Molecular structure lead enolates

In this nonplanar geometry the intramolecular cc resonance is weakened at the expense of the inter-molecular Uq oh interaction, and the radical reverts to a much more localized enol-like form. The NRT description of the relaxed A-state complex exhibits these effects, as shown in the leading a-spin structures 21a to 21c... 

In the majority of dehydration reactions, heterocyclic compounds are formed, rather than carbocyclic compounds. Many possibilities for formation of carbocyclic compounds exist, but these are important only if (a) the heterocyclic or acyclic tautomers cannot undergo further elimination reactions, or (b) the conditions of reaction greatly favor the formation of an acyclic tautomer capable of affording only the carbocyclic compound. Both five- and six-membered carbocyclic compounds have been isolated, with reductic acid being the compound most frequently reported. Ring closure occurs by an inter-molecular, aldol reaction that involves the carbonyl group and an enolic structure. Many examples of these aldol reactions that lead to formation of carbocyclic rings have been studied.47 As both elimination and addition of a proton are involved, the reaction occurs in both acidic and basic solutions. As examples of the facility of this reaction, pyruvic acid condenses spontaneously to a dibasic acid at room temperature in dilute solution, and such 8-diketones as 29 readily cyclize to form cyclohexenones, presumably by way of 30, either in acid or base. 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

Metal enolate solutions consist of molecular aggregates (6) such as dimers, trimers and tetramers in equilibrium with monomeric covalently bonded species (7), contact ion pairs (8) and solvent-separated ion pairs (9), as shown in Scheme 1. The nature of the metal cation, the solvent and, to a degree, the structure of the enolate anion itself may significantly influence the extent of association between the anion and the metal cation. In general, the factors that favor loose association, e.g. solvent-separated ion pairs, lead to an increase in the nucleophilicity of the enolate toward alkylating agents and also its ability to function as a base, i.e. to participate in proton transfer reactions. 

Following an early lead from the Meyers group [94,95], Paterson used the readily available diisopinocampheyl (Ipc) boron triflate to make Z(0)-boron enolates of 3-pentanone [93] and other ketones [96], which add to aldehydes to produce syn adducts in 83 - 96% es (Scheme 5.16 and Table 5.6). Based on molecular mechanics calculations [55,56], the transition structure analysis shown in Scheme 5.16 was suggested to rationalize the enantioselectivity. The axial boron ligand rotates so that the C-H bond is over the top of the Zimmerman-Traxler six-membered ring, and the equatorial ligand orients with its C-H bond toward the... 

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