Trisilaallene is a subclass of silene derivatives where a central silicon atom forms double bonds with each of two terminal silicon atoms, with the generic formula R2Si=Si=SiR2. Trisilaallene is a silicon-based analog of an allene, but their chemical properties are markedly different.
The first isolable trisilaallene compound was reported by Kira et al. in 2003, synthesized by reductive dehalogenation of tetrachlorosilane using potassium graphite. [1] This tetraalkyl-substituted trisilallene showed thermal stability up to its melting point around 200 °C, but decomposed in contact with air. Its remarkable stability is attributable to bulky substituents providing kinetic protection at the terminal silicon atoms. [2] The 29Si-NMR shifts of the central silicon and terminal silicon atoms were observed at 157.0 ppm and 196.9 ppm respectively. Two Si=Si bond lengths were determined to be 2.177 Å and 2.188 Å by X-ray crystallography, which are within the typical range of Si=Si double bonds. [3]
Sekiguchi et al. synthesized a silyl-substituted trisilaallene from a reaction between a N-heterocyclic carbene (NHC) adduct of SiCl2 and 1,1-dilithiosilane (t-Bu2MeSi)2SiLi2. [4] [5] Although crystallographic analysis of the product was not successful, the formation of trisilaallene was confirmed by 1H-, 13C-, and 29Si-NMR spectroscopy, high-resolution mass spectrometry (HRMS), and reactivity study. The low electronegativity of silyl substituents compared to alkyl substituents resulted in a more upfield 29Si-NMR shift for the terminal silicon atoms (44.6 ppm) and a downfield shift for the central atom (418.5 ppm).
In contrast to its linear carbon analog, trisilaallene is characterized by bent geometry. For example, Kira's trisilaallene had a Si=Si=Si bond angle of 136.5º. [1] While the bulky and electropositive silyl ( tBu2MeSi-) substituents widened the bond angle to 164.3º (calculated), no linear trisilaallene has been reported yet. [6] The two planes that each terminal silicon atom and attached substituents lie on tend to be perpendicular to each other, which is analogous with allene. The central silicon atom shows fluxional behavior in that its relative position varies with respect to the substituents planes, and the distribution of resultant isomers depends on the temperature.
The bent structure of trisilaallene is explained by the second-order Jahn-Teller distortion. [7] Unlike the 2s and 2p orbitals of carbon, where the orbital radii of maximum electron density are similar, the 3s orbital of silicon is significantly smaller than 3p orbitals (rnp– rns= -0.2 pm for n = 2 and > 20 pm for n > 2). Therefore, the σ-overlap between the 3s orbital of the central silicon atom and a set of 3pz orbitals of the terminal atoms (when z-axis is the molecular axis) is poor compared to in allene, resulting in a relatively low-lying σ*-orbital. The energy gap between σ*-orbital and π-orbitals originated from 3px and 3py orbitals is small enough to induce considerable mixing between the σ*-orbital and one of the π-orbitals with appropriate symmetry. This orbital mixing removes degeneracy between the two π-orbitals, accompanied by geometric distortion.
The ab initio and density functional theory (DFT) calculations predict that Si3R4 molecules with smaller substituents (R = H or Me) adopt zwitterionic structures with C2v or Cs symmetry and with drastically smaller bond angles (~70º for R = H, ~90º for R = Me). [1] [2] [7] [8] However, the steric congestion required for isolation prevent these highly bent structures. [9]
DFT calculation at B3LYP/6-31+G(d, p) level suggests that the frontier molecular orbitals of the tetraalkyl-substituted trisilaallene are markedly different from those of allene. [1] [7] The calculation was performed on a model trisilaallene compound with methyl substituents and the experimentally observed bond angle from Kira's compound (136.5º). According to the calculation, the alkyl-substituted trisilaallene has nondegenerate HOMO-1 and HOMO based on π-interaction and also nondegenerate LUMO and LUMO+1 based on π*-interaction, as a direct result of Jahn-Teller distortion. These orbitals correspond to in-plane and out-of-plane twisted overlaps of p-orbitals, which are delocalized over the Si=Si=Si unit. These frontier orbitals make a striking contrast to those of all-carbon allenes, whose frontier orbitals consist of degenerate pairs of π-bonding orbitals (HOMO) and π*-antibonding orbitals (LUMO) localized between two carbons.
