Lead atoms, asymmetric

Esters are alkylated in the presence of strong bases in aprotic solvents. A common combination is LDA in tetrabydrofuran at low temperatures. Equimolar amounts of base are sufficient and only the mono-carbanion Js formed. After addition of one mole of alkyl halide the products form rapidly, and no dialkylation, which is a problem in the presence of excess base, is possible. Addition of one more mole of LDA and of another alkyl halide leads to asymmetric dialkylation of one or-carbon atom in high yield (R.J. Cregge, 1973). 

The analogous reactions of PbCl2 with lithium l-(dimethylamino)-3,5-dimethylboratabenzene or l-[bis(trimethyl-silyl)amino]-3,5-dimethylboratabenzene yield the corresponding sandwich compounds 44 and 45. The lead atom of 45 displays a rather asymmetric bonding to the boratabenzene rings as evidenced by the fact that the lead-to-carbon/ boron distances span the large range from 2.519 to 3.033 A.7... 

Abstract The use of Co building block in presence of water or an alcohol to functionalise alkenes by hydroxycarbonylation or alkoxycarbonylation reactions is reviewed in this chapter. The hydroxyl groups can be present in the substrate itself so cascade reactions can occur. Palladium precursors are largely involved in these reactions and this analysis of the literature focuses on the mechanisms involving Pd(0), Pd(H)(X)L2, PdX2 and an oxidant like CuCl systems. Introduction of chiral L or L2 ligands or even the presence of chiral carbon atoms in the substrate lead to asymmetric carbonylation reactions. 

The ligands are asymmetrically bound such that both a short and a long M-S bond arise from each ligand. The short bonds are positioned trans to each other in a PbS4 polyhedron that resembles a distorted square pyramid with Pb at the apex. This structure, which appears to result from lone pair-bonding pair repulsions, is quite similar for the Pb(II) and Sn(II) complexes (Table IV). In the Pb(i-PrXant)2 Py complex, the pyridine molecule is located above the lead atom with a Pb-N distance of 2.55(4) A (288). 

The Pb- -S mediated chains observed in the structure of polymeric Pb(S2COEt)2 are disrupted upon the addition of base. The immediate coordination geometry about the lead atom in the adduct Pb(S2CO—i-Pr)2(py) (231) is shown in Fig. 145(a). The lead atom is coordinated by the pyridine nitrogen atom and two xanthate ligands, one that forms asymmetric Pb—SI and Pb—S2 distances of 2.72(2) and 3.06(2) A, and one that forms symmetric bonds of... 

Figure 12. Arrangement of lead atoms in the unit cell (dotted) of a-lead azide. The crystal morphology is shown by the solid lines and the direction of decomposition by the arrow. The axis of the most asymmetric azide radicals lie approximately in this direction.

The imine (101) derived from a 2-aryltryptamine and ethyl glyoxylate undergoes a diastereo-selective spirocyclization to the indolenine (102) (Equation (27)). Replacement of the ethyl ester by chiral esters, such as the 8-phenylmenth-3-yl ester, leads to asymmetric induction at the spiro carbon atom <90HCA439>. Imines derived from tryptophan esters have also been studied as substrates for the Pictet-Spengler cyclization (e.g., <87tlii31 . Enantioselective cyclizations have made use of enantiomerically pure aldehydes and tryptophan esters <92H(34)517, 93JCS(pi)43i, 93T8589>. 

Generally, the model in which all lead atoms in the solid solution are statically shifted from their equilibrium positions in the ideal strueture is considered in the first-order approximation [10],[13]. In each unit cell, lead atoms are shifted in the same directions and at the same distance . In the first-order approximation models, shifts in asymmetric directions are sometimes chosen [[8]]. In this case, the model acquires three additional fitting parameters ... 

Non-enzymatic cyclizations of educts containing chiral centres can lead to products with additional "asymmetric centres. The underlying effect is called "asymmetric induction . Its systematic exploration in steroid syntheses started when G. Saucy discovered in 1971 that a chiral carbon atom in a cyclic educt induces a stereoselective Torgov condensation several carbon atoms away (M. Rosenberger, 1971, 1972). 

Anilino vinyl derivatives of thiazolium (30, R = H) or acetanilido (30, R = C0CH3), as well as formyl methylene 30b (methods E-G), give asymmetrical dyes when condensed with a methyl reactive group of another species (Scheme 42). Mesosubstituted symmetrical or unsymmet-rical thiazolocyanines are obtainable via /S-alkylmercaptovinyl thiazolium derivatives (32) (methods H and I) (Scheme 43). a or /S carbon atoms of the trimethine chain can be substituted by acetyl when a dye is treated with acetic anhydride (method L). The hydrolysis of neocyanines lead to trimethine cyanine by fractional elimination of a composant chain (method K). 

The electronic structure of a trimethine asymmetrical cyanine, controls the attack of a ketomethylene (Scheme 54). There is a condensation of the nucleophilic carbon on the electrophilic central carbon atom of the methine chain, leading to a neutrodimethine cyanine and simultaneously elimination of the more basic nucleus. 

On the other hand, the rate constant k does not depend on the changing steric influence of substitutents in the 8-position, but correlates surprisingly well with the Hammett-Brown constant cr. This result indicates that the formation of an sp3-hybridized carbon atom (at the 1-position of the o-complex) leads to a compound without significant steric interaction of the electrophile with substituents in the 8-position. The o-complex cannot be planar and is asymmetric. The preferred conformation of a o-complex of this type is illustrated in Figure 12-6. The pseudoax-ial position of the electrophile E reduces the steric interaction between this group and the peri substituent R. 

Summary.—The assumption that atomic nuclei consist of closely packed spherons (aggregates of neutrons and protons in localized Is orbitals—mainly helions and tritions) in concentric layers leads to a simple derivation of a subsubshell occupancy diagram for nucleons and a simple explanation of magic numbers. Application of the close-packed-spheron model of the nucleus to other problems, including that of asymmetric fission, will be published later.13... 

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