Lead, tris stereochemistry

Lead, tris(ethyldithiocarbonato)-stereochemistry. 1,82 structure, 1, 82 Lead bromide, 3.194 Lead bromide hydrate, 3, 195 Lead carboxylatcs, 3,222 Lead complexes, 3,183-223 r 6-arene, 3,220 bivalent... 

Lead, tris(ethyldithiocarbonato)-stereochemistry, 82 structure, 82 Ligand exchange neutral complexes, 287 solid state, 468 Ligand field theory, 213-274... 

Figure 89 shows that at b = 1.2 an additional feature has appeared on the potential energy surface, which at b = 1.3 has formed a deep minimum (Figure 90). The two minima correspond to the two optical isomers of the Z)4 square antiprism (Figure 93). It may be noted that in contrast to the three-bladed propeller , which is the dominant stereochemistry for tris(bidentate) complexes (Section 2.3.3), this four-bladed propeller is only expected in tetrakis(bidentate) complexes where the bidentate ligands have exceptionally large normalized bites. At b = 1.26, the angle of twist 0 = 22.5° and the two square faces are staggered with respect to each other (Figure 93). In an analogous way to the behaviour of tris(bidentate) complexes, a decrease in b leads to a decrease in 9.

The variable-temperature NMR spectra help to explain the catalytic properties of the dppp complex system which were outlined previously in Table I. The reduced catalytic activity compared with the tris(triphenylphosphine) complex system is apparently due to the reduced dissociation of the cyclic complexes. For example, the 90°C spectra of Figures 3 and 13, clearly show that the ligand-exchange rate is much slower in the case of dppp. However, temperature-dependent ligand exchange of the monocyclic complex occurs and leads to cis-bisphosphine species that catalyze the hydroformylation of olefins at minimal partial pressures of CO. The hydroformylation rate of the dppp system is faster at 1 atm CO pressure than that of the dppe system. Of course, such hydroformylations are nonselective due to the cis-stereochemistry. 

P-Keto esters and -keto amides, each substituted between the two carbonyl units with a 2-[2-(tri-methylsilyl)methyl] group, also undergo Lewis acid catalyzed, chelation-controlled cyclization. When titanium tetrachloride is used, only the product possessing a cis relationship between the hydroxy and ester (or amide) groups is product yields range from 65 to 88% (Table 8). While loss of stereochemistry in the product and equilibration of diastereomers could have occurred via a Lewis acid promoted retro aldol-aldol sequence, none was observed. Consequently, it is assumed that the reactions occur under kinetic, rather than thermodynamic, control. In contrast to the titanium tetrachloride promoted process, fluoride-induced cyclization produces a 2 1 mixture of diastereomeric products, and the nonchelating Lewis acid BF3-OEt2 leads to a 1 4.8 mixture of diastereomers. 

Acyclic 1,2-disubstituted alkenes from sulfoxide pyrolyses are obtained with the trans geometry because of increased torsional interaction in the transition state leading to the cis isomer (e.g. equation 22). The stereochemistry of tri- and tetra-substituted alkenes is determined by the stereochemistry of the precursor sulfoxides and is predictable on the basis of the syn elimination mechanism. In practice the stereospeciflc synthesis of such alkenes is limited by the availability of isomerically pure starting materials. 

Common error alert Be certain to preserve the stereochemistry about all three it bonds When a diene is given in its s-trans conformation, as in the preceding example, students often isomerize the it bonds inadvertently when they try to redraw it in its s-cis conformation. A correct application of the out-endo-cis rule then leads to an incorrect answer. This common error can be prevented by obeying Grossman s rule. 

A radical cyclization has been used by Hart, at The Ohio State University, to prepare the manzamine tricyclic core (139). Utilizing tri-w-butyltin hydride to mediate the cyclization of selenide 171, the desired stereochemistry of the octahydroisoquinoline ring (172) was established (Scheme 13) electrophilic catalysis generated the tricyclic intermediate 174. An anionic cyclization leading to a tricyclic intermediate is being developed by Marko, at the University of Sheffield, toward which he has recently reported a model study (140). 

The trialdehyde 38 was obtained in four steps in 60-65% overall yield from trimesic acid (34, Scheme 3). Esterification of 34 with 1-propanol in excess, by refluxing with hydrogen chloride catalyst, leads to triester 35 in quantitative yield. Hydrogenation of 35 in acetic acid solvent (Pt catalyst) yields pure cij,ds-cyclohexane-l,3,5-tricarboxylate ester 36, also in quantitative yield. Reduction of ester 36 with lithium aluminum hydri in tetrahydrofuran solvent produces c ,cis-l,3,5-tris(hydroxymethyl)cyclo-hexane cis,cis-37) in 90-95% yields. Swern oxidation of triol 37 led to cij,c -l,3,5-triformylcyclohexane 38 in 70% yield. The stereochemistry of 38, as well as that of precursors 36 and 37, was established as ca,cis in each case by high resolution H NMR. 

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