Unraveling Diastereoselectivity: Insights into Asymmetric Synthesis

The world of asymmetric synthesis is rich with nuances, particularly when it comes to diastereoselection. A fascinating aspect of this process involves the interplay between simple diastereoselection and substrate-induced diastereoselectivity. When one of the reactants possesses a stereogenic center near the reactive site, the outcome can be significantly influenced, as demonstrated through various reactions involving α-chiral enolborinates.

Using in situ generated enolborinates from ketones and dialkylchloroboranes, researchers can produce either (E)- or (Z)-enolborinates depending on specific conditions. The formation of anti-adducts from (E)-enolborinates and syn-adducts from (Z)-enolborinates is notably influenced by the Zimmerman-Traxler transition state. This distinction highlights the importance of the structural configuration of the enolborinate in determining the diastereoselectivity of the reaction.

In contrast, traditional lithium enolates display significantly lower induced diastereoselectivity, particularly when the stereogenic center is situated within the aldehyde. Notably, if stereogenic centers exist in both components, the one in the enolborinate predominantly dictates the outcome, showcasing the complexity of stereochemical influences in these reactions.

Moreover, the addition of allyl- and crotylchromium(II) species to aldehydes introduces a different dimension to diastereoselectivity. The formation of anti-arrangements in the resulting products, due to a unique chromium complex, further complicates the landscape of diastereoselective processes. Although substrate-induced diastereoselectivities can be significant, they generally do not exceed ratios of 3:1 to 4:1. Conversely, the presence of stereogenic units in the allylic component often leads to more pronounced induction, overwhelming the effects from the aldehyde itself.

The crotylstannane reaction presents a notably unique case of diastereoselectivity. Here, the stereochemical outcome pivots on the presence or absence of a Lewis acid. The open transition state resulting from Lewis acid catalysis contrasts sharply with the closed transition state of thermal reactions, illustrating the profound impact of reaction conditions on stereochemical outcomes.

As researchers continue to explore these intricacies, the field of asymmetric synthesis will undoubtedly deepen our understanding of diastereoselectivity and its implications for the development of complex molecules.