The electronic structure of trisilaallene is highly affected by its substituents. On the contrary to the alkyl-substituted trisilaallene, according to DFT calculation at B3LYP/6-31G(d) level, tBu2MeSi-substituted trisilaallene has almost degenerate π (Si=Si) and π* (Si=Si) orbitals, localized between the central silicon atom and only one of the two terminal silicon atoms. [4] The two π-bonding orbitals and the two π*-antibonding orbitals are perpendicular to each other. These features are analogous to all-carbon allenes, which is justified by the close-to-linear geometry of the silyl-substituted trisilaallene.
Trisilaallene readily reacts with alcohol to generate dialkoxytrisilane. The regioselectivity of the alcohol addition reaction depends on the type of substituents. In 2007, Kira et al. reported that the alkyl-substituted trisilaallene gives rise to 1,3-dialkoxytrisilane in the presence of excess ROH (R = H, Me, Et). [1] [10] Larger alcohols such as isopropanol and tert-butanol did not react due to the steric congestion arising from the bulky substituents of trisilaallene. In contrast, two methoxy groups were added to the central silicon atom of the silyl-substituted trisilaallene from the reaction with methanol. [4]
The different regioselectivity between the alkyl-substituted trisilaallene and the silyl-substituted trisilaallene is explained by charge distribution within Si=Si=Si unit. Calculation at the B3LYP/6-31+G(d,p) level for the methyl-substituted trisilaallene predicted >Siδ+=Siδ-=Siδ+< charge distribution with -0.408 on the central atom and +1.016 / +1.026 on the terminal atoms. [10] Therefore, the nucleophilic addition of alcohol occurs at the terminal atoms. This charge distribution is in accordance with the frontier molecular orbitals of the alkyl-substituted trisilaallene. HOMO-1 and HOMO have the largest orbital coefficient on the central atom, while LUMO and LUMO+1 have larger coefficients on the terminal two. On the other hand, the silyl-substituted trisilaallene is expected to have greater negative charges on the terminal atoms (-0.36 / -0.37) than on the central atom (-0.08), calculated at B3LYP/6-31G(d) level. [4]
Reactions of trisilaallene with haloalkanes generate halogenated silane products. [10] Treating trisilaallene with methyl iodide breaks the Si=Si double bonds, yielding two molecules of iodo(methyl)silane. Meanwhile, the reaction of trisilaallene with tetrachloromethane gives rise to the complete chlorination of the two Si=Si bonds without bond breaking, yielding tetrachlorotrisilane.
Trisilaallene also reacts with acetone to form a strained bicyclic adduct, whose structure was confirmed by X-ray crystallography. [10] This reaction is supposed to be initiated by an ene reaction of one of the Si=Si double bond with acetone, followed by [2 + 2] cycloaddition of the other Si=Si bond and a C=C bond to give the product.
The silyl-substituted trisilaallene goes through thermal rearrangement when heated to 120 °C in benzene. [4] The resultant isomer, tetrakis(di-tert-butylmethylsilyl)cyclotrisilene, was calculated to be thermodynamically more stable than the parent trisilaallene compound by 10.5 kcal/mol.
In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms. Allenes are classified as cumulated dienes. The parent compound of this class is propadiene, which is itself also called allene. An group of the structure R2C=C=CR− is called allenyl, where R is H or some alkyl group. Compounds with an allene-type structure but with more than three carbon atoms are members of a larger class of compounds called cumulenes with X=C=Y bonding.
