Roush allylboronation

The synthesis and use of tartrate-modified allylboronate 1 was first reported by Roush and co-workers in 1985. The synthesis and use of the corresponding (E)- and (. -crotylboronate reagents 2 and 3 was published by Roush and co-workers shortly thereafter. The ease of synthesis, stability and efficient reactivity of these reagents offers advantages over many other allyl- and crotylmetal reagents. Roush and co-workers have extensively explored the enantioselective allylations with achiral aldehydes as well as the 

Roush and co-workers have shown that the nature of the substituents on the dioxaborolane auxiliary profoundly affects the rate and enantioselectivity of the allylboration reaction reagents with ester substituents are far more reactive and more selective than reagents with alkyl substituents. The enantioselectivity of reagents 1-3 is thought to arise more from electronic attraction and repusion than from steric hindrance. 

The stereochemical outcome of the enantioselective allylation is rationalized via formation of the major product via transition state A, where the (i ,R)-DIPT crotylboronate 1 adds to the Si face of the aldehyde s carbonyl carbon. Conversely, transition state B leads to the minor enantiomer via Re face addition. A number of theories have been put forth to 

The relative stereochemistry observed in the reactions of these crotylboronates with aldehydes is consistent with cyclic Zimmerman-Traxler transition states where the aldehyde positions its R group in the pseudo-equatorial position about the six-membered ring transition state. Thus, the (4)-reagent 2 leads to the anti diastereomer via C while the (. -reagent 3 leads to the syn diastereomer via D. 

The reactions of reagents 1-3 with a variety of achiral aldehydes have been reported by Roush and co-workers. A few of these reactions are summarized in Table 1. The diastereoselectivity is uniformally high ( 95 5) while the enantioselectivity varies from moderate to excellent (55-91% ee), depending upon the aldehyde and reagent structure. The (jE)-crotylboronate 2 is generally the most enantioselective while the (. -crotylboronate 3 is 

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Evans aldol reaction Hajos-Wiechert reaction Keck stereoselective allylation Roush allylboronation... 

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