Which element is fused immediately before the star develops an iron core?

Silicon

Massive stars undergo a progressive process of layered nuclear burning after their core hydrogen is exhausted, creating successively heavier elements in concentric shells around the center, much like an onion structure. This fusion chain involves helium burning to carbon, followed by carbon fusion, then neon, and then oxygen. The final thermonuclear stage before reaching the roadblock of iron involves the fusion of silicon. Silicon fuses under extreme temperature and pressure conditions, producing iron as its ash in the innermost shell of the active fusion region. Once silicon burning ceases and the core is pure iron, the star has reached the end of its energy-producing life, as iron fusion is energetically unfavorable.