In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a pericyclic reaction with a concerted mechanism. More specifically, it is classified as a thermally-allowed [4+2] cycloaddition with Woodward–Hoffmann symbol [π4s + π2s]. It was first described by Otto Diels and Kurt Alder in 1928. For the discovery of this reaction, they were awarded the Nobel Prize in Chemistry in 1950. Through the simultaneous construction of two new carbon–carbon bonds, the Diels–Alder reaction provides a reliable way to form six-membered rings with good control over the regio- and stereochemical outcomes. Consequently, it has served as a powerful and widely applied tool for the introduction of chemical complexity in the synthesis of natural products and new materials. The underlying concept has also been applied to π-systems involving heteroatoms, such as carbonyls and imines, which furnish the corresponding heterocycles; this variant is known as the hetero-Diels–Alder reaction. The reaction has also been generalized to other ring sizes, although none of these generalizations have matched the formation of six-membered rings in terms of scope or versatility. Because of the negative values of ΔH° and ΔS° for a typical Diels–Alder reaction, the microscopic reverse of a Diels–Alder reaction becomes favorable at high temperatures, although this is of synthetic importance for only a limited range of Diels-Alder adducts, generally with some special structural features; this reverse reaction is known as the retro-Diels–Alder reaction.
In organic chemistry, the ene reaction is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond, in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.
Diphosphene is a type of organophosphorus compound that has a phosphorus–phosphorus double bond, denoted by R-P=P-R'. These compounds are not common but are of theoretical interest. Normally, compounds with the empirical formula RP exist as rings. However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by a large steric hindrance from the substitutions. The first isolated diphosphene bis(2,4,6-tri-tert-butylphenyl)diphosphene was exemplified by Masaaki Yoshifuji and his coworkers in 1981, in which diphosphene is stabilized by two bulky phenyl group.
The Fleming–Tamao oxidation, or Tamao–Kumada–Fleming oxidation, converts a carbon–silicon bond to a carbon–oxygen bond with a peroxy acid or hydrogen peroxide. Fleming–Tamao oxidation refers to two slightly different conditions developed concurrently in the early 1980s by the Kohei Tamao and Ian Fleming research groups.
The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula is C
2H+
3. More generally, a vinylic cation is any disubstituted, trivalent carbon, where the carbon bearing the positive charge is part of a double bond and is sp hybridized. In the chemical literature, substituted vinylic cations are often referred to as vinyl cations, and understood to refer to the broad class rather than the C
2H+
3 variant alone. The vinyl cation is one of the main types of reactive intermediates involving a non-tetrahedrally coordinated carbon atom, and is necessary to explain a wide variety of observed reactivity trends. Vinyl cations are observed as reactive intermediates in solvolysis reactions, as well during electrophilic addition to alkynes, for example, through protonation of an alkyne by a strong acid. As expected from its sp hybridization, the vinyl cation prefers a linear geometry. Compounds related to the vinyl cation include allylic carbocations and benzylic carbocations, as well as aryl carbocations.
Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.
Phosphinidenes are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The "free" form of these compounds is conventionally described as having a singly-coordinated phosphorus atom containing only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties. In the last few decades, several strategies have been employed to stabilize phosphinidenes, and researchers have developed a number of reagents and systems that can generate and transfer phosphinidenes as reactive intermediates in the synthesis of various organophosphorus compounds.
Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation C5H+5(Cp+) and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.
The nitrone-olefin (3+2) cycloaddition reaction is the combination of a nitrone with an alkene or alkyne to generate an isoxazoline or isoxazolidine via a [3+2] cycloaddition process. This reaction is a 1,3-dipolar cycloaddition, in which the nitrone acts as the 1,3-dipole, and the alkene or alkyne as the dipolarophile.
A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic residue and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
Decamethylsilicocene, (C5Me5)2Si, is a group 14 sandwich compound. It is an example of a main-group cyclopentadienyl complex; these molecules are related to metallocenes but contain p-block elements as the central atom. It is a colorless, air sensitive solid that sublimes under vacuum.
