A hypervalent pentacoordinate boron compound with an N-B-N three-center four-electron bond

For the purpose of synthesizing and characterizing hypervalent boron compounds with strong hypervalent interaction, we have prepared a boron compound with a tridentate ligand bearing two pyrimidine rings as nitrogen donors. X-ray analysis and molecular orbital calculations suggested that the boron compound was of hypervalent pentacoordinate structure with an N-B-N hypervalent bond. Thus, we have prepared the first hypervalent second row element compound with apical N coordination. A breakdown of energy contributions by DFT calculations revealed that the N-B-N bond energy of the pentacoordinate state ground state (13) was 2.8 kcal mol(-1). Implications were that the conjugation energy difference of 6.6 kcal mol(-1) (14.2-7.6 kcal mol(-1)) with the tetracoordinate state was a crucial factor for shifting stability toward the pentacoordinate structure.     

氢膦烷的合成及其反应

氢膦烷能参与多种化学反应,可以衍生出许多类型的五配位磷化合物,而且在有机合成中也有重要的应用价值.本文结合作者的研究工作,对近年来氢膦烷研究的重要进展进行了评述.     

Synthesis of stable silicon heterocycles by reaction of organic substrates with a chlorosilylene [PhC(NtBu)2SiCl

Heteroleptic chlorosilylene (PhC(NtBu)(2)SiCl) (1) reacts with unsaturated organic compounds under oxidative addition. Reactions of 1 with cyclooctatetraene (COT) and a diimine afford [1+4]-cycloaddition products 3 and 6, respectively. In the case of COT, one Si-N bond of the amidinato ligand is cleaved, resulting in tetracoordinate silicon, whereas with a diimine a pentacoordinate silicon is formed. Treatment of 1 with ArN=C=NAr (Ar=2,6-iPr(2)C(6)H(3)) yields silaimine complex 4 with cleavage of one of the C=N bonds. The facile isolation of silaimine complexes is probably due to the kinetic protection afforded by the bulky Ar moiety. When 1 is treated with tert-butyl isocyanate, cleavage of the C=O bond is observed, which leads to formation of the four-membered Si(2)O(2) cycle 5. The same product is formed when 1 is allowed to react with trimethylamine N-oxide. When 1 is treated with diphenyl disulfide, cleavage of the S-S bond occurs to form 7. All products have been characterized by multinuclear NMR spectroscopy, EI mass spectrometry, and elemental analysis. In addition, the molecular structures of 3-6 have been determined by single-crystal X-ray diffraction studies. Collectively, these results suggest that owing to the presence of the lone pair of electrons, the propensity of 1 to undergo oxidative addition is very high.     

Synthesis and structural characterisation of neutral pentacoordinate silicon(IV) complexes with a tridentate dianionic N,N,S chelate ligand

A series of novel neutral pentacoordinate silicon(IV) complexes with SiClSN(2)C, SiBrSN(2)C, SiSN(3)C, SiSON(2)C, SiS(2)N(2)C, SiSeSN(2)C and SiTeSN(2)C skeletons (compounds 1-12) was synthesised, starting from PhSiCl(3), PhSiBr(3), PhSi(NCO)(3), MeSiCl(3) or C(6)F(5)SiCl(3). Compounds 1-12 contain (i) a tridentate dianionic N,N,S chelate ligand (derived from 2-{[(pyridin-2-yl)methyl]amino}benzenethiol), (ii) a phenyl, methyl or pentafluorophenyl group and (iii) a monodentate monoanionic ligand (Cl, Br, NCO, NCS, N(3), OS(O)(2)CF(3), OPh, SPh, SePh, TePh). The pentacoordinate silicon(iv) complexes 1-12 were characterised by elemental analyses, NMR spectroscopic studies in the solid state and in solution and crystal structure analyses. These experimental investigations were complemented by computational studies.     

Pentacoordinate spirosilicate anion, bis(2,2′-biphenyldiyl)methylsilicate, synthesized by the lithium cleavage of dimethoxyethane

The reaction of spirobisilafluorene (1) with lithium in dimethoxyethane produces lithium 1-methyl-spirobisilafluorenide (2), a stable pentacoordinate silicon compound with five carbon ligands, and lithium 2-methoxyethoxide, which was identified by trapping with Ph₃SiCl to give Ph₃Si-OCH2CH2OCH3 (4). The X-ray crystal structure of 2 shows that the geometry at silicon is an idealized trigonal bipyramid, slightly distorted toward a square pyramid. Methanolysis of 2 cleaves a Si-aryl bond producing a methyl biphenylsilafluorene, 3. Crystal structures are reported for 3 and 4.     

