Theoretical study of tetramethyl- and tetra-tert-butyl-substituted cyclobutadiene and tetrahedrane

The tetramethyl and tetra-tert-butyl derivatives of cyclobutadiene and tetrahedrane have been studied with ab initio and density functional methods. The ring in tetra-tert-butylcyclobutadiene displays very unequal bond lengths (1.354, 1.608 Angstrom) and confirms the earlier suspicion that the low-temperature X-ray structure was distorted. The C-C single bonds have the longest separations found to date between sp(2)-hybridized carbons. Tetra-rert-butyltetrahedrane, which prefers T over Td symmetry, is calculated to be 1.5 kcal/mol more stable than tetra-tert-butylcyclobutadiene (B3LYP/6-311+G(d)//B3LYP/6-3 1G(d)). The dications, C4R42+, dianions, C4R42-, and dilithiated species, Li2C4R4 (R = H, Nle, t-Bu), also were studied to determine the effect of substitution on structure and stability. Finally, NICS (nucleus-independent chemical shifts) values were calculated and showed, as expected, the dications and dianions (4n + 2 pi-electrons) to be aromatic (negative NICS(0)) and the cyclobutadienes to be antiaromatic (positive NICS(0))

Methylzinc tetrahydroborate: investigation of the vapour phase by spectroscopic and quantum chemical techniques

Methylzinc tetrahydroborate vapour, as shown by its mass and infrared spectra, consists of an equilibrium mixture of monomeric and dimeric species; the proportion of the timer increases as the temperature is raised from 243 to 268 K (at pressures < 10-1 Torr). Consistent with the results of Density Functional Theory (DFT) calculations, the pattern of infrared bands of the matrix-isolated monomer points to a bidentate BH4 group and a tri-coordinate zinc centre. Close agreement between computed and experimentally determined infrared frequencies adds credence to the calculated structural parameters. The nature of the dimer is less certain. The spectroscopic results are consistent with a cyclic structure of the type MeZn(μ2,η2-HB4)2ZnMe; the minimum energy DFT structure features C(2v) symmetry and a puckered eight-membered Zn(μ-H)B(H)2(μ-H)Zn(μ-H)B(H)2(μ-H) ring

METHYLSILYLHYDROXYLAMINES - PREPARATIVE, SPECTROSCOPIC AND AB-INITIO STUDIES

Mitzel NW, HOFMANN M, WATERSTRADT E, SCHLEYER PV, SCHMIDBAUR H. METHYLSILYLHYDROXYLAMINES - PREPARATIVE, SPECTROSCOPIC AND AB-INITIO STUDIES. JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS. 1994;(17):2503-2508.Methylsilylhydroxylamines [(MeH2Si)2NOMe, (MeH2Si)MeNOMe, Me2NOSiH2Me] have been prepared from bromo(methyl)silane and the corresponding methylhydroxylamines in the presence of an auxiliary base (triethylamine or N,N,N',N'-tetramethylethylenediamine). The compounds were studied by NMR spectroscopy of all elements present (H-1, C-13, N-15, O-17, Si-29). The magnitude of the one-bond coupling constants (J(NSi))-J-1-N-15-Si-29 is interpreted in terms of the hybridization associated with the pyramidal co-ordination of nitrogen, a unique structural feature in Si/N chemistry. Ab initio studies confirmed these structural predictions. Singly silylated hydroxylamines have been shown to be more strongly pyramidal than doubly silylated ones. Calculations on the model compound (H3Si)2NOMe gave a barrier to inversion at nitrogen of 9.7 kcal mol-1. This inversion is accompanied by a partial rotation around the N-O bond. The NMR chemical shifts of the compounds have been calculated and the results are in good agreement with the experimental data. The unusually low chemical shifts delta(O-17) of hydroxylamines have thus been confirmed by theory. A comparison of the calculated normal modes of vibration with experimental data leads to a complete assignment of the IR spectra

Quantitative evaluation of hyperconjugation in the cyclopropylcarbinyl cation and in cyclopropylborane

