The results for the five datasets mentioned in Table were checked manually by TLP, the first author, an experienced chemist. We reviewed all reactions to determine their chemical validity, typically focusing on whether the reaction center or bond changes were valid. The results presented in Section 3 provide a good indicator of how many of the imputations should be correct. However, validating individual outcomes necessitates the expertise of a domain specialist. Predicting a confidence for results from the MCS-based method can be used to filter out potentially wrong imputations and increase the accuracy of the method. We observed that the accuracy strongly depends on the complexity of the reaction center, for example on the number of bonds involved in the reaction. We therefore developed a machine learning model using the