1, 4‐Additions of HCl and HBr to a Tetrasilabutadiene: Formation of Unsymmetrically Substituted Disilenes [1]

The reactions of hexakis(2,4,6‐triisopropylphenyl)tetrasilabuta‐1,3‐diene R₂Si=SiR—SiR=SiR₂ ( 1 ) with HCl and HBr, slowly generated from HSiCl₃ or LiBr and CF₃COOH, respectively, furnish the unsymmetrically substituted disilenes R₂XSi—SiR=SiR—SiHR₂, X = Cl ( 2 ), Br ( 3 ), by formal 1,4‐addition of the hydrogen halides to 1 . However, passing gaseous hydrogen halides over the solution of 1 yields the 1,4‐dihalotetrasilanes by two‐fold 1,2‐additions to the double bonds of 1 . The structures of 2 and 3 which crystallize isotypically with one another have been determined by X‐ray crystallography.     

Theoretical prediction of vertical transition energies of diaminosilylenes and aminosubstituted disilenes

Vertical electronic transition energies of diaminosilylenes and their dimers (disilenes and nitrogen‐bridged) were investigated by ab initio and density functional calculations. A good linear correlation was found between the observed UV transition energies of various silylenes and disilenes and those of model compounds calculated using the CIS and TD–DFT methods. On the basis of these computations the experimental UV absorption maximum observed for the dimer of (i‐Pr₂N)₂Si: (λ_max 439 nm at 77 K), could be assigned to an Si? Si bonded dimer with an unusually long Si? Si distance of 2.472 Å, and the isomeric amino‐bridged cyclic dimer could be discarded. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1536–1541, 2001     

The Reaction of Transient 1,1‐Bis(trimethylsilyl)silenes with Tris(trimethylsilyl)silyllithium – Synthesis and Structure of Sterically Congested 1‐Trimethylsilylalkylpolysilanes

Tris(trimethylsilyl)silyllithium ( 3 ) reacted with aldehydes and ketones (molar ratio 2 : 1) according to a modified Peterson mechanism under formation of transient silenes, which were immediately trapped by excess 3 to give the organolithium derivatives (Me₃Si)₃SiSi(SiMe₃)₂C(Li)R¹R² ( 7 ). Hydrolysis of 7 afforded the alkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂CHR¹R² ( 8 ). Depending on the substituents R¹ and R², 7 proved to be rather unstable in THF solution and underwent a rapid rearrangement, involving a 1,3‐Si,C‐trimethylsilyl migration, resulting in the formation of the lithium silanides (Me₃Si)₂Si(Li)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 9 ), which were hydrolized during the aqueous workup to give the H‐silanes (Me₃Si)₂Si(H)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 10 ). Reaction of 9 with chlorotrimethylsilane produced the 1‐trimethylsilylalkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂C(SiMe₃)R¹R² ( 11 ). The structures of the products described were elucidated by comprehensive spectral analyses. The results of X‐ray crystal structure analyses, performed for 8 l (R¹ = H, R² = 2,4,6‐(MeO)₃C₆H₂), 10 d (R¹ = H, R² = Mes) and 11 d (R¹ = H, R² = Mes) are discussed and confirm the expected extreme sterical congestion of the molecules.     

A Silyliumylidene Cation Stabilized by an Amidinate Ligand and 4‐Dimethylaminopyridine

The synthesis and reactivity of a silyliumylidene cation stabilized by an amidinate ligand and 4‐dimethylaminopyridine (DMAP) are described. The reaction of the amidinate silicon(I) dimer [ L Si:]₂ ( 1 ; L =PhC(NtBu)₂) with one equivalent of N‐trimethylsilyl‐4‐dimethylaminopyridinium triflate [4‐NMe₂C₅H₄NSiMe₃]OTf and two equivalents of DMAP in THF afforded [ L Si(DMAP)]OTf ( 2 ). The ambiphilic character of 2 is demonstrated from its reactivity. Treatment of 2 with 1 in THF afforded the disilylenylsilylium triflate [ L′ ₂( L )Si]OTf ( 3 ; L′ = L Si:) with the displacement of DMAP. The reaction of 2 with [K{HB(iBu)₃}] and elemental sulfur in THF afforded the silylsilylene [ L SiSi(H){(NtBu)₂C(H)Ph}] ( 4 ) and the base‐stabilized silanethionium triflate [ L Si(S)DMAP]OTf ( 5 ), respectively. Compounds 2 , 3 , and 5 have been characterized by X‐ray crystallography.     

