The selective functionalization of strong, typically inert carbon-hydrogen (C–H) bonds in organic molecules is changing synthetic chemistry. However, the undirected functionalization of primary C–H bonds without competing functionalization of secondary C–H bonds is rare. The borylation of alkyl C–H bonds has occurred previously with this selectivity, but slow rates required the substrate to be the solvent or in large excess. We report an iridium catalyst ligated by 2-methylphenanthroline with activity that enables, with the substrate as limiting reagent, undirected borylation of primary C–H bonds and, when primary C–H bonds are absent or blocked, borylation of strong secondary C–H bonds. Reactions at the resulting carbon-boron bond show how these borylations can lead to the installation of a wide range of carbon-carbon and carbon-heteroatom bonds at previously inaccessible positions of organic molecules.

Scheme 2. Experiments to compare relative ease of C-CN oxidative addition/reductive elimination at benzylic and non-benzylic positions. Scheme 3. Development of novel HCN donors for regioselective transfer hydrocyanation. Following further optimization of the reaction conditions, the applicability of this HCN-free, branched-selective transfer hydrocyanation was examined (Scheme 4). A variety of functional groups were well tolerated in the reaction including both electron donating and electron withdrawing moieties. Besides the high branched selectivity of this new reaction, which is complementary to previous HCN transfer protocols, 8d,9a we were also interested to see if it exhibits greater functional group tolerance, given that the current reaction conditions do not require co-catalytic Lewis acid. Indeed, substrates possessing acidic protons (1r-1w) or a Boc-protected amine (1v) were well tolerated. Finally, vinylheteroarenes were also investigated and several common heterocyclic cores (1x-1z) were found to successfully undergo transfer hydrocyanation with high selectivity. Scheme 4. Substrate scope for the regioselective, HCN-free transfer hydrocyanation. a,b a Yields are given for isolated and purified material. Reactions were conducted on a 0.5 mmol scale. b Regioselectivity was determined by 1 H NMR analysis of the crude reaction mixture. c Aldehyde 1k was also isolated in 33% yield.

17O NMR spectroscopy probes specific electronic features decisive for reactivity of peroxo species in electrophilic epoxidation.

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5− 15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV−vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5−2.0. Notably, solid-state 95 Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded 95 Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that 95 Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO−Mo(O) 2 −OSi) angles and low coordination numbers at Mo lead to highly deshielded 95 Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the 95 Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The 95 Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

<p><i>Typically, reversible catalytic reactions operate under thermodynamic control and thus establishing a selective catalytic system poses a considerable challenge. In this manuscript, we report a reversible yet kinetically selective transfer hydrocyanation protocol. Selectivity is achieved by exploiting the lower barrier for C–CN oxidative addition and reductive elimination at benzylic positions in the absence of co-catalytic Lewis acid. The design of a novel type of HCN donor was crucial to realizing this practical, branched-selective, HCN-free transfer hydrocyanation. The synthetically useful resolution of a mixture of branched and linear nitrile isomers was also demonstrated to underline the value of reversible and selective transfer reactions. In a broader context, this work demonstrates that high kinetic selectivity can be achieved in reversible transfer reactions, thus opening new horizons for their synthetic applications.</i></p>