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Chirality centers proceeding with retention

Consiglio and Morandini and co-workers (67) have investigated the stereochemistry involved in the addition of acetylenes to chiral ruthenium complexes. Reaction of propyne with the separated epimer of the chiral ruthenium phosphine complex 34 at room temperature results in the chemo- and stereospecific formation of the respective propylidene complex 64. An X-ray structure of the product (64) proves that the reaction proceeds with retention of configuration at the ruthenium center. The identical reaction utilizing the epimer with the opposite configuration at ruthenium (35) also proceeded with retention of configuration at the metal center, proving that the stereospecificity of the reaction in not under thermodynamic control [Eq. (62)]. [Pg.35]

A reaction is said to proceed with retention of configiuation at a chirality center if no bonds to the chirality center are broken. This is true even if the R,S designation for the chirality center changes because the relative priorities of groups around it changes as a result of the reaction. [Pg.227]

The chiral catalyst is boronate X, described in the previous example. Note that absolute configuration in 31-34 is maintained in the cyclic products 35-38. On protonation a vinylic methoxy group became the better leaving group, and cyclization proceeded with retention of the configuration on the chiral center. [Pg.78]

Furthermore, Kamigata and co-workers have shown an example of the conversion of a chiral selenoxide, obtained by the optical resolution of a diastereomeric mixture, into the corresponding enantiomerically pure selenimide, ascertaining the detailed stereochemistry of this compound [39] (Scheme 23). The transformation of the selenoxide into the selenimide proceeded with an overall retention of configuration at the selenium center in the presence of dicy-clohexylcarbodiimide (DCC). [Pg.222]

Chemical transformations in which the chiral silicon center is not implicated allow one to correlate the configuration of two different compounds. Step iii in Scheme 18 is a case in point (58). Since this step does not change the configuration at silicon and steps iv and v involve the same stereochemistry (retention), the propyl and allyl alkoxysilanes correspond in configuration and since steps i and ii involve displacement of different ligands, they must proceed with opposite stereochemistry. On the basis of other, related results, retention and inversion were assigned to steps i and ii respectively. [Pg.80]

Inversion of configuration in the above reaction is in contrast to retention of stereochemistry generally observed in the enantioselective reaction with carbonyl compounds. There are a few reports on the Sg2 reaction with a carbonyl compound proceeding with inversion [32,44]. Recently, Hoppe and coworkers have observed that the configurationally stable chiral a-thio carbanion derived from chiral thiocarbamates reacted with carbonyl compounds to create the quaternary center with complete inversion of the carbanionic center [Eq. (8)] [32], whereas the reaction of the corresponding a-oxy carbanion with electrophiles (including carbonyl compounds) proceeds with retention of configuration [45]. [Pg.191]

Carbenes and carbenoids constitute a class of reactive intermediates traditionally intimately associated with the synthesis of cyclopropanes, and more recently with products derived from C-H insertion processes. The reactions of diazoacetic acid esters with aromatic hydrocarbons such as toluene to give cyclopropanes with the liberation of N2, date back to the seminal experiments by Buchner and Curtius reported in 1885 [11]. Much later, in 1942, Meerwein reported that carbenes generated from diazomethane undergo insertion into C-H bonds [12], Seminal experiments that had significant impact for the utilization of such reactions in synthesis were performed by Stork, who demonstrated that carbenes prepared by photolysis of diazoketones participated in stereospecific intramolecular cycloadditions with al-kenes to form bicyclic cyclopropanes (Equations 1 and 2) [13]. Julia reported that intramolecular C-H insertion reactions of a chiral substrate 5 including a stereogenic methine center proceeded stereospecifically with retention of configuration (Equation 3) [14]. [Pg.483]


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