From Disilene (SiSi) to Phosphasilene (SiP) and Phosphacumulene (PCN)

The generation of heavier double‐bond systems without by‐ or side‐product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N‐alkyl substituent R, the reaction of disilenide Ar₂Si?Si(Ar)Li (Ar=2,4,6‐iPr₃C₆H₂) with ClP(NR₂)₂ either affords the phosphinodisilene Ar₂Si?Si(Ar)P(NR₂)₂ (for R=iPr) or P‐amino functionalized phosphasilenes Ar₂(R₂N)Si? Si(Ar)?P(NR₂) (for R=Et, Me) by 1,3‐migration of one of the amino groups. In case of R=Me, upon addition of one equivalent of tert‐butylisonitrile a second amino group shift occurs to yield the 1‐aza‐3‐phosphaallene Ar₂(R₂N)Si? Si(NR₂)(Ar)? P?C?NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single‐crystal X‐ray diffraction and DFT calculations in selected cases.     

Synthese und Struktur eines Pentaalkylchlorohexastibans Sb6R5Cl [R = (Me3Si)2CH]

Synthesis and Structure of Pentaalkylchlorohexastibane Sb₆R₅Cl [R = (Me₃Si)₂CH] The reaction of RSbCl₂ [R = (Me₃Si)₂CH] with Na‐K alloy in tetrahydrofuran gives besides the known rings Sb_nR_n (n = 3, 4), (Me₃Si)₂CH₂ and the pentaalkylchlorohexastibane Sb₆R₅Cl ( 1 ). 1 was characterized by spectroscopic methods (MS, ¹H‐, ¹³C‐NMR, X‐ray diffraction). The structure of 1 consists of a folded four membered antimony ring in the all‐trans configuration with three alkyl groups and one Sb(R)—Sb(R)Cl fragment as substituents.     

P‐Aminophosphaalkenes with C‐Isopropyldimethylsilyl Groups

Amination of the C‐isopropyldimethylsilyl P‐chlorophosphaalkene (iPrMe₂Si)₂C=PCl ( 1 ) leads to the P‐aminophosphaalkenes (iPrMe₂Si)₂C=PN(R)R′ (R, R′ = Me ( 2 ), R = H, R′ = nPr ( 3 ), R = H, R′ = iPr ( 4 ), R = H, R′ = tBu ( 5 ), R = H, R′ = 1‐Ada ( 6 ), R = H, R′ = CPh₃ ( 7 ), R = H, R′ = Ph ( 8 ), R = H, RR′ = 2,6‐iPr₂Ph (= DIP) ( 10 ), R = H, R′ = 2,4,6‐Me₃Ph (= Mes) ( 11 ), R = H, R′ = 2,4,6‐tBu₃Ph (= Mes*)] ( 12 ), R = H, R′ = SiMe₃ ( 13 ), and R, R′ = SiMe₂Ph (1 4 ). ³¹P‐NMR spectra confirm that phosphaalkenes 2 – 7 and 10 – 14 are monomeric in solution; the structures of 7 , 10 , and 12 were determined by X‐ray crystallography. Freshly prepared (iPrMe₂Si)₂C=PN(H)Ph ( 8 ) is a monomer that dimerizes with (N→C) proton migration within several hours to the stable diazadiphosphetidine [(iPrMe₂Si)₂CHPNPh]₂ ( 9 ). NMR‐scale reactions of deprotonated 5 and 13 with tBuiPrPCl provide by P–P bond formation the P‐phosphanyl iminophosphoranes [(iPrMe₂Si)₂C=](RN=)PPtBu(iPr) [R = tBu ( 15 ), R = Me₃Si ( 17 )]. Deprotonated 5 and Me₃GeCl deliver by N–Ge bond formation the aminophosphaalkene (iPrMe₂Si)₂C=PN(tBu)GeMe₃ ( 20 ), which with elemental selenium 5 undergoes (N→C) proton migration to form the alkyl(imino)(seleno)phosphorane [(iPrMe₂Si)₂CH](tBuN=)P=Se ( 21 ), which is a selenium‐bridged cyclic dimer in the solid state.     

29Si NMR Shielding Tensors in Triphenylsilanes – 29Si Solid State NMR Experiments and DFT‐IGLO Calculations

²⁹Si NMR shielding tensors of a series of triphenylsilanes Ph₃SiR with R = Ph, Me, F, Cl, Br, OH, OMe, SH, NH₂, SiPh₃, C≡CPh were determined from ²⁹Si CP/MAS spectra recorded at low spinning rates. In addition the principal components of the shielding tensor were calculated employing the DFT‐IGLO method. For most silanes experimental and calculated values are in good accordance. Larger differences were observed for systems with hydrogen bridge forming substituents and the halides bromide and chloride. In some of the spectra the shielding information interfered with residual dipolar couplings. The different contributions of the various substituents to the principal components of the shielding tensor and the orientation of the tensor within the molecules are discussed and compared for the compounds under investigation.     

