  The Diels-Alder reaction between 2,3-dibromo-1,3-butadiene and maleic
anhydride has been studied by means of multisurface adiabatic reactive
molecular dynamics and the PhysNet neural network architecture. This system is
used as a prototype to explore the concertedness, synchronicity and possible
ways of promotion of Diels-Alder reactions. Analysis of the minimum dynamic
path indicates that rotational energy is crucial ($\sim$ 65 %) to drive the
system towards the transition state in addition to collision energy ($\sim$ 20
%). Comparison with the reaction of butadiene and maleic anhydride shows that
the presence of bromine substituents in the diene accentuates the importance of
rotational excitation to promote the reaction. At the high total energies at
which reactive events are recorded, the reaction is found to be direct and
mostly synchronous.
