Isomeric metamorphosis: Si3E (E = S, Se, and Te) bicyclo[1.1.0]butane and cyclobutene

Group 14 and 16 hybrid heavy bicyclo[1.1.0]butanes (tBu2MeSi)4Si3E (E = S, Se, and Te) 2a-c have been prepared by the [1 + 2] cycloaddition reaction of trisilirene 1 and the corresponding chalcogen. Bicyclo[1.1.0]butanes 2 have exceedingly short bridging Si-Si bonds (2.2616(19) A for 2b and 2.2771(13) A for 2c), a phenomenon explained by the important contribution of the trisilirene-chalcogen pi-complex character to the overall bonding of 2. Photolysis of 2a and 2b produced their valence isomers, the heavy cyclobutenes 3a and 3b, featuring flat four-membered Si3E rings and a planar geometry of the Si=Si double bond. The mechanism of such isomerization was studied using deuterium-labeled 2a-d6 to ascertain the preference of the pathway, involving the direct concerted symmetry-allowed transformation of bicyclo[1.1.0]butane 2 to cyclobutene 3.     

Donor‐Stabilized 1,3‐Disila‐2,4‐diazacyclobutadiene with a Nonbonded Si⋅⋅⋅Si Distance Compressed to a Si=Si Double Bond Length

A donor‐stabilized 1,3‐disila‐2,4‐diazacyclobutadiene presents an exceptionally short nonbonded Si???Si distance (2.23 Å), which is as short as that of Si=Si bonds (2.15–2.23 Å). Theoretical investigations indicate that there is no bond between the two silicon atoms, and that the unusual geometry can be related to a significant coulomb repulsion between the two ring nitrogen atoms. This chemical pressure phenomenon could provide an alternative and superior way of squeezing out van der Waals space in highly strained structures, as compared to the classical physical methods.     

Sterically crowded bicyclo[1.1.0]butane radical cations

The variability of carbon-carbon single bonds by steric and electronic effects is probed by DFT calculations of sterically crowded bicyclo[1.1.0]butanes and their radical cations. The interplay of sterics and electronics on the gradual weakening and breaking of bonds was studied by investigating bridgehead substitution in 1,3-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetramethyl-1,3-di-tert-butylbicyclo[1.1.0]butane and geminal substitution in 2,2'-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetra-tert-butylbicyclo[1.1.0]butane. Bridgehead substitution leads to a lengthening of the central bond, whereas bisubstitution on the geminal carbon leads to a shortening of this bond due to a Thorpe-Ingold effect. Although the character of the central bond can be modulated by substitution and electron transfer over a range of 0.35 A, the state forbidden ring planarization does not occur. Sterically crowded bicyclo[1.1.0]butane radical cations are therefore promising candidates for the investigation of extremely long carbon-carbon single bonds.     

A Mixed Heavier Si=Ge Analogue of a Vinyl Anion

The versatile reactivities of disilenides and digermenide, heavier analogues of vinyl anions, have significantly expanded the pool of silicon and germanium compounds with various unexpected structural motifs in the past two decades. We now report the synthesis and isolation of a cyclic heteronuclear vinyl anion analogue with a Si=Ge bond, potassium silagermenide as stable thf‐solvate and 18‐c‐6 solvate by the KC₈ reduction of germylene or digermene precursors. Its suitability as synthon for the synthesis of functional silagermenes is proven by the reactions with chlorosilane and chlorophospane to yield the corresponding silyl‐ and phosphanyl‐silagermenes. X‐ray crystallographic analysis, UV/Vis spectroscopy and DFT calculations revealed a significant degree of π‐conjugation between N=C and Si=Ge double bonds in the title compound.     

SiP 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‐tBu₃C₆H₂) was prepared by SiMe₃Cl 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.     

9‐Phenyl‐3,4,4a,9a‐tetrahydrotri­ptycene and 9‐phenyl‐1,2,3,4,4a,9a‐hexa­hydro­triptycene

The structure of 9‐phenyl‐3,4,4a,9a‐tetra­hydro­triptycene, C₂₆H₂₂, (I), exhibits regiochemistry consistent with a stepwise mechanism for its formation from photo­cyclo­addition of 1,3‐cyclo­hexa­diene and 9‐phenyl­anthracene. Bond distances involving the bridgehead C atoms are similar in (I) and the hydrogenated derivative, 9‐phenyl‐1,2,3,4,4a,9a‐hexa­hydro­triptycene, C₂₆H₂₄, (II), with bonds to the quaternary‐C atoms exhibiting significant elongation [1.581 (2) Å in (I) and 1.585 (2) Å in (II)]. The molecular geometry precludes significant σ–π overlap between the phenyl groups and the interannular bonds in both compounds, indicating that the origin of the bond lengthening is steric in nature.     

