Enantioselectivity Roush allylboronation

Roush et al. discovered that the tartrate ester-modified allylboronates, such as diisopropyl tartrate allylboronate (.S, .S )-41, react with achiral aldehydes to give the homoallylic alcohols 42 in good yields and high levels of enantioselectivity of up to 87% ee when the reaction is carried out in toluene in the presence of 4-A molecular sieves20 (Scheme 3.1q). To rationalize the asymmetric induction realized by 41, two six-membered transition states were compared (Scheme 3.1r). It was reasoned that transition state A was favored over transition state B due mainly to the nonbonded electronic repulsive interactions of the lone-pair electrons of the aldehyde oxygen and the carbonyl oxygen of the tartrate ester. 

Roush, W. R., Walts, A. E., Hoong, L. K. Diastereo- and enantioselective aldehyde addition reactions of 2-allyl-1,3,2-dioxaborolane-4,5-dicarboxylic esters, a useful class of tartrate ester modified allylboronates. J. Am. Chem. Soc. 1985,107, 8186-8190. 

Roush, W. R., Grover, P. T. Diisopropyl tartrate (E)-Y-(dimethylphenylsilyl)allylboronate, a chiral allylic alcohol 3-carbanion equivalent for the enantioselective synthesis of 2-butene-1,4-diols from aldehydes. Tetrahedron Lett. 1990, 31,7567-7570. 

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... 

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