Where silylene–silicon centres matter in the activation of small molecules

Small molecules such as H2, N2, CO, NH3, O2 are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry. The last few decades have witnessed significant advances in the chemistry of heavy low-coordinate main-group elements, with a plethora of newly synthesised functional compounds, behaving like transition-metal complexes with respect to facile activation of such small molecules. Among them, silylenes have received particular attention in this vivid area of research showing even metal-free bond activation and catalysis. Recent striking discoveries in the chemistry of silylenes take advantage of narrow HOMO–LUMO energy gap and Lewis acid–base bifunctionality of divalent Si centres. The review is devoted to recent advances of using isolable silylenes and corresponding silylene–metal complexes for the activation of fundamental but inert molecules such as H2, COx, N2O, O2, H2O, NH3, C2H4 and E4 (E = P, As).DFG, 390540038, EXC 2008: UniSysCatTU Berlin, Open-Access-Mittel - 202

Synthesis and Reactivity of an Anti-van’t Hoff/Le Bel Compound with a Planar Tetracoordinate Silicon(II) Atom

For a long time, planar tetracoordinate carbon (ptC) represented an exotic coordination mode in organic and organometallic chemistry, but it is now a useful synthetic building block. In contrast, realization of planar tetracoordinate silicon (ptSi), a heavier analogue of ptC, is still challenging. Herein we report the successful synthesis and unusual reactivity of the first ptSi species of divalent silicon present in 3, supported by the chelating bis(N-heterocyclic silylene)bipyridine ligand, 2,2′-{[(4-tBuPh)­C(NtBu)]2­SiNMe}2­(C5N)2, 1]. The compound resulted from direct reaction of 1 with Idipp-SiI2 [Idipp = 1,3-bis(2,6-diisopropyl­phenyl)­imidazol-2-ylidene]. Alternatively, it can also be synthesized by a two-electron reduction of the corresponding Si(IV) precursor 2 with 2 molar equiv of KC10H8. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3pz orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound 4, with the methyl group located in an apical position. Remarkably, the reaction of 2 with [CuOtBu] leads to the regeneration of the bis(silylene) arms via Si–Si bond scission and induces the Si(II) → Si(IV) oxidation of the central Si(II) atom and concomitant two-electron reduction of the bipyridine moiety to form the neutral bis(silylene)silyl Cu(I) complex 5

Zn(OTf)₂‑Catalyzed Phosphinylation of Propargylic Alcohols: Access to γ‑Ketophosphine Oxides

The first facile and efficient Zn­(OTf)₂-catalyzed direct coupling of unprotected propargylic alcohols with arylphosphine oxides has been developed, affording a general, one-step approach to access structurally diverse γ-ketophosphine oxides via sequential Meyer–Schuster rearrangement/phospha-Michael reaction along with new C­(sp³)P and CO bond formations, operational simplicity, and complete atom economy under ligand-free and base-free conditions

Zn(OTf)₂‑Catalyzed Phosphinylation of Propargylic Alcohols: Access to γ‑Ketophosphine Oxides

The first facile and efficient Zn­(OTf)₂-catalyzed direct coupling of unprotected propargylic alcohols with arylphosphine oxides has been developed, affording a general, one-step approach to access structurally diverse γ-ketophosphine oxides via sequential Meyer–Schuster rearrangement/phospha-Michael reaction along with new C­(sp³)P and CO bond formations, operational simplicity, and complete atom economy under ligand-free and base-free conditions

Copper-Catalyzed Direct Coupling of Unprotected Propargylic Alcohols with P(O)H Compounds: Access to Allenylphosphoryl Compounds under Ligand- and Base-Free Conditions

The first facile and efficient copper-catalyzed direct C–P cross-coupling of unprotected propargylic alcohols with P­(O)H compounds has been developed, providing a general, one-step approach to construct valuable allenylphosphoryl frameworks with operational simplicity and high step- and atom-economy under ligand-, base-, and additive-free conditions

SDS-PAGE analysis of the recombinant BglPC28 after purification using the GST•bind agarose resin.

<p>Lane: 1, uninduced crude extract; 2, soluble fraction of the crude extract of the induced recombinant BL21 (DE3) cells; 3, precipitated fraction of the crude extract of the induced recombinant BL21 (DE3) cells; 4, GST-BglPC28 after purification with the GST<b>•</b>bind agarose resin; 5, purified recombinant BglPC28 after cleavage via thrombin.</p

Phylogenetic analysis of characterized glycoside hydrolases family 3 (GH3).

<p>Amino acid sequences were obtained from the NCBI/EMBL database and CAZy database (accession numbers are indicated on the tree). This tree was made using the neighbor-joining method with a poisson model and pairwise deletion. Bootstrap values expressed as percentages of 1,000 replications greater than 65% are shown at the branch points. The bar represents 20 amino acid residues substitutions per 100 amino acid residues.</p

Sequence alignment of ginsenoside-transforming BglPC28 and Bgp1 or structure-determined glycoside hydrolase family 3 enzymes.

<p>The sequence alignment was created using ClustalW at the EBI-server using default settings and visualized with Jalview (<a href="http://www.ebi.ac.uk/Vmichele/jalview/" target="_blank">http://www.ebi.ac.uk/Vmichele/jalview/</a>). Regions of identity or high similarity among sequences are shown as black or gray columns, respectively. The conserved active site residues (general acid/base and nucleophile residues) were marked by asterisk. The PA14 domains of <i>S.ven</i> and <i>K.mar</i> are boxed. Genbank IDs of the glycoside hydrolases family 3 are as follows, <i>Pseudonocardia</i> sp. Gsoil 1536 β-glucosidase [<i>BglPC28</i> (This study)], JX960416; <i>Microbacterium esteraromaticum</i> KACC 16318 β-glucosidase (<i>Bgp1</i>), AEX88466; <i>Streptomyces venezuelae</i> β-glycosidase (<i>S.ven</i>), AAC68679; <i>Thermotoganeapolitana</i>DSM 4359^T β-glycosidase (<i>T.nea</i>), ABI29899; <i>Kluyveromycesmarxianus</i> β-glucosidase (<i>K.mar</i>), ACY95404.</p

Electrospray negative ion mass spectrum of produced ginsenoside Rg₂(<i>S</i>) of which m/z was 783.8 [M−H]⁻. Glc, glucose moiety; Rha, rhamnose moiety.

<p>Electrospray negative ion mass spectrum of produced ginsenoside Rg₂(<i>S</i>) of which m/z was 783.8 [M−H]⁻. Glc, glucose moiety; Rha, rhamnose moiety.</p

TLC analyses of time course of ginsenosides bioconversion by BglPC28.

<p>(a), transformation of ginsenoside Re; (b), transformation of ginsenoside Rg₁. Developing solvent: CHCl₃-CH₃OH-H₂O (65∶35∶10, v/v, lower phase). Lanes S, ginsenosidestandards (PPT type ginsenoside mixtures).</p