Chelate complexes, enolates

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

Further variations of the Claisen rearrangement protocol were also utilized for the synthesis of allenic amino acid derivatives. Whereas the Ireland-Claisen rearrangement led to unsatisfactory results [133b], a number of variously substituted a-allenic a-amino acids were prepared by Kazmaier [135] by chelate-controlled Claisen rearrangement of ester enolates (Scheme 18.47). For example, deprotonation of the propargylic ester 147 with 2 equiv. of lithium diisopropylamide and transmetallation with zinc chloride furnished the chelate complex 148, which underwent a highly syn-stereoselective rearrangement to the amino acid derivative 149. 

Although the results are easily rationalised in the case of the a-alkylation (attack of the electrophile at the Re face, i.e., attack from the less hindered a face), in the aldol condensation it is somewhat more difficult to rationalise and several factors should be considered. According to Evans [14] one possible explanation for the diastereofacial selection observed for these chiral enolates is illustrated in Scheme 9.14. In the aldol reactions, the more basic carbonyl group of the aldehyde partner interacts with the chelated boron enolate 45 to give the "complex" A which may... 

Base-catalyzed transformations can be carried out elsewhere on a complex molecule in the presence of such protected -dicarbonyl magnesium chelate. For example, the chelated magnesium enolate of a /3-ketoester such as 71 prevents the carbonyl keto group becoming an acceptor in aldol condensations. However, in the presence of excess of magnesium methanolate, exchange of the acetyl methyl protons can occur via a carbanion 72 stabilized by delocalization into the adjacent chelate system (equation 99). 

In the first step ketone 6 is deprotonated with LDA. Based on the Ireland model,19 it is the Z enolate 39 that forms. This attacks the aldehyde carbon atom in 9 such that the Z lithium enolate leads to the syn isomer. Because of intramolecular chair-like chelate complex 39 the freedom of the transition state is limited and attack on aldehyde 9 is so directed that only the desired diastereomer 10 arises. 

The decarboxylation of simple /f-ketoacids, such as acetoacetic acid, is not metal promoted (Fig. 5-22) - this is in part due to formation of the chelate complex, which is in the enolate form. Mechanistic studies have indicated that the enol or enolate is inactive in the decarboxylation reaction. The mechanism indicated in Fig. 5-21 is not applicable to the metal complex. 

The presence of S-carbonyl groups with at least one proton on the carbon between them allows a keto/enol tautomerism to occur and, under appropriate conditions, the eno-lic proton can be removed. The S-5-tricarbonyl compounds are the higher analogues of the / -diketonates and can take triketone, monoenol and dienol forms in their tautomeric equilibrium (equation 86) accordingly, they can behave as bidentate or tridentate ligands to form metal chelate complexes. ... 

Low-valent Ru(II) [150] and Rh(I) complexes catalyze aldol and Michael reactions of 2-nitrilo esters. The sequence is thought to be initiated by nitrile complexation to the transition metal. This Lewis acid-activation is followed by an oxidative addition to give a metal hydride and a nitrile complexed enolate as shown in Sch. 36. Examples including diastereoselective Ru(II) catalyzed reactions [151] and enantioselective Rh(I)-catalyzed reactions [152-154] with the large trans-chelating chiral ligand PhTRAP are shown in Tables 8 and 9. 

The capability of the highly oxygenated carbohydrate auxiliaries to coordinate the counter-ion of the enolate allows the formation of chiral chelate complexes with a restricted flexibility of the enolate moiety. The cation complexation also increases the tendency of the carbohydrate to react as a leaving group. It has been found [153] that the enolate 204 generated by deprotonation of the carbohydrate linked ester 203 with LDA underwent an elimination of the carbohydrate moiety generating the alcohol chelate complex 205 and the ketene 206 (Scheme 10.67). 

