Ethylene oligomerization to produce 1-alkenes is a cornerstone of organometallic research. The original α-olefin commercial catalyst system, triethylaluminum, has grown to become one of the highest ...volume organometallics. This tutorial covers the developmental arc of ethylene oligomerization research and explains the ongoing technological shift from full-range (C4–C30) to selective (C6, C8) catalytic systems. Catalyst design determines product selectivity, and the differing mechanisms underpinning catalyst performance, linear versus metallacyclic, are covered in detail. Despite the field’s maturity, there are still significant opportunities for exploration and discovery which are discussed at the conclusion.
A new (N‐phosphinoamidinate)manganese complex is shown to be a useful pre‐catalyst for the hydrosilative reduction of carbonyl compounds, and in most cases at room temperature. The Mn‐catalyzed ...reduction of tertiary amides to tertiary amines, with a useful scope, is demonstrated for the first time by use of this catalyst, and is competitive with the most effective transition‐metal catalysts known for such transformations. Ketones, aldehydes, and esters were also successfully reduced under mild conditions by using this new Mn catalyst.
Hey Man(ganese): A newly prepared (N‐phosphinoamidinate)manganese pre‐catalyst (see Scheme) has been shown to be effective for the hydrosilative reduction of a diverse scope of carbonyl compounds, and in most cases can be used at room temperature. The reaction proceeds under reaction conditions that are competitive with the most effective transition‐metal catalysts known for such transformations, and thereby establishes a new class of synthetically useful Mn‐catalyzed transformations.
Computational design of molecular homogeneous organometallic catalysts followed by experimental realization remains a significant challenge. Here, we report the development and use of a density ...functional theory transition-state model that provided quantitative prediction of molecular Cr catalysts for controllable selective ethylene trimerization and tetramerization. This computational model identified a general class of phosphine monocyclic imine (P,N)-ligand Cr catalysts where changes in the ligand structure control 1-hexene versus 1-octene selectivity. Experimental ligand and catalyst synthesis as well as reaction testing quantitatively confirmed predictions.
The synthesis and characterization of a series of structurally varied N-phosphinoamidinate-ligated cobalt complexes is described, along with the successful application of these and a related iron ...complex as precatalysts in the isomerization–hydroboration of terminal, geminal, and internal alkenes. These reactions proceed under mild conditions (23–65 °C), at relatively low base-metal loadings (1–5 mol %), typically without cosolvent, and with high terminal hydroboration selectivity across a broad spectrum of branched alkenes. With some of the alkene substrates examined, the N-phosphinoamidinate-ligated precatalysts employed herein are shown to provide alternative terminal selectivity versus other previously reported precatalyst classes for such transformations. Reports of terminal-selective metal-catalyzed alkene isomerization–hydroboration disclosed thus far in the literature employ pinacolborane (HBPin); while effective in the system herein, we also report the first examples of such transformations employing either 1,3-dimethyl-1,3-diaza-2-boracyclopentane or benzo-1,3,2-diazaborolane. The application of these 1,3,2-diazaborolanes in place of HBPin in some instances enables novel terminal selectivity in the isomerization–hydroboration of branched alkenes.
Herein we establish the utility of a three‐coordinate (N‐phosphinoamidinate)cobalt(amido) pre‐catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room ...temperature, leading to the selective terminal addition of the boron group.
Chain‐walking hydroborations: A three‐coordinate (N‐phosphinoamidinate)cobalt(amido) pre‐catalyst is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.
The use of data science tools to provide the emergence of non-trivial chemical features for catalyst design is an important goal in catalysis science. Additionally, there is currently no general ...strategy for computational homogeneous, molecular catalyst design. Here, we report the unique combination of an experimentally verified DFT-transition-state model with a random forest machine learning model in a campaign to design new molecular Cr phosphine imine (Cr(P,N)) catalysts for selective ethylene oligomerization, specifically to increase 1-octene selectivity. This involved the calculation of 1-hexene : 1-octene transition-state selectivity for 105 (P,N) ligands and the harvesting of 14 descriptors, which were then used to build a random forest regression model. This model showed the emergence of several key design features, such as Cr-N distance, Cr-α distance, and Cr distance out of pocket, which were then used to rapidly design a new generation of Cr(P,N) catalyst ligands that are predicted to give >95% selectivity for 1-octene.
The use of data science tools to provide the emergence of non-trivial chemical features for catalyst design is an important goal in catalysis science.
