Smart investments in sustainable food production: revisiting mixed crop-livestock systems

Farmers in mixed crop-livestock systems produce about half of the world's food. In small holdings around the world, livestock are reared mostly on grass, browse, and nonfood biomass from maize, millet, rice, and sorghum crops and in their turn supply manure and traction for future crops. Animals act as insurance against hard times and supply farmers with a source of regular income from sales of milk, eggs, and other products. Thus, faced with population growth and climate change, small-holder farmers should be the first target for policies to intensify production by carefully managed inputs of fertilizer, water, and feed to minimize waste and environmental impact, supported by improved access to markets, new varieties, and technologies.

Manganese(I)-Catalyzed C-H Activation: The Key Role of a 7-Membered Manganacycle in H-Transfer and Reductive Elimination

Manganese‐catalyzed C−H bond activation chemistry is emerging as a powerful and complementary method for molecular functionalization. A highly reactive seven‐membered Mn^I intermediate is detected and characterized that is effective for H‐transfer or reductive elimination to deliver alkenylated or pyridinium products, respectively. The two pathways are determined at Mn^I by judicious choice of an electron‐deficient 2‐pyrone substrate containing a 2‐pyridyl directing group, which undergoes regioselective C−H bond activation, serving as a valuable system for probing the mechanistic features of Mn C−H bond activation chemistry.

Structures and anion-binding properties of M4L6 tetrahedral cage complexes with large central cavities

Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =[BF(4)](-), [ClO(4)](-), [PF(6)](-) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =[BF(4)](-) and [PF(6)](-) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds.

Bioactive properties of iron-containing carbon monoxide-releasing molecules

Carbon monoxide-releasing molecules (CO-RMs) are compounds capable of delivering controlled amounts of CO within a cellular environment. Ruthenium-based carbonyls [tricarbonyldichloro ruthenium(II) dimer and tricarbonylchloro-(glycinato)ruthenium(II)] and boronacorbonates (sodium boranocarbonate) have been shown to promote vasodilatory, cardioprotective, and anti-inflammatory activities in a variety of experimental models. Here, we extend our previous studies by showing that eta-4-(4-bromo-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F3), an irontricarbonyl complex that contains a 2-pyrone motif, liberates CO in vitro and exerts pharmacological actions that are typical of CO gas. Specifically, CORM-F3 caused vasorelaxation in isolated aortic rings and inhibited the inflammatory response (e.g., nitrite production) of RAW264.7 macrophages stimulated with endotoxin in a dose-dependent fashion. By analyzing the rate of CO release, we found that when the bromide at the 4-position of the 2-pyrone CORM-F3 is substituted with a chloride group [eta-4-(4-chloro-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F8)], the rate of CO release is significantly decreased (4.5-fold), and a further decrease is observed when the 4- and 6-positions are substituted with a methyl group [eta-4-(4-methyl-6-methyl-2-pyrone)tricarbonyl iron (0) (CORM-F11)] or a hydrogen [eta-4-(4-chloro-2-pyrone)tricarbonyl iron (0) (CORM-F7)], respectively. Interestingly, the compounds containing halogens at the 4-position and the methyl at the 6-position of the 2-pyrone ring (CORM-F3 and CORM-F8) were found to be less cytotoxic compared with other CO-RMs when tested in RAW246.7 macrophages. Thus, iron-based carbonyls mediate pharmacological responses that are achieved through liberation of CO and the nature of the substituents in the organic ligand have a profound effect on both the rate of CO release and cytotoxicity.

η4-Pyrone iron(0)carbonyl complexes as effective CO-releasing molecules (CO-RMs)

The CO-releasing properties of iron(0)tricarbonyl complexes bearing a 2-pyrone ligand have been evaluated. In this report, we demonstrate that the intrinsic stability of the (eta4-2-pyrone)Fe(CO)3 complex influences the extent and rate of CO release, which is affected by the presence of a halogen substituent on the 2-pyrone ring. The cell viability index has been highlighted for the active carbon monoxide-releasing molecules (CO-RMs), demonstrating that these complexes and related derivatives are a promising new class of compounds with potential therapeutic applications.

