Leibniz algebra $L$ is defined as a vector spaces that satisfies the Leibniz rule. In fact, every Lie algebra is a Leibniz algebra. In the case of Lie algebras we know that Lie algebras inject to the enveloping algebra. What can we mention that about Leibniz algebras? Can we construct an deal in universal enveloping algebras of Leibniz algebra $L$ generated by $L$? We recall that the associative related in the case of Leibniz algebras is called universal enveloping dialgebra which has the structure of a dialgebra. A diassociative algebra or a dialgebra is a ${K}$-module $D$ equipped with two associative ${K}$-linear products $\dashv, \vdash \colon D \times D \to D$, called respectively, the left product and the right product, such that the products satisfy the following associative laws: $x \dashv (y \vdash z)= x \dashv (y \dashv z)$, $(x \dashv y) \vdash z= x \vdash (y \vdash z)$ and $x \vdash (y \dashv z)= (x \vdash y) \dashv z$.
for all $x,y,z \in D$.