Exotherm is killing your selectivity. The correct way is to add a slightly sub-stoechiometric DIBAL (for example eq.) and use an oversized reaction flask (. 1L round flask for 100mL of react. mixture) with a large stirbar. Immerse the flask into a dry ice cooling mix (do not use liq. N2 mixtures) - all the way to the flask neck and canulate the DIBAL solution in very slowly ALONG THE FLASK WALL so that the added DIBAL solution cools on its way down. Do not drip DIBAL into the vortex because it wont be chilled when it hits the reaction mix.