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Securines and securamines are cytotoxic alkaloids that contain reactive alkene and heterocyclic residues embedded in skeletons comprising four to six oxidized rings. This structural complexity imparts a rich chemistry to the isolates but has impeded synthetic access to the structures in the nearly three decades since their isolation. We present a flexible route to eight isolates that exemplify the three skeletal classes of metabolites. The route proceeds by the modular assembly of the advanced azides38and49(13 steps, 6 to 10% yield), sequential oxidative photocyclizations, and late-stage functional group manipulations. With this approach, the targets were obtained in 17 to 19 steps, 12 to 13 purifications, and 0.5 to 3.5% overall yield. The structure of an advanced intermediate was elucidated by microcrystal electron diffraction (MicroED) analysis. The route will support structure-function and target identification studies of the securamines. 

The synthesis of a range of PBP supported palladium pincer complexes with different alkyl ligands is described. The rates of CO₂insertion into the alkyl group are quantified and rationalized based on the identity of the alkyl ligand. 

Porphyrin complexes are well-known in O 2 and CO 2 reduction, but their application to N 2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N 2 reduction to ammonia, verified by 15 N 2 labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N–H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol −1 ). We place these results in the context of other work on homogeneous N 2 reduction catalysis. 

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type ( R PBP)M(CH 3 ) ( R PBP = B(NCH 2 PR 2 ) 2 C 6 H 4 − ; R = Cy or t Bu; M = Ni or Pd) to generate κ 1 -acetate complexes of the form ( R PBP)M{OC(O)CH 3 } is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual R PBP ligand, which features a central boryl donor that exerts a strong trans -influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into ( R PBP)M(CH 3 ) is facile and occurs at room temperature because of the high trans -influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using ( t Bu PBP)Pd(CH 3 ). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into ( t Bu PBP)M(CH 3 ) (M = Ni or Pd) proceeds via an S E 2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization. 

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N 2 splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium( iii ) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH 2 ) 2 Cl] + , or results in dehydrohalogenation to the rhenium( iii ) amido complex, (PNP)Re(NH 2 )Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol −1 , while DFT computations indicate a substantially weaker N–H bond of the putative rhenium( iv )-imide intermediate (BDE = 38 kcal mol −1 ). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH 3 generation. 

Here, we report the quantitative electroreduction of CO 2 to CO by a PNP-pincer iridium( i ) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker within the ligand. The complex 3-N is far superior to the analogues with methylene and oxygen linkers, showing higher activity and better selectivity for CO 2 over proton reduction.