Theoretical study on the reaction mechanisms of ethylene with Cp2TiR, Cp2Ti(Cl)R, and Cp2Ti(Cl:AlH2Cl)R (R=H and CH3)

The potential energy surfaces for reactions of ethylene with Cp₂Ti⁺R, Cp₂Ti(Cl)R, and Cp₂Ti(Cl:AlH₂Cl)R (R=H and CH₃) were calculated by ab initio molecular orbital methods. These six reaction mechanisms were compared. Of the two possible reaction paths, attack of ethylene at Ti and the Cl and R ligands (path IN) and that from the opposite side of the Cl ligand (path OUT), the former is found to be more favorable, with a very low activation energy for reaction of ethylene with Cp₂Ti(Cl)H. For reaction of ethylene with Cp₂Ti(Cl)CH₃, the insertion transition states on both paths have almost the same energy barrier height above the reactants. For reaction of ethylene with Cp₂Ti(Cl:AlH₂Cl)R, the bond alternation between Ti–Cl and Cl–Al plays an important role in the mechanisms.     

Selective formation of bis(diphenylphosphino)-methanido- or -phosphido-bridged dihydridoruthenium-rhodium complexes in reactions of RuRhH2Cl(C8H12)(Ph2PCH2PPh2)2 with trialkylborohydrides as a function of solvent and reagent basicity

RuRhH₂Cl(COD)(dppm)₂ (1) (COD = 1,5-C₈H₁₂, DPPM = Ph₂PCH₂PPh₂) does not react with LiBHEt₃ in THF, but in toluene RuRhH₂(PhPCH₂PPh₂)(dppm)(COD) (3) is formed quantitatively. In contrast, the reaction of 1 with KBH(sec-Bu)₃ in THF selectively gives RuRhH₂(CH(PPh₂)₂)(dppm)(COD) (4), whereas in toluene a mixture of 3 and 4 is obtained. Mechanisms are proposed for these reactions. The X-ray crystal structure of 4 · PhCH₃ reveals direct coordination of a methanido group to rhodium and a long metal-metal distance (3.3006(6) Å).     

Cyclopropanation of alkenes with CH2I2/Et3Al by the phase-vanishing method based on fluorous phase screen

Phase-vanishing (PV) method using perfluorohexanes as a screen phase was applied to cyclopropanation reactions with CH₂I₂/Et₂Zn and CH₂I₂/Et₃Al. When Et₃Al was used as a carbenoid generator, the reaction proceeded smoothly and desired cyclopropane derivatives were obtained in high yield. The PV cyclopropanation took 2 or 3 days to complete, however, reduction of reaction time by a factor of 2-3 was also achieved by vigorous stirring after the bottom CH₂I₂ layer disappeared.     

On the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions

An investigation into the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C(1) centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C(1) centre of R configuration. The Lewis base group directs the cyclization of carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C(1) centre in carbenoid 2. This relationship of the stereochemistry to the chiral character of the carbenoid has implications for the design of new efficient carbenoid reagents for stereoselective cyclopropanation.     

Soluble Ni alkoxides based on dimethylaminoisopropoxide ligands: molecular structure of [Li(PrOH)Ni (η-OR)2Cl]2 and of cis-NiCl2(ROH)2 (R = CHMeCH2NMe2)

Various routes to Ni^II aminoalkoxides have been investigated. A nickel isopropoxide derivative 1 was prepared by anodic dissolution of the metal in the presence of LiCl as electrolyte. Alcoholysis reactions of 1 with 1-dimethylamino-2-propanol afforded the homoleptic nickel(II) aminoalkoxide 2 together with a Ni---Li species 3. 2 was also obtained by metathesis reactions between sodium alkoxide and the nickel hexammine complex whereas the reaction between the latter and the aminoalcohol led to an halide solvate, cis-NiCl₂(η²-ROH)₂, 4. The various compounds were characterized by elemental analysis, FT-IR and UV-vis spectroscopies as well as by X-ray diffraction for 3 and 4. 3 corresponds to [Li(PrⁱOH)Ni(η²-OCHMeCH₂NMe₂)Cl]₂ and the overall structure can be seen as two [Ni(η²-OCHMeCH₂NMe₂)₂Cl]⁻ moieties assembled by Li(PrⁱOH)⁺. The lithium atom is 4-coordinate due to its interaction with the oxygen atoms of the aminoalkoxide ligands. Nickel is 5-coordinate with a distorted tetragonal pyramidal stereochemistry, one nitrogen being in the apical position. The metal displays a distorted octahedral surrounding for the NiCl₂ adduct 4. The bond distances vary in the order Ni---OR < Ni---N ≈ Ni---O(H)R < Ni---Cl for 3 and 4. The various compounds (except 1) are soluble in organic media.     