An orbital deletion procedure (ODP) at HF/6-311G{*}{*} have been used to evaluate the hyperconjugation effects in the cyclopropylcarbinyl cation (1) and in cyclopropylborane (2), as well as the conjugation effects in the allyl cation (3) and in vinylborane (4). The hyperconjugation (or conjugation) energies have been quantified by ODP in which the critical "vacant" carbocation (or boron) p orbital is "deactivated". Comparisons between the bisected conformations of 1 with 3, and 2 with 4 demonstrate that cyclopropane can be just as effective as a pi-electron donor as a C=C double bond. (C) 1997 Elsevier Science B.V

Application of the orbital deletion procedure (ODP) to planar carbocations

An orbital deletion procedure (ODP), which inactivates a critical pi orbital and therefore interrupts cyclic conjugation, and the nucleus independent chemical shifts (NICS) have been employed to evaluate the aromaticity of the tropylium, cyclopropenyl and benzyl cations, as well as the antiaromaticity of the singlet cyclopentadienyl cation, compared with their reference cations: 1,3,5-heptatrienyl, allyl, 1,3,5-hexatrienyl-3-carbinyl and 1,3,5-pentadienyl. The aromatic stabilization energies, defined as the difference in the delocalization energies (deduced from ODP) between the cyclic and the acyclic reference cation, are in general agreement with the values from appropriate isodesmic equations. (C) 1998 Published by Elsevier Science B.V. All rights reserved

CYCLIC SILYLHYDRAZINES AND THEIR BORANE ADDUCTS

Mitzel NW, HOFMANN M, ANGERMAIER K, SCHIER A, SCHLEYER PV, SCHMIDBAUR H. CYCLIC SILYLHYDRAZINES AND THEIR BORANE ADDUCTS. INORGANIC CHEMISTRY. 1995;34(19):4840-4845.A series of five-, six-, and seven-membered cyclic silylhydrazines have been prepared from the reactions of 1,2-bis(bromosilyl)ethane and 1,3-bis(bromosilyl)propane with 1,1-dimethylhydrazine [leading to 1-(dimethylamino)-1-aza-2,5-disilacyclopentane (3) and 1-(dimethylamino)-1-aza-2,6-disilacyclohexane (4)] and 1,2-dimethylhydrazine [leading to 1,2-dimethyl-1,2-diaza-3,6-disilacyclohexane (7) and 1,2-dimethyl-1,2-diaza-2,7-disilacycloheptane (8)] in the presence of triethylamine, respectively. The compounds with endocyclic Si-N-Si units (3, 4) are found to be stable for long periods of time, while those with Si-N-N-Si units (7, 8) decompose within a few days at ambient temperature. Compounds 3 and 4 have been reacted with the Lewis acid BH3 to give the dimethylamine-borane adducts 5 and 6. All compounds have been fully characterized by spectroscopic data [IR, MS, NMR (H-1, C-13, N-15, Si-29)]. Single crystals of 5 [6] grown from the melt and studied by low-temperature X-ray diffraction analyses are orthorhombic, space group Pbca (No. 61), with a = 11.385(1) [13.300 (1)] Angstrom, b = 9.938(1) [9.837(1)] Angstrom, c = 17.156(1) [16.364(1)] Angstrom, d(calc) = 1.096 (1.081) g cm(-3), and Z = 8 [8]. In both compounds, the BH3 unit is bound to the nonsilylated nitrogen atom, indicating the reduction of the basicity of nitrogen by Si substitution. The silylated nitrogen atoms show planar coordination, while the borylated amine unit is tetrahedrally coordinated. From a comparison of the ring geometries of 5 and 6 with known open-chain structures, it appears that the C2Si2N ring system of 5 is clearly more strained than that of 6 (C3Si2N) This argument also offers an explanation for the preferred formation of the compounds 1,6-diaza-2,5,7,10-tetrasila-[4.4.0]bicyclodecane (1) and bi(1-aza-2,6-disilacyclohexyl) (2) as compared to their isomers with different ring sizes. The relative stabilities of these isomers in question have been quantified by ab initio (MP2(fc)/6-31G*) calculations of geometries and energies of the systems [(CH2)(n)(SiH2)]N-N[(SiH2)(CH2)(n)] versus the annelated molecules (CH2)(n)(SiH2)N-2(CH2)(n)(SiH2) with n = 1-3. These results show the annelated isomers (ring enlarged) to be lower in energy for n = 1 and 2, while for n = 3 the N-N bridged nonannelated isomer is preferred

Synthesis of volatile cyclic silylamines and the molecular structures of two 1-aza-2,5-disilacyclopentane derivatives

