In a typical size-controlled synthesis of colloidal gold nanoclusters, monodispersed ultra-small gold clusters stabilized by hydrophilic polymer, poly(N-vinyl-2-pyrrolidone) (PVP), are prepared in this research by rapid reduction of Au(III) ions of aqueous HAuCl4 with NaBH4 in the presence of PVP at 0 °C under vigorous stir for conventional batch mixing or in a microfluidic reactor to give gold clusters with average diameters of 1.3 and 1.1 nm, respectively. No surface plasmon band was observed at 520 nm in UV/Vis spectrum indicating that there is a negligible population of gold clusters larger than 2 nm. These Au:PVP were first used for N-formylation of amines which were carried out through the oxidation of methanol at reflux temperature or formalin at room temperature (27 °C) with excellent yield of corresponding amides. In the second part, selectivity of α-oxygenation of benzo-fused cyclic amines and dehydrogenation of amines were investigated to furnish corresponding lactams and imines, respectively. The optimized conditions for N-formyaltion of amines and α-oxygenation and also dehydrogenation of cyclic amines were disclosed together with the mechanisms of these reactions. The reaction could not proceed without Au:PVP or base additive or using a large size of gold clusters. In addition, no reaction was observed under anaerobic condition.