Ana Torvisco

The Role of 2,6-Diaminopyridine Ligands in the Isolation of an Unprecedented, Low-Valent Tin Complex

Stabilization of the central atom in an oxidation state of zero through coordination of neutral ligands is a common bonding motif in transition-metal chemistry. However, the stabilization of main-group elements in an oxidation state of zero by neutral ligands is rare. Herein, we report that the transamination reaction of the DAMPY ligand system (DAMPY=2,6-[ArNH-CH2 ]2 (NC5 H3 ) (Ar=C6 H3 -2,6-iPr2 )) with Sn[N(SiMe3 )2 ]2 produces the DIMPYSn complex (DIMPY=(2,6-[ArNCH]2 (NC5 H3 )) with the Sn atom in a formal oxidation state of zero. This is the first example of a tin compound stabilized in a formal oxidation state of zero by only one donor molecule. Furthermore, three related low-valent Sn(II) complexes, including a [DIMPYSn(II) Cl](+) [SnCl3 ](-) ion pair, a bisstannylene DAMPY{Sn(II) [N(SiMe3 )2 ]2 }2 , and the enamine complex MeDIMPYSn(II) , were isolated. Experimental results and the conclusions drawn are also supported by theoretical studies at the density functional level of theory and (119) Sn Mössbauer spectroscopy.

A Sequential Ugi Multicomponent/Cu-Catalyzed Azide−Alkyne Cycloaddition Approach for the Continuous Flow Generation of Cyclic Peptoids

The development of a continuous flow multistep strategy for the synthesis of linear peptoids and their subsequent macrocyclization via Click chemistry is described. The central transformation of this process is an Ugi four-component reaction generating the peptidomimetic core structure. In order to avoid exposure to the often toxic and malodorous isocyanide building blocks, the continuous approach was telescoped by the dehydration of the corresponding formamide. In a concurrent operation, the highly energetic azide moiety required for the subsequent intramolecular copper-catalyzed azide-alkyne cycloaddition (Click reaction) was installed by nucleophilic substitution from a bromide precursor. All steps yielding to the linear core structures can be conveniently coupled without the need for purification steps resulting in a single process generating the desired peptidomimetics in good to excellent yields within a 25 min reaction time. The following macrocyclization was realized in a coil reactor made of copper without any additional additive. A careful process intensification study demonstrated that this transformation occurs quantitatively within 25 min at 140 °C. Depending on the resulting ring strain, either a dimeric or a monomeric form of the cyclic product was obtained.

Bismuth sulphide–polymer nanocomposites from a highly soluble bismuth xanthate precursor

Bismuth sulphide nanocrystal–polymer hybrid layers are of interest for various optoelectronic, thermoelectric or sensing applications. In this work, we present a ligand-free in situ route for the formation of Bi2S3 nanorods directly within a polymer matrix. For this purpose, we introduce a novel bismuth xanthate (bismuth(III) O-3,3-dimethylbutan-2-yl dithiocarbonate), which is highly soluble in non-polar organic solvents. The analysis of the crystal structure revealed that the prepared bismuth xanthate crystallises in the monoclinic space group C2/c and forms dimers. The bismuth xanthate can be converted into nanocrystalline Bi2S3 with an orthorhombic crystal structure via a thermally induced solid state reaction at moderate temperatures below 200 °C. In combination with the high solubility in non-polar solvents this synthetic route for Bi2S3 is of particular interest for the preparation of Bi2S3–polymer nanocomposites as exemplarily investigated on Bi2S3–poly(methyl methacrylate) and Bi2S3–poly(3-hexylthiophene-2,5-diyl) (P3HT) nanocomposite layers. Atomic force and transmission electron microscopy revealed that Bi2S3 nanorods are dispersed in the polymer matrix. Photoluminescence experiments showed a quenching of the P3HT fluorescence with increasing Bi2S3 content in the hybrid layer.

Tetraacylgermanes: Highly Efficient Photoinitiators for Visible‐Light‐Induced Free‐Radical Polymerization

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R]4 (R=mesityl (1 a), phenyl (1 b)), a previously unknown class of highly efficient Ge-based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one-pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The efficiency of 1 a,b as photoinitiators is demonstrated in photobleaching (UV/Vis), time-resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo-DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long-wavelength visible-light photoinitiators for free-radical polymerization.

Concise synthesis of C-1-cyano-iminosugars via a new Staudinger/aza Wittig/Strecker multicomponent reaction strategy.

A new Staudinger/aza Wittig/Strecker multicomponent reaction sequence to C-1-cyano iminoalditols has been developed. When applied to 5-azidodeoxy-d-xylose and -d-glucose as substrates the method leads smoothly in good yield and with excellent stereoselectivity to respectively, 1,5-dideoxy-1,5-imino-d-idurono nitrile and 2,6-didesoxy-2,6-imino-d-glycero-d-ido-heptononitrile.

Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes.

Previously unknown 1,1,4-tris(trimethylsilyl)-4-acyldodecamethylcyclohexasilanes (Me3Si)2Si6Me12(Me3Si)COR (16a, R = tert-butyl; 16b, R = 1-adamantyl) have been synthesized by the reaction of the potassium silanides (Me3Si)2Si6Me12(Me3Si)K with acid chlorides ClCOR, and their photochemical rearrangement reactions have been studied. The molecular structures of 16a,b as determined by single-crystal X-ray diffraction analysis exhibit an unusual twist-boat conformation of the cyclohexasilane ring. When 16a,b were photolyzed with λ >300 nm radiation, they underwent Brook type 1,3-Si → O migration reactions to generate the cyclohexasilanes 17a,b with exocyclic Si=C bonds along with smaller amounts of the ring-enlarged species 19a,b with endocyclic Si=C double bonds. While 17a,b were stable enough to allow characterization by NMR and UV absorption spectroscopy, the less stable products 19a,b could only be observed in the form of their methanol adducts.

Stable Silenolates and Brook-Type Silenes with Exocyclic Structures.

The first silenolates with exocyclic structures [(Me3Si)2Si(Si2Me4)2SiC(R)O]−K+ (2a: R = 1-adamantyl; 2b: mesityl; 2c: o-tolyl) were synthesized by the reaction of the corresponding acylcyclohexasilanes 1a–c with KOtBu. NMR spectroscopy and single-crystal X-ray diffraction analysis suggest that the aryl-substituted silenolates 2b,c exhibit increased character of functionalized silenes as compared to the alkyl-substituted derivative 2a due to the different coordination of the K+ counterion to the SiC(R)O moiety. 2b,c, thus, reacted with ClSiiPr3 to give the exocyclic silenes (Me3Si)2Si(Si2Me4)2Si=C(OSiiPr3)R (3b: R = Mes; 3c: o-Tol), while 2a afforded the Si-silylated acylcyclohexasilane 1d. The thermally remarkably stable compound 3b, which is the first isolated silene with the sp2 silicon atom incorporated into a cyclopolysilane framework, could be fully characterized structurally and spectroscopically.

The Staudinger/aza-Wittig/Grignard reaction as key step for the concise synthesis of 1-C-Alkyl-iminoalditol glycomimetics

The scope of a one-pot tandem approach for the synthesis of C-1 alkyl iminoalditol derivatives with a Staudinger/aza-Wittig/Grignard cascade has been evaluated. The reaction conditions have been optimized for two azidodeoxy aldose substrates and a range of Grignard reagents. The nature of both, substrate as well as nucleophile, was found to control the stereoselectivity of the alkyl addition to the cyclic iminium intermediate at position C-1. This approach enabled the synthesis of a collection of C-alkyl iminoalditols, which were biologically evaluated as inhibitors against a set of standard glycoside hydrolases. All compounds were found to exhibit highly selective inhibition of β-glucosidase activity.

From mono- to tetraacylgermanes: extending the scope of visible light photoinitiators

We have investigated the inititiaton efficiency of carefully selected germanium-based photointiators for radical polymerization. To establish a systematic relationship between structure and reactivity, we have developed a convenient synthetic protocol for the preparation of a trisacylgermane, closing the gap from mono- to tetraacylgermane photoinitiators. The studied acylgermanes display distinct, wavelength-dependent photobleaching upon irradiation up to 470 nm. In particular, tetraacylgermanes featuring ortho-alkyl substituents reveal red-shifted n–π* bands, in line with excellent photobleaching upon visible light irradiation. Quantum yields of decomposition (determined at 385 nm) have been found to be highest for bisacylgermanes. Germyl radicals produced upon triplet-state α-cleavage of the acylgermanes react remarkably fast with monomers. Addition rate constants to (meth)acrylates range from 0.4–4.5 × 108 M−1 s−1, depending on the substitution pattern. These values are clearly higher than those reported for related phosphorus-centered radicals derived from acylphosphane oxides. We have further established the nature of the products and side-products formed at initial stages of the polymerizations using chemically induced dynamic nuclear polarization (CIDNP) experiments.

Synthesis and characterization of cyclic acylsilanes

GRAPHICAL ABSTRACT ABSTRACT Acylcyclohexasilanes are interesting starting materials for the formation of cyclic silenes. Employing standard cyclopolysilane synthetic procedures, nine previously unknown acylcyclohexasilanes were synthesized and characterized by NMR, and UV-Vis spectroscopy and X-ray crystallography to elucidate substituent influences of the R-group attached to the carbonyl C-atom.