Let $U$ be a simple right $R$ module, and $D$ be the ring of endomorphisms of $U$. If $A$ is any $D$ linear operator and $X$ is any finite subset of $R$, there exists $r\in R$ such that $xr=xA$ for all $x\in X$. As a corollary every right (left) primitive ring is a dense subring of a ring of linear transformations.
Link: https://en.wikipedia.org/wiki/Jacobson_density_theorem