The salt metathesis reaction of chromium(III) chloride tris-tetrahydrofuran with 3 equivalents of sodium benzoate in THF produces insoluble Cr(benzoate)3. Pyridine dissolves the complex and ...coordinates the chromium center to yield mer-Cr(η1-O2CPh)3(py)3 which was structurally characterized. Attempts to produce an anionic tris-benzoate complex via MII salt precursors were unsuccessful. Cr2(μ-O2CPh)4(THF)2 was isolated from the reaction of sodium benzoate and CrCl2(THF)2, while CoCl2 and MnCl2 starting materials produced the isostructural polymers Na(THF)M3(O2CPh)7(THF)2∞. Sodium complexation prevented polymerization but could not quell oligomerization yielding the trimeric Na(15-crown-5)2Co3(O2CPh)8.
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The salt metathesis reaction of chromium(III) chloride tris-tetrahydrofuran with 3 equivalents of sodium benzoate in THF produces insoluble Cr(benzoate)3. Pyridine dissolves the complex and coordinates the chromium center to yield mer-Cr(η1-O2CPh)3(py)3 which was structurally characterized. Attempts to produce an anionic tris-benzoate complex via MII salt precursors were unsuccessful. Cr2(μ-O2CPh)4(THF)2 was isolated from the reaction of sodium benzoate and CrCl2(THF)2, while CoCl2 and MnCl2 starting materials produced the isostructural polymers Na(THF)M3(O2CPh)7(THF)2∞. Sodium complexation prevented polymerization but could not quell oligomerization yielding the trimeric Na(15-crown-5)2Co3(O2CPh)8.
The synthesis and structural characterization of three-coordinate iron(II) and cobalt(II) complexes supported by new N-phosphinoamidinate ligands is reported, along with the successful application of ...these complexes as precatalysts for the challenging room-temperature hydrosilylation of carbonyl compounds to afford alcohols upon workup. Under the rigorous screening conditions employed (0.015 mol % Fe) for the reduction of acetophenone, the well-defined iron(II) amido precatalyst 2b, featuring bulky N-2,6-diisopropylphenyl and di-tert-butylphosphino moieties within the N-phosphinoamidinate ligand, exhibited exceptional catalytic performance. Further experimentation revealed that the yield achieved in the hydrosilylation of acetophenone employing 2b was unaltered when conducting reactions in the absence of light, in the presence of excess mercury, or under solvent-free conditions. Notably, precatalyst 2b was found to exhibit the broadest substrate scope reported to date for such room-temperature iron-catalyzed carbonyl hydrosilylations en route to alcohols, enabling the chemoselective reduction of structurally diverse aldehydes and ketones, as well as for the first time esters, at remarkably low loadings (0.01–1.0 mol % Fe) and using only 1 equiv of phenylsilane reductant.
Cr phosphine catalysts are uniquely suited for industrial selective ethylene trimerization to 1-hexene. We recently introduced a Cr N-phosphinoamidine catalyst ((P,N)Cr) transition-state model for ...selectivity, and here, we use density functional theory calculations to address catalyst reactivity for ethylene trimerization. This is particularly important because there are currently no empirical parameters or design principles that provide prediction of high catalyst activity while maintaining trimerization selectivity. Specifically, using transition states and the energetic span model, we examined the ethylene trimerization catalytic cycle with the bidentate (P,N)Cr catalyst 1a and compared this highly productive catalyst to the surprisingly inactive tridentate (P,N,N)Cr catalyst. For (P,N)Cr 1a, this analysis revealed that for the high-spin CrI/III chromacycle mechanism, there are multiple CrI ethylene-coordinated resting states and multiple turnover-controlling transition states, which is consistent with previous experimental rate studies and can account for a partial rate order in ethylene. Based on the calculated energy landscape, the calculated 1-hexene productivity of 6.5 mol s–1 and mass of 2.0 × 106 g h–1 is close to the experimental value. This analysis also revealed that the tridentate (P,N,N)Cr catalyst has a much larger energy span and is ∼107 slower, which results from the stabilization of the energy landscape around the chromacyclopentane intermediate. In addition to this reactivity/inactivity comparison, we also calculated and compared the reactivity of several other experimentally reported 1-hexene Cr tridentate catalysts. Based on the catalytic energy spans, our calculations were able to qualitatively and semi-quantitatively replicate relative catalyst reactivity.
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