From a Si3‐Cyclopropene to a Si3S‐Bicyclo[1.1.0]butane to a Si3S‐Cyclopropene to a Si3S2‐Bicyclo[1.1.0]butane: Back‐and‐Forth,and In‐Between

Compact and highly reactive bicyclo[1.1.0]butanes constitute one of the most fascinating classes of organic compounds. Furthermore, interplay of bicyclo[1.1.0]butanes with their valence isomers, such as buta‐1,3‐dienes and cyclobutenes, is among the fundamental pericyclic transformations in organic chemistry. Herein we report the back‐and‐forth interconversion between the cyclotrisilenes and thiatrisilabicyclo[1.1.0]butanes, allowing for the synthesis of novel representatives of such classes of highly reactive organometallics. The peculiar structural and bonding features of the newly synthesized compounds, as well as the mechanism of their isomerization, were verified both experimentally and computationally.     

From a Si3‐Cyclopropene to a Si3S‐Bicyclo[1.1.0]butane to a Si3S‐Cyclopropene to a Si3S2‐Bicyclo[1.1.0]butane: Back‐and‐Forth,and In‐Between

Compact and highly reactive bicyclo[1.1.0]butanes constitute one of the most fascinating classes of organic compounds. Furthermore, interplay of bicyclo[1.1.0]butanes with their valence isomers, such as buta‐1,3‐dienes and cyclobutenes, is among the fundamental pericyclic transformations in organic chemistry. Herein we report the back‐and‐forth interconversion between the cyclotrisilenes and thiatrisilabicyclo[1.1.0]butanes, allowing for the synthesis of novel representatives of such classes of highly reactive organometallics. The peculiar structural and bonding features of the newly synthesized compounds, as well as the mechanism of their isomerization, were verified both experimentally and computationally.     

An Isolable Potassium Salt of a Borasilene–Chloride Adduct

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon–boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([ 2 ]⁻), which was obtained as an orange crystalline solid. Single‐crystal X‐ray diffraction analysis and reactivity studies on [ 2 ]⁻ confirmed the double‐bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl⁻ to the boron center. A thermal reaction of [ 2 ]⁻ afforded a bicyclic product by formal intramolecular C−H insertion across the Si=B bond of 1 , which was corroborated by a theoretical study.     

1,3‐Diazasilabicyclo[1.1.0]butane with a Long Bridging N−N Bond

A 1,3‐diazasilabicyclo[1.1.0]butane ( 1 ) is synthesized as thermally stable crystals by using the cycloaddition reaction of an isolable dialkylsilylene with aziadamantane. The bridge N?N bond length of 1 (1.70 Å) is the longest among those of known N?N singly‐bonded compounds, including side‐on bridged transition‐metal dinitrogen complexes. The compound 1 is intact in air but moisture sensitive. No reaction occurs with hydrogen, even under pressure at 0.5 MPa. Irradiation of 1 with light gives an isomer quantitatively by N?N and adamantyl C?C bond cleavage. The origin of the remarkable N?N bond elongation is ascribed to significant interaction between a Si?C σ* and Ν?Ν π and σ orbitals as determined by DFT calculations of model compounds.     

Building up Strain in One Step: Synthesis of an Edge‐Fused Double Silacyclobutene from an Extensively Trichlorosilylated Butadiene Dianion

The exhaustive trichlorosilylation of hexachloro‐1,3‐butadiene was achieved in one step by using a mixture of Si₂Cl₆ and [nBu₄N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt [nBu₄N]₂[ 1 ] was isolated in 36 % yield after recrystallization. The negative charges of [ 1 ]^2? are mainly delocalized across its two carbanionic (Cl₃Si)₂C termini (α‐effect of silicon) such that the central bond possesses largely C=C double‐bond character. Upon treatment with 4 equiv of HCl, [ 1 ]^2? is converted into neutral 1,2,3,4‐tetrakis(trichlorosilyl)but‐2‐ene, 3 . The Cl^? acceptor AlCl₃, induces a twofold ring‐closure reaction of [ 1 ]^2? to form a six‐membered bicycle 4 in which two silacyclobutene rings are fused along a shared C=C double bond (84 %). Compound 4 , which was structurally characterized by X‐ray crystallography, undergoes partial ring opening to a monocyclic silacyclobutene 2 in the presence of HCl, but is thermally stable up to at least 180 °C.     

