Substituent effects on the carbon-silicon double bond. Monosubstituted silenes

Apeloig, Yitzhak; Karni, Miriam

doi:10.1021/ja00334a036

Cited by 144 publications

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“…The calculated structures are similar to the X-ray crystal structure recently determined by us. [9] The Si atomic charges of the 2-silenolates (0.06 e (28 A), 0.62 e (28 B), and À 0.13 e (28 C)) also resemble those of the silyl anions H 2 MeSi À (0.02 e), Me 3 Si À (0.59 e), and (H 3 Si) 2 MeSi À (À 0.11 e). Although the agreement is excellent in q(Si) of the silyl anions and 2-silenolates, this comparison is not fully justified, since it assumes that the extent of inductive electron withdrawal from Si by a CHO group and by a methyl group are similar.…”

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confidence: 96%

“…This neglects differences in the inductive electron withdrawal from Si by the various CXY units, and also the change in electronegativity of Si when going from an sp 2 -hybridized Si in a planar silene to an sp 3 -hybridized Si in a pyramidalized one. Bergman and Hinze determined the sp 2 and sp 3 hybrid orbital electronegativities of Si according to the Pauling scale as 2.44 and 2.23, respectively. [43] This small difference may justify neglect of the difference in inductive electron withdrawal from Si exerted by, for example, a methyl group in a silene with a planar versus pyramidal Si atom.…”

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Zwitterionic Silenes: Interesting Goals for Synthesis?

Ottosson

2003

Chemistry A European J

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Properties of silenes, as a function of increased reversal of the Si=C bond polarity, have been examined through quantum-chemical calculations. The aim of this study was to identify silenes that can be of general interest for organic synthesis. The calculations were carried out primarily with the B3LYP hybrid density functional method, but also with the CASSCF, MP2, MP4(SDQ), and CCSD(T) methods. The study was performed on Z(2)Si=CXY compounds which were divided into three sets that differ with regard to their Si substituents (Z), and with their C substituents (X and Y) varying from weakly to strongly pi-electron-donating groups. The charge at the Si atom (q(Si)) was used as a measure of the extent of reversed silicon-carbon bond polarity. For each of the three sets, the variation in silicon-carbon bond lengths (r(Si=C)) and extent of Si pyramidalization (SigmaSi) in relation to q(Si) follow three separate curves. Silenes with strongly pi-electron-donating X and Y groups are completely described by zwitterionic (reverse-polarized) resonance structures. Such zwitterionic silenes are singly (Si=C) rather than doubly bonded (Si=C), and have a distinctly pyramidal Si atom due to negative charge localization. These silenes also have much lower heats of dimerization than the parent silene. Finally, inversion barriers of zwitterionic silenes are increased by electron-withdrawing substituents, and this enables computational design of silenes with their Si atoms as chiral centers. It is hoped that such chiral zwitterionic silenes can find use in organic synthesis.

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Zwitterionic Silenes: Interesting Goals for Synthesis?

Ottosson

2003

Chemistry A European J

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“…The enhanced SiQC character is not surprising, and C-silyl-substitution at the SiQC bond is known to have a stabilizing effect. 17 Altogether, the Si lone pair is heavily involved in the p-system. The C-C distances (Fig.…”

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confidence: 99%

Planar lithium silolide: aromaticity, with significant contribution of non-classical resonance structures

