Enolate complexes structure

The chiral auxiliary methodology using boron enolates has been successfully applied to many complex structures (see also Scheme 2.6). 

Nitroalkenes are shown to be effective Michael acceptor B units in three sequential reactions (A + B + C coupling) in one reaction vessel. The sequence is initiated by enolate nucleophiles (A) and is terminated by aldehydes or acrylate electrophiles (C). The utility of this protocol is for rapid assembly of complex structures from simple and readily available components. A short total synthesis of a pyrrolizidine alkaloid is presented in Scheme 10.16.114... 

The more complex structures are inappropriate for consideration here, but the two compounds orsellinic acid and phloracetophenone exemplify nicely the enolate anion mechanisms we have been considering, as well as the concept of keto-enol tautomerism. 

Essentially the same sort of enolate anion aldol and Claisen reactions occur in the production of the more complex structures mycophenolic acid, griseofulvin, and tetracycline. However, the final structure is only obtained after a series of further modifications. 

The chemistry of enolates has provided excellent routes to highly complex structures, in particular in the total synthesis of natural products. Because of the highly oxygenated structures of carbohydrates, enolate formation could easily result in p-elimination of a suitably located oxygenated group (ethers, esters, and such) to provide enone. For these reasons, the chemistry of carbohydrate enolates has been poorly documented. 

Alkylation Alkylation of the phenylindanone 31 with catalyst 3a by the Merck group demonstrates the reward that can accompany a careful and systematic study of a particular phase-transfer reaction (Scheme 10.3) [5d,5f,9,36], The numerous reaction variables were optimized and the kinetics and mechanism of the reaction were studied in detail. It has been proposed that the chiral induction step involves an ion-pair in which the enolate anion fits on top of the catalyst and is positioned by electrostatic and hydrogen-bonding effects as well as 71—71 stacking interactions between the aromatic rings in the catalyst and the enolate. The electrophile then preferentially approaches the ion-pair from the top (front) face, because the catalyst effectively shields the bottom-face approach. A crystal structure of the catalyst as well as calculations of the catalyst-enolate complex support this interpretation [9a,91]. Alkylations of related active methine compounds, such as 33 to 34 (Scheme 10.3), have also appeared [10,11]. 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

Another mechanistically interesting example was reported by Ikariya et al. in 2003 (Scheme 10) [12], The authors focused on the basic character of the Ru-amido complex 21. The reaction of dimethyl malonate with 21 afforded a C-bound Ru-enolate, the structure of which was supported by X-ray analysis. It was considered that the N-H moiety plays a role in bringing the enones to the optimum position by hydrogen bonding, as shown in 22 C-C bond formation then occurs at relatively high reaction temperature, affording the desired adduct in 97 % ee. Before this report appeared, a related catalyst system had been examined by Suzuki et al. for the Type I reaction [4f]. 

D. Seebach, Structure and Reactivity of Lithium Enolates. From Pinacolone to Selective C-Alkylations of Peptides. Difficulties and Opportunities Afforded by Complex Structures, Angew. Chem. Int. Ed. Engl 1988, 27, 1624-1654. 

D. Seebach, Structure and reactivity of lithium enolates From pinacolone to selective C-alkyla-tions of peptides. Difficulties and opportunities afforded by complex structures, Angew. Chem., Int. Ed. Engl. 1988, 27,1624. 

In a model reaction used to support the structure identification of a neutral polyaza cleft for enolate complexations, the disubstituted dihydropyrrolo[2,3-/i]quinoline (51) was prepared by alkylation of 7-bromo-5,6,7,8-tetrahydro-8-quinolone with ethyl 3,3-diamino-2-propenoate (Equation (24)) <91JA9687>. The regioisomeric residence of the two substituents in this product was supported by 13C—13C shift-correlated NMR spectroscopy. 

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. 

Bidentate /3-diketonates usually have symmetric structure, and many crystal structures show sets of equal M—O, C—C and C—O bonds. Alkali metal enolate structures are symmetric as well as those for the Pd, Rh and A1 enolates. Few structures show unequal M—O distances these enolate complexes (M = Ge, Sn and Sb) are asymmetric as the metal atom is not located at equal distances from the nearest oxygen atoms . 

Consecutive nuclear Overhauser enhancement spectroscopy (NOESY) experiments allowed one to calculate the distance between the enolate proton and other protons in the octahedrally coordinated titanium enolate 13. The H -tf and H -H- distances calculated from NMR spectra are 0.2 A longer or 0.5 A shorter, respectively, than those in the model structures. Using modeling studies, the same prediction can be observed, indicating that one side of the Ti enolate complex is less sterically hindered than the other side. This is due to the efficient diffusion pathway formed by the enolate H (H ), the protons on the i-Pr group (H, Me) and the protons on the selone heterocycle (H, H ). These distances confirm that the enolate oxygen atoms are cis to each other and their orientation should promote a strong facial preference upon subsequent reaction with an aldehyde or ketone. 

Some very interesting structural features exist for neutral jS-keto-enolate complexes (18) of the type M(AA)nLm. Adducts with nitrogenous bases or water have yielded a 5-coordinate irregular Zn(AA)2 H20 complex and trans 6-coordinate cobalt(II) and nickel (II) complexes M(AA)2 2B, with B = H2O or pyridine. The vanadyl complex, VO(AA)2, contains a square pyramid of oxygens about the metal, while U02(AA)2-H2O shows a pentagonal bipyramidal structure. The anhydrous material is dimeric. 

Figure 13 Enol resonance structure of BF3/enal complexes...

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