Stereochemistry, lead compound

It is the purpose of this chapter to summarize what is currently known about the stereochemistry and conformation of organogermanium, tin and lead compounds. Coverage is selective rather than exhaustive. The first section deals with compounds in which substitution by four different groups causes the metal atom to be stereogenic. We have limited our discussion to those cases in which at least three of the four substituents are alkyl or aryl. In this section we also briefly discuss pentacoordinated triorgano halostannanes. 

Two factors combine to lend a greater diversity in the stereochemistries exhibited by bivalent germanium, tin and lead compounds, the increased radius of Mn compared with that of Mw and the presence of a non-bonding pair of electrons. When the non-bonding pair of electrons occupies the isotropic valence level s orbital, as in, for example, the complex cations Pb[SC(NH2)2]6+ and Pb[antipyrine]6+, or when they are donated to conductance band levels, as in the binary tin and lead selenides or tellurides or the perovskite ternary phases CsMX3 (M = Sn, Pb X = Cl, Br, I), then the metal coordination is regular. However, in the majority of compounds an apparent vacancy in the coordination sphere of the metal is observed, which is usually ascribed to the presence of the non-bonding pair of electrons in a hybrid orbital and cited as evidence for a stereochemically active lone pair . 

Gielen, M., Dehouck, C., Mokhtar-Jamai, H., Topart, J., Stereochemistry of Four-, Five- and Six-Coordinate Complexes of Group IV Metals, Rev. Silicon Germanium Tin Lead Compounds 1 [1972] 9/33. 

Among group IV elements it is the most electropositive, and has the most stable divalent state. The coordination number for divalent lead compounds ranges from 2 to 7, and for tetravalent ones from 4 to 8. The stereochemistry is usually octahedral or tetrahedral. 

During the time to develop hexythiazox, we established a novel method of biological tests involving the observation of life cycles of mites. Furthermore, we could overcome some problems in stereochemistry by means of the establishment of stereoselective processes in the synthesis. The success in the development of hexythiazox should depend on such efforts in biological, toxicological and synthetic studies after the finding of lead compound. 

Special mention should be made of the two extreme values of e.e. An e.e. of 100% corresponds to an enantiomerically pure compound (thetermhomochiral which is also sometimes used is not favoured). A reaction which gives a product of 100% e.e. is enantiospedflc. Since this represents an ideal situation which is rarely attainable in practice, the term enantioselective should generally be used. An e.e. of 0% corresponds to a 1 1 mixture of enantiomers known as a racemic mixture or racemate (this is denoted by the prefix ( )-). The process by which the stereogenic unit in a chiral compound is destroyed and then reformed with random stereochemistry leading to a fall in the e.e., eventually to zero, is... 

Belloli, R., Resolution and Stereochemistry of Asymmetric Silicon, Germanium, Tin, and Lead Compounds, J. Chem. Educ. 46 [1969] 640/4. 

When we compare the nucleophilic addition mechanism and the experimental results in this experiment, the first oddity is referred to the stereochemistry of compound 3. The experimental result indicates that deuterated ester 3 is obtained as a single diastereomer, which after cyclization (promoted by ZnCU) leads to 4 with c -stereochemistry. However, following the nucleophilic addition mechanism previously discussed, product 9 (structurally related to 3) should be obtained, but as a mixture of diastereomers. 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

The smooth intramolecular nucleophilic displacement of biphenyl carboxylic acids leading to benzocoumarins (See Section II.A.) inspired also investigation of the behavior of similar diphenyl ether, diphenyl sulfide and A-methyldiphenyl amine derivatives 458 under similar conditions. However, all these attempts to achieve cyclization to tricyclic compounds 459 were unsuccessful, probably due to the unfavorable stereochemistry for the formation of the required seven-mem-bered transition states and also to the presence of the deactivating bridge groups X (Eq. 42) [68JCS(C)1030]. 

I-Oialkoxy carbonyl compounds are a special class of chiral alkoxy carbonyl compounds because they combine the structural features, and, therefore, also the stereochemical behavior, of 7-alkoxy and /i-alkoxy carbonyl compounds. Prediction of the stereochemical outcome of nucleophilic additions to these substrates is very difficult and often impossible. As exemplified with isopropylidene glyceraldehyde (Table 15), one of the most widely investigated a,/J-di-alkoxy carbonyl compoundsI0S, the predominant formation of the syn-diastereomer 2 may be attributed to the formation of the a-chelate 1 A. The opposite stereochemistry can be rationalized by assuming the Felkin-Anh-type transition state IB. Formation of the /(-chelate 1C, which stabilizes the Felkin-Anh transition state, also leads to the predominant formation of the atm -diastereomeric reaction product. 

This contrary stereochemistry in the Bucherer - Bergs reaction of camphor has been attributed to steric hindrance of e.w-attack of the cyanide ion on the intermediate imine. Normally, equatorial approach of the cyanide ion is preferred, giving the axial (t>Mr/o)-amino nitrile by kinetic control. This isomer is trapped under Bucherer-Bergs conditions via urea and hydan-toin formation. In the Strecker reaction, thermodynamic control of the amino nitrile formation leads to an excess of the more stable compound with an equatorial (e.w)-amino and an axial (endo)-cyano (or carboxylic) function13-17. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

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