Recent advances in the catalytic hydroboration of carbonyl compounds

Catalyst-free and solvent-free hydroboration of aldehydes

The well-defined heavy rare-earth ytterbium iodide complex 1 (L2YbI) has been successfully employed as an efficient catalyst for the hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) at room temperature. The protocol requires low catalyst loadings (0.1–0.5 mol %) and proceeds rapidly (>99% conversion in <10 min). Additionally, catalyst 1 shows a good functional group tolerance even toward the hydroxyl and amino moieties and displays chemoselective hydroboration of aldehydes over ketones under mild conditions.

Ytterbium-Catalyzed Hydroboration of Aldehydes and Ketones

Four amidate-functionalized N-heterocyclic carbene (NHC) rare-earth metal amido complexes [(κ2-N,O-κ1-L)2REN(SiMe3)2] (L = 1-(C6H5C═ONCH2CH2)-3-(CH3)3C6H2(N(CH)2NC)) [RE = Er (1), Y (2), Dy (3), Gd...

Well-Defined Amidate-Functionalized N-Heterocyclic Carbene -Supported Rare-Earth Metal Complexes as Catalysts for Efficient Hydroboration of Unactivated Imines and Nitriles.

The first examples of the hydroboration of imines with HBpin catalyzed by the commercially available n-BuLi are reported herein. Good functional group tolerance under mild conditions and short reaction time have been achieved. Further investigation showed that n-BuLi could also serve as an efficient catalyst for the hydroboration of alkynes with HBpin. High chemoselectivity toward imines over alkynes is presented. A possible mechanistic pathway of the n-BuLi catalyzed hydroboration of imines with HBpin is proposed based on computational studies.

n-Butyllithium catalyzed hydroboration of imines and alkynes

The first examples for the rhenium-catalysed hydroboration of aldehydes, ketones and aldimines, including heteroaromatic quinoline, are reported herein Reactions are remarkably chemoselective and tolerant of several functional groups A wide array of rhenium complexes were efficient pre-catalysts for these hydroborations, including new low-valent complexes of the formula [Re(N–N)(CO)3(L)]X (N–N = bipy derivative, L = labile ligand/solvent, and X = [BArF4]− and [B(3,5-di-tBu-cat)2]−), which have been characterized fully including an X-ray diffraction study for [Re(bipy)(CO)3(quin)][BArF4] (2) A new silver spiroboronate ester Ag[B(3,5-di-tBu-cat)2](NCCH3)3 (3) was prepared and characterized fully, including an X-ray diffraction study, and used to make one of the new rhenium complexes

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Rhenium-catalysed hydroboration of aldehydes and aldimines

The interplay between the primary and secondary coordination spheres in biological metal sites plays an essential role in controlling their properties. Some of the clearest examples of this are from copper sites in blue and purple copper proteins. Many such proteins contain methionine (Met) in the primary coordination sphere as a weakly bound ligand to Cu. While the effects of replacing the coordinated Met are understood, less so is the importance of its second-sphere interactions. In this combined informatics and experimental study, we first present a bioinformatics investigation of the second-sphere environments in biological Met-Cu motifs. The most common interaction is between the Met-CH3 and the π-face of a phenylalanine (Phe) (81% of surveyed structures), tyrosine (Tyr) (11%), and tryptophan (Trp) (8%). In most cases, the Met-CH3 also forms a contact with a π-face of one of a Cu-ligating histidine-imidazole. Such interactions are widely distributed in different Cu proteins. Second, to explore the impact of the second-sphere interactions of Met, a series of artificial Pseudomonas aeruginosa azurin proteins were produced where the native Phe15 was replaced with Tyr or Trp. The proteins were characterized using optical and magnetic resonance spectroscopies, X-ray diffraction, electrochemistry, and an investigation of the time-resolved electron-transfer kinetics of photosensitizer-modified proteins. The influence of the Cu-Met-Aro interaction on azurin's physical properties is subtle, and the hallmarks of the azurin blue copper site are maintained. In the Phe15Trp variant, the mutation to Phe15 induces changes in Cu properties that are comparable to replacement of the weak Met ligand. The broader impacts of these widely distributed interactions are discussed.

The Impact of Second Coordination Sphere Methionine-Aromatic Interactions in Copper Proteins.

Oxidation of a series of CrV nitride salen complexes (CrVNSalR) with different para-phenolate substituents (R = CF3, tBu, NMe2) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords CrVI nitrides ([CrVINSalR]+; R = CF3, tBu) and a localized CrV nitride phenoxyl radical for the more electron-donating NMe2 substituent ([CrVNSalNMe2]•+). The facile nitride homocoupling observed for the MnVI analogues was significantly attenuated for the CrVI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the CrV ligand radical derivative ([CrVNSalNMe2]•+) in comparison to the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu). As a result, [CrVNSalNMe2]•+ reacts with B(C6F5)3, thus exhibiting similar nucleophilic reactivity to the neutral CrV nitride derivatives. In contrast, the CrVI derivatives ([CrVINSalR]+; R = CF3, tBu) act as electrophiles, displaying facile reactivity with PPh3 and no reaction with B(C6F5)3. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to CrVI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Spatial characterization of redox processes and speciation of Ru(III) anticancer complexes by 19F magnetic resonance imaging.

The dinuclear title compound, [In2(CH3)2(C7H7S)4] or [Me(2-MeC6H4S)In-μ-(2-MeC6H4S)2InMe(2-MeC6H4S)], was prepared from the 1:2 reaction of Me3In and 2-MeC6H4SH in toluene. Its crystal structure exhibits a four-membered In2S2 ring core via bridging (2-MeC6H4S) groups. The dimeric units are further associated into a one-dimensional polymeric structure extending parallel to the a axis via inter­molecular In⋯S contacts. The In atoms are then in distorted trigonal–bipyramidal CS4 bonding environments.

https://journals.iucr.org/e/issues/2017/04/00/lh5837/lh5837.pdf

Gregory A. MacNeil

Papers

Rhenium-catalysed hydroboration of aldehydes and aldimines

The Impact of Second Coordination Sphere Methionine-Aromatic Interactions in Copper Proteins.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Spatial characterization of redox processes and speciation of Ru(III) anticancer complexes by 19F magnetic resonance imaging.

Synthesis and crystal structure of bis(μ-2-methylbenzenethiolato-κ2S:S)bis[methyl(2-methylbenzenethiolato-κS)indium(III)]

Gregory A. MacNeil

Papers

Rhenium-catalysed hydroboration of aldehydes and aldimines

The Impact of Second Coordination Sphere Methionine-Aromatic Interactions in Copper Proteins.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation.

Spatial characterization of redox processes and speciation of Ru(III) anticancer complexes by 19F magnetic resonance imaging.

Synthesis and crystal structure of bis­(μ-2-methyl­benzene­thiol­ato-κ2S:S)bis­[meth­yl(2-methyl­benzene­thiol­ato-κS)indium(III)]

Synthesis and crystal structure of bis(μ-2-methylbenzenethiolato-κ2S:S)bis[methyl(2-methylbenzenethiolato-κS)indium(III)]