Reductive Insertion of Elemental Chalcogens into Boron–Boron Multiple Bonds

The syntheses of sulfur‐ and selenium‐bridged cyclic compounds containing boron stabilized by N‐heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron–boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six‐electron reduction reactions, resulting in the formation of [2.2.1]‐bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron–boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC‐stabilized trisulfidodiborolane. The demonstration of these six‐ and four‐electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.     

Diradicaloid or Zwitterionic Character: The Non‐Tetrahedral Unsaturated Compound [Si4{N(SiMe3)Dipp}4] with a Butterfly‐type Si4 Substructure

The reduction of the tribromoamidosilane {N(SiMe₃)Dipp}SiBr₃ (Dipp=2,6‐i Pr₂C₆H₃) with potassium graphite or magnesium resulted in the formation of [Si₄{N(SiMe₃)Dipp}₄] ( 1 ), a bicyclo[1.1.0]tetrasilatetraamide. The Si₄ motif in 1 does not adopt a tetrahedral substructure and exhibits two three‐coordinate and two four‐coordinate silicon atoms. The electronic situation on the three‐coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as ²⁹Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si₄S₃{N(SiMe₃)Dipp}₄] ( 2 ).     

Photochemical Strain‐Release‐Driven Cyclobutylation of C(sp3)‐Centered Radicals

A new photoredox‐catalyzed decarboxylative radical addition approach to functionalized cyclobutanes is described. The reaction involves an unprecedented formal Giese‐type addition of C(sp³)‐centered radicals to highly strained bicyclo[1.1.0]butanes. The mild photoredox conditions, which make use of a readily available and bench stable phenyl sulfonyl bicyclo[1.1.0]butane, proved to be amenable to a diverse range of α‐amino and α‐oxy carboxylic acids, providing a concise route to 1,3‐disubstituted cyclobutanes. Furthermore, kinetic studies and DFT calculations unveiled mechanistic details on bicyclo[1.1.0]butane reactivity relative to the corresponding olefin system.     

Facile Access to an NHC‐Coordinated Silicon Ring Compound with a Si=N Group and a Two‐Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄{N(SiMe₃)Dipp}₄ 1 (Dipp=2,6‐diisopropylphenyl) with 5 equiv of the N‐heterocyclic carbene NHC^Me4 (1,3,4,5‐tetramethylimidazol‐2‐ylidene) affords a bifunctional carbene‐coordinated four‐membered‐ring compound with a Si=N group and a two‐coordinate silicon atom Si₄{N(SiMe₃)Dipp}₂(NHC^Me4)₂(NDipp) 2 . When 2 reacts with 0.25 equiv sulfur (S₈), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two‐coordinate silicon atom in 2 .     

1,3-Disila-2-gallata- and -indataallenic Anions [>SiMSi<]-.Li+ (M = Ga, In): compounds featuring double bonds between elements of groups 13 and 14

The first compounds containing Si=Ga or Si=In double bonds, 1,3-disila-2-gallata- (2-) and -indataallenic anions (3-), were obtained in the form of lithium salts as dark red crystals by the reaction of bis(di-tert-butylmethylsilyl)dilithiosilane 1 with GaCl3 or InCl3. X-ray crystallographic analysis of 2-.[Li(thf)4]+ and 3-.[Li(thf)4]+ showed that the >SiMSi< frameworks (M = Ga, In) are not linear and the two Si=M bond lengths are about 9% shorter than typical Si-M single bonds. In the 29Si NMR spectra, the signals of the terminal Si atoms in 2- and 3- are greatly shifted upfield (2-: -79.9; 3-: -77.6 ppm). The NPA charge distribution for the model compound, [(Me3Si)2SiGaSi(SiMe3)2]- (4-) showed a large part of the negative charge localizing on the two terminal silicon atoms (-0.736 and -0.729). In addition, 2-.[Li(thf)4]+ reacted with MeI to give the corresponding iodogallane 5 nearly quantitatively. This result confirms that the allenic [>SiMSi<]- fragments are highly polarized.     