et al. 2017

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The first planar lithium silolide characterized by X-ray crystallography is reported. The structural data, NMR and DFT results reveal high aromatic character, with the unusual dominance of non-classical resonance structures.The cyclopentadienide ion is a textbook example for a planar aromatic compound alongside benzene. 1a The replacement of a carbon atom by silicon results in the silacyclopentadienyl (silolide) anion, 1b-f for which the planarity and the apparently related aromaticity are far from being trivial. the structural similarity for the isoelectronic silolide anion is also expected. Computational studies have also predicted that silolide is pyramidal; 5 however, even with the non-planar silicone a moderate aromaticity was reported for the parent compound, 5a furthermore the presence of the countercation has a flattening effect on silicon. 5 A noteworthy feature of the calculated planar silolide structure was the equalization of the C-C distances, the Cb-Cb 0 bond being slightly (by 0.008 Å) shorter than Ca-Cb. Accordingly, for the planar silolide the ''b'' resonance structure had larger (30%) weight than the classical ''a'' (17%) according to a natural resonance theory (NRT) study (Fig. 1 Si and upfield in 13 C NMR) with respect to the neutral precursor disilane was attributed to cyclic delocalization. 7 Unfortunately, for this particular silolide, no X-ray structure could be obtained, The situation is further complicated by the fact that for 1-Me-tetraphenylsilolide the dimerization of the anion was reported, yielding tricyclic systems with a central 1,3-disilacyclobutane ring. 8While no planar silolide has been reported to date, Sekiguchi's 1,2,3-trisilacyclopentadienide and 1,2-disila-3-germacyclopentadienide anions have been shown to exhibit planar X-ray structures with equalized bond lengths, and with 29 Si NMR data indicating aromaticity.9 Furthermore, nearly planar transition metal (Zr, Hf or Ru) complexed silolides were also synthetized. 10In a computational search for planar silolide anions we revealed that due to the charge stabilizing effect, p-acceptor SiH 3 groups at the a carbons enhance the planarity and aromaticity of the naked silolide ring considerably. 5d This effect was already noted on the related phosphole with SiH 3 11 and BH 2 12 substituents, and the charge stabilizing effect was also shown in the case of the silyl-substituted stannole dianion. 13Furthermore, the a-silyl groups enhanced the relative weight of the b type resonance structure. 5d Z 5 -Coordination of the Li Fig. 1 Resonance structures for planar heteroaromatic five membered rings, with different heteroelements. E can be S, PR, or SiR À (R:H, alkyl, aryl, etc.). The numbering of the ring atoms is shown on resonance structure a.

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“…The Si=C distances of the kinetically stabilized silenes (Me 3 Si) 2 Si=C-(OSiMe 3 )Ad (1.764 Å, Ad = 1-adamantyl) [13] and (Me 3 Si)-(tBuMe 2 Si)=AdЈ (1.741 Å, AdЈ = 2-adamantylidene) [14] are almost the same as those of the thermodynamically stabilized silenes, but these elongations were accounted for by the electronic effects of the substituents. [14,15] For compound 3 the situation is quite comparable to compound 1, as the results in Table 2 show. Only one of the two dimethylamino groups interacts with the tetra-coordinated silene silicon atom.…”

Section: Resultsmentioning

confidence: 58%

Ab initio Study of Some Intramolecularly Donor‐Stabilized Silenes

Wandschneider¹,

Michalik²,

Oehme³

et al. 2005

Eur J Inorg Chem

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Keywords: Silenes / Ab initio calculations / Donor-acceptor systems Ab initio calculations were performed to predict geometric structures, stabilizing energies and rotational barriers of some intramolecularly donor-stabilized silenes. Calculations were performed using the HF, MP2, and B3LYP method with the 6-31G(d) and 6-31++G(d,p) basis set. Results were compared to experimentally available X-ray structures and energy barriers known from temperature dependent 1 H NMR

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Substituent effects on the carbon-silicon double bond. Monosubstituted silenes

Abstract: A series of isomeric silenes H2C=SiHR (1) and H2Si=CHR (2) where R = CH3, SiH3, F, OH, OSiH3, CN, and N02 were studied at the RHF/3-21G and 6-31G* levels. The two basis sets give in general similar results. The calculated r(C=Si) are (3-21G, values (in A) for 1 followed by 2) 1.

Cited by 144 publications

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Zwitterionic Silenes: Interesting Goals for Synthesis?

Zwitterionic Silenes: Interesting Goals for Synthesis?

Planar lithium silolide: aromaticity, with significant contribution of non-classical resonance structures

Ab initio Study of Some Intramolecularly Donor‐Stabilized Silenes

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