code: public static IReactionSet getRelevantReactionsAsReactant(IReactionSet reactSet, IAtomContainer molecule) {
          IReactionSet newReactSet = reactSet.getBuilder().newInstance(IReactionSet.class);
          for (IReaction reaction : reactSet.reactions()) {
              for (IAtomContainer atomContainer : reaction.getReactants().atomContainers())
                  if (atomContainer.equals(molecule)) newReactSet.addReaction(reaction);
          }
          return newReactSet;
      }
docs: Get all Reactions object containing a Molecule as a Reactant from a set
  of Reactions.
  
  @param reactSet The set of reaction to inspect
  @param molecule The molecule to find as a reactant
  @return         The IReactionSet