Conspectus
In the domain of reaction development,
one aims to obtain higher
efficacies as measured in terms of yield and/or selectivities. During
the empirical cycles, an admixture of outcomes from low to high yields/selectivities
is expected. While it is not easy to identify all of the factors that
might impact the reaction efficiency, complex and nonlinear dependence
on the nature of reactants, catalysts, solvents, etc. is quite likely.
Developmental stages of newer reactions would typically offer a few
hundreds of samples with variations in participating molecules and/or
reaction conditions. These “observations” and their
“output” can be harnessed as valuable labeled data for
developing molecular machine learning (ML) models. Once a robust ML
model is built for a specific reaction under development, it can predict
the reaction outcome for any new choice of substrates/catalyst in
a few seconds/minutes and thus can expedite the identification of
promising candidates for experimental validation. Recent years have
witnessed impressive applications of ML in the molecular world, most
of them aimed at predicting important chemical or biological properties.
We believe that an integration of effective ML workflows can be made
richly beneficial to reaction discovery.
As with any new technology,
direct adaptation of ML as used in
well-developed domains, such as natural language processing (NLP)
and image recognition, is unlikely to succeed in reaction discovery.
Some of the challenges stem from ineffective featurization of the
molecular space, unavailability of quality data and its distribution,
in making the right choice of ML model and its technically robust
deployment. It shall be noted that there is no universal ML model
suitable for an inherently high-dimensional problem such as chemical
reactions. Given these backgrounds, rendering ML tools conducive for
reactions is an exciting as well as challenging endeavor at the same
time. With the increased availability of efficient ML algorithms,
we focused on tapping their potential for small-data reaction discovery
(a few hundreds to thousands of samples).
In this Account, we
describe both feature engineering and feature
learning approaches for molecular ML as applied to diverse reactions
of high contemporary interest. Among these, catalytic asymmetric hydrogenation
of imines/alkenes, β-C(sp3)–H bond functionalization,
and relay Heck reaction employed a feature engineering approach using
the quantum-chemically derived physical organic descriptors as the
molecular featuresall designed to predict the enantioselectivity.
The selection of molecular features to customize it for a reaction
of interest is described, along with emphasizing the chemical insights
that could be gathered through the use of such features. Feature learning
methods for predicting the yield of Buchwald–Hartwig cross-coupling,
deoxyfluorination of alcohols, and enantioselectivity of N,S-acetal formation are found to offer excellent
predictions. We propose a transfer learning protocol, wherein an ML
model...