Chelation stereochemistry

When the reaction of acetaldehyde with a 6-diazopenicillanate is catalyzed by BFs-EtiO, an epoxide of unknown stereochemistry is obtained (79H( 13)227). With ZnCh catalysis, however, the 6a-acetyl derivative is obtained, which can be stereospecifically reduced as the Mg chelate to the 6a-(/ -l-hydroxyethyl) derivative as part of an elegant synthesis of the carbapenem thienamycin (Scheme 40) (8UA6765). 

Compounds analogous to the cobaltammines may be similarly obtained using chelating amines such as ethythenediamine or bipyridyl, and these too have played an important role in stereochemical studies. Thus ct5-[Co(cn)2(NH3)Cl] was resolved into d(+) and /(—) optical i.so-mers by Werner in 1911 thereby demonstrating. to all but the most determined doubters, its octahedral stereochemistry. More recently, the absolute configuration of one of the optical isomers of [Co(en)3] was determined (.sec Panel on p, 1125),... 

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. 

Thus chelation control " may lead to either product, depending on the relative stabilities of the respective ot- and /(-chelates. In cases with predominant formation of the anri-diastereomer, it is often difficult to establish whether the formation of a /(-chelate or an open-chain Felkin - Anh transition state is responsible for the observed stereochemistry the decision usually rests on plausibility considerations. Thus, with regard to the results obtained for a-alkoxy carbonyl... 

On the other hand, in the presence of Lewis acids such as titanium(lV) chloride or eerium(TIT) chloride, the (S)-e s-conformer predominates via chelation of the two carbonyl groups and a reversed stereochemistry of the addition reaction is observed1 °. 

In the case of the amide 11 (R = CI13) derived from 2-oxopropanoic amid and amine G the chelation-controlled product is predominantly formed with all organometallic reagents. No reversal of the stereochemistry is observed, presumably for the same steric reason as with the corresponding pyruvic amides derived from amines E and F. 

The syn selectivity in the titanium(IV) chloride mediated reactions can be explained by an intermolecular chelation, with transition state 21A being sterically favored over 21B. On the other hand, nonchelation control governs the stereochemistry of the boron trifluoride mediated reactions. Thus, the sterically favored transition state 21 C leads to the observed anf/ -diastereo-mer12. 

The Ireland-Claisen reaction of ( )-vinylsilanes has been applied to the stereoselective synthesis of syn- and c/nti-2-substituted 3-silyl alkcnoic acids. a R-2-Alkyl-3-silyl acids are prepared by rearrangement of ( )-silyl ketene acetals which are generated in situ from the kinetically formed (Z)-enolate of the corresponding propionate ester40. Chelation directs the stereochemistry of enolization of heteroelement-substituted acetates in such a way that the syn-diastereomers are invariably formed on rearrangement403. 

Stereochemistry of four coordinate chelate compounds of Schiff bases and their analogues. G. V. Panova, N. K. Vikulova and V. M. Potapov, Russ. Chem. Rev. (Engl. Transl.), 1980, 49, 655-667 (188). 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

Enantiomerically pure 3-tolyl-2-sulfinyl-2-cyclopentenone 37 undergoes smooth, mild and diastereoselective conjugate hydride addition with lithium tri(sec-butyl)borohydride to afford ultimately 3-tolylcyclopentanone 38 in 93% enantiomeric purity (equation 35)78. The absolute stereochemistry of product 38 is consistent with a chelated intermediate directing hydride addition from that diastereoface containing the sulfoxide lone pair. 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

Browning, J Bushnell, G.W., Dixon, KR. and Pidcock, A. (1983) [PtCl(PEt3)(CH (PPhjSjj)], A Novel C,S-Bonded Chelate With Dynamic Stereochemistry Controlled By A Metal-Ligand... 

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