Herein, we have developed a NNO-manganese complex featuring a relatively low-congestion metal-ligand cooperative environment, one enabling the alkylation of bulky α-branched ketones with bulky alcohols. We demonstrated that while the efficiency of the catalyst in the dehydrogenation or transfer hydrogenation step is essential, chelation assistance is equally crucial for suppressing retro-aldol reactions, thereby ensuring the success of the process.