Organolithium reagents stereochemistry

Coleman established the hydroxypropyl stereochemistry via addition of a homochiral a-alkoxyalkyl organometallic species. This reagent was prepared in high enantiomeric excess using a Noroyi BINAL-H reduction of organostannane 33, which was transmetallated with ra-BuLi to achieve the desired organolithium reagent 35 (Scheme 7.5). Both enantiomers of 35 could be obtained via this route. 

Solvent and temperature effects on the diastereoselective addition of n-BuLi to 2-phenylpropanal have been reassessed95 and the stereochemistry of addition of organolithium reagents to carbohydrate enones has been studied.96 No compelling evidence has been found to support the suggestion that addition of n-BuLi to benzoic acid might compete with formation of the lithium salt and thereby provide an alternative to the sequential route to PhCOBu".97... 

Alkenyllithium compounds can also be prepared by metallation of alkenes, particularly when alkenyl hydrogens are rendered acidic by an a-substituent (equation 22). Transmetallation of alkenyl stannanes with organolithium reagents gives alkenyllithium compounds with retention of alkene stereochemistry (equation 23). Tin-lithium transmetallation has been used to prepare 1,4-dilithio-l,3-butadiene. Monosubstituted alkenyllithium compounds RHC=CHLi, can also be prepared from the corresponding diorganotel-luride, RHC=CHTeBu, by reaction with butyllithium in... 

Once again, the stereochemistry of reactions involving organolithium reagents has been investigated using substrates based on the 1,3,2-oxazaphospholidine skeleton. The reactions between the diastereoisomeric 2-methyl-1,3,2-oxazaphospholidine 2-oxides... 

These observations, added to the fact that addition of polar solvents to polymerization reactions of butadiene initiated by organolithium reagents in hydrocarbon solvents can change the stereochemistry from one of predominantly rtr-1,4 to that of 1,2 and trans-, , suggests that the lithium may form a tt complex with the olefin, and if this is sufficiently strong, orient the monomer prior to the incorporation step. 

Here the driving force is provided by the formation of a carbanion adjacent to a stabilizing group (RjSi— or dithioacetal). Organolithium reagents, however, add to conjugated dienes and hence initiate their polymerization. In hydrocarbon solvents n-butyl lithium and butadiene result in mostly 1,4-addition. The polymer contains a mixture of cis and trans stereochemistry about the double bonds. 

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