Anions, configurationally labile

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

Only a few examples were reported where cationic transition metal complexes contained chiral borate anions. Chiral borate anions were first reported in 1925 when Boeseken and Mijs made the bis(catecholato) anions 4.30 and 4.31 (Figure 4.13) which were configurationally labile which limited their applications.Later on, several borate anions 4.32 and 4.33 were prepared derived from the previous examples but most of these were configurationally labile. 

A powerful access to lithiiun carbanions is from phenylthio ethers and acetals using lithium radical anions such as lithium naphthalenide (42a, LN), S- N,N-dimethylamino)-naphthalenide (42b, LDMAN) or 4,4 -di(ferf-butyl)biphenyle-nide (42c, LDBB) [Eq. (15)] [30-32]. The reaction is not usually stereospecific since a configurationally labile radical 43 is involved [Eq. (15)] [33]. 

The preparation, stabilization, and characterization of buckybowl anions, ranging from archetypal corannulene to large hemifullerenes, have been reviewed. The synthetic potential and configurational stability of configurationally labile chiral carbanions next to electron-withdrawing groups has been summarized. ... 

Reaction proceeds in the presence of pyridine, but in some cases the ben-zotriazole anion, formed in the course of the reaction, acts as a base. Reaction is stereospecific halogenation of compounds with a prochiral a-carbon yields only one of two possible diastereoisomers. Chlorination of optically active sulfoxides takes place with preservation of configuration, but addition of silver ions causes the inversion of configuration at sulfur. The lability of hydrogen at the a-carbon atom does not considerably affect the direction of chlorination. On chlorination of benzyl methyl sulfoxide, benzyl chloromethyl and a-chlorobenzyl methyl derivatives are formed in approximately equal amounts (Scheme 66). 

The enantiomers of trans-2-ACPC were separated by Yamazaki et al. [78]. The Boc-protected racemic amino acid was acylated with / -(+)- -methylbenzylamine by use of a mixed anhydride (Scheme 5). The diastereomers 34 and 35 were separated by silica gel column chromatography after removal of the acid-labile Boc group. The absolute configuration was proved by X-ray diffraction of 35. The amino acid enantiomers 36 and 37 were obtained after strong acid treatment of 34 and 35, and anion-exchange desalting of the product [78]. 

A series of thio- 46 [51,168] and seleno-carboxylato complexes 47 [175] with tmns-configurations have been synthesized from the reaction of NiCl2(R3P)2 with alkali metal or 0-trimethylsilyl selenocarboxylates (Scheme 4). Telluro-carboxylato nickel complexes 48 Ni(RCOTe)2(R3P)2 are too labile to isolate, although their formation has been confirmed by their H, and Te NMR and IR spectral data [175]. The anionic thiocarboxylato nickel complex 49 is... 

Of the three 6B group [(/r-H)M2(CO)io] complexes, the molybdenum derivative is least stable both toward dimer dissociation and toward CO lability. X-Ray diffraction studies have revealed that the [HMo2(CO)io] anion can adopt either a linear/eclipsed ([Et4N] salt) or an appreciably bent/staggered ([(Ph3P)2N] salt) configuration. ... 

---