Preparation method of sitagliptin phosphate

The invention discloses a preparation method of sitagliptin phosphate. The preparation method comprises the following steps: taking 4-(2, 4, 5-trifluorophenyl)-3-methyl oxobutyrate I as a raw material; the reaction is mainly divided into five steps: carrying out asymmetric reduction on the raw material I through an N-heterocyclic carbene palladium catalyst to obtain chiral alcohol ester s-4-(2, 4,5-trifluorophenyl)-3-methyl hydroxybutyrate II; carrying out intramolecular condensation cyclization on the II to obtain chiral lactam quaternary ring (R)-N-benzyloxy-4-[1-methyl-(2, 4, 5-trifluorophenyl)]-2-azetidinone III; carrying out ring opening on the III under an alkaline condition to obtain IV; performing condensation reaction to form amide to obtain V; carrying out catalytic reduction onthe V through recycled N-heterocyclic carbene palladium to remove benzyloxy and form phosphate to obtain sitagliptin phosphate VI, wherein the N-heterocyclic carbene palladium catalyst is Pd (IPr-NHC) (acac) Cl or Pd (IPr-NHC