Stereoselective Allylation with Allylboron: Roush and Brown Reagents

Allylations involving allylboron (Type I) reagents are often highly stereoselective, which can be explained by invoking a transition state similar to a Zimmerman-Traxler chair conformation. The metal in this class of nucleophile is sufficiently Lewis acidic for the reaction to occur without the use of an external activatior. Since the reaction occurs faster than metallotropic rearrangement, efficient translation of stereochemical information is possible.[1,2,3]

Stereoselective Allylation with Allylboron Reagents

It should be noted that B–O and B–C bonds are shorter than analogous bonds with most metals, resulting in a tighter transition state and concomitant higher stereoselectivity. (Z)-Crotylmetals from this class of nucleophiles generally result in the formation of syn-products and (E)-crotylmetals result in the formation of anti-products. This can be rationalised by assuming the large substituent of the aldehyde preferentially occupies the pseudoequatorial position in the six-membered cyclic transition state to minimise 1,3-diaxial interactions.[2]

Stereoselctive allylation with crotylboron reagents

Roush and Brown Reagents

Chiral ligands on the metal can be used to impart enantioselectivity, with reagents designed by Brown (pinene-derived),[1] and Roush (tartrate-derived)[3] being the most commonly employed. Some examples of the use of these reagents are shown below.

Enantioselectivity allylations using chiral ligands on boron

References

1. H. C. Brown, K. J. Parabhakar, J. Am. Chem. Soc. 1983, 105, 209.

2. C. Diner, K. J. Szabo, J. Am. Chem. Soc. 2017, 139, 2.

3. W. R. Roush, A. E. Walts, L. K. Hoong, J. Am. Chem. Soc. 1985, 107, 8186.