Complexation of tris(pentafluorophenyl)silanes with neutral Lewis bases

A detailed analysis of monodentate and bidentate complexation of tris(pentafluorophenyl)silyl (TPFS) derivatives with neutral Lewis bases was performed. The NMR spectroscopy and X-ray diffraction analysis (11 structures) were the key methods to characterize tetra- or pentacoordinate silicon compounds, whereas the peculiarities of crystal packing were analyzed by means of DFT calculations. The interaction of TPFS-X (X = F, Cl, OTf) with strong Lewis bases (HMPA, N-methylpyrrolidinone) may afford three different species: neutral pentacoordinate TPFS(X)-L, cationic tetracoordinate TPFS-L⁺ X⁻, and cationic pentacoordinate TPFS-(L)⁺₂X⁻, representatives of each type were characterized by X-ray diffraction. A variety of complexes with bidentate complexation, featuring the trigonal bipyramidal geometry with apical C₆F₅-group was prepared and structurally characterized. The extent of Si-C_apical bond elongation depends on the donating ability of the coordinating ligand, with the longest Si-C bond of 1.981(1) Å observed for six-membered complex of TPFS-ether of N-(2-hydroxybenzoyl)pyrrolidine.     

Synthesis and properties of stereoregular cyclic polysilanols: cis-[PhSi(O)OH](4), cis-[PhSi(O)OH](6), and tris-cis-tris-trans-[PhSi(O)OH](12)

New stereoregular cyclic polysilanols of the general formula [PhSi(O)OH]n (n = 6 and 12) have been selectively obtained in high yields by the reaction of cagelike oligophenylmetallasiloxanes with dilute solutions of hydrochloric acid at low temperatures. An alternative method was used to prepare cis-[PhSi(O)OH](4) from sodium phenylsiloxanolate, cis-[(Na(+))(4)[PhSi(O)O(-)](4)].(1-butanol)(x). All compounds were fully characterized by NMR and IR spectroscopy and molecular weight determinations. The structure of cis-[PhSi(O)OH](6) was confirmed by single-crystal X-ray analysis. Furthermore, a series of stereoregular cyclosiloxanes containing triorganylsiloxy groups at each silicon atom was prepared by the reactions of the cyclic polysilanols with triorganylchlorosilanes Me(3)SiCl, Me(2)ViSiCl, and Me(2)(CH(2)Cl)SiCl.     

Syntheses and structures of hypervalent pentacoordinate carbon and boron compounds bearing an anthracene skeleton--elucidation of hypervalent interaction based on X-ray analysis and DFT calculation

Pentacoordinate and tetracoordinate carbon and boron compounds (27, 38, 50-52, 56-61) bearing an anthracene skeleton with two oxygen or nitrogen atoms at the 1,8-positions were synthesized by the use of four newly synthesized tridentate ligand precursors. Several carbon and boron compounds were characterized by X-ray crystallographic analysis, showing that compounds 27, 56-59 bearing an oxygen-donating anthracene skeleton had a trigonal bipyramidal (TBP) pentacoordinate structure with relatively long apical distances (ca. 2.38-2.46 A). Despite the relatively long apical distances, DFT calculation of carbon species 27 and boron species 56 and experimental accurate X-ray electron density distribution analysis of 56 supported the existence of the apical hypervalent bond even though the nature of the hypervalent interaction between the central carbon (or boron) and the donating oxygen atom was relatively weak and ionic. On the other hand, X-ray analysis of compounds 50-52 bearing a nitrogen-donating anthracene skeleton showed unsymmetrical tetracoordinate carbon or boron atom with coordination by only one of the two nitrogen-donating groups. It is interesting to note that, with an oxygen-donating skeleton, the compound 61 having two chlorine atoms on the central boron atom showed a tetracoordinate structure, although the corresponding compound 60 with two fluorine atoms showed a pentacoordinate structure. The B-O distances (av 2.29 A) in 60 were relatively short in comparison with those (av 2.44 A) in 59 having two methoxy groups on the central boron atom, indicating that the B-O interaction became stronger due to the electron-withdrawing nature of the fluorine atoms.     

