Gregor Schnakenburg

Stable N‐Heterocyclic Carbene Adducts of Arylchlorosilylenes and Their Germanium Homologues

The first N-heterocyclic carbene adducts of arylchlorosilylenes are reported and compared with the homologous germanium compounds. The arylsilicon(II) chlorides SiArCl(Im-Me(4)) [Ar=C(6)H(3)-2,6-Mes(2) (Mes=C(6)H(2)-2,4,6-Me(3)), C(6)H(3)-2,6-Trip(2) (Trip=C(6)H(2)-2,4,6-iPr(3))] were obtained selectively on dehydrochlorination of the arylchlorosilanes SiArHCl(2) with 1,3,4,5-tetramethylimidazol-2-ylidene (Im-Me(4)). The analogous arylgermanium(II) chlorides GeArCl(Im-Me(4)) were prepared by metathetical exchange of GeCl(2)(Im-Me(4)) with LiC(6)H(3)-2,6-Mes(2) or addition of Im-Me(4) to GeCl(C(6)H(3)-2,6-Trip(2)). All compounds were fully characterized. Density functional calculations on ECl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)), where E=Si, Ge, at different levels of theory show very good agreement between calculated and experimental bonding parameters, and NBO analyses reveal similar electronic structures of the two aryltetrel(II) chlorides. The low gas-phase Gibbs free energy of bond dissociation of SiCl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)) (Delta(calcd) degrees=28.1 kJ mol(-1)) suggests that the carbene adducts SiArCl(Im-Me(4)) may be valuable transfer reagents of the arylsilicon(II) chlorides SiArCl.

Silicon–Oxygen Double Bonds: A Stable Silanone with a Trigonal‐Planar Coordinated Silicon Center

SiO in a complex: The first silanone that is stable at room temperature (3) is reported. The two-step synthesis involves carbonylation of the silylidyne complex 1 to give the chromiosilylene 2, followed by oxidation of 2 with N2 O. Silanone 3 features a polar, short SiO bond (1.526(3) Å) to a trigonal-planar-coordinated silicon center and reacts with water to give the dihydroxysilyl complex.

Self‐Discriminating Self‐Assembly of Dinuclear Heterochiral Rhombs from Tröger’s Base Derived Bis(pyridyl) Ligands

Five racemic dissymmetric bis(pyridyl) ligands based on 2,8- or 3,9-difunctionalised Trögers base derivatives have been synthesised. Only those derived from a 2,8-difunctionalised scaffold were found to undergo selective self-assembly to discrete self-assembled dinuclear metallosupramolecular aggregates of rhomboid shape upon coordination to cis-protected Pd(2+) or Pt(2+) ions, as evidenced by ESI mass spectrometry, NMR spectroscopy and single-crystal X-ray diffraction. Interestingly, these processes were found to be highly diastereoselective leading to the formation of C(2v)-symmetric heterochiral assemblies in a self-discriminating manner.

Anodic Coupling of Guaiacol Derivatives on Boron-Doped Diamond Electrodes

The anodic treatment of guaiacol derivatives on boron-doped diamond electrodes (BDD) provides a direct access to nonsymmetrical biphenols, which would require a multistep sequence by conventional methods. Despite the destructive nature of BDD anodes they can be exploited for chemical synthesis.