A density functional theory investigation of the Simmons-Smith cyclopropanation reaction: examination of the insertion reaction of zinc into the C-I bond of CH(2)I(2) and subsequent cyclopropanation reactions.

The insertion reaction of zinc into the C-I bond of CH(2)I(2) and subsequent cyclopropanation reactions with CH(2)CH(2) have been investigated using B3LYP level density functional theory calculations. The Simmons-Smith cyclopropanation reaction of olefins does not proceed easily due to the relatively large barriers on the insertion and cyclopropanation pathways. The computed results indicate that the IZnCH(2)I molecule is the active reagent in the Simmons-Smith reaction. This is consistent with the IZnCH(2)I reactive species being generated from diiodomethane and a Zn-Cu couple as proposed by several other research groups. The Simmons-Smith IZnCH(2)I carbenoid and CH(2)I-I carbenoid cyclopropanation reactions with olefins are compared. The reactions of olefins with the radicals from the decomposition of the IZnCH(2)I and CH(2)I-I species were also compared. We found that the chemical reactivity of the carbenoid species is dependent on its electrophilic behavior, steric effects, the leaving group character and the mechanism of the cyclopropanation reactions.     

Theoretical studies of samarium carbenoid promoted cyclopropanation reaction with allylic alcohol on the reaction pathways

The density functional theory was employed to investigate the mechanism for the cyclopropanation reactions of samarium carbenoid with an allylic alcohol. Seven competitive reaction pathways were investigated. Analysis of the calculated results shows that the models 4 and 6 have relatively low reaction barriers which suggested that the deprotonation of allylic alcohol promoted by CH₃SmCH₂I plays a significant important role in the cyclopropanation reaction via a samarium carbenoid. The methylene transfer and carbometalation pathways are involved in both intermolecular and intramolecular reaction pathways. On the basis of the energetics of the reaction pathways, the methylene transfer pathway is favored over the carbometalation pathway in the whole reactions. Our computational results are in good agreement with the experimental results performed by G.A. Molander and L.S. Harring.     

Anchimeric assistance by γ-substituents Z, Z=MeO, PhO, MeS or PhS, in reactions of the bromides (Me3Si)2(ZMe2Si)CSiMe2Br with AgBF4

The new organosilicon bromides (Me₃Si)₂(ZMe₂Si)CSiMe₂Br with Z=PhO or MeS have been prepared and new spectroscopic data obtained for the previously reported compounds with Z=H, F, Br, Me, Ph, MeO or PhS. Competitions between pairs of bromides for a deficiency of AgBF₄ in Et₂O, with the determination of the ratio of the fluoride products by ¹⁹F-NMR spectroscopy, have led to the following approximate relative reactivities of the bromides and so to the relative abilities of the γ-Z groups to provide anchimeric assistance to the leaving of Br⁻ in this reaction: Me, 1; Ph, 40; PhO, 3400; PhS, 5000; MeS, 7000; MeO, 54 000. In methanolysis in CH₂Cl₂, (Me₃Si)₂(MeOMe₂Si)CSiMe₂Cl has been found to be roughly 120 times as reactive as (Me₃Si)₂(PhOMe₂Si)CSiMe₂Cl. Combination of the results with previously available information suggests the following approximate order of ability of γ-groups Z to provide anchimeric assistance in reactions at the Si---X bonds in compounds (Me₃Si)₂(ZMe₂Si)CSiMe₂X: OCOMe>OMe>OCOCF₃>MeS>PhS, PhO>N₃, Cl>NCS>Ph>CH=CH₂>Me.     

Absolute dipole differential oscillator strengths for inner shell spectra from high resolution electron energy loss studies of the freon molecules CF4, CF3Cl, CF2Cl2, CFCl3 and CCl4

Absolute differential photoabsorption oscillator strengths (cross sections) for the F 1s, C 1s, and Cl 2p, 2s inner shells of the freon molecules CF₃Cl, CF₂Cl₂ and CFCl₃ have been derived from high resolution electron energy loss spectra obtained under dipole dominated conditions of high impact energy (3 keV) and zero degree mean scattering angle. Differential oscillator strengths have also been obtained from earlier reported electron energy loss spectra for CF₄ and CCl₄. The spectra are analyzed using the MO picture and the potential barrier model.     

Theoretical study of samarium (II) carbenoid (ISmCH2I) promoted cyclopropanation reactions with ethylene and the effect of THF solvent on the reaction pathways

A computational study of the cyclopropanation reactions of divalent samarium carbenoid ISmCH(2)I with ethylene is presented. The reaction proceeds through two competing pathways: methylene transfer and carbometalation. The ISmCH(2)I species was found to have a "samarium carbene complex" character with properties similar to previously investigated lithium carbenoids (LiCH(2)X where X = Cl, Br, I). The ISmCH(2)I carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons-Smith (IZnCH(2)I) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The ISmCH(2)I/(THF)(n)() (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10.7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (>15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species.     

