Hydrogenation of (Hetero)aryl Boronate Esters with a Cyclic (Alkyl)(amino)carbene–Rhodium Complex: Direct Access to cis ‐Substituted Borylated Cycloalkanes and Saturated Heterocycles

Access to Unexplored 3D Chemical Space: cis ‐Selective Arene Hydrogenation for the Synthesis of Saturated Cyclic Boronic Acids

A new class of saturated boron‐incorporated cyclic molecules has been synthesized employing an arene‐hydrogenation methodology. cis‐Selective hydrogenation of easily accessible, and biologically important molecules comprising benzoxaborole, benzoxaborinin, and benzoxaboripin derivatives is reported. Among the various catalysts tested, rhodium cyclic(alkyl)(amino)carbene [Rh‐CAAC] (1) pre‐catalyst revealed the best hydrogenation activity confirming turnover number up to 1400 with good to high diastereoselectivity. A broad range of functional groups was tolerated including sensitive substituents such as −F, −CF₃, and −silyl groups. The utility of the synthesized products was demonstrated by the recognition of diols and sugars under physiological conditions. These motifs can have a substantial importance in medicinal chemistry as they possess a three‐dimensional structure, are highly stable, soluble in water, form hydrogen bonds, and interact with diols and sugars.

Enantio- and Diastereoselective, Complete Hydrogenation of Benzofurans by Cascade Catalysis

We report an enantio- and diastereoselective, complete hydrogenation of multiply substituted benzofurans in a one-pot cascade catalysis. The developed protocol facilitates the controlled installation of up to six new defined stereocenters and produces architecturally complex octahydrobenzofurans, prevalent in many bioactive molecules. A unique match of a chiral homogeneous ruthenium-N-heterocyclic carbene complex and an in situ activated rhodium catalyst from a complex precursor act in sequence to enable the presented process.

Interrupted Pyridine Hydrogenation: Asymmetric Synthesis of δ-Lactams

Metal-catalyzed hydrogenation is an effective method to transform readily available arenes into saturated motifs, however, current hydrogenation strategies are limited to the formation of C-H and N-H bonds. The stepwise addition of hydrogen yields reactive unsaturated intermediates that are rapidly reduced. In contrast, the interruption of complete hydrogenation by further functionalization of unsaturated intermediates offers great potential for increasing chemical complexity in a single reaction step. Overcoming the tenet of full reduction in arene hydrogenation has been seldom demonstrated. In this work we report the synthesis of sought-after, enantioenriched δ-lactams from oxazolidinone-substituted pyridines and water by an interrupted hydrogenation mechanism.

Catalytic Transfer Hydrogenation of Arenes and Heteroarenes

Transfer hydrogenation reactions are of great interest to reduce diverse molecules under mild reaction conditions. To date, this type of reaction has only been successfully applied to alkenes, alkynes and polarized unsaturated compounds such as ketones, imines, pyridines, etc. The reduction of benzene derivatives by transfer hydrogenation has never been described, which is likely due to the high energy barrier required to dearomatize these compounds. In this context, we have developed a catalytic transfer hydrogenation reaction for the reduction of benzene derivatives and heteroarenes to form complex 3-dimensional scaffolds bearing various functional groups at room temperature without needing compressed hydrogen gas.

Polar Organofluorine Substituents: Multivicinal Fluorines on Alkyl Chains and Alicyclic Rings

This Review outlines the progression, primarily of our own work, but with important contributions from other labs, on the synthesis and properties of multiple vicinally fluorinated alkyl chains and rings. Chain conformations of individual diastereoisomers with -CHF- at adjacent carbons are influenced by stereoelectronic factors associated with the polar C-F bond and the polarised geminal hydrogens. Generally, the chain will prefer a conformation which acts to minimise overall molecular polarity, and where the C-F bonds orient away from each other. However, when vicinal fluorine atoms are positioned on a ring then conformations are more constrained. The ring will adopt optimal conformations such as a chair in cyclohexane and then C-F bonds can be introduced with a stereochemistry that forces parallel (axial) orientations. In the case of cyclohexane, 1,3-diaxial arrangements of C-F bonds impart considerable polarity to the ring, resulting in an electronegative 'fluorine face' and an electropositive 'hydrogen face'. For all-syn 1,2,3,4,5,6-hexafluorocyclohexane, this arrangement generates an unusually polar aliphatic ring system. Most recently the concept has been extended to the preparation of all-syn 1,2,3-trifluorocyclopropanes, a rigid ring system with fluorine atoms on one face and hydrogens on the other. Lipophilicity Log P values of such compounds indicate that they are significantly more polar than their parent alicyclic hydrocarbons and give some positive indication for a future role of such substituents in medicinal chemistry. Expanding to such a role will require access to improved synthesis methods to these motifs and consequently access to a broader a range of building blocks, however some exciting new methods have emerged recently and these are briefly reviewed.