η1-2-Pyrone metal carbonyl complexes as CO-releasing molecules (CO-RMs): A delicate balance between stability and CO liberation

An evaluation of the CO releasing ability of iron(II) and molybdenum(II) complexes has facilitated the discovery of the most rapid CO releaser, namely [Mo(CO)(3)(eta(5)-C(5)H(5))(eta(1)-{O}-C{=O}-O-CMe=CH-COMe=CBr)]BF(4) (CORM-F10), reported to date. The rate of CO release is related to the overall solution phase stability of the transition metal carbonyl complex. The cytotoxicity and vasodilatory properties of CORM-F10 have been determined.

Rhodium-Promoted Linear Tetramerization and Cyclization of 3,3-Dimethylbut-l-yne

With tetrahydrofuran as the solvent [Rh(thf)(2)(cod)](+) promotes the selective coupling of tBuC(2)H to form triene-yne 1; at high alkyne concentrations, however, an unusual C-H activation process intervenes, leading to the formation of complex 2, which contains two linked five-membered rings. cod=1,5-cyclooctadiene.

Accelerated syntheses of amine-bis(phenol) ligands in polyethylene glycol or “on water” under microwave irradiation

Pure amine-bis(phenol) ligands are readily accessible in high yield, often >90%, when the Mannich condensation reactions are performed “on water” or in poly(ethyleneglycol) (PEG). Microwave-assisted synthesis dramatically reduces the time and energy required to prepare these molecules, typically from 24 h to 5 min. The approach seems to be widely applicable (7 amines and 5 phenols were tested to yield a diverse set of bis(phenol) ligands). Significant improvements in yield were observed for ligands derived from di-tert-amyl and di-tert-butyl phenols, possibly resulting from a hydrophobic effect. Single crystal X-ray diffraction data for the ligand derived from p-cresol and N,N′-dimethylethylenediamine is reported.Key words: amine-phenol, Mannich condensation, on water, microwave, ligand, high-throughput.

Mechanistic Insight into Catalytic Redox-Neutral C-H Bond Activation Involving Manganese(I) Carbonyls: Catalyst Activation, Turnover, and Deactivation Pathways Reveal an Intricate Network of Steps

Manganese(I) carbonyl-catalyzed C-H bond functionalization of 2-phenylpyridine and related compounds containing suitable metal directing groups has recently emerged as a potentially useful synthetic methodology for the introduction of various groups to the ortho position of a benzene ring. Preliminary mechanistic studies have highlighted that these reactions could proceed via numerous different species and steps and, moreover, potentially different catalytic cycles. The primary requirement for typically 10 mol % catalyst, oftentimes the ubiquitous precursor catalyst, BrMn(CO)₅, has not yet been questioned nor significantly improved upon, suggesting catalytic deactivation may be a serious issue to be understood and resolved. Several critical questions are further raised by the species responsible for providing a source of protons in the protonation of vinyl-manganese(I) carbonyl intermediates. In this study, using a combination of experimental and theoretical methods, we provide comprehensive answers to the key mechanistic questions concerning the Mn(I) carbonyl-catalyzed C-H bond functionalization of 2-phenylpyridine and related compounds. Our results enable the explanation of alkyne substrate dependencies, i.e., internal versus terminal alkynes. We found that there are different catalyst activation pathways for BrMn(CO)₅, e.g., terminal alkynes lead to the generation of Mn^I-acetylide species, whose formation is reminiscent of Cu^I-acetylide species proposed to be of critical importance in Sonogashira cross-coupling processes. We have unequivocally established that alkyne, 2-phenylpyridine, and water can facilitate hydrogen transfer in the protonation step, leading to the liberation of protonated alkene products.

Gold-alkynyls in catalysis: alkyne activation, gold cumulenes and nuclearity

The use of cationic gold(i) species in the activation of substrates containing C[triple bond, length as m-dash]C bonds has become a valuable tool for synthetic chemists. Despite the seemingly simple label of 'alkyne activation', numerous patterns of reactivity and product structure are observed in systems employing related substrates and catalysts. The complications of mechanistic determination are compounded as the number of implicated gold(i) centres involved in catalysis increases and debate about the bonding in proposed intermediates clouds the number and importance of potential reaction pathways. This perspective aims to illustrate some of the principles underpinning gold-alkynyl interactions whilst highlighting some of the contentious areas in the field and offering some insight into other, often ignored, mechanistic possibilities based on recent findings.