Mitzel NW, Schmidbaur H, Rankin DWH, Smart BA, Hofmann M, Schleyer PV. Synthesis of volatile cyclic silylamines and the molecular structures of two 1-aza-2,5-disilacyclopentane derivatives. INORGANIC CHEMISTRY. 1997;36(20):4360-4368.An optimized synthetic procedure for alpha,omega-bis (bromosilyl)alkanes, BrH2Si(CH2)(w)SiH2Br (with n = 2 and 3), is proposed. I,2-Bis(bromosilyl)ethane reacts with ammonia to give 1,4-bis(1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane, traces of 1,6-diaza-2,5,7,10,11,14-hexasilabicyclo[4.4.4]tetradecane and nonvolatile products. The primary reaction products undergo slow redistribution reactions whereby (1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane is formed as the major product. Reactions of alpha,omega-bis(bromosilyl)alkanes, BrH2Si(CH2)(n)SiH2Br (with n = 2 and 3), with isopropylamine afford the heterocycles 1-isopropyl-1-aza-2,5-disilacyclopentane and 1-isopropyl-1-aza-2,6-disilacyclohexane, whereas the analogous reaction with bis(bromosilyl)methane gives 1,5-diisopropyl-1,5-diaza-2,4,6,8-tetrasilacyclooctane rather than a four-membered ring compound. All compounds have been characterized by elemental analysis, mass spectrometry, and IR and NMR spectroscopy [H-1, C-13, N-15 and Si-29 including the measurement of (1)J((SiN)-Si-29-N-15) coupling constants]. The molecular structure of 1-isopropyl-1-aza-2,5-disilacyclopentane, determined by analysis of gas-phase electron-diffraction data augmented by restraints derived from ab initio calculations, is compared with the molecular structure of the isoelectronic 1-(dimethylamino)-1-aza-2,5-disilacyclopentane . The latter also was determined by gas-phase electron-diffraction (Supported by nb initio calculations) and by low-temperature crystallography. The presence of a beta-donor Si...N interaction in the tatter compound, leading to a narrow Si-N-N angle, is apparent from a significant distortion of the molecular structure as compared with the isoelectronic reference compound

Strain and reactivity: Electrophilic addition of bromine and tribromide salts to cyclic allenes

The kinetics and the products of the bromination of several cyclic allenes, from C-9 to C-13 (1a-e), with tetrabutylammonium tribromide (TBAT) and Br-2 have been investigated in 1,2-dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br-2 is a 1:1 pi complex. The stability constant of this complex, determined at 25degreesC for allene 1a, is 7.4m(-1). The comparison of this value with those reported for several alkenes and alkynes further support the hypothesis of the existence of sizeable structural effects on the stability of these complexes. The negative values of the apparent activation energy for the reaction of allenes 1a-e with Br-2 in DCE demonstrate the involvement of these complexes as essential intermediates along the reaction coordinate. Different stereochemical behavior was observed in the bromine addition on going from the strained 1,2-cyclononadiene to the larger compounds. Furthermore, a solvent-dependent stereochemistry has been observed for each compound. The kinetic and product distribution data have been interpreted in terms of the influence of the strain on the nature of the intermediate and by considering the competition between pre-association and ion-pair pathways on going from aprotic to nuclophilic solvents or when nucleophilic bromide ions are added. Ab initio (MP2/6-311 + G**) and density functional (B3LYP/6-311 + G**) computations of 1:1 Br-2 complexes showed that the association energies of allene Br-2 and ethene Br-2 complexes are nearly the same but are greater than that of acetylene Br-2 complexes. Allene 2Br(2) complexes are more stable than their ethene 2Br(2) counterparts. Br-2 allene Br-2 structures, in which the bromine molecules interact either with a single allene double bond or individually with both double bonds, are not preferred significantly over alternatives with (Br2Br2)-Br-... interactions. As a result of the entropy, the association of bromine with unsaturated hydrocarbons is usually unfavorable in the gas phase (except at extremely low temperatures); complexes are observed in solution (under ambient conditions), since the entropy loss is reduced as a result of restricted translation and rotation and possible association to the solvent. The 1,2-cycloheptadiene Br-2 > 1,2-cyclononadiene Br-2 > 1,3-dimethylallene Br-2 association energies increase with ring strain