Alkene Isomerisation Catalysed by a Superbasic Sodium Amide.

Typically catalysed by transition metals, alkene isomerisation is a powerful  methodology for preparation of internal olefins. In contrast, the use of more earth abundant main group reagents is limited to activated substrates, requiring high temperatures and excess stoichiometric amounts. Opening a new portal for progressing this field, here we report applications of bulky sodium amide NaTMP (TMP = 2,2,6,6-tetramethylpiperidide) when partnered by tridentate Lewis donor PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine) in catalytic alkene isomerisation of terminal olefins under mild reaction conditions. An array of distinct olefins could successfully be isomerised, including unactivated olefins, allylamines and allylethers, showing the high activity of this partnership. In-depth mechanistic insights provided by X-ray crystallography, real-time nuclear magnetic resonance (NMR) monitoring, and density functional theory (DFT) calculations have unveiled the crucial role of in-situ-generated TMP(H) in facilitating efficient isomerisation and the choice of alkali-metal. Additionally, theoretical studies shed light on the observed E/Z selectivity, particularly accounting for selective formation of Z-vinyl ethers. The versatility of our method is further demonstrated through isomerisation of unactivated cycloalkenes, which undergo hydrogen isotope exchange to produce deuterated compounds.

Unlocking the Metalation Applications of TMP-powered Fe and Co(II) bis(amides): Synthesis, Structure and Mechanistic Insights.

Typified by LiTMP and TMPMgCl.LiCl, (TMP=2,2,6,6-tetramethylpiperidide), s-block metal amides have found widespread applications in arene deprotonative metalation. On the contrary, transition metal amides lack sufficient basicity to activate these substrates. Breaking new ground in this field, here we present the synthesis and full characterisation of earth-abundant transition metals M(TMP)2 (M=Fe, Co). Uncovering a new reactivity profile towards fluoroarenes, these amide complexes can promote direct M-H exchange processes regioselectively using one or two of their basic amide arms. Remarkably, even when using a perfluorinated substrate, selective C-H metalation occurs leaving C-F bonds intact. Their kinetic basicity can be boosted by LiCl or NBu4Cl additives which enables formation of kinetically activated ate species. Combining spectroscopic and structural studies with DFT calculations, mechanistic insights have been gained on how these low polarity metalation processes take place. M(TMP)2 can also be used to access ferrocene and cobaltocene by direct deprotonation of cyclopentadiene and undergo efficient CO2 insertion of both amide groups under mild reaction conditions.

Insights into the Palladium(II)-Catalyzed Wacker-Type Oxidation of Styrene with Hydrogen Peroxide and tert-Butyl Hydroperoxide.

Wacker oxidations are ubiquitous in the direct synthesis of carbonyl compounds from alkenes. While the reaction mechanism has been widely studied under aerobic conditions, much less is known about such processes promoted with peroxides. Here, we report an exhaustive mechanistic investigation of the Wacker oxidation of styrene using hydrogen peroxide (H2O2) and tert-butyl hydroperoxide (TBHP) as oxidants by combining density functional theory and microkinetic modeling. Our results with H2O2 uncover a previously unreported reaction pathway that involves an intermolecular proton transfer assisted by the counterion [OTf]- present in the reaction media. Furthermore, we show that when TBHP is used as an oxidant instead of H2O2, the reaction mechanism switches to an intramolecular protonation sourced by the HOtBu moiety generated in situ. Importantly, these two mechanisms are predicted to outcompete the 1,2-hydride shift pathway previously proposed in the literature and account for the level of D incorporation in the product observed in labeling experiments with α-d-styrene and D2O2. We envision that these insights will pave the way for the rational design of more efficient catalysts for the industrial production of chemical feedstocks and fine chemicals.

Size-dependent activity of carbon dots for photocatalytic H2 generation in combination with a molecular Ni cocatalyst.

