Carbanions configuration

One of the more important approaches to 1-azirines involves a similar base-induced cycloelimination reaction of a suitably functionalized ketone derivative (route c. Scheme 1). This reaction is analogous to route (b) (Scheme 1) used for the synthesis of aziridines wherein displacement of the leaving group at nitrogen is initiated by a -carbanionic center. An example of this cycloelimination involves the Neber rearrangement of oxime tosylate esters (357 X = OTs) to 1-azirines and subsequently to a-aminoketones (358) (71AHC-(13)45). The reaction has been demonstrated to be configurationally indiscriminate both syn and anti ketoxime tosylate esters afforded the same product mixture of a-aminoketones... 

While a planar configuration characterizes the last monomeric unit of a polymeric chain growing by a radical or carbonium ion mechanism, a tetrahedral configuration should be attributed to the end of a growing polymeric carbanion. Hence an isotactic or a... 

The tetrahedral configuration is further stabilized if the gegen ion is close to the carbanion. 

D Chiral or achiral. v/H-carbanions with an additional, configurationally stable stereo-genic center. Very often the additional stereogenic center is in close vicinity (a- or -position) to the anionic center. 

D Achiral non-.s/C-carbanions with a configurationally stable stereogenic center somewhere in the reagent. 

One major task is the selection of the optimal cation" for a given carbanion . It determines to a great extent the mechanism of the reaction and the positional and configurational stability of the allyl moiety, and thus, the regioselectivity, EjZ selectivity, and diastereoselectivity. Some reviews cover these topics in general1 " 5. 

Carbanionic additions usually do not provide significant simple synjanti diastereoselectivities. The EjZ geometry is determined by the configuration of the reacting ally lie carbanion. The attack at position 3 conserves the configuration of the 1,2-partial double bond and vice versa. 

A simple experimental test of the potential configurational stability of carbanions was recently published26 48 it is based on double stereodifferentiation3 and can be carried out with the racemates. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

Solladie and coworkers545 confirmed the earlier result of Nishihata and Nishio546 that the carbonation of the a-sulphinyl carbanion proceeds under kinetic control with retention of configuration at the metallated carbon atom. However, they also found that the stereochemical outcome of this reaction depends on other factors. They observed that 90% of asymmetric induction may be achieved under kinetic control (reaction time < 0.5 min) by using a base with low content of lithium salts, a result consistent with an electrophilic assistance by the lithium cation (equation 286)545. 

Optically pure (S)-benzyl methyl sulfoxide 139 can be converted to the corresponding a-lithio-derivative, which upon reaction with acetone gave a diastereomeric mixture (15 1) of the /S-hydroxysulfoxide 140. This addition reaction gave preferentially the product in which the configuration of the original carbanion is maintained. By this reaction, an optically active epoxy compound 142 was prepared from the cyclohexanone adduct 141181. Johnson and Schroeck188,189 succeeded in obtaining optically active styrene oxide by recrystallization of the condensation product of (+ )-(S)-n-butyl methyl sulfoxide 143 with benzaldehyde. 

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