Stereoselective Synthesis of Alcohols, XXXVII. Origins of Stereoselectivity in Reactions of α‐Substituted Allylboronates with Aldehydes

Rate constants for the addition of α‐substituted allylboronates 4 (XH, Cl, Br, OCH3, SCH3] to aldehydes have been measured. The results have been used to differentiate between different models that have been advanced to account for the high proportion of (Z)‐configurated homoallyl alcohols 5 formed in allylboration reactions. Attempts have been made in this context to detect ate‐complex formation between benzylboronates and aldehydes.