Arseni Kostenko

Formation of Three New Bonds and Two Stereocenters in Acyclic Systems by Zinc-Mediated Enantioselective Alkynylation of Acylsilanes, Brook Rearrangement, and Ene-Allene Carbocyclization Reactions

Diastereoisomerically pure (dr > 99:1) and enantiomerically enriched (er up to 98:2) substituted propargyl diols possessing a tertiary hydroxyl group were synthesized in a single-pot operation from simple acylsilanes through a combined catalytic enantioselective alkynylation of acylsilanes, followed by an allenyl-Zn-Brook rearrangement and Zn-ene-allene (or Zn-yne-allene) cyclization reaction. Two remarkable features of these reactions are the near complete transfer of chirality in the allenyl-Zn-Brook rearrangement and the highly organized six-membered transition state of the Zn-ene-allene carbocyclization found by DFT calculations. In this process, three new bonds and two new stereogenic centers are created in a single-pot operation in excellent diastereo- and enantiomeric ratios. DFT calculations show that the allenyl-Zn-Brook rearrangement occurs in preference to the classic [1,2]-Zn-Brook rearrangement owing to its significantly lower activation barrier.

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is, the SiSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350-410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet-triplet energy gap of only 7.3 kcal mol(-1) .

Facile Access to NaOC≡As and Its Use as an Arsenic Source to Form Germylidenylarsinidene Complexes

A facile, one-pot synthesis of [Na(OC≡As)(dioxane)x ] (x=2.3-3.3) in 78 % yield is reported through the reaction of arsine gas with dimethylcarbonate in the presence of NaOt Bu and 1,4-dioxane. It has been employed for the synthesis of the first arsaketenyl-functionalized germylene [LGeAsCO] (2, L=CH[CMeN(Dipp)]2 ; Dipp=2,6-i Pr2 C6 H3 ) from the reaction with LGeCl (1). Upon exposure to ambient light, 2 undergoes CO elimination to form the 1,3-digerma-2,4-diarsacyclobutadiene [L2 Ge2 As2 ] (3), which contains a symmetric Ge2 As2 ring with ylide-like Ge=As bonds. Remarkably, the CO ligand located at the arsenic center of 2 can be exchanged with PPh3 or an N-heterocyclic carbene iPr NHC donor (iPr NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) to afford the novel germylidenylarsinidene complexes [LGe-AsPPh3 ] (4) and [LGe-As(iPr NHC)] (5), respectively, demonstrating transition-metal-like ligand substitution at the arsinidene-like As atom. The formation of 2-5 and their electronic structures have been studied by DFT calculations.

Intriguing Physical and Chemical Properties of Phosphorus Corroles

The fluorescence intensity of phosphorus corroles increases upon meso-aryl C-F/C-H and P-OH/P-F substitutions, the latter affects corrole-centered redox processes more than C-H/C-F substitution on the corroles skeleton, and the presence of F atoms allows for the first experimental insight into the electronic structures of oxidized corroles. Experimental and theoretical methodologies reveal that mono- but not bis-chlorosulfonation of the corrole skeleton is under kinetic control. Selective introduction of heavy atoms leads to complexes that are phosphorescent at room temperature.

Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins

The first chelating bis(N-heterocyclic silylene)xanthene ligand [SiII(Xant)SiII] as well as its Ni complexes [SiII(Xant)SiII]Ni(η2-1,3-cod) and [SiII(Xant)SiII]Ni(PMe3)2 were synthesized and fully characterized. Exposing [SiII(Xant)SiII]Ni(η2-1,3-cod) to 1 bar H2 at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni2Si2 core. Exchange of the 1,3-COD ligand by PMe3 led to [SiII(Xant)SiII]Ni(PMe3)2, which could activate H2 reversibly to afford the first SiII-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [SiII(Xant)SiII]Ni(η2-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H2 pressure at room temperature. DFT calculations reveal a novel mode of H2 activation, in which the SiII atoms of the [SiII(Xant)SiII] ligand are involved in the key step of H2 cleavage and hydrogen transfer to the olefin.

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

Tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1) was subjected to a temperature-dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene (2). From the temperature dependent EPR absorption area we derive a singlet→triplet (1→2) energy gap, EST , of 13.9 kcal mol-1 , in agreement with calculated values. The zero-field splitting parameters D=0.171 cm-1 , E=0 cm-1 are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.

Activation of Homolytic Si-Zn and Si-Hg Bond Cleavage, Mediated by a Pt(0) Complex, via Novel Pt-Zn and Pt-Hg Compounds.

The thermally stable [(tBuMe2 Si)2 M] (M=Zn, Hg) generate R3 Si(.) radicals in the presence of [(dmpe)Pt(PEt3 )2 ] at 60-80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn (2 a and 2 b), M=Hg (3 a and 3 b)) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt-MSiR3 bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn (5), Hg (6)) whose structures were determined by EPR spectroscopy and DFT calculations.

Chelate Silylene-Silyl Ligand Can Boost Rhodium-Catalyzed C−H Bond Functionalization Reactions

The first N-heterocyclic silylene (NHSi)-silane scaffold LSi-R-Si(H)Mes2 (1) (L=PhC(NtBu)2 ; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me3 C6 H2 ) was synthesized and used to form the unique rhodium(III) complex (LSi-R-SiMes2 )Rh(H)Cl 2 through its reaction with 0.5 molar equivalents of [Rh(coe)2 Cl]2 (coe=cyclooctene). An X-ray diffraction analysis revealed that 2 has a (SiII SiIV )Rh(H)Cl core with three short Rh⋅⋅⋅H-C contacts with Me groups of the ligand 1, which cause a distorted pentagonal bipyramidal coordination of the Rh center. Unexpectedly, the reaction of 2 with tBuONa gives the new bis(silyl)hydridorhodium(III) complex 4. Due to the strong donor ability of the chelate SiII -SiIV ligand, 2 and 4 can act as highly efficient pre-catalysts in the Rh-mediated selective C-H functionalization of 2-phenylpyridines with C-C unsaturated organic substrates under mild reaction conditions.

Generation and Characterization of the First Persistent Platinum(I)-Centered Radical

The first persistent platinum(I)-centered radical was generated by homolytic cleavage of a Pt-HgSiR3 bond of a mercury-substituted platinum(II) complex. The PtI radical was characterized by EPR spectroscopy, chemical trapping experiments, and density functional theory (DFT) calculations.