The `cyclophene' [2.2.2](1,2,4)cyclophan‐9‐ene

In the title compound, C₁₈H₁₆, the [2.2]paracyclophane geometry is restrained to a considerable extent despite the introduction of the extra C=C bridge; typical paracyclophane features, such as the elongated C—C bridges, are still observed. However, the bridgehead atoms of the C=C bridge are forced into unusually close proximity [2.657 (3) Å], which in turn causes the rings to be rotated to an interplanar angle of 13.7 (2)°. The packing involves hexagonally close‐packed layers of molecules parallel to the xy plane, corresponding to the known `7,11' pattern of paracyclophanes, but without significant short intermolecular contacts.     

Bridgehead Substitution via Putative Norborn‐1‐en‐3‐ones: Application in the Synthesis of Complex Molecules

The base‐mediated formation of a bridgehead double bond in a bicyclo[2.2.1]heptane system (anit‐Bredt molecules) is described. The synthesis of exocyclic norbornyl enones by Wittig reaction of α‐diketones is reported. These enones and their Michael adducts are used as substrates for the generation of transient bridgehead enones and their trapping with MeOH and H₂O. Bridgehead alcohols are easily synthesized from norbornyl enones and are exploited for the diversity oriented synthesis of frameworks of natural and unnatural products.     

Dry synthesis of triple cumulative double bonds (C=C=C=N) on Si(111)-7 x 7 surfaces

The interactions of cyanoacetylene and diacetylene with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of unsaturated organic molecules to diradical-like silicon dangling bonds. Vibrational studies show that cyanoacetylene mainly binds to the surface through a diradical reaction involving both cyano and C[triple bond]C groups with an adjacent adatom-rest atom pair at 110 K, resulting in an intermediate containing triple cumulative double bonds (C=C=C=N). On the other hand, diacetylene was shown to the covalently attached to Si(111)-7 x 7 only through one of its C[triple bond]C groups, forming an enynic-like structure with a C=C-C[triple bond]C skeleton. These chemisorbed species containing triple cumulative double bonds (C=C=C=N) and C=C-C[triple bond]C may be employed as precursors (or templates) for further construction of bilayer organic films on the semiconductor surfaces.     

Dimeric Calixarenes: A New Family of Major‐Groove Binders

A new class of potent DNA binding agents is presented. Dimeric calix[4]arenes with cationic groups at their upper rims and flexible alkyl bridges can be synthesized from triply acyl‐protected calix[4]arene tetramines in relatively short synthetic sequences (3–5 steps). The compounds attach themselves to double‐stranded nucleic acids in a noncovalent fashion, with micro‐ to nanomolar affinities. Guanidinium headgroups with their extended hydrogen‐bonding “fingers” are more powerful than ammonium groups, and the benzylamine series is superior to the anilinium series (see below). The new ligands easily distinguish between RNA and various DNA types, and produce characteristic changes in UV/Vis, fluorescence, CD, as well as NMR spectra. Especially extended oligonucleotides of more than 100 base pairs are bound with affinities increasing from RNA (10 μM K_d)<AT‐rich (1 μM )<GC‐rich DNA double strands (100–10 nM ). Ethidium bromide displacement studies confirm this order. CE₅₀ values are remarkably low (1–4 μM ), and are more than 300 times lower than that of spermine, which is a typical backbone binder. Stoichiometries are rather high (one calixarene dimer per two BP), suggesting a potential aggregation of bound ligands inside the major groove. Most UV/Vis melting curves display an inverted shape, and start from drastically enhanced absorption intensities for the DNA complexes. DAPI displacement studies prove that up to one equivalent of calixarene dimer can be accommodated in the dye‐loaded DNA. RNA complexation by calixarene dimers is accompanied by a drastic CD spectral transition from the typical A‐form to a perfect B‐signature, providing further experimental evidence for major‐groove binding. The orientation of the ligands can be deduced from NMR titrations and is reproduced in Monte‐Carlo simulations on 1:1 complexes in water.