B. de Bruin

Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst

A direct route from acids to alcohols Making alcohols via hydrogen addition to C=O bonds is among the most widely applied reactions in chemistry. The transformation has also garnered renewed interest for generating commodity chemicals from biomass. Korstanje et al. now show that a cobalt compound can catalyze hydrogenation of the C=O bonds in carboxylic acids. These constitute a particularly challenging substrate class, given the propensity of many other catalysts to degrade under acidic conditions. The cobalt catalyst tolerates a versatile substrate range, and the Earth abundance of the metal bodes well for long-term utility. Science, this issue p. 298 Earth-abundant cobalt catalyzes a broadly useful chemical conversion of acids to alcohols. The reduction of esters and carboxylic acids to alcohols is a highly relevant conversion for the pharmaceutical and fine-chemical industries and for biomass conversion. It is commonly performed using stoichiometric reagents, and the catalytic hydrogenation of the acids previously required precious metals. Here we report the homogeneously catalyzed hydrogenation of carboxylic acids to alcohols using earth-abundant cobalt. This system, which pairs Co(BF4)2·6H2O with a tridentate phosphine ligand, can reduce a wide range of esters and carboxylic acids under relatively mild conditions (100°C, 80 bar H2) and reaches turnover numbers of up to 8000.

Remote supramolecular control of catalyst selectivity in the hydroformylation of alkenes

The supramolecular interactions between a Rh phosphine catalyst equipped with an anion-binding pocket and alkenes that contain anionic functionalities (see picture) provide an excellent design concept to achieve remote control of the regioselectivity in hydroformylation reactions. The 4-pentenoate and 3-butenylphosphonate, which fit tightly between the Rh center and the pocket, were hydroformylated with unprecedented selectivity.

Synthesis and Reactivity of a Transient, Terminal Nitrido Complex of Rhodium

Irradiation of rhodium(II) azido complex [Rh(N3){N(CHCHPtBu2)2}] allowed for the spectroscopic characterization of the first reported rhodium complex with a terminal nitrido ligand. DFT computations reveal that the unpaired electron of rhodium(IV) nitride complex [Rh(N){N(CHCHPtBu2)2}] is located in an antibonding Rh-N π* bond involving the nitrido moiety, thus resulting in predominant N-radical character, in turn providing a rationale for its transient nature and observed nitride coupling to dinitrogen.

Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a Rh-II metalloradical: A combined experimental and theoretical study

Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [Rh(II)(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh(III)(tmp)Me] (3) and [Rh(III)(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh(II)(tmp)] is regenerated from [Rh(III)(tmp)H] with formation of 2,2,6,6-tetramethyl-piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [Rh(III)(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the Rh(II) radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [Rh(II)(por)] producing the [TEMPO]+ [RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh(I) with formation of [Rh(III)(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3,4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh(I)(por)]- at one of the C(Me)-C(ring) sigma* orbitals of [TEMPO]+. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (SN2) pathway have barriers which are low enough to explain the experimental kinetic data.

Reactivity of a Mononuclear Iridium(I) Species Bearing a Terminal Phosphido Fragment Embedded in a Triphosphorus Ligand

The first example of an iridium(I) species bearing a terminal phosphido (PR(2)(-)) ligand is reported. This stable compound shows well-behaved reactivity toward various electrophiles, owing to its exposed phosphorus lone pair, allowing reversible protonation, selective alkylation, isolation of a phosphidoborane of iridium, and generation of a phosphido-bridged iridium(I)-gold(I) dinuclear species.

A Phosphorus Analogue of Bis(η4‐cyclobutadiene)iron(0)

P makes it possible: The convenient oxidative synthesis of the 16-electron organophosphorus iron sandwich complex [Fe(eta(4)-P(2)C(2)tBu(2))(2)] (see structure) suggests that the elusive all-carbon complex [Fe(eta(4)-C(4)H(4))(2)] is a viable synthetic target.

Amplified Vibrational Circular Dichroism as a Probe of Local Biomolecular Structure

We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to 2 orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling with low-lying electronic states. We find that the enhancement factor is strongly mode dependent and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion and to zoom in on such local structure within larger biomolecular systems.

Reversible cyclometalation at RhI as a motif for metal–ligand bifunctional bond activation and base-free formic acid dehydrogenation

Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined RhI complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (LH = PNCH = 2-di(tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh–CPh bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal–ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H2 evolution from putative Rh(CO)(H)(LH) complex A is very facile, proposedly encompassing formal C–H oxidative addition at Rh to give Cvia agostic intermediate B and subsequent reductive elimination of H2. Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H2, supporting a cooperative pathway. Reversible C–H activation is thus a viable mechanism for metal–ligand bifunctional catalysis.

Bioinspired Nonheme Iron Complexes Derived from an Extended Series of N,N,O-Ligated BAIP Ligands

A series of mononuclear Fe(II) triflate complexes based on the 3,3-bis(1-alkylimidazole-2-yl)propionate ester (BAIP) ligand scaffold are reported. In these complexes, the tripodal N,N,O-BAIP ester ligand is varied by (i) changing the ester moiety (i.e., n-Pr, tert-Bu esters, n-Pr amide), (ii) changing the methylimidazole moieties to methylbenzimidazole moieties, and (iii) changing the methylimidazole moieties to 1-ethyl-4-isopropylimidazole moieties. The general structure of the resulting complexes comprises two facially capping BAIP ligands around a coordinatively saturated octahedral Fe(II) center, with either a transoid or cisoid orientation of the N,N,O-donor manifold that depends on the combined steric and electronic demand of the ligands. In the case of the sterically most encumbered ligand, a four-coordinate all N-coordinate complex is formed as well, which cocrystallizes with the six-coordinate complex. In combination with the catalytic properties of the new complexes in the epoxidation/cis-dihydroxylation of cyclooctene with H2O2, in terms of turnover number and cis-diol formation, these studies provide a number of insights for further ligand design and catalyst development aimed at Fe-mediated cis-dihydroxylation.

Rhodium catalysed conversion of carbenes into ketenes and ketene imines using PNN pincer complexes

Ketene synthesis involving catalytic carbonylation of carbenes is an interesting alternative to traditional synthetic protocols, offering milder conditions to diversified products. Analogous catalytic ketene imine production from carbenes and isocyanides is also a promising reaction. However, both methods are underdeveloped. Rhodium carbonyl complexes B and E, based on (6-(phosphinomethyl)pyridin-2-yl)methan-sec-amine type PNN ligand scaffolds, reveal good catalytic activities in ketene and ketene imine production using ethyl diazoacetate (EDA, 1) or sodium 2-benzylidene-1-tosylhydrazin-1-ide (5) as the carbene precursors, as demonstrated by in situ amide/imidamide and β-lactam synthesis. DFT calculations suggest that diazo activation is the rate-determining step and that NH-deprotonation of the ligand produces a more active rhodium complex. The ketene formation step likely proceeds via an outer-sphere CO insertion mechanism. Subsequent stepwise and concerted [2 + 2] cyclization mechanisms have comparable barriers. The complexes are the first rhodium catalysts reported for catalytic ketene/ketene imine production from carbenoids. The higher affinity of rhodium for binding ketene or ketene imine intermediates as compared to other reported metal catalysts (i.e. Pd, Co) may provide opportunities for future enantioselective reactions when using chiral ligands.