Carbon dots (CDs) are low-cost light-absorbers in photocatalytic multicomponent systems, but their wide size distribution has hampered rational design and the identification of the factors that lead to their best performance. To address this challenge, we report herein the use of gel filtration size exclusion chromatography to separate amorphous, graphitic, and graphitic N-doped CDs depending on their lateral size to study the effect of their size on photocatalytic H2 evolution with a DuBois-type Ni cocatalyst. Transmission electron microscopy and dynamic light scattering confirm the size-dependent separation of the CDs, whereas UV-vis and fluorescence spectroscopy of the more monodisperse fractions show a distinct response which computational modelling attributes to a complex interplay between CD size and optical properties. A size-dependent effect on the photocatalytic H2 evolution performance of the CDs in combination with a molecular Ni cocatalyst is demonstrated with a maximum activity at approximately 2-3 nm CD diameter. Overall, size separation leads to a two-fold increase in the specific photocatalytic activity for H2 evolution using the monodisperse CDs compared to the as synthesized polydisperse samples, highlighting the size-dependent effect on photocatalytic performance.

Sodium mediated deprotonative borylation of arenes using sterically demanding B(CH2SiMe3)3: unlocking polybasic behaviour and competing lateral borane sodiation.

The deprotonative metalation of organic molecules has become a convenient route to prepare functionalised aromatic substrates. Amongst the different metallating reagents available, sodium bases have recently emerged as a more sustainable and powerful alternative to their lithium analogues. Here we report the study of the sterically demanding electrophilic trap B(CH2SiMe3)3 for the deprotonative borylation of arenes using NaTMP (TMP = 2,2,6,6-tetramethylpiperidide) in combination with tridentate Lewis donor PMDETA (PMDETA = N,N,N',N'',N''-pentamethyldiethylenetriamine). Using anisole and benzene as model substrates, unexpected polybasic behaviour has been uncovered, which enables the formal borylation of two equivalents of the relevant arene. The combination of X-ray crystallographic and NMR monitoring studies with DFT calculations has revealed that while the first B-C bond forming process takes place via a sodiation/borylation sequence to furnish [(PMDETA)NaB(Ar)(CH2SiMe3)3] species, the second borylation step is facilitated by the formation of a borata-alkene intermediate, without the need of an external base. For non-activated benzene, it has also been found that under stoichimetric conditions the lateral sodiation of B(CH2SiMe3)3 becomes a competitive reaction pathway furnishing a novel borata-alkene complex. Showing a clear alkali-metal effect, the use of the sodium base is key to access this reactivity, while the metalation/borylation of the amine donor PMDETA is observed instead when LiTMP is used.

Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes.

While cobalt complexes have already shown their potential for C-H and C-F bond activation of fluoroarenes, their reactivity as metalating agents via Co-H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS)2 [HMDS=N(SiMe3 )2 ] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo- and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na2 CoAr4 ] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel C-H metalation. Depending on the reaction stoichiometry, this process can occur inter- or intramolecularly, furnishing transient [NaCo(HMDS)2 Ar] intermediates which can undergo ligand rearrangement to afford [Na2 CoAr4 ] with concomitant formation of Co(HMDS)2 and [NaCo(HMDS)3 ].

Facilitating the Ferration of Aromatic Substrates through Intramolecular Sodium Mediation.

Exploiting cooperative effects between Na and FeII centres present in tris(amide) ferrate complexes has led to the chemoselective ferration of pentafluorobenzene, benzene, toluene, anisole, and pyridine being realised at room temperature. The importance of this bimetallic partnership is demonstrated by neither the relevant sodium amide (NaHMDS or NaTMP) nor the FeII amide Fe(HMDS)2 efficiently metallating these substrates under the conditions of this study. By combining NMR studies with the isolation of key intermediates and DFT calculations, we offer a possible mechanism for how these reactions take place, uncovering a surprising reaction pathway in which the metals cooperate in a synchronised manner. Although the isolated products are formally the result of Fe-H exchange, theoretical calculations indicate that the aromatic substrates undergo Na-H exchange followed by fast intramolecular transmetallation to Fe, thus stabilizing the newly generated aryl fragment.