Yitzhak Apeloig

The chemistry of organic silicon compounds

Historical overview and comparison of silicon with carbon, J.Y.Corey theoretical aspects of organosilicon compounds, Y.Apeloig structural chemistry of organic silicon compounds, W.S.Sheldrick dynamic stereochemistry at silicon, R.J.P.Corriu et al thermochemistry, R.Walsh analysis of organosilicon compounds, T.R.C.Crompton positive and negative ion chemistry of silicon-containing molecules in the gas phase, H.Schwarz NMR spectroscopy of organosilicon compounds, E.A.Williams photoelectron spectra of silicon compounds, H.Bock and B.Solouki general synthetic pathways to organosilicon compounds, L.Birkofer and O.Stuhl recent synthetic application of organosilanes, G.L.Larson acidity, basicity and complex formation of organosilicon compounds, A.R.Bassindale and P.G.Taylor reaction mechanisms of nucleophilic attacks at silicon, A.R.Bassindale and P.G.Taylor activating and directive effects of silicon, A.R.Bassindale and P.G.Taylor the photochemistry of organosilicon compounds, A.G.Brook trivalent silyl ions, J.B.Lambert and W.J.Schulz Jr multiple bonds to silicon, G.Raabe and J.Michl bio-organic chemistry, R.Tacke and J.Linoh polysilanes, R.West hypervalent silicon compounds, R.J.P.Corriu and J.C.Young siloxane polymers and copolymers, T.C.Kendrick organosilicon derivatives of phosphorus arsenic, antimony and bismuth, D.A.Armitage chemistry of compounds with silicon-sulphur, silicon-selenium and silicon-tellurium bonds, D.A.Armitage transition-metal silyl derivatives, T.D.Tilley the hydrosilylation reaction, I.Ojima.

Si═X Multiple Bonding with Four-Coordinate Silicon? Insights into the Nature of the Si═O and Si═S Double Bonds in Stable Silanoic Esters and Related Thioesters: A Combined NMR Spectroscopic and Computational Study

The electronic structures and nature of silicon-chalcogen double bonds Si=X (X = O, S) with four-coordinate silicon in the unique silanoic silylester 2 and silanoic thioester 3 have been investigated for the first time, by (29)Si solid state NMR measurements and detailed DFT and ab initio calculations. (29)Si solid state NMR spectroscopy of the precursor silylene 1 was also carried out. The experimental and computational study of 2 and 3, which was also supported by a detailed computational study of smaller model systems with Si=O and Si=S bonds, provides a deeper understanding of the isotropic and tensor components of their NMR chemical shifts. The general agreement between the experimental NMR spectra and the calculations strongly support our previous NMR assignment deduced from experiment. The calculations revealed that in 2 delta((29)Si(=O))(iso) is shifted upfield relative to H(2)Si=O by as much as 175 ppm; the substituents are responsible for ca. 100 ppm of this shift, while the remaining upfield shift is caused by change in the coordination number from three to four at the Si=O moiety. The change in coordination number leads to a nearly cylindrical symmetry in the plane which is perpendicular to the Si=O molecular axis (delta(11) approximately delta(22)), in contrast to the significant anisotropy found in this plane in typical doubly bonded compounds. The change in r(Si=O) or in the degree of pyramidality at the Si=O center which accompanies the change in coordination number has practically no effect on the chemical shift. delta((29)Si(=S))(iso) in 3 is shifted downfield significantly relative to that in 2, and a similar trend is found in smaller models with Si=S vs those with Si=O subunits. This downfield shift can be explained by the smaller sigma-pi* energy difference in the Si=S bond, relative to that of the Si=O bond. The NMR measurements of 2 and 3 having a four-coordinate silicon-chalcogen moiety, and the calculations of their tensor components, their bond polarities, and their Wiberg bond indices revealed that the Si=X moieties in both 2 and 3 have a significant pi(Si=X) character; yet, in both molecules there is a substantial contribution from a zwitterionic Si(+)-X(-) resonance structure, which is more pronounced in 2.

