Miriam Karni

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).

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.

Ab‐Initio Calculations and Ideal Gas Thermodynamic Functions of Cyclopentadiene and Cyclopentadiene Derivatives

Structures, frequencies and energies, ideal gas thermodynamic properties and values, have been calculated for cyclopentadiene, cyclopentadienols, and a number of radicals derived from them. The necessary molecular information for these calculaions was found by ab‐initio molecular orbital calculations. The gometries, vibrational frequencies and moments of inertia of 8 species are reported. In order to estimate the accuracy of the computations the molecular parameters were compared with known values reported in the literature whenever those were available.

The stabilities of α-oxy and α-thio carbenium ions: the importance of the ground-state energies of the neutral precursors

The abilities of oxy and thio substituents to stabilize an adjacent carbenium ion centre have been evaluated by ab initio methods, up to the MP3/6-31G* level of theory. The relative stabilities of RXCH2+(X = O or S; R = H or CH3) have been calculated by using the hydride-transfer equation ROCH2++ CH3SR → CH3OR + RSCH2+. HOCH2+ is calculated to be more stable than HSCH2+ by 2.3 kcal mol–1, and CH3SCH2+ is more stable than CH3OCH2+ by 0.7 kcal mol–1. On the other hand, when chlorides are used as the precursors, the equation ROCH2++ RSCH2Cl → RSCH2++ ROCH2Cl is exothermic, e.g. by 2.9 kcal mol–1 for R = H (MP3/6-31G*) and ca. 2–3 kcal mol–1(estimated for MP3/6-31G*) for R = CH3. The latter value is in excellent agreement with recent ion cyclotron resonance experiments. The apparent contrast between the conclusions from the foregoing equations regarding the relative stabilities of RSCH2+ and ROCH2+ results from significant ground-state stabilization of ROCH2Cl relatively to RSCH2Cl. The energies of the two isodesmic equations show a strong dependence on the basis set, and reliable results are obtained only when both d-functions and correlation energy are included in the calculations. The possible correlation of the π-donation abilities and other charge-related criteria of the RS and RO substituents with the stabilities of RSCH2+ and ROCH2+ is analysed and critically discussed. It is concluded that such correlations should be treated with great caution, in particular when first-and second-row substituents are compared.

Formation of Three New Bonds and Two Stereocenters in Acyclic Systems by Zinc-Mediated Enantioselective Alkynylation of Acylsilanes, Brook Rearrangement, and Ene-Allene Carbocyclization Reactions

Diastereoisomerically pure (dr > 99:1) and enantiomerically enriched (er up to 98:2) substituted propargyl diols possessing a tertiary hydroxyl group were synthesized in a single-pot operation from simple acylsilanes through a combined catalytic enantioselective alkynylation of acylsilanes, followed by an allenyl-Zn-Brook rearrangement and Zn-ene-allene (or Zn-yne-allene) cyclization reaction. Two remarkable features of these reactions are the near complete transfer of chirality in the allenyl-Zn-Brook rearrangement and the highly organized six-membered transition state of the Zn-ene-allene carbocyclization found by DFT calculations. In this process, three new bonds and two new stereogenic centers are created in a single-pot operation in excellent diastereo- and enantiomeric ratios. DFT calculations show that the allenyl-Zn-Brook rearrangement occurs in preference to the classic [1,2]-Zn-Brook rearrangement owing to its significantly lower activation barrier.

A Bis(silylenyl)pyridine Zero‐Valent Germanium Complex and Its Remarkable Reactivity

The synthesis, reactivity, and electronic structure of the unique germylone iron carbonyl complex [SiNSi]Ge0 →Fe(CO)4 is reported. The compound was obtained in 49 % yield from the reaction of the bis(N-heterocyclic silylenyl)pyridine pincer ligand SiNSi (1,6-C5 NH3 -[EtNSi(Nt Bu)2 CPh]2 ) with GeCl2 ⋅(dioxane) to give the corresponding chlorogermyliumylidene chloride precursor [SiNSi]GeII Cl+  Cl- , which was further reduced with K2 Fe(CO)4 . Single-crystal X-ray diffraction analysis of [SiNSi]Ge→Fe(CO)4 revealed that the Ge0 center adopts a trigonal-pyramidal geometry with a Si-Ge-Si angle of 95.66(2)°. Remarkably, one of the SiII donor atoms in the complex is five-coordinated because of additional (pyridine)N→Si coordination. Unexpectedly, the reaction of [SiNSi]Ge→Fe(CO)4 with GeCl2 ⋅(dioxane) (one molar equivalent) yielded the first push-pull germylone-germylene donor-acceptor complex, [SiNSi]Ge→GeCl2 →Fe(CO)4 through the insertion of GeCl2 into the dative Ge0 →Fe bond. The electronic features of the new compounds were investigated by DFT calculations.

Rearrangement vs. dissociation of gaseous silicon-containing cations: a combined experimental–theoretical approach

The dissociative ionizations of p-MeC6H4O[CH2]2SiMe3, MeCH(Cl)SiMe2Cl, and Me[CH2]2SiMe2Cl were studied in the gas phase, experimentally by using tandem mass spectrometric techniques (MS/MS) and computationally by ab initio M.O. methods, revealing a complex scheme of rearrangements that interconvert the resulting isomeric C5H13Si+ ions, including silicenium ions and α- and β-silyl substituted carbocations.