Pentacoordinate 1H-phosphirenes: reactivity, bonding properties, and substituent effects on their structures and thermal stability

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a very high degree of p character, which results from their being both basal bonds in the SP structure and endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations, it can be considered that a sigma-pi interaction between the sigma orbital of the apical bond and the pi orbital of the C=C bond in the three-membered ring is operative in pentacoordinate phosphirenes. The sigma-pi interaction is found to lower the reactivity of the C=C bond of the three-membered ring. The reactivities of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-membered ring.     

Beiträge zur Chemie der Halogensilan-Addukte. XXII. Wasserstoffbrückenbindung und Ionisierung Pentakoordinierter SiCl-Verbindungen. Darstellung,Eigenschaften, Kristall- und Molekülstruktur von 2,2-Dimethyl-2,3-dihydro-1H-[1,4,2]diazasilolo[4,5-α]pyridiniumchlorid-Chloroform(1/1),

Contributions to the Chemistry of Halosilane Adducts. XXII. Hydrogen Bonding and Ionization of Pentacoordinated Si-Compounds. Preparation, Properties, Crystal and Molecular Structure of 2,2-Dimethyl-2,3-dihydro-1H-[1,4,2]diazasilolo[4,5-α]pyridinium-chloride-Chloroform(1/1), The title compound ( 1 ) is obtained by the reaction of 2-trimethylsilylaminopyridine and Me₂CH₂ClSiCl in CHCl₃. The SiCl bond is ionic (X-ray structure determination). 1 crystallizes monoclinic space group P2₁/c (Z = 8). The asymmetric unit contains two symmetrically nonequivalent molecular cations of 1 , two Cl anions and two CHCl₃. Silicon is tetracoordinate with Si? N-bond distances of 1.766 Å and 1.777 Å. The shortest SiCl distances (3.908 and 4.110 Å) correspond to ionic interactions. Both Cl anions are hydrogen bonded to CHC1₃. Additionally one C1_? is hydrogen bonded to the NH groups of two neighbouring cations. Structural comparison with related compounds suggests these interactions to be responsible for the transition from pentacoordination to tetracoordination at Si (ionization of the Sic1 bond). ¹H- and ²⁹Si-NMR investigations of 1 show this transition to be dependent on the combined effect of NH…?C1^? and C1₃CH…?C1_? interactions. 1 is completely ionized in CDCl₃ in the temperature range ?80° to +80°C while in the weaker acceptor solvent benzonitrile a temperature dependent shift from the ionic tetracoordinate to the pentacoordinate structure is observed with increasing pentacoordination with increase of temperature.     

Synthesis and Characterization of New Pyrospirophosphoranes Containing a P‐O‐P Bond by the Atherton–Todd Reaction

The classical Atherton–Todd reaction has been successfully applied to the synthesis of pentacoordinate pyrospirophosphoranes containing a P‐O‐P bond under mild conditions. The absolute configurations of the products were confirmed by X‐ray diffraction analysis and spectroscopic identification. And ³¹P NMR chemical shifts of the products were correlated with stereochemistry. A possible mechanism for explaining the stereochemistry of the reaction was proposed, and it could well explain the results of the reactions of different pentacoordinate hydrospirophosphoranes.     

星型液晶的合成、相行为与应用研究

通过SiCl4的Si-Cl键与偶氮苯类液晶基元末端OH的有效联接，合成了三种新的 星型液晶，利用红外光谱（IR）、核磁共振（^1H NMR)对其分子结构进行了鉴定， 利用偏光显微镜（POM）和示差扫描量热法（DSC）对 其液晶相行为进行了表征， 并对其在毛细管气相色谱分析中的应用进行了初步探讨。     

Structural Characterization of a Pentacoordinate Monohydrosilane with the Apical Si-H Bond

A pentacoordinate phenyldibenzosilazocine containing the apical Si-H bond has been prepared and characterized by ¹H and ²⁹Si NMR spectra and X-ray analysis.     

