E. Lukevics

A multinuclear NMR spectroscopy study of alkoxysilanes

Abstract 17 O, 29 Si, and 13 C NMR spectra of more than 100 mono-, di-, tri- and tetra-alkoxysilanes R 4− n Si(OR′) n ; R = C n H 2 n +1 , Ph, CH 2 Cl, CH 2 Br; R′ = C n H 2 n +1 , CH 2 Ph, CH 2 CH 2 Cl, CH 2 CHue5fbCH 2 , CH 2 Cue5fcCH, CH 2 CF 3 . (CH 2 ) 3 Cl, (CH 2 ) 3 CN have been studied. Linear relationships between the chemical shifts of 17 O, 29 Si, 13 C in alkoxysilanes and the inductive and steric constants of substituents R and R′ were observed. Different transmission of electronic effects along the Siue5f8O bond in various directions was revealed by means of 13 C, 29 Si, 17 O NMR spectroscopy and correlation analysis. The results are discussed in terms of ( pue5f8d ) π -bonding between the oxygen and silicon atoms in compounds containing an Siue5f8O bond.

The conformations of n-substituted 2,2-diaryl-1,3-dioxa-6-aza-2-silacyclooctanes in solution

Abstract 1 H, 13 C and 29 Si NMR data obtained for 2,2-diaryl-1,3-dioxa-6-aza-2-silacyclooctanes Ar 2 Si(OCH 2 CH 2 ) 2 NR indicate the occurrence in solution of an equilibrium between the conformations boat-boat/af chair-chair which are determined by the presence or absence of a transannular N → Si bond. The equilibrium is greatly affected by the electronic and steric effects of the N-and Si-substituents.

The conflict between the conformational properties of the eight-membered heterocycle and trigonal bipyramid of five-coordinated silicon in 1,3-dioxa-6-aza-2-silacyclooctanes

Abstract The 1 H, 13 C and 29 Si NMR spectra of 1,3-dioxa-6-aza-2-silacyclooctanes (I) containing a transannular N → Si bond have been studied at different temperatures in solution. The sterically less crowded equatorial location of the Si substituent at the trigonal bipyramid (TBP) corresponds to the energetically unfavourable axial position in the eight-membered heterocycle (C 8 ). For this reason, the preferred orientation of the Si substituent and the conformational equilibrium of C 8 strongly depend on the energy of the N → Si bond. The C 8 of I in solution has the boat-chair (BC) and/or chair-chair (CC) conformations. The weakening of the N → Si bond leads to an increase in dihedral angle ϕ ON and shifts the conformational equilibrium of C 8 to the CC form. The exchange between the Si substituent positions at the TBP, observed at low temperatures proceeds through the inversion of C 8 and requires dissociation of the N → S bond.

Hydrosilylation of carbonyl compounds catalysed by alkali metal fluorides in the presence of crown ethers

Abstract Hydrosilylation of the C=O bond with dimethylphenylsilane proceeds readily in low-polarity solvents (dichloromethane, benzene, THF) in the presence of the catalytic pair MF/18-crown-6 (M = Cs, Rb, K), with caesium and rubidium fluorides being the most active. Using the CsF/18-crown-6 pair, the hydrosilylation of a number of aromatic and heteroaromatic aldehydes and ketones has been carried out. Silyl ethers of the corresponding alcohols have been prepared in good yields.

Correlation of one-bond 15N13C, 15NM, 29SiX (M = 11B, 29Si, 119Sn, X = 1H, 13C) nuclear spin-spin coupling constants with the structure of metallatranes in solution