4‐exo Cyclizations by Template Catalysis

The cyclobutane ring is an important structural motif in many natural products. A number of synthetic approaches to fourmembered rings have been described, but none of them is general. This is also the case for radical cyclizations, even though they are in principle amongst the most powerful reactions for the construction of carbocyclic rings. Difficulties encountered in the preparation of cyclobutanes are caused by their inherent strain and by the low rate constant of the cyclization of the archetypal pentenyl radical. Therefore, the few efficient examples of 4-exo cyclizations in classical free radical chemistry, and in metal-mediated and metal-catalyzed radical reactions using SmI2 [3] and titanocene(III) reagents have to rely on special structural features. In these cases, the presence of gem-dialkyl or gemdialkoxyl substitution adjacent to the radical center or the incorporation of the cyclization into transannular tandem sequences was necessary for obtaining useful yields of the desired products. A general access to cyclobutanes by the 4-exo cyclization is still elusive. Herein, we address this issue by using cationic functionalized titanocenes as template catalysts for such reactions. In these complexes the pendant amide ligand as well as the chloride ligand can be replaced by polar groups. For a suitably substituted radical this results in an energetically favorable two-point binding to the template in which the radical center and radical acceptor are forced into close spatial proximity. As a consequence, the 4-exo cyclization becomes a transannular transformation, which is thermodynamically and kinetically more favorable. With careful adjustment of the steric interactions this template effect can lead to highly ordered intermediates and transition states, and hence to a stereoselective cyclization (Scheme 1). We have realized the two-point binding with unsaturated epoxides as radical precursors. Reductive electron transfer to the epoxide generates a radical which is covalently attached to titanium through a Ti O bond. The pivotal second binding can be enforced by a donor group displacing the amide ligand. To this end, we chose 1 as the substrate, because a,bunsaturated carbonyl groups usually constitute better ligands than the corresponding saturated carbonyl groups. The reaction of a tertiary radical renders the process thermodynamically unfavorable. Our results of the first 4-exo cyclization without gem disubstitution are summarized in Scheme 2 and Table 1 (Coll = collidine = 2,4,6-trimethylpyridine).

Metal–Silicon Triple Bonds: Nucleophilic Addition and Redox Reactions of the Silylidyne Complex [Cp(CO)2MoSi‐R]

Recent developments in the chemistry of low-valent maingroup element compounds have revealed that N-heterocyclic carbenes (NHC) are particularly suitable to stabilize silicon centers in low oxidation states. Remarkable achievements include the syntheses of stable NHC adducts of Si2, [2] SiX2 (X = Cl, Br), silanones, and Si(R)Cl (R = m-terphenyl). The latter compounds were shown to be useful precursors for the preparation of complexes containing metal–silicon multiple bonds as exemplified by the synthesis of the first silylidyne complex, [Cp(CO)2Mo Si-R] (1). Compound 1 offers, as silicon analogue of metal alkylidyne complexes, new perspectives in organosilicon chemistry. The high synthetic potential of 1 is demonstrated here by a series of reactions providing access to new complexes containing metal–silicon double and single bonds. Complex 1 contains an electrophilic silicon center and reacts smoothly with various anionic nucleophiles to give unprecedented anionic silylidene complexes (Scheme 1). For example, treatment of 1 with (NMe4)Cl in 1,2dimethoxyethane (DME) afforded selectively the bright orange chlorosilylidene complex salt 2 in 72% yield, and reaction of 1 with (NEt4)N3 gave the orange azidosilylidene complex salt 3 in quantitative yield (Scheme 1). Compounds 2 and 3 were isolated as very air-sensitive solids, which decompose on heating at 122–126 8C and 143 8C, respectively. Chlorosilylidene complexes are very rare and azidosilylidene complexes as 3 have not been reported to date highlighting the synthetic potential of 1. Moreover, complex 1 undergoes selective addition reactions with carbon-centered nucleophiles. For example, addition of one equivalent of LiMe to a solution of 1 in diethylether at 60 8C was accompanied by a rapid color change from redbrown to orange and afforded cleanly the methylsilylidene complex salt 4, which was isolated as a yellow, air-sensitive, thermolabile diethylether solvate in 75 % yield. 11] The crystal structures of 2 and 3 were determined by single-crystal X-ray diffraction analyses and revealed the presence of well separated tetraalkylammonium cations and silylidene complex anions. The most striking bonding features of the

SiP Double Bonds: Experimental and Theoretical Study of an NHC‐Stabilized Phosphasilenylidene

An experimental and theoretical study of the first compound featuring a Si=P bond to a two-coordinate silicon atom is reported. The NHC-stabilized phosphasilenylidene (IDipp)Si=PMes* (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, Mes*=2,4,6-tBu3 C6 H2 ) was prepared by SiMe3 Cl elimination from SiCl2 (IDipp) and LiP(Mes*)SiMe3 and characterized by X-ray crystallography, NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroscopy. It has a planar trans-bent geometry with a short Si=P distance of 2.1188(7)Å and acute bonding angles at Si (96.90(6)°) and P (95.38(6)°). The bonding parameters indicate the presence of a Si=P bond with a lone electron pair of high s-character at Si and P, in agreement with natural bond orbital (NBO) analysis. Comparative cyclic voltammetric and UV/Vis spectroscopic experiments of this compound, the disilicon(0) compound (IDipp)Si=Si(IDipp), and the diphosphene Mes*P=PMes* reveal, in combination with quantum chemical calculations, the isolobal relationship of the three double-bond systems.