Spectroscopic modifications of Pippard relations: first-order and second-order phase transitions in NH4Cl

In this study we introduce the spectroscopic modifications of Pippard relations and apply them to the disorder-induced Raman modes of NH₄Cl in the first-order (P=0) and second-order (2.8 kbar) phase regions in this crystalline system. We obtain linear variations of the specific heat C_P with our observed frequency shifts [(1/ν)(∂ν/∂T)_P] of those Raman modes studied for the first-order and second-order phase transitions in NH₄Cl. This will be discussed in detail.     

钐(II)类卡宾CH_3SmCH_2I与乙烯环丙烷化反应及溶剂化效应(THF)对反应途径影响的理论研究

用密度泛函B3LYP方法研究了过渡金属钐类卡宾与乙烯的环丙烷化反应的机理.对钐类卡宾试剂CH3SmCH2I和CH2CH2反应的反应物、中间体、过渡态和产物构型的全部结构几何参数进行了优化,并计算了THF溶液的溶剂化效应,用内禀反应坐标(IRC)计算和频率分析方法,对过渡态进行了验证.结果表明:CH3SmCH2I与CH2CH2环丙烷化反应按亚甲基转移机理(通道A)和卡宾金属化机理(通道B)都可以进行,与锂类卡宾的反应机理相同,通道A比通道B反应的势垒降低了14.65kJ/mol.溶剂化效应使通道B比通道A的反应势垒大幅度提高,更有利于反应沿通道A进行,而不利于通道B.     

Arylkomplexe des typs trans-[Ir(Ar)(CO)(PPh3)2]

The preparation and properties as well as some reactions of a series of arylcarbonylbis(triphenylphosphine)iridium(I) complexes [Ir(Ar)(CO)(PPh₃)₂] (Ar = C₆H₅, C₆F₅, 2-C₆H₄CH₃, 3-C₆H₄CH₃, 4-C₆H₄CH₃, 2-C₆H₄OCH₃, 2,6-C₆H₃-(OCH₃)₂, 4-C₆H₄N(CH₃)₂, 3-C₆H₄Cl, 4-C₆H₄Cl, 4-C₆H₄Cl, 3-C₆H₄CF₃, 4-C₆H₄CF₃) are described, and the most important IR data as well as the ³¹P NMR parameters of these, without exception trans-planar, compounds are given.

Some of the complexes react with molecular oxygen to form well defined dioxygen adducts [Ir(Ar)(O₂)(CO)(PPh₃)₂] (Ar = C₆H₅, 3-C₆H₄CH₃, 4-C₆H₄CH₃). Complexes with ortho-substituted aryl ligands are not oxygenated. This effect is referred to as a steric shielding of the metal center by the corresponding ortho-substituents. With SO₂ the similar irreversible addition compound [Ir(4-C₆H₄CH₃)-(SO₂)(CO)(PPh₃)₂] is obtained. Sulfur dioxide insertion into the Ir---C bond cannot be observed.

The first step of the reaction between [Ir(4-C₆H₄CH₃)(CO)(PPh₃)₂] and hydrogen chloride involves an oxidative addition of HCl to give [Ir(H)(Cl)(4-C₆-H₄CH₃)(CO)(PPh₃)₂]. Ir---C bond cleavage by reductive elimination of toluene from the primary adduct does not occur except at elevated temperature.     

Theoretical study of the elimination kinetics of several 2-substituted ethyl N,N-dimethylcarbamates in the gas phase, [(CH3)2NCOOCH2CH2Z, Z=CH2Cl, C≡CH, C≡N]

The theoretical studies of the gas-phase elimination of 2-substituted ethyl N,N-dimethylcarbamates (Z=CH₂Cl, C≡CH, C≡N) were performed using ab initio MP2/6-31G and MP2/6-31G(d) levels of theory. The gas phase elimination reaction of these carbamates yields N,N-dimethylcarbamic acid and the corresponding substituted olefin in a rate-determining step. The intermediate N,N-dimethylcarbamic acid is unstable and rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO₂ gas. The results of these calculations suggest a mechanism to be concerted, asynchronous, and a six-membered cyclic transition state structure. Plotting the relative theoretical rate coefficients against Taft's σ* values gave an approximate straight line (ρ*=0.4057, r=0.9894 at 360 °C). The correlation between experimental log k_rel vs. theoretical log k_rel. for these 2-substituted ethyl N,N-dimethylcarbamates gave an approximate straight line (r=0.9715 at 360 °C), suggesting the same type of mechanism.     

