Readily available and bench-stable Cu(acac) 2 (1) addresses many challenges in exploratory hydrophosphination catalysis. Mechanistic investigations were performed to answer questions that remain about the reactivity of 1, the role of light in catalysis, and to provide direction for further study. A divergent Hammett plot indicates different mechanisms based on electron density at the alkene substrate. A radical process was eliminated based on trapping reactions and in situ electron paramagnetic resonance experiments. Isotopic labeling experiments, a zwitterionic trapping experiment, stoichiometric model reactions, and catalytic reactions using proxy intermediates indicate that both the conjugate addition and insertion-based mechanistic pathways occur with this system, depending on the unsaturated substrate. Computational analysis indicates that the lowest energy transition is a ligand-to-metal charge transfer from the phosphido ligand where the LUMO has significant Cu−P antibonding character, suggesting that a weakened Cu−P bond accelerates insertion under photocatalytic conditions. This hypothesis explains the greater activity of 1 compared to prior copper-catalyzed hydrophosphination reports and appears to be a general phenomenon for copper(I) catalysts. These results have been leveraged to achieve heretofore unknown catalytic hydrophosphination reactivity, namely the diastereoselective hydrophosphination of a tri-substituted styrene substrate.
An examination of several catalytic reactions among the group 15 elements is presented. The connections between the chemistry of the pnictogens can sometimes be challenging, but aspects of metal–pnictogen reactivity...
Titanium compounds supported by the triamidoamine ligand (N3N, N(CH2CH2NSiMe3 3–)3) have been investigated for hydrophosphination catalysis. The simple titanium alkyl compound (N3N)TiMe (2) demonstrates modest activity as a precatalyst for the photocatalytic hydrophosphination of styrene, but the terminal phosphido compound, (N3N)TiPHPh (4), is inactive. Analysis of these reactions by EPR spectroscopy indicates that (N3N)TiMe undergoes homolytic Ti–C bond cleavage to achieve radical hydrophosphination. This pathway was further supported in a radical trapping experiment. The phosphido derivative 4 does not produce radicals under similar conditions, despite undergoing facile migratory insertion reactions with polar substrates featuring C–N and C–O multiple bonds. Both nitrile and isonitrile substrates insert with ultimate formation of phosphaalkene products, a change in reactivity as compared to the zirconium congener. Molecular structures of (N3N)TiPHPh (4), (N3N)TiNBnCPPh (5), and (N3N)TiN(H)C(Ph)PPh (6) are reported.
Chromium-bearing tourmalines are rare. Chromium-rich tourmaline from the northwestern part of the Adirondack Mountains in the Adirondack Lowlands is among the most chromium-rich tourmalines found to date. The mineral, with >21.0 wt. % Cr2O3, is from the marble-hosted talc–tremolite–cummingtonite schist in the #1 mine in Balmat, St. Lawrence County, New York. The atomic arrangement of the sample (a = 16.0242(3) Å, c = 7.3002(2) Å) was refined to R1 = 0.0139. The composition, from chemical analyses and optimization of the formula, is X(Ca0.22Na0.69K0.01) Y(Cr3+1.68Mg0.80Ti0.13V0.06Mn0.02Fe0.02Li0.29) Z(Al3.11Cr3+1.18Mg1.70Fe0.01) T(Si5.93Al0.07) B3O27 OH3.99 F0.01. There has been extensive debate over the ordering of Cr3+ between the tourmaline Y and Z octahedral sites. Recent work has suggested that, at low concentrations (<~1.03 apfu), the substituent Cr3+ is ordered into the Y-site, whereas, at greater concentrations, the substituent is disordered over both octahedral sites. An analysis of nine recently published, high-precision structures of chromium-bearing tourmaline, in combination with the Adirondack tourmaline, suggests that structural changes to the Y-site at low concentrations of Cr3+ induce changes in the Z-site that make it more amenable to incorporation of the Cr3+ substituents by increasing <Z–O>. The bond lengths change to lower the bond-valence sum of Cr3+ in the Z-site of the chromium-dravite, making that site more amenable to the substituent. Calculations suggest that the Z-site begins to accept substituent Cr3+ when the bond valence sum of that ion in Z reduces to a value of ~3.36 valence units.
Readily available and bench-stable Cu(acac)2 (1) addresses many challenges in exploratory hydrophosphination catalysis. Mechanistic investigations were performed to answer questions that remain about the reactivity of 1, the role of light in the catalysis, and to provide direction for further study. A divergent Hammett plot indicates differing mechanisms based on electron density at the alkene substrate. A radical process was eliminated based on trapping reactions and in-situ EPR experiments. Isotopic labeling experiments, a zwitterionic trapping experiment, stoichiometric model reactions, and catalytic reactions using proxy intermediates indicate that both conjugate addition and insertion-based mechanistic pathways occur with this system, depending on the unsaturated substrate. Computational analysis indicates that the lowest energy transition is a ligand-to-metal charge transfer (LMCT) from the phosphido ligand where the LUMO has significant Cu–P antibonding character, suggesting that a weakened Cu–P bond accelerates insertion under photocatalytic conditions. This hypothesis explains the greater activity of 1 compared to copper-catalyzed hydrophosphination reports and appears to be a general phenomenon for copper(I) catalysts. These results have been leveraged to achieve heretofore unknown catalytic hydrophosphination reactivity, namely the diastereoselective hydrophosphination of a tri-substituted styrene substrate.
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