Design of asymmetric catalysts generally involves time- and resource-intensive heuristic endeavors. In view of the steady increase in interest toward efficient catalytic asymmetric reactions and the rapid growth in the field of machine learning (ML) in recent years, we envisaged dovetailing these two important domains. We selected a set of quantum chemically derived molecular descriptors from five different asymmetric binaphthyl-derived catalyst families with the propensity to impact the enantioselectivity of asymmetric hydrogenation of alkenes and imines. The predictive power of the random forest (RF) built using the molecular parameters of a set of 368 substrate–catalyst combinations is found to be impressive, with a root-mean-square error (rmse) in the predicted enantiomeric excess (%ee) of about 8.4 ± 1.8 compared to the experimentally known values. The accuracy of RF is found to be superior to other ML methods such as convolutional neural network, decision tree, and eXtreme gradient boosting as well as stepwise linear regression. The proposed method is expected to provide a leap forward in the design of catalysts for asymmetric transformations.
In this article we discussed synthesis and catalytic applications of heterogeneous catalysts based on lacunary polyoxometalates (LPOMs) and transition metal substituted polyoxometalates (TMSPOMs), however exclusively the focus has been given to the mono LPOMs and mono-TMSPOMs. As the field of supported LPOMs/TMSPOMs is upcoming important field, some aspects about choice of support as well as methods of supporting have also been described. We also include a few of our new research results in order to understand the effect of support and active species as well as choice of organic transformation.
ARTICLE HISTORY
The
first example of free amine γ-C(sp3)–H
fluorination is realized using 2-hydroxynicotinaldehyde as the transient
directing group. A wide range of cyclohexyl and linear aliphatic amines
could be fluorinated selectively at the γ-methyl and methylene
positions. Electron withdrawing 3,5-disubstituted pyridone ligands
were identified to facilitate this reaction. Computational studies
suggest that the turnover determining step is likely the oxidative
addition step for methylene fluorination, while it is likely the C–H
activation step for methyl fluorination. The explicit participation
of Ag results in a lower energetic span for methylene fluorination
and a higher energetic span for methyl fluorination, which is consistent
with the experimental observation that the addition of silver salt
is desirable for methylene but not for methyl fluorination. Kinetic
studies on methyl fluorination suggest that the substrate and PdL
are involved in the rate-determining step, indicating that the C–H
activation step may be partially rate-determining. Importantly, an
energetically preferred pathway has identified an interesting pyridone-assisted
bimetallic transition state for the oxidative addition step in methylene
fluorination, thus uncovering a potential new role of the pyridone
ligand.
A Pd(II)-catalyzed
protocol for highly regioselective distal γ-C–H
silylation and germanylation of aliphatic carboxylic acids is reported.
Bidentate 8-aminoquinoline as the directing group was found to stabilize
the six-membered palladacycle. A variety of aliphatic carboxylic acids
and amino acids were silylated and germanylated in good yields and
high diasteroselectivities. Detailed mechanistic studies involving
isolation of a Pd(II) intermediate, determination of the reaction
rate and order, control experiments, and isotopic labeling and DFT
studies were found to be crucial for elucidating the elementary steps
involved in this distal aliphatic functionalization.
We disclose an intriguing and a potentially general role for one of the most commonly used silver salt additives whose molecular understanding continues to remain rather vague in the contemporary practice of palladium catalysis.
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