zr(NEt2)4 and Bu OH, and the Vlv and CrIV r-butoxides are also readily obtained via this pathway (equation 11).74... 

The alcoholysis of a lower valent metal dialkylamide can also lead to oxidation of the metal, typically with evolution of hydrogen (equations 13 and 14).74,81... 

Supposedly, lanthanide chemistry is lagging behind main group and d-transition metal chemistry. The concluding statement which Bradley made with respect to transition metal dialkylamides and silylamides almost 20 years ago [19b] is a current topic in lanthanide amide chemistry To date homogenous transition-metal catalysis has been restricted to hydrocarbon systems involving the facile formation and rupture of M-H, M-C, C-H and C-C bonds. An extension to include M-N, C-N, M-O, and C-O bonds seems plausible and could lead to substantial advances in transition-metal catalysis. ... 

Carbamate complexes are synthesized by carbonation of metal dialkylamides (equation 20), or by reaction of the appropriate amine, carbon dioxide, and a metal halide (equation 21). In both cases, it has been suggested that the reaction involves combination of the amine and carbon dioxide to form the carbamate anion, followed by anion exchange. ... 

The two main problems in the preparation of silyl enol ethers are control of regios-electivity, kinetic and thermodynamic, and stereoselectivity, (E) and (Z). Although many useful procedures are now available for the kinetic deprotonation of ketones by use of alkali metal dialkylamides, there are few practical procedures for thermodynamic deprotonation. Recently, the author and Yamamoto et al. found that the regio- and stereoselective isomerization of a kinetic silyl enol ether to a thermodynamic ether was catalyzed by LBA [138]. 

The compounds M N [81(013)3] 2)3 may be considered as a special class of transition metal dialkylamides. The bulky trimethylsilyl groups eliminate inter-molecular association and lead to unusually low coordination around the metal atom. These coordinatively unsaturated molecules are highly reactive and may have catalytic applications. 

Although carboxylic acids and their derivatives are somewhat weaker carbon acids than aldehydes and ketones, it is generally possible to quantitatively convert them to the corresponding metal enolates with dialkylamide bases, the most popular of which is LDA. - - Thus, monoanions of saturated esters, lactones, nitriles, /VA -dialkylamides and V-alkyllactams and dianions of carboxylic acids and V-unsub-stituted amides and lactams are easily prepared in aprotic solvents such as THF and C-alkylated with a variety of simple and functionalized SN2-reactive alkylating agents at room temperature or below. When more-hindered systems are involved, the basicity of the metal dialkylamide and the reactivity of the metal enolate can be enhanced by the addition of HMPA. Of course, many of the indirect methods used for the generation of aldehyde and ketone enolates are also applicable to the preparation of enolates of carboxylic acid derivatives (Section 1.1.2.1). O-Alkylations or dialkylations at carbon generally are of minimal importance with metal enolates of carboxylic acid derivatives. 

E. Reactions of Metal Dialkylamides and Bis(trimethylsilyl)amides with... 

Transition-metal dialkylamides and disilylamides (D. C. Bradley and M. H. Chisholm, Accounts Chem. Res. 1976,9, 273). 

Besides binary compounds (MLa.) containing the uninegative ligand L = XRa-, a number of ternary (MLa-L y) and quaternary (ML L L" ) complexes were also isolated by use of supplementary ligands U and L", and there is scope for further exploitation of the field. In this account, we restrict the discussion to metal alkoxides and metal dialkylamides since the metal alkyls are the subject of papers by Lappert et al, (6, 7). 

During the period 1965-1975 the chemistry of the 1,2-dithiolene complexes of the transition metals was the subject of considerable study. However, during this period of great activity few complexes of the early transition metals were reported aside from those of vanadium. The problem had much to do with synthetic procedures, since reaction of, say, the anhydrous metal chlorides with the dithiolene or its sodium salt did not prove successful. However, the use of metal dialkylamides did result in clean reactions (e.g. equation 21). 

Sterically Hindered Base for Enolate Formation. Like other metal dialkylamide bases, sodium bis(trimethylsilyl)amide is sufficiently basic to deprotonate carbonyl-activated carbon acids and is sterically hindered, allowing good initial kinetic vs. thermodynamic deprotonation ratios. The presence of the sodium counterion also allows for subsequent equilibration to the thermodynamically more stable enolate. More recently, this base has been used in the stereoselective generation of enolates for subsequent alkylation or oxidation in asymmetric syntheses. As shown in eq 1, NaHMDS was used to selectively generate a (Z)-enolate alkylation with lodomethane proceeded with excellent diastereoselectivity. In this case, use of the sodium enolate was preferred as it was more reactive than the corresponding lithium enolate at lower temperatures. 

Other electrophilic heterocumulenes (e.g., CS2, RNCO, RNCS, SO2) are also subject to insertion reactions. Accordingly, the insertion of CS2 into the M-N bond in metal dialkylamides produces dithiocarbamate derivatives and the reaction of SO2 with M-C bonds gives sulfinate complexes resulting from either 1,1-insertion [M-0S(0)R] or 1,2-insertion [M-S(R)02]. Rare examples of insertion of dioxygen into Pt-H bonds to generate remarkably stable hydroperoxide derivatives L Pt-OOH have also been reported. 

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states. 

Reactions of Metal Dialkylamides M(NR2)x (R = Me, Et, SiMes) with Alcohols (Method I)... 

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