Palladium(II)‐Catalyzed Synthesis of Sulfinates from Boronic Acids and DABSO: A Redox‐Neutral,Phosphine‐Free Transformation

A redox‐neutral palladium(II)‐catalyzed conversion of aryl, heteroaryl, and alkenyl boronic acids into sulfinate intermediates, and onwards to sulfones and sulfonamides, has been realized. A simple Pd(OAc)₂ catalyst, in combination with the sulfur dioxide surrogate 1,4‐diazabicyclo[2.2.2]octane bis(sulfur dioxide) (DABSO), is sufficient to achieve rapid and high‐yielding conversion of the boronic acids into the corresponding sulfinates. Addition of C‐ or N‐based electrophiles then allows conversion into sulfones and sulfonamides, respectively, in a one‐pot, two‐step process.     

Iron(II) Complexes of Chiral Tridentate Nitrogen Donors and their Application in Catalytic Hydrosilylation Reactions

Enantiomerically pure, C₂-symmetric 2,6-bis(pyrazol-3-yl) pyridine ligands were obtained by treatment of diethyl-2,6-pyridinedicarbonate with (1R,4R)-(+)-camphor in the presence of NaH followed by ring closure with hydrazine. After twofold N-alkylation at the pyrazole rings, the addition of iron(II) chloride led to the according pentacoordinate dichloridoiron(II) complexes. All intermediates of the ligand synthesis, the ligands bearing NCH₃ and NCH₂C₆H₅ groups and the derived iron(II) complexes were structurally characterized by means of X-ray structure analysis. In-situ reaction with iron(II) carboxylates resulted in the formation of iron(II) carboxylate complexes, which turned out to be highly active in the hydrosilylation of acetophenone. However, even at room temperature, the enantiomeric excess of the product 1-phenylethanol is poor. ⁵⁷Fe Mössbauer spectroscopy gave an insight into the species formed during catalysis.     

Neodymium(III) Complexes Capable of Multi‐Electron Redox Chemistry

A family of neodymium complexes featuring a redox‐active ligand in three different oxidation states has been synthesized, including the iminoquinone (L⁰) derivative, (^dippiq)₂NdI₃ ( 1‐iq ), the iminosemiquinone (L¹⁻) compound, (^dippisq)₂NdI(THF) ( 1‐isq ), and the amidophenolate (L²⁻) [K(THF)₂][(^dippap)₂Nd(THF)₂] ( 1‐ap ) and [K(18‐crown‐6)][(^dippap)₂Nd(THF)₂] ( 1‐ap crown ) species. Full spectroscopic and structural characterization of each derivative established the +3 neodymium oxidation state with redox chemistry occurring at the ligand rather than the neodymium center. Oxidation with elemental chalcogens showed the reversible nature of the ligand‐mediated reduction process, forming the iminosemiquinone metallocycles, [K(18‐crown‐6)][(^dippisq)₂Nd(S₅)] ( 2‐isq crown ) and [K(18‐crown‐6)(THF)][(^dippisq)₂Nd(Se₅)] ( 3‐isq crown ), which are characterized to contain a 6‐membered twist‐boat ring.     

Synthesis of PCP‐Supported Nickel Complexes and their Reactivity with Carbon Dioxide

The Ni amide and hydroxide complexes [(PCP)Ni(NH₂)] ( 2 ; PCP=bis‐2,6‐di‐tert‐butylphosphinomethylbenzene) and [(PCP)Ni(OH)] ( 3 ) were prepared by treatment of [(PCP)NiCl] ( 1 ) with NaNH₂ or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H₂O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2‐Me‐imidazole)] ( 4 ), [(PCP)Ni(dimethylmalonate)] ( 5 ), [(PCP)Ni(oxazole)] ( 6 ), and [(PCP)Ni(CCPh)] ( 7 ), respectively. The hydroxide compound 3 , could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN₃ generated [(PCP)Ni(CN)] ( 8 ) or [(PCP)Ni(N₃)] ( 9 ), respectively. Compounds 3–7 , and 9 were characterized by X‐ray crystallography. Although 3 , 4 , 6 , 7 , and 9 are all four‐coordinate complexes with a square‐planar geometry around Ni, 5 is a pseudo‐five‐coordinate complex, with the dimethylmalonate ligand coordinated in an X‐type fashion through one oxygen atom, and weakly as an L‐type ligand through another oxygen atom. Complexes 2–9 were all reacted with carbon dioxide. Compounds 2 – 4 underwent facile reaction at low temperature to form the κ¹‐O carboxylate products [(PCP)Ni{OC(O)NH₂}] ( 10 ), [(PCP)Ni{OC(O)OH}] ( 11 ), and [(PCP)Ni{OC(O)‐2‐Me‐imidazole}] ( 12 ), respectively. Compounds 10 and 11 were characterized by X‐ray crystallography. No reaction was observed between 5 – 9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2 – 9 to form a κ¹‐O carboxylate product and understand the pathways for carbon dioxide insertion into 2 , 3 , 6 , and 7 . The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7 , but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.     

Concise Synthesis of Potassium Acyltrifluoroborates from Aldehydes through Copper(I)‐Catalyzed Borylation/Oxidation

Potassium acyltrifluoroborates (KATs) were prepared through copper(I)‐catalyzed borylation of aldehydes and subsequent oxidation. This synthetic route is characterized by the wide range of aldehydes accessible, favorable step economy, mild reaction conditions, and tolerance of various functional groups, and it enables the facile generation of a range of KATs, for example, bearing halide, sulfide, acetal, or ester moieties. Moreover, this method was applied to the three‐step synthesis of various α‐amino acid analogues that bear a KAT moiety on the C‐terminus by using naturally occurring amino acids as the starting material.