Diphosphagermylenes are a class of compounds containing a divalent germanium atom bound to two phosphorus atoms. While these compounds resemble diamidocarbenes, such as N-heterocyclic carbenes (NHC), diphosphagermylenes display bonding characteristics distinct from those of diamidocarbenes. In contrast to NHC compounds, in which there is effective N-C p(π)-p(π) overlap between the lone pairs of planar nitrogens and an empty p-orbital of a carbene, systems containing P-Ge p(π)-p(π) overlap are rare. Until 2014, the geometry of phosphorus atoms in all previously reported diphosphatetrylenes are pyramidal, with minimal P-Ge p(π)-p(π) interaction. It has been suggested that the lack of p(π)-p(π) in Ge-P bonds is due to the high energetic barrier associated with achieving a planar configuration at phosphorus, which would allow for efficient p(π)-p(π) overlap between the phosphorus lone pair and the empty P orbital of Ge. The resulting lack of π stabilization contributes to the difficulty associated with isolating diphosphagermylene and the Ge-P double bonds. However, utilization of sterically encumbering phosphorus centers has allowed for the isolation of diphosphagermylenes with a planar phosphorus center with a significant P-Ge p(π)-p(π) interaction.
A phosphetane is a 4-membered organophosphorus heterocycle. The parent phosphetane molecule, which has the formula C3H7P, is one atom larger than phosphiranes, one smaller than phospholes, and is the heavy-atom analogue of azetidines. The first known phosphetane synthesis was reported in 1957 by Kosolapoff and Struck, but the method was both inefficient and hard to reproduce, with yields rarely exceeding 1%. A far more efficient method was reported in 1962 by McBride, whose method allowed for the first studies into the physical and chemical properties of phosphetanes. Phosphetanes are a well understood class of molecules that have found broad applications as chemical building blocks, reagents for organic/inorganic synthesis, and ligands in coordination chemistry.
Silylones are a class of zero-valent monatomic silicon complexes, characterized as having two lone pairs and two donor-acceptor ligand interactions stabilizing a silicon(0) center. Synthesis of silylones generally involves the use of sterically bulky carbenes to stabilize highly reactive Si(0) centers. For this reason, silylones are sometimes referred to siladicarbenes. To date, silylones have been synthesized with cyclic alkyl amino carbenes (cAAC) and bidentate N-heterocyclic carbenes (bis-NHC). They are capable of reactions with a variety of substrates, including chalcogens and carbon dioxide.
Phosphasilenes or silylidenephosphanes are a class of compounds with silicon-phosphorus double bonds. Since the electronegativity of phosphorus (2.1) is higher than that of silicon (1.9), the "Si=P" moiety of phosphasilene is polarized. The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or by modifying the electronic properties of the substituents. The phosphasilene Si=P double bond is highly reactive, yet with the choice of proper substituents, it can be stabilized via donor-acceptor interaction or by steric congestion.
Tellurophenes are the tellurium analogue of thiophenes and selenophenes.
An N-Heterocyclic silylene (NHSi) is an uncharged heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.
9-borafluorenes are a class of boron-containing heterocycles consisting of a tricyclic system with a central BC4 ring with two fused arene groups. 9-borafluorenes can be thought of as a borole with two fused arene rings, or as a trigonal planar boron atom with an empty p orbital bridging two biphenyl rings. However, 9-borafluorenes are generally less reactive than boroles due to less antiaromatic character and Lewis acidity. Containing highly conjugated π systems, 9-borafluorenes possess interesting photophysical properties. In addition, 9-borafluorenes are good Lewis acids. This combination of properties enables potential uses such as in light-emitting materials, solar cells, and sensors for some molecules.
Carbones are a class of molecules containing a carbon atom in the 1D excited state with a formal oxidation state of zero where all four valence electrons exist as unbonded lone pairs. These carbon-based compounds are of the formula CL2 where L is a strongly σ-donating ligand, typically a phosphine (carbodiphosphoranes) or a N-heterocyclic carbene/NHC (carbodicarbenes), that stabilises the central carbon atom through donor-acceptor bonds. Carbones possess high-energy orbitals with both σ- and π-symmetry, making them strong Lewis bases and strong π-backdonor substituents. Carbones possess high proton affinities and are strong nucleophiles which allows them to function as ligands in a variety of main group and transition metal complexes. Carbone-coordinated elements also exhibit a variety of different reactivities and catalyse various organic and main group reactions.