An early reference teaches us that even trimethylaluminum can cause deprotonation of a specialized ketone to generate the aluminum enolate under rather drastic conditions (toluene, reflux) [42]. As expected, the reaction proceeded under thermodynamic control, in which aldol and retro-aldol reactions occurred reversibly, to give a high level of anti diastereoselectivity, with concomitant removal of chelation complex 46 from the solvent (Scheme 6.22). 

At pH 6.5-S.5 the enol form of dibenzoylmethane (formula 54.1), a reagent of the P-diketone group, forms a yellow uranyl chelate complex which has been used for determination of uranium [76,77]. It dissolves in aqueous ethanol medium containing pyridine, or may be extracted with ethyl acetate, butyl acetate, or CHCI3 [78]. 

In many instances the complexing enolate anion forms neutral chelates with metals whose preferred coordination number is twice their oxidation state the resulting complexes are effectively coordinatively saturated, thus precluding further... 

In all cases except (2) the nitrogen is part of an imide system which forms a chelate complex after deprotonation to the enolate. We used a different approach by using the N,O-acetals 35/36. This was first performed in racemic form to test the diastereose-lectivity of the enolate alkylation (Scheme 8). The diastereomers 35 and 36 are readily separable by chromatography. Analogously, amides 35a and 36a are prepared from O-jV-Boc-amino benzaldehyde. Derivative 36a is crystalline and was submitted to an X-ray crystal structure analysis (Fig. 1). 

A second type of oxygen-chelated complex that can be formed with acetylacetone is the simple Lewis acid-base adduct. In these complexes acetylacetone does not lose its acidic proton to form an enolate anion, but rather as the neutral molecule in the keto tautomer donates electrons from the oxygens of each carbonyl to an acceptor or acidic species. Examples of this type of complex are the six-coordinate adducts formed between typically strong Lewis adds as tin tetrachloride or titanium... 

A very efficient approach to 1-aryl-substituted l-lithio-2-alkenyl carbamates was very recently found by Seppi [172]. Two (from many more) examples are depicted in Eq. (75). On treating the (Z)-enol carbamate (Z)-261 with -butyllith-ium/(-)-sparteine, one of the enantiotopic y-protons or Hjj is removed with complete selectivity, leading to a highly enriched, configurationally stable, five-membered chelate complex 262. Trapping the reaction mixture with ketones or acid chlorides produces products 263 in essentially enantiomerically pure form, having the absolute configuration shown. 

Keim and co-workers have found that the treatment of Ni(COD)2 with both triphenylphosphine and the phosphorane, Ph3PCHC(0)Ph, affords a nickel(II) chelate complex formally derived from the enolate of Ph2PCH2C(0)Ph (Figure 7.10). ° This crystalline compound, which can be conveniently prepared on a large scale, has been characterized by single crystal X-ray diffraction. Much like o-diphenylphosphinobenzoate, the novel enolate ligand functions as an anionic P-O donor. What is particularly intriguing is that its nickel complex also efficiently catalyzes the formation of linear a-olefins from ethylene. 

Exceptions can only be observed if steric interactions either between the substituents in the allyl substrate [14] or between the allyl moiety and the ligands [15] destabilize the syn,syn-complex. However, selective palladiumotalyzed conversions of (Z)-allyl substrates with retention of the alkene geometry is not a trivial issue. A transfer of the (Z)-configuration from the allyl substrate to the product would only be possible, if the reaction could be carried out at low temperatures (below —60 °C) at which isomerization reactions do not yet take place. This can only be achieved with highly reactive nucleophiles such as chelated ester enolates, but not with the generally used stabilized soft C-nucleophiles [16]. 

All beta-ditetone compounds exist in the enol form and readily form stable anions with electron delocalization over the O—C—CH—C—O chain, which chelates the cations via the two oxygen atoms. The "oxygen-loving" metals whose cations are hard acids form stable chelate complexes of the form [M(n)(ACAC)J(Figure 4). Beryllium is particularly well extracted as the neutral [) ACAC)2] complex because the combination of the tetrahedral [BeO I coordination geometry and the small radius of the ion results in a v y stable and compact molecule. 

---