Bildung siliciumorganischer Verbindungen. XXXVII1,3,5,7-Tetrasila-adamantane aus der Pyrolyse des (CH3)3SiCl und Auftrennung des Pyrolysegemisches

Further separation of the pyrolysis products of (CH₃)₃SiCl can be achieved by reaction with LiAlH₄/LiH (transfer of SiCl to SiH groups). By means of adsorptions chromatography a separation is obtained into 4 groups of components. By application of gel chromatography (sephadex LH 20) separation is improved, thus fractions of carbosilanes are found with average molecular weights between 5000 and 200. A given mixture of the compounds [5], [9], [10] has been separated by means of gel chromatography so that pure compounds were obtained. The mixture of the 1,3,5,7-Tetrasila-adamantanes, which are formed in the pyrolysis of (CH₃)₃SiCl, is separated by gel chromatography (efficiency control of separation is performed by NMR and mass spectrography of the different fractions), a concentration of some compounds is obtained, some of them are isolated purely by further operations. The ratio of the compounds [1], [2], [3], [4], found in the pyrolysis products, is 170:26:3:1. Derivatives are formed with SiH, SiCl, and SiCH₃ groups by complete or respectively partial hydrogenation. Comparing the values of the chemical shift of the CH₃-protones [measured in τ) a linear decrease is found in the compounds [9], [4], [3], [2].     

Saturated Heavier Group 14 Compounds as ‐Electron‐Acceptor (Z‐Type) Ligands

This review article describes the chemistry of transition‐metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ‐electron‐acceptor (Z‐type) ligands and discusses the characteristics of bonds between the transition metal and Z‐type ligand. Moreover, we review the iridium hydride mediated cleavage of E–X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.     

Difluorinated Danishefsky's diene: a versatile C(4) building block for the fluorinated six-membered rings

[reaction: see text] A Mg(0)/Me(3)SiCl system was found to be effective for the preparation of difluoro Danishefsky's dienes 2, which involves selective C-F bond cleavage of trifluoromethyl ketones 3. Subsequent hetero Diels-Alder reactions of 2 with aldehydes and imines gave a variety of fluorinated six-membered heterocycles.     

Silylation products of cyclic tri-aminal carbanions and their lithiation

The structure of 1,3,5-trimethyl-1,3,5-triaza-cyclohexane (TMTAC) was determined by single crystal X-ray diffraction and compared with earlier gas-phase data. It shows a preference for an aee-conformation in all phases. Lithiated TMTAC, [(RLi)(2)·(RH)] (1) (R = 2,4,6-trimethyl-2,4,6-triaza-cyclohex-1-yl), was reacted with Et(3)SiCl, Ph(3)SiCl and PhMe(2)SiCl to afford the substituted silanes Et(3)SiR (1), Ph(3)SiR (2) and PhMe(2)SiR (3) in moderate yields. They were characterised by NMR spectroscopy ((1)H, (13)C, (29)Si). 1 reacts with Me(2)SiCl(2) and Ph(2)SiCl(2) to give Me(2)SiR(2) (5) and Ph(2)SiR(2) (6) which were characterised by NMR spectroscopy. 5 was also identified by crystal structure determination. Analogous triple substitution could not be observed by employing trichlorosilanes. Quantumchemical calculations explain this by sterical overcrowding of the silicon atom. The reaction of 1 with SiCl(4) did not yield fourfold substitution but a formal insertion product of SiCl(2) into a C-N bond of the TMTAC ring (2,4,6-trimethyl-2,4,6-triaza-1,1-dichloro-1-sila-cycloheptane, 7) in very small quantities. It was identified by X-ray crystallography and shows an intramolecular Si···N dative bond. The reactions of (3) and (5) with n-butyl lithium afforded lithiation of the silicon bound methyl groups in both cases. The products, 8 and 9, were characterised by NMR spectroscopy ((1)H, (13)C, (29)Si), 8 was also characterised by X-ray crystallography.     

Regiospecific and highly stereoselective allylation of aldehydes with allyltrifluorosilane activated by fluoride ions.

Allyltrifluorosilane/CsF systems form pentacoordinate allylsiliconates which undergo chemoselective, regiospecific and highly stereoselective allylation of aldehydes, presumably via six-membered cyclic transition states.     

Mg-promoted double silylation of trifluoroacetimidoyl chlorides. A new entry to the fluorinated dianion equivalents

[reaction: see text] A Mg(0)/Me(3)SiCl system was found to be effective for the preparation of a novel fluorinated dianion equivalent. A one-pot reaction sequence involving reductive C-F and C-Cl bond cleavage reactions of trifluoroacetimidoyl chlorides afforded bis-silylated difluoroenamines. Subsequent carbon-carbon bond-forming reactions of the bis(silyl)enamines with two kinds of electrophiles gave a variety of difluorinated imines.     

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1’-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [L^PhSiCl] (L^Ph=PhC(NtBu)₂) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO₂ and HCl. The reaction with S₈ led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO₂ resulted in the cleavage of the CO₂ molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO₂ and HCl to a Si=C bond is unprecedented.