From lithium ketazides to isomeric silylketazine-rings - imine-enamine tautomerism

Di(tert-butylmethyl)ketazine (I) reacts with n-BuLi in a 1:1 molar ratio to give a monolithium salt (II). The reaction of II with ^tBu₂SiF₂ in n-hexane leads, even in a 1:1 molar ratio, to the formation of the isomeric five- and four-membered ring compounds 1 and 2. Compound 1 has an endocyclic imine and an exocyclic enamine unit. The opposite is found for 2. The acyclic monosubstitution product, ^tBu₂SiFCH₂-C^tBuN-NC^tBuCH₃ (III) could not be isolated. It reacts with the lithium ketazide to give 1 or 2. I is reformed. The reaction in THF yields only the four-membered ring 2. In a comparable reaction of the lithium ketazide and (H₃C)₂SiF₂, the substitution product 3 could be isolated. A possible formation mechanism for 2 includes an intermediate silene IV. Both compounds 1 and 2 react with H₃C-OH under cleavage of the endocyclic Si-N-bond to give the addition product 5. The reaction mechanism includes a hydrogen shift from a nitrogen atom to a carbon atom via an imine-enamine tautomerism. In a 2:1 molar ratio, n-BuLi and the di(tert-butylmethyl)-ketazine (I) form the dilithium salt, 6. Compound 6 crystallizes from THF as trimer with four imine and two enamine units. A seven-membered ring (7) isomeric to 1 and 2 is the result of the reaction of 6 with ^tBu₂SiF₂. Compound 7 contains one imine and one enamine unit in the ring skeleton.The comparable reaction of the (CH₃)₃Si-substituted dilithium-di(tert-butylmethyl)ketazide and ^tBu₂SiF₂ yields the five-membered ring compound 8 with one endocyclic imine and one exocyclic enamine unit.Quantum chemical calculations of 1, 2, 7 and the intermediate silene IV have been carried out and show a low energy difference between the cyclic silyl-ketazine isomers.     

Mono- and bis(hydroxylamino)silanes – synthesis, isomerisation and quantum chemical calculations

Silylhydroxylamines can undergo anionic, neutral and thermal rearrangements. Lithium derivatives of silylhydroxylamines have been used for more than 30 years in such synthesis. They are formed by the reaction of N,O-bis(silyl)hydroxylamines with n-butyl-lithium and crystallize as O-lithium-N,N-bis(silyl)hydroxylamides under silyl group migration from the oxygen to the nitrogen atom. Depending on the reaction conditions and the bulkiness of the substituents, dimeric, trimeric and tetrameric oligomers are isolated. Lithium is bonded end on to the oxygen atom in the dimeric and trimeric silylhydroxylamides and side on to the N-O bond in the tetrameric oligomer.Fluorofunctional bis(silyl)hydroxylamines are excellent precursors for ring systems. In the reactions of dihalosilanes and hydroxylamine the first bis(hydroxylamino)silanes, R₂Si(O-NH₂)₂, areobtained.O-Fluorosilyl- and O-stannyl-N,N-bis(trialkylsilyl)hydroxylamines undergo irreversible dyotropic rearrangements to N-fluorosilyl-N,O-bis(trialkylsilyl)hydroxylamines and N-stannyl-N,O-bis(trialkylsilyl)hydroxylamines, respectively. Thermal rearrangement of tris(silyl)hydroxylamines leads to the formation of silylaminodisiloxanes.Quantum chemical calculations for model compounds demonstrate the course of the dyotropic and thermal rearrangements. The results of these calculations allow the prediction of the resulting isomeric silylaminodisiloxane.     

State-selected dynamics of the complex-forming bimolecular reaction Cl- +CH3 Cl'-->ClCH3+Cl'-: a four-dimensional quantum scattering study

