No, you can't.
All numerical integrators, ode45
and friends included, use some form of iterative scheme to solve the user-implemented (coupled) non-linear (partial) differential equations.
Each new step in the iterative schemes of ode45/15s/..
(to compute the new state of the system) depends on the previous step (the old state of the system), therefore, these numerical integrators cannot be parallelized effectively.
The only speedup you can do that's likely to have a big impact is to optimize your implementation of the differential equation.