AbstractSubstituent effects were measured for the reactions of substituted benzaldehydes with allytributylin (1) and allyltriethyllead (2) reagents in the presence of BF3.OEt2in CH2Cl2. The Hammettpvalues were small and positive at 25 °C and negative at −78 °C for both 1 and 2. These could be interpreted in terms of the contribution of electrophilic complexation between the aldehyde function and BF3as a rate‐limiting step. A large negativepvalue was observed for the complex‐formation equilibria between substituted benzaldehydes