ChromiumSilicon Multiple Bonds: The Chemistry of Terminal N-Heterocyclic-Carbene-Stabilized Halosilylidyne Ligands

An efficient method for the synthesis of the first N-heterocyclic carbene (NHC)-stabilized halosilylidyne complexes is reported that starts from SiBr(4). In the first step, SiBr(4) was treated with one equivalent of the N-heterocyclic carbene 1,3-bis[2,6-bis(isopropyl)phenyl]imidazolidin-2-ylidene (SIdipp) to give the 4,5-dihydroimidazolium salt [SiBr(3)(SIdipp)]Br (1-Br), which then was reduced with potassium graphite to afford the silicon(II) dibromide-NHC adduct SiBr(2)(SIdipp)(2-Br) in good yields. Heating 2-Br with Li[CpCr(CO)(3)] afforded the complex [Cp(CO)(2)Cr=SiBr(SIdipp)] (3-Br) upon elimination of CO. Complex 3-Br features a trigonal-planar-coordinated silicon center and a very short CrSi double bond. Similarly, the reaction of SiCl(2)(SIdipp) (2-Cl) with Li[CpCr(CO)(3)] gave the analogous chloro derivative [Cp(CO)(2)Cr=SiCl(SIdipp)] (3-Cl). Complex 3-Br undergoes an NHC exchange with 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene (IMe(2)iPr(2)) to give the complex [Cp(CO)(2)CrSiBr(IMe(2)iPr(2))(2)] (4-Br). Compound 4-Br features a distorted-tetrahedral four-coordinate silicon center. Bromide abstraction occurs readily from 4-Br with Li[B(C(6)F(5))(4)] to give the putative silylidene complex salt [Cp(CO)(2)Cr=Si(IMe(2)iPr(2))(2)][B(C(6)F(5))(4)], which irreversibly dimerizes by means of an Si-promoted electrophilic activation of one carbonyl oxygen atom to yield the dinuclear siloxycarbyne complex [Cp(CO)Cr{(μ-CO)Si(IMe(2)iPr(2))(2)}(2-)Cr(CO)Cp][B(C(6)F(5))(4)](2) (5). All compounds were fully characterized, and the molecular structures of 2-Br-5-Br were determined by single-crystal X-ray diffraction. DFT calculations of 3-Br and 3-Cl and their carbene dissociation products [Cp(CO)(2)Cr=Si-X] (X=Cl, Br) were carried out, and the electronic structures of 3-Br, 3-Cl and [Cp(CO)(2)Cr=Si-X] were analyzed by the natural bond orbital method in combination with natural resonance theory.

Synthesis, Resolution, and Absolute Configuration of Difunctionalized Tröger's Base Derivatives

Two racemic derivatives of Trögers base, the 2,8-diboronic acid ester 6 and the 3,9-dibromo-substituted derivative 5, were synthesized and successfully resolved by HPLC on a chiral stationary Whelk-01 phase on a semipreparative scale, thereby giving rise to both enantiomers in a pure form. These functionalized C(2)-symmetric building blocks are valuable precursors for a variety of further applications. Their absolute configurations were determined by comparison of their quantum chemically calculated CD and UV/Vis spectra with the experimental ones and were independently confirmed by X-ray diffraction analysis.

SiSi Double Bonds: Synthesis of an NHC-Stabilized Disilavinylidene†

An efficient two-step synthesis of the first NHC-stabilized disilavinylidene (Z)-(SIdipp)Si=Si(Br)Tbb (2; SIdipp=C[N(C6H3-2,6-iPr2)CH2]2, Tbb=C6H2-2,6-[CH(SiMe3)2]2-4-tBu, NHC=N-heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr2(SIdipp) with the 1,2-dibromodisilene (E)-Tbb(Br)Si=Si(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC-stabilized bromo(silyl)silylene, namely SiBr(SiBr2Tbb)(SIdipp) (1). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr2(SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C8K to give the NHC-stabilized disilavinylidene 2. Both low-valent silicon compounds were comprehensively characterized by X-ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum-chemical methods.