Time-independent quantum scattering calculations have been carried out on the Walden inversion S(N)2 reaction Cl(-)+CH(3)Cl(')(v(1),v(2),v(3))-->ClCH(3)(v(1) ('),v(2) ('),v(3) ('))+Cl('-). The two C-Cl stretching modes (quantum numbers v(3) and v(3) (')) and the totally symmetric internal modes of the methyl group (C-H stretching vibration, v(1) and v(1) ('), and inversion bending vibration, v(2) and v(2) (')) are treated explicitly. A four-dimensional coupled cluster potential energy surface is employed. The scattering problem is formulated in hyperspherical coordinates using the exact Hamiltonian and exploiting the full symmetry of the problem. Converged state-selected reaction probabilities and product distributions have been calculated up to 6100 cm(-1) above the vibrational ground state of CH(3)Cl, i.e., up to initial vibrational excitation (2,0,0). In order to extract all scattering resonances, the energetic grid was chosen to be very fine, partly down to a resolution of 10(-12) cm(-1). Up to 2500 cm(-1) translational energy, initial excitation of the umbrella bending vibration, (0,1,0), is more efficient for reaction than exciting the C-Cl stretching mode, (0,0,1). The combined excitation of both vibrations results in a synergic effect, i.e., a considerably higher reaction probability than expected from the sum of both independent excitations, even higher than (0,0,2) up to 1500 cm(-1) translational energy. Product distributions show that the umbrella mode is strongly coupled to the C-Cl stretching mode and cannot be treated as a spectator mode. The reaction probability rises almost linearly with increasing initial excitation of the umbrella bending mode. The effect with respect to the C-Cl stretch is five times larger for more than two quanta in this mode, and in agreement with previous work saturation is found. Exciting the high-frequency C-H stretching mode, (1,0,0), yields a large increase for small energies [more than two orders of magnitude larger than (0,0,0)], while for translational energies higher than 2000 cm(-1), it becomes a pure spectator mode. For combined initial excitations including the symmetric C-H stretch, the spectator character of the latter is even more pronounced. However, up to more than 1500 cm(-1) translational energy, the C-H vibration does not behave adiabatically during the course of reaction, because only 20% of the initial energy is found in the same mode of the product molecule. The distribution of resonance widths and peak heights is discussed, and it is found that individual resonances pertinent to intermediate complexes Cl(-)...CH(3)Cl show product distributions independent of the initial vibrational state of the reactant molecule. The relatively high reactivity, of resonance states with respect to excitation of any mode, found in previous work is confirmed in the present calculations. However, reactivity of intermediate states and reactivity with respect to initial vibrational excitation have to be distinguished. There is a strong mixing between the vibrational states reflected in numerous avoided crossings of the hyperspherical adiabatic curves.     

Cyclization and Isomerization Reactionsin Silylhydrazine Chemistry

Summary.  This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring expansion of the three-membered Si(SiN₂) to the four-membered (SiN)₂ ring by lithiation of the (SiN₂) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN₂) to a five-membered (CSi₂N₂) ring by SiCH₂ insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN₂)₂ rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N₂Si₂N)N ring to the isomeric six-membered (SiNN)₂ ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by crystal structure determinations. Received February 23, 2001. Accepted (revised) April 24, 2001     

Temperature variation of the magnetic cluster structures in an iron-nickel invar alloy

Small-angle scattering of long wavelength neutrons $\text{(λ = 6.42}\text{A}\text{?}\text{)}$ from an Fe₆₅Ni₃₅ single crystal has been measured with the applied magnetic field (6.2 kG) parallel and perpendicular to the scattering vector $\text{K}$ of the elastic scattering over the temperature range from 25 to 422°C (T_c = 227°C). The scattering cross sections due to the longitudinal spin fluctuation have been analyzed by means of Guinier's approximation $\text{(dσ/dω)}{}_0\text{exp}\text{(?κ}{}^2\text{R}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{3}\text{)}$, where the forward cross section (dσ/dω)₀ is proportional to n, which is the number of atoms in a paramagnetic cluster, and R_g is the radius of gyration of the cluster. The empirical relation between n and R_g is = 0.298 × R_g^2.34 to be compared with that calculated for a simple spherical cluster model n = 1.274 R_g³.     

Determination of the origin of stereoselectivity in multiple-transition-state reactions using DFT calculations: enantioselective synthesis of homoallylic alcohols from aliphatic methyl ketones via an auxiliary-mediated allylation

Computational investigations on the highly stereoselective allylation of butanone in the presence of a chiral norpseudoephedrine-derived auxiliary have been performed. They suggest an SN1-type mechanism via the attack of allyltrimethylsilane to an intermediately formed oxocarbenium ion. The identification of preferred transition states (TSs) leads to a straightforward rationalization of the observed selectivity which can be extended to analogues of the auxiliary. A screening process has been devised to select 61 potentially relevant TSs from a total of almost 300 theoretically possible TSs. Final results were obtained from gas-phase calculations employing the B3LYP/6-31+G(d) level of theory as well as in dichloromethane solution using the B3LYP/6-311++G(2d,p)//B3LYP/6-31+G(d) level of theory in combination with polarizable continuum model and the UAKS set of radii. The agreement of theoretically predicted and experimentally observed selectivities is very good in both cases. However, the relative energy differences for several relevant TSs differ significantly when going from gas phase to solution, thus illustrating the necessity of performing calculations in solution to draw correct conclusions.     

KMgF3:Ni2+晶体的占位、局部结构、吸收光谱和顺磁g因子的统一解释

利用完全对角化方法和强场耦合方案,采用半自洽场(semi-SCF)自由Ni2+的d轨道模型和Ni2+-6X-(x=F,Cl,Br,I)络合物的μ-κ-α模型研究,建立了含有过渡族金属离子的晶体的局域结构与吸收光谱和顺磁g因子之间的定量关系,对KMgF3:Ni2+晶体的占位、局域结构、吸收光谱和顺磁g因子作出了统一解释,预测了KMgF3:Ni2+晶体的光谱精细结构.所得理论计算结果与实验值符合得很好.