Transition metal enolates structure

The position of the lithium cation in the enolates has been the object of much debate. It is now well established that the cations of the strongly electropositive metals of groups I, II and III stand closer to the oxygen than to the carbon atom while this metalotropy is more balanced with transition metal enolates. The structure of the lithium enolates in vacuum, in the solid state as well as in solution is discussed in detail in the next section of this chapter. 

Not all aldol additions exhibit a dependence of product configuration on enolate geometry. Acid catalyzed aldols [45], some base catalyzed aldols [58], and aldols of some transition metal enolates [63,64] show no such dependency. For example, zirconium enolates afford syn adducts ( / topicity) independent of enolate geometry for a number of propionates [63,64]. As shown in Scheme 5.9, two explanations have been proposed to explain the behavior of zirconium enolates. One explanation (Scheme 5.9a) is that the closed transition structure changes from a chair for the Z(0)-enolate to a boat for the (0)-enolate [16,63,65]. Another hypothesis is that these additions occur via an open transition structure. Although the original authors... 

This section includes the synthesis and structures of bothC-bound and Obound transition metal enolate complexes. Reviews of reactions catalyzed by Lewis acidic, early metal enolate intermediates have been published. Cross-couphng of enolates is described in Qiapter 19. Additional reactions of enolate nucleophiles, such as enolate allylation, are ttiought to occur by external attack of the enolate nucleophile on -ir-ligands and are described in Chapters 11 and 20. Late transition metal enolate complexes undergo a number of the classes of stoichiometric reactions presented in Chapters 3-12 of this text, including 3-hydride elimination to produce enones, - insertion of unsaturated ir-systems, and reductive eluninations. - - - ... 

Triketones are homologues of 1,3-diketones and in these, too, keto-enol tautomerism has been probed by H NMR spectroscopy.567,568 Triketones and tetraketones may coordinate to one or more metal ions per molecule. In the latter case the metal centers are held in such close proximity that, in some cases, interesting magnetic effects may be observed. Structural and magnetic properties of polynuclear transition metal jS-polyketonates have been thoroughly reviewed.569,570... 

Another impressive example of the transition metal-catalyzed Michael reaction was reported by Sawamura and Ito in 1992 (Scheme 6) [7]. a-Methylcyanoacetate was treated with enones using 1 mol% Rh-TRAP (12) complex, and the corresponding adduct 13 was formed in up to 93 % ee. For this reaction, the trans-coordination mode of the chiral diphosphine 12 was essential for high asymmetric induction. It was proposed that coordination of the nitrile group to Rh, then oxidative addition of the active methine C-H bond gave not the a-C-bound enolate, but the nitrile-coordinating enolate 14, which was considered to be a reactive intermediate. The unique structure of this enolate was supported by X-ray analysis of a similar achiral Ru-cyanoacetate complex [8]. 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Transition metals in a high oxidation state are often capable of extracting an electron from electron-rich organic substances. Ketones, esters, nitriles and various other carbon acids that can form enols, enolates and related structures are by far the most commonly used substrates. Their oxidation can lead to a free radical, which then follows one or more of the pathways deployed in Scheme 8.3. Its important to take into account that the rate of radical production will depend on the exact structure of the substrate, its propensity to exist as the corresponding enol or enolate in the medium, the pH, the solvent, the temperature and, of course, the redox potential of the metallic salt (which can be strongly affected by the nature of the ligand around the metal) and the exact mechanism by which electron transfer actually occurs (i.e. inner or outer sphere)... 

To facilitate an analysis of enolate reactivity and as an aid to the rationalization of the stereochemical outcome of the aldol reaction, a consideration of the enolate structure is necessary. For convenience, the following classification of transition metal and lanthanide metal enoiates will be used here T) -0-bound metal enoiates (1) of carbonyl compounds T) -C-bound metal enoiates (2) and ij -metal enoiates (3) of... 

An important modification of the aldol reaction involves the use of boron enolates. The boron enolates react with aldehydes to give aldols. A cyclic transition state is believed to be involved, and, in general, the stereoselectivity is higher than for lithium or magnesium enolates. The O—B bond distances are shorter than those in metal enolates, and this leads to a more compact structure for the transition state. This should magnify the steric interactions which control stereoselectivity. 

The third species in Scheme 1.1, the oxallyl enolate 3, featuring an // -metal bond is also typical for transition metals and may coexist with the O- and C-bound species in equilibria. Enolates with oxallyl structure 3 were obtained by directed preparation and characterized [24] and also postulated as reactive intermediates [25]. The unambiguously characterized rhodium complex 16 (Scheme 1.6) may serve as an illustrative example. According to several theoretical calculations, lithium enolates may form an bond, resulting from a 3t(CC)-Li bond in addition to the OLi bond [26]. 

In summary, the relatively electropositive, highly Lewis acidic early transition metals form O-bound enolates, as expected according to their oxophilic character. When switching to the so-called late transition metals, located to the right of group 5 in the periodic table, the clear preference for O-bound enolate structures vanishes, and the C-bound tautomers become involved but also the i/ -bound oxallyl type. A general rule on which tautomer is favored by which metal cannot be deduced, because the type of enolate structure is mainly determined by the oxidation state, the coordination number, and in particular the individual ligands at the transition metal. 

---