The in-situ preparation of [(Me 2C) 2O 2BCH 2]Cu(CN)ZnI ( 3) from Knochel's (dialkoxyboryl)methylzinc reagent ( 2) and CuCN•2LiCl in THF, followed by its addition to aldehydes in the presence of boron trifluoride etherate yielded rather stable β-hydroxyalkylboronates ( 5). The thermal dehydroxyboronation or the alkaline hydrogen peroxide oxidation of 5 gave the corresponding alkenes ( 6) or 1,2-alkanediols ( 7) in high yields. The reaction provides a simple procedure for the olefination or the hydroxymethylation of aldehydes.