Abstract The 15 Nue5f8 13 C, 15 Nue5f8 11 B, 15 Nue5f8 29 Si, 29 Siue5f8 1 H and 29 Siue5f8 13 C one-bond coupling constants in 15 N-labelled boratranes, silatranes and germatranes were measured and considered together with the literature data on 15 Nue5f8 119 Sn and 119 Snue5f8 13 C couplings in stannatranes. The quantitative pattern of structural variations in metallatranes revealed by analysis of X-ray data was used for the interpretation of the results. 1 J ( 15 Nue5f8 13 C) coupling constants quantitatively describe the increase in nitrogen pyramidality with the strengthening of the donor—acceptor (DA) bond N → M in solution. The coupling constants of the DA bond N → M were interpreted in terms of the Fermi contact interaction. 1 J ( 15 Nue5f8 29 Si) values were used to predict the bond order and the length of the N → Si bond in silatranes. In silocanes, bicyclic analogues of silatranes, a linear correlation was found between the 1 J ( 15 Nue5f8 29 Si) values and the free activation energy for DA bond cleavage in solution. The latter was used to estimate the energy of the DA bond N → Si in silatranes (50–70 kJ/mol). The coupling constant 2J ( 15 Nue5f8Siue5f8 1 H) depends greatly on the size of the corresponding valence angle, whose increase from 90 to 180° results in a sharp reduction of the coupling constant 2 J ( 15 Nue5f8Siue5f8 1 H) from ∼ 8 to 0 Hz. The effect of the substituents electronegativity on the coupling constant through the covalent and DA bonds was found to be the opposite.

73Ge NMR spectroscopic studies of organogermanium compounds

Abstract The literature, and the authors own data, on the 73Ge NMR spectra of organogermanium compounds are critically assessed. The experimental techniques for the observation of 73Ge NMR spectra the mechanisms for 73Ge nuclear relaxation, as well as relationships between the 73Ge chemical shifts and those of other Group 14 elements or substituent electronegativities, are discussed. 73Ge coupling constants and similar couplings, including other Group 14 elements, are compared.

13C, 15N and 29Si NMR spectra of triazasilatranes

Abstract 13C, 15N and 29Si chemical shifts and 29Siue5f81H, 29Siue5f813C and 29Siue5f815N coupling constants as well as Siue5f8H bond stretching frequencies in the triazasilatranes (I) (2,5,8,9-tetraaza-1-silatricyclo[3.3.3.01,5] undecanes) and model compounds, tris(alkylamino)silanes with RSi = H, Me, CH2ue5fbCH (Vi) and C6H5 (Ph) were measured. A stronger intramolecular N → Si bonding was revealed in I compared with their oxygen analogues, silatranes (II). This was assumed to be caused by the higher polarity of the equatorial Siue5f8X bonds in I (X = NH) in comparison with II (X = O).

1H, 13C and 29Si NMR study of α- and β-silylstyrenes and their adducts with dichlorocarbene

Abstract 1 H, 13 C and 29 Si NMR spectra for the α- and β-silylstyrenes ( E )-PhCHue5fbCHSiR 3 (I) and PhC(SiR 3 )ue5fbCH 2 (II) (R = Cl, Me, Ph), and those for some dichlorocarbene adducts of I and II (R = Me, Ph), were examined. From the 13 C NMR data, the phenyl substituent in the molecules I and II enhances the electronic effects of the organosilicon substituent at Cα, and weakens these effects on the C α resonance. The degree to which polarization of the vinyl Cue5fbC bond is polarized increases with increased electron-withdrawing properties of substituent R in the SiR 3 group in compounds I and II, and correlates with the reduced reactivity of the bond toward electrophilic dichlorocarbene. Several long-range coupling constants (CC) in the molecules I, II and in their adducts with :CCl 2 were measured. The estimated CC is a useful aid for the study of electronic effects in organosilicon compounds.

Synthesis and 1H, 13C, 15N, 29Si NMR spectra of sil- and germ-atranones

Abstract A novel technique using trimethylsilyl aminoacetic acid derivatives and trialkoxy or trichloro derivatives of silicon and germanium has been employed to prepare sil- and germ-atranones. The 1H, 13C, 15N and 29Si NMR spectroscopy data obtained for the synthesized compounds indicate that an increase in the number of carbonyl groups in the atrane framework enhances charge transfer along the donor-acceptor N → M bond. Because of the prominent electron-acceptor properties of the central atom, the atrantriones tend to bind electron-donor solvents. This is accompanied by an increase in the coordination number of silicon and germanium in these complexes, reaching six. A substitution in the equatorial position of the title compounds has been found to affect more readily charge transfer along the N → M bond than in the axial one. The steric structure of the compounds under study is discussed.

Reduction of alkoxysilanes, halo-silanes and -Germanes with lithium aluminium hydride under phase-transfer conditions

Abstract In the presence of phase-transfer catalysts, silicon and germanium organohydrides were obtained in high yield by reduction of the corresponding halo and alkoxy derivatives with lithium aluminium hydride in the solid LiAlH 4 /hydrocarbon two-phase system.