Mechanism of keto Å enol tautomerism of ionized vinyl alcohol versus acetaldehyde and their dissociation to C2H3O+ and H·. An ab initio molecular orbital study

Abstract In complete accord with experiment, ab initio molecular orbital calculations provide a detailed description of the potential energy surface for some isomers of the C2H4O+· ions. In particular, it is predicted that the ionized hydroxy(methyl)carbene, H3CCOH+·, is a stable C2H4O+· isomer and serves as the key intermediate in the isomerization/dissociation processes of the cation radical of gaseous vinyl alcohol. A comparison between the results of semi-empiric (MINDO/3 and MNDO) and ab initio calculations at various levels of theory suggests that (i) MINDO/3 fails to describe properly the central features of the C2H4O+· energy surface, (ii) MNDO gives results which are qualitatively similar to those obtained by the more elaborate ab initio procedures and (iii) inclusion of the effects of correlation and zero-point energies, as well as the use of large basis sets, are essential for obtaining a reliable insight into the gas phase chemistry of these and related cation radicals.

Trisilaallene and the Relative Stability of Si3H4 Isomers

A theoretical quantum-mechanical study of trisilaallene, H2Si [Formula: see text] Si [Formula: see text] SiH2, and of 15 other Si3H4 isomers was carried out using ab initio and DFT methods with a variety of basis sets. Values given below are at B3LYP/6-31G(d,p). Unlike H2C [Formula: see text] C [Formula: see text] CH2 which is linear, H2Si [Formula: see text] Si [Formula: see text] SiH2 is highly bent at the central silicon atom, with a SiSiSi bending angle of 69.4°. The Si [Formula: see text] Si bond length is 2.269 Å, longer than a regular Si [Formula: see text] Si double bond (2.179 Å) but shorter than a Si-Si single bond (2.351 Å). The distance between the terminal silicon atoms is 2.583 Å, significantly longer than a Si-Si single bond. The geometry and electronic properties of H2Si [Formula: see text] Si [Formula: see text] SiH2 are similar to those of the corresponding trisilacyclopropylidene, which is only 2.7 kcal/mol higher in energy. A barrier of only 0.1 kcal/mol separates trisilacyclopropylidene and trisilaallene which can be described as bond-stretch isomers. Sixteen minima were located on the Si3H4 PES, most of them within a narrow energy range of ca. 10 kcal/mol. Six of the Si3H4 isomers are analogous to the classic C3H4 minima structures; however, the other Si3H4 isomers do not have carbon analogues, and they are characterized by hydrogen-bridged structures.

HCSiF and HCSiCl: The First Detection of Molecules with Formal C≡Si Triple Bonds

Neutralization of [C,H,Si,X]  .+ radical cations (X=F, Cl) in conjunction with electronic structure calculations provides the first experimental evidence for the formation of the neutral silynes HC≡SiF and HC≡SiCl, which have nonlinear structures (see picture).

Radical Reactions of a Stable N-Heterocyclic Germylene: EPR Study and DFT Calculation

The first radical adducts of a stable N-heterocyclic germylene were investigated. Novel radical species were produced from a variety of precursors and studied by EPR spectroscopy. DFT (B3LYP) calculations of radical adducts of different (C, Si, Ge) unsaturated N-heterocyclic divalent species with phenoxyl radical show that in the radicals studied the unpaired electron is delocalized over the five-membered ring and the spin density on the central atoms decreases in the following order: C > Si > Ge. These trends can be understood in terms of zwitterionic structure of radical adducts.

The Synthesis and Molecular Structure of the First Two-Coordinate, Dinuclear σ-Bonded Mercury(I) RHgHgR Compound.

A linear Si-Hg-Hg-Si arrangement and a Hg-Hg distance of 265.69 pm are exhibited by the first two-coordinate, dinuclear σ-bonded organomercury(I) compound 1. It was formed unexpectedly in the reaction of two equivalents of the silane (Me3 SiMe2 Si)3 SiH with tBu2 Hg. In contrast if the reagents are allowed to react in a 1:1 ratio the expected mercury(II) compound (Me3 SiMe2 Si)3 SiHgtBu is obtained.

One‐Pot Zinc‐Promoted Asymmetric Alkynylation/Brook‐Type Rearrangement/Ene–Allene Cyclization: Highly Selective Formation of Three New Bonds and Two Stereocenters in Acyclic Systems

Its as easy as 1, 2, 3: In a one-pot sequence, two stereocenters and three new bonds were created with high selectivity through an asymmetric alkynylation of acyl silanes, a tandem Brook-type rearrangement and Zn-ene-allene cyclization, the addition of an electrophile, and finally oxidation. The straightforward nature of the synthetic procedure contrasts strongly with the complexity of the densely functionalized products obtained.