A General Diversified Synthesis of Carbazoles and the First Synthesis of Karapinchamine A

[IPrAuCl]/AgSbF₆‐catalyzed cyclization of the readily available 4‐benzoxyl‐1‐(indol‐2‐yl)‐2‐alkynols occurred smoothly in 1,2‐dichloroethane (DCE) in the presence of 4 Å MS to form a series of differently polysubstituted 2‐oxygenated carbazole derivatives efficiently. Based on mechanistic study, a possible mechanism involving 1,3‐migration of a benzoate group to form the allene, Au⁺‐mediated cyclization–elimination to form a gold–carbene intermediate, and subsequent highly selective 1,2‐migration has been proposed for the formation of carbazoles. Highly selective 1,2‐migration referring to the two substituents R³ and R⁴ (R⁴=H, alkyl, and aryl group) was observed: (1) In the presence of both H and alkyl groups, 1,2‐hydrogen migration is exclusive; (2) in the presence of a methyl group (R³), propyl, isopropyl, 4‐methylphenyl, and 4‐chlorophenyl groups (R⁴) migrate exclusively. Finally, the first total synthesis of the recently isolated naturally occurring carbazole alkaloid karapinchamine A in 5.2 g scale has been realized in 6 steps from commercially available chemicals without need for any protection–deprotection.     

Heavy Carbene Analogues: Donor‐Free Bismuthenium and Stibenium Ions

Kinetically stabilized congeners of carbenes, R₂C, possessing six valence electrons (four bonding electrons and two non‐bonding electrons) have been restricted to Group 14 elements, R₂E (E=Si, Ge, Sn, Pb; R=alkyl or aryl) whereas isoelectronic Group 15 cations, divalent species of type [R₂E]⁺ (E=P, As, Sb, Bi; R=alkyl or aryl), were unknown. Herein, we report the first two examples, namely the bismuthenium ion [(2,6‐Mes₂C₆H₃)₂Bi][BAr^F₄] ( 1 ; Mes=2,4,6‐Me₃C₆H₂, Ar^F=3,5‐(CF₃)₂C₆H₃) and the stibenium ion [(2,6‐Mes₂C₆H₃)₂Sb][B(C₆F₅)₄] ( 2 ), which were obtained by using a combination of bulky meta‐terphenyl substituents and weakly coordinating anions.     

Isolable silylene, disilenes, trisilaallene, and related compounds

Our recent studies of synthesis, structure, and reactions of an isolable silylene, stable novel cyclic and conjugated disilenes, a trisilaallene are summarized. Due to the distinctive electronic and steric effects of trialkylsilyl substituents, tetrakis(trialkylsilyl)disilenes showed interesting structural features around SiSi bonds, electronic spectra, and reactions. The tetrasilyldisilenes were useful reagents for the synthesis of novel types of organosilicon compounds such as η²-disilene transition metal complexes and a 1,3-disilabicyclo[1.1.0]butane. Photochemical and thermal interconversion among Si₄R₆ isomers including a cyclotetrasilene, a silylcyclotrisilene, and a bicyclo[1.1.0]tetrasilane occured without apparent participation of the corresponding tetrasila-1,3-diene. The first spiropentasiladiene was thermally very stable and showed remarkable spiroconjugation between the two ring π systems. An isolable dialkylsilylene was found to be well-protected sterically from dimerization but least perturbed electronically. Using the silylene, a trisilaallene, the first stable compound with formal sp-hybridized silicon atom, was synthesized. In contrast to carbon allenes, the skeleton of the trisilaallene was significantly bent and remarkably fluxional.     

Diverse Reactivity of an Electrophilic Phosphasilene towards Anionic Nucleophiles: Substitution or Metal–Amino Exchange

The reaction of MesLi (Mes=2,4,6‐trimethylphenyl) with the electrophilic phosphasilene R₂(NMe₂)Si‐RSi=PNMe₂ ( 2 , R=Tip=2,4,6‐triisopropylphenyl) cleanly affords R₂(NMe₂)Si‐RSi=PMes and thus provides the first example of a substitution reaction at an unperturbed Si=P bond. In toluene, the reaction of 2 with lithium disilenide, R₂Si=Si(R)Li ( 1 ), apparently proceeds via an initial nucleophilic substitution step as well (as suggested by DFT calculations), but affords a saturated bicyclo[1.1.0]butane analogue as the final product, which was further characterized as its Fe(CO)₄ complex. In contrast, in 1,2‐dimethoxyethane the reaction of 1 with 2 results in an unprecedented metal–amino exchange reaction.     