The anomeric effect at silicon

The anomeric effect at silicon has been studied by ab initio calculations. The geometries and energies of H2Si(XH)2, X = O, S were optimized with the basis sets 3-21G, 3-21G(*), and 6-31G *. Single point MP3/6-31G * //6-31G * calculations were also carried out. H2Si(XCH3)2, X = O, S, were studied with the 3-21G basis set. All compounds are most stable in the gauche, gauche conformation, pointing to the operation of an anomeric effect. The total anomeric interactions given by the energies of the equations: H2Si(XH)2 + SiH4 → 2H3SiXH are (MP3/6-31G *): 8.6 and 2.2 kcal/mol for X = O and X = S, respectively. Rotation barriers on going from the (g,g), to the (g,a) and to the (a,a) conformations are (MP3/6-31G *, kcal/mol): 2.5 and 3.8 in H2Si(OH)2 and 2.1 and 3.2 in H2Si(SH)2. Thus, the anomeric effect in H2Si(OH)2 is large, although smaller than in H2C(OH)2. In H2Si(SH)2 and H2C(SH)2 the anomeric effects are comparable, and both relatively small. The anomeric effect is predicted to be important in determining the conformations of compounds with silicon bonded to 2 oxygens such as R2Si(OR′)2, disilaoxiranes, and related molecules.

On the Question of Cyclopropylidene Intermediates in Cyclopropene-to-Allene Rearrangements − Tetrakis(trimethylsilyl)cyclopropene, 3-Alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and Related Model Compounds

Several tetrasubstituted cyclopropenes have been prepared and their pyrolyses and photolyses have been investigated. Tetrakis(trimethylsilyl)cyclopropene (10), which was obtained in 25% yield from tris(trimethylsilyl)cyclopropenylium hexachloroantimonate (9), gave tetrakis(trimethylsilyl)allene (12) as the sole product both thermally and photochemically. Kinetic studies in [D8]toluene indicated first-order behavior with Arrhenius parameters log(A/s−1) = 11.75 ± 1.20 and Ea = (37.5 ± 2.5) kcal mol−1. All three new 3-alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes (17a−c, with C1-, C2-, and C3-alkenyl groups as tethers, respectively) gave allenes upon irradiation, but thermally only two (17a, 17c) gave allenes, whilst 17b yielded a bicyclo[4.1.0]hept-3-ene derivative 22 as a result of an intramolecular ene reaction. Photolyses of two further cyclopropenes (33a,b) bearing 1,2-bis(alkenyldimethylsilyl) substituents also gave the corresponding allenes as the sole products. For none of these tethered cyclopropenes was a product found that could have originated from intramolecular trapping of a cyclopropylidene intermediate. Quantum mechanical (ab initio) calculations have been carried out on the silyl-substituted cyclopropene model compounds 3,3-dimethyl-1-silyl- (36a), 3,3-dimethyl-1,2-disilyl- (37), and tetrasilylcyclopropene (38) at the QCISD(T)/6-311G*//B3LYP/6-311G* + ZPVE level of theory, and on 3,3-dimethyl-1-(trimethylsilyl)cyclopropene (36b) at the B3LYP/6-311G*//B3LYP/6-311G* + ZPVE level. These calculations provided us with detailed energy surfaces for the potential pyrolysis pathways. Although the potential cyclopropylidene species in these rearrangements are significantly stabilized, for none of the systems was this sufficient to permit isomerization via these intermediates. 36b is calculated to rearrange via a vinylidene intermediate to give 3-methyl-1-trimethylsilyl-1-butyne (47), in agreement with experiment. Comparison of the calculations for 36a and 36b shows that H3Si− is a poor model for an Me3Si− substituent in these rearrangements. When an appropriate correction is applied, the calculations on disilyl- (37) and tetrasilylcyclopropenes (38) are consistent with the experimental findings that the trimethylsilyl-substituted cyclopropenes 48 and 10 form allenes 49 and 12, respectively, via vinylcarbene-type intermediates. These findings considerably extend our understanding of silyl group substituent effects on the various intermediates involved in cyclopropene rearrangements.