Dialkylboryl-Substituted Cyclic Disilenes Synthesized by Desilylation-Borylation of Trimethylsilyl-Substituted Disilenes

π-Electron systems of silicon have attracted attention because of their narrow HOMO-LUMO gap and high reactivity, but the structural diversity remains limited. Herein, new dialkylboryl-substituted disilenes were synthesized by the selective desilylation-borylation of the corresponding trimethylsilyl-substituted disilenes. The dialkylboryl-substituted disilenes were fully characterized by a combination of NMR spectroscopy, MS spectrometry, single-crystal X-ray diffraction analysis, and theoretical calculations. The longest-wavelength absorption bands of boryldisilenes were bathochromically shifted compared to the corresponding silyl-substituted disilenes, indicating a substantial conjugation between π(Si=Si) and vacant 2p(B) orbitals. In the presence of 4-(dimethylamino)pyridine (DMAP), the dialkylboryl groups in the boryl-substituted disilenes were easily converted to trimethylsilyl groups, suggesting the dialkylboryl-substituted disilenes in the presence of a base serve as the surrogates of disilenyl anions (disilenides).     

Oxygen and Water Additions to a Tetrasilabuta‐1,3‐diene

The reaction of hexakis(2,4,6‐triisopropylphenyl)tetrasilabuta‐1,3‐diene R₂Si=SiR–SiR=SiR₂ ( 1 ) with atmospheric oxygen furnishes the oxidation product R₂Si(O)₂SiROSiR(O)₂SiR₂ ( 5 ) by oxygen insertion into all Si–Si bonds. However, treatment of 1 with meta‐chloroperoxobenzoic acid provides R₂Si(O)₂SiR–SiR(O)₂SiR₂ ( 7 ) with retention of the Si–Si single bond. Reaction of 1 with traces of water gives the oxatetrasilacyclopentane derivative 10 analogous to THF. With excess water a tetrasilane‐1,4‐diol is formed. The structures of 5 , 7 , and 10 were determined by X‐ray crystallography.     

Hydroalumination and Hydrogallation Reactions with Tri(ethynyl)silanes – Generation of Compounds with up to Three Coordinatively Unsaturated Aluminium Atoms

Hydroalumination or hydrogallation of tri(ethynyl)silanes, RSi(C≡C‐Ar)₃ ( 1a , R = Ph, Ar = Ph; 1b , R = Me, Ar = Ph; 1c , R = Me, Ar = C₆H₄Me), with the element hydrides H‐EtBu₂ (E = Al, Ga) in stoichiometric ratios of 1:1 to 1:3 at ambient temperature yielded the addition products (PhC≡C)₂(R)Si[(tBu₂E)C=C(H)Ph] ( 2 , R = Ph, E = Ga; 3a , R = Me, E = Al; 3b , R = Me, E = Ga), (PhC≡C)(Me)Si[(tBu₂E)C=C(H)Ph]₂ ( 4a , E = Al, 4b , E = Ga) and (Me)Si[(tBu₂Al)C=C(H)Ar]₃ ( 5 , Ar = Ph; 6 , Ar = C₆H₄Me). Compounds 2 – 4 show a relatively close interaction between the coordinatively unsaturated aluminium or gallium atoms and one of the C_α(≡C) atoms of unreacted alkyne substituents [245 (E = Al) and 266 pm (E = Ga)] that stabilises the kinetically favoured cis addition products with E and hydrogen on the same side of the resulting C=C double bonds. In the absence of these stabilising effects the compounds were found to isomerise to the thermodynamically favoured trans isomers.     

Synthesis,Crystal Structure,and Spectroscopic Characterization of the Borazine Derivatives [B{CH2(SiCl3)}NH]3 and [B{CH2(SiCl2CH3)}NH]3

The borazine derivatives B, B′, B″‐tris[(trichlorosilyl)methyl]borazine [B{CH₂(SiCl₃)}NH]₃ ( 1 ), and B, B′, B″‐tris[{dichloro(methyl)silyl}methyl]borazine [B{CH₂(SiCl₂CH₃)}NH]₃ ( 2 ) were prepared by reacting (Cl₃Si)CH₂(BCl₂) ( 3 ) and [Cl₂(CH₃)Si]CH₂(BCl₂) ( 4 ) with hexamethyldisilazane (hmds), respectively. Both compounds, 1 and 2 crystallize in space group R3c with a = 1712.53(4), c = 1230.33(4) pm, Z = 6, R₁ = 0.030, and a = 1713.8(2), c = 1258.7(2) pm, Z = 6, R₁ = 0.034, respectively. According to the single crystal X‐ray diffraction analyses, the title compounds show a planar B₃N₃ six‐membered ring with B—N distances of 142.3(3) pm (point symmetry C₃) and synfacial oriented substituents. The borazine derivatives have also been characterized by NMR and IR spectroscopy as well as by MS spectrometry.     

Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reaction)

Part I: Kinetic data for the static system silane pyrolysis (from 640–703 K, 60–400 torr) are presented. For conversion from 3–30%, first-order kinetics are obtained, with silane loss rates equal to half the hydrogen formation rates. At conversions greater than 40%, rate inhibition attributable to the back reaction of hydrogen with silylene occurs. Overall reaction rates are not surface sensitive, but disilane and trisilane yield maxima under some conditions are. A nonchain mechanism capable of describing quantitatively all stages of the silane pyrolysis is proposed. Post 1.0% initiation is both homogeneous (gas phase) and heterogeneous (on the walls), and reaction intermediates are silylenes and disilenes. Free radicals are not involved at any stage of the reaction. Rate data at high conversions and with added hydrogen provide kinetics for the addition of silylene to hydrogen [reaction (?1)1] relative to its addition to silane [reaction (2)]: k_?1,/k₂ = 10^?0.65 × e^{?3200 cal/RT}. With E₂ = 1300 cal, this gives a high pressure activation energy for silylene insertion into hydrogen of E_?1 = 8200 cal. Part II: An analysis is made of each rate constant of the silane mechanism and the modeling results are compared with experimental results. Agreement is excellent. It is concluded that the dominant sink reaction for silylene intermediates is 1,2—H₂ elimination from disilane (followed by Si₂H₄ polymerization and wall deposition). The model is in accord with slow isomerization between disilene and silylsilylene and near exclusive 1,2—H₂ elimination from Si₂H₆. It is also concluded that disilene is about 10 kcal/mol more stable than silylsilylene and that the activation energy for isomerization of silylsilylene to disilene is greater than 26 kcal/mol.     

A Fairly Stable Crystalline Silanone

Silanones 2 substituted by bulky amino‐ and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron‐donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t _1/2=0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me₂Si=O owing to the strong π‐electron donation from the phosphonium ylide substituent.     

Das reduzierte Nitridosilicat BaSi6N8

The Reduced Nitridosilicate BaSi₆N₈ The reduced nitridosilicate BaSi₆N₈ has been synthesized starting from barium nitride and silicon diimide in a radio‐frequency furnace at temperatures of about 1650 °C. The structure has been determined from X‐ray powder diffraction data and was refined by the Rietveld method (Imm2 (no. 44), a = 793.16(1), b = 934.37(2), c = 483.57(1) pm, V = 358.38(1) ·10⁶ pm³, Z = 2, wR_p = 0.0353, R_p = 0.0238, R_F² = 0.0660, 452 observed reflections, 42 parameters). BaSi₆N₈ crystallizes isotypically with SrSi₆N₈. The three‐dimensional Si‐N‐network consists of corner‐sharing SiN₄ tetrahedra and single bonds Si‐Si forming N₃Si‐SiN₃ building units. ²⁹Si solid‐state NMR spectra of BaSi₆N₈ resemble those of SrSi₆N₈ exhibiting two resonances at δ = ?54.3 and ?28.0 ppm. Their observed intensity ratio of approximately 2 : 1 can be attributed to the S iN₄ tetrahedra and the S i₂N₆ units, respectively. This observation is in accordance with the results from the X‐ray structure determination (Si at Wyckoff positions 8e ( S iN₄) and 4d ( Si ₂N₆)).     

Ab initio MO and DFT study for the isomerisation of bicyclo[1.1.0]tetrasilane and the germanium analogues

The mechanism of the unimolecular isomerisation reaction of the silicon and germanium analogues of bicyclo[1.1.0]butane with various kinds of substituents (X₄R₆; X?=?Si and Ge, R=H, CH₃, t-Bu and SiH₃) to the corresponding cyclobutene analogues has been investigated by ab initio molecular orbital and DFT methods. Several reaction mechanisms were considered. They are roughly divided into two types; (1) skeletal rearrangement and (2) substituent migration. It was found that substituents (R) have the leading effect on the reaction mechanism but the partial or full replacement of the skeletal silicon atoms by germanium atoms has some important effects as well. Furthermore, the character of the bridge bond of the long-bond and short-bond isomers of these bicyclic compounds was investigated and discussed in comparison with the ?? bond in ethene and disilene by the CiLC analysis.