Samarium(III) carbenoid as a competing reactive species in samarium-promoted cyclopropanation reactions

The trivalent samarium carbenoid I2SmCH2I-promoted cyclopropanation reactions with ethylene have been investigated and are predicted to be highly reactive, similarly to the divalent samarium carbenoid ISmCH2I. The methylene transfer and carbometalation pathways were explored and compared with and without coordination of THF solvent molecules to the carbenoid. The methylene transfer was found to be favored, with the barrier to reaction going from 12.9 to 9.2 kcal/mol compared to barriers of 15.4-17.5 kcal/mol for the carbometalation pathway upon the addition of one THF molecule.     

[Ir(3,4,7,8-Me4phen) (CH2=CH2)2Cl]: A very active catalyst precursor for hydrogen transfer from alcohols to unsaturated organic substrates

Some new complexes of the type Ir¹chel(CH₂=CH₂)₂Cl (chel = bipy; 4,4′-Me₂bipy; 4,4′-Ph₂bipy; phen; 5,6-Me₂phen; 4,7-Ph₂phen; 3,4,7,8-Me₄phen) behave as catalyst precursors for the hydrogen transfer from alcohols to ketones and Schiff bases. The most active of the complexes is the 3,4,7,8-Me₄phen derivative, which, at 83°C, gave turnovers of up to 2850 cycles/min with cyclohexanone, 2700 cycles/min with 4-tert-butylcyclohexanone and 5000 cycles/min with benzylidenaniline, at a catalyst concentration of 4 X 10⁻⁵ M and a KOH (cocatalyst) concentration of 8 X 10⁻⁴ M. Good catalytic activity was observed also at room temperature. Some catalytic activity was found at low substrate concentrations, even in the absence of KOH. The maximum stereoselectivity reached in the reduction of 4-tertbutylcyclohexanone was 97%, the trans-alcohol being formed.     

Organopolysilane oligomers bearing transition-metal moieties: Synthesis, reactivities and molecular structure of [η-Me3SiMe2SiC5H4(CO)Fe(CO)2Fe(CO)-η-C5H4SiMe2SiME3]

Reaction of Me₃SiMe₂SiC₅H₅ (4), prepared from Me₃SiMe₂SiCl and C₅H₅Na, with Fe(CO)₅ in refluxing xylene afforded the title compound (3). The silicon-silicon bond in 3 is exceptionally stable in refluxing xylene and also in succeeding reactions to prepare a series of its derivatives. Thus, 3 reacted with I₂ in either chloroform or benzene, giving [η⁵-Me₃SiMe₂SiC₅H₄Fe(CO)₂I] (6). Compound 3 was reduced by sodium amalgam and reacted subsequently with CH₃I, PhCH₂Cl, CH₃COCl, PhCOCl, Cy₃SnCl (Cy = cyclohexyl) and Ph₃SnCl, producing [η⁵-Me₃SiMe₂SiC₅H₄Fe(CO)₂R][7 : R = CH₃ (a), PhCH₂ (b), CH₃CO (c), PhCO (d), Cy₃Sn (e) and Ph₃Sn (f), respectively]. The molecular structure of 3 has been determined by X-ray diffraction crystallography. It was found that 3 has a trans-configuration with a symmetrical centre located at the middle of the Fe---Fe bond. It is abnormal that the conformation of the disilane part around the Si---Si bond is almost eclipsed rather than staggered.     

Tert-butylbenzamidate diruthenium(II, III) compounds. Crystal structure of [Ru2(μ-HNOCC6H4-p-CMe3)4(OPPh3)2]BF4

The reaction between Ru₂Cl(μ-O₂CCH₃)₄ and molten p-tert-butylbenzamide led to the formation of Ru₂Cl(μ-HNOCC₆H₄-p-CMe₃)₄. The polymeric structure of this insoluble compound was broken with AgBF₄, in anhydrous thf, giving [Ru₂(μ-HNOCC₆ H₄-p-CMe₃)₄(thf)₂]BF₄. The reaction of this cationic complex with OPPh₃ gave [Ru₂(μ-HNOCC₆H₄-p-CMe₃)₄(OPPh₃)₂]BF₄. The compounds have been characterized by elemental analysis, spectroscopic data and magnetic measurements and the crystal structure of [Ru₂(μ-HNOCC₆H₄-p-CMe₃)₄(OPPh₃)₂]BF₄ was determined by X-ray crystallography. The asymmetric unit is composed of halves of two different crystallographically independent centrosymmetric cations. Each ruthenium(II,III) dimer is bonded to four bridging p-tert-butylbenzamidate ligands and to two axial triphenylphosphine oxide molecules. The Ru---Ru distances in the two dimeric cations of the unit cell are 2.281(2) and 2.280(2) Å. The compound has a non-polar 2 : 2 arrangement of the tert-butylbenzamidate ligands.     

钐(II)类卡宾CH3SmCH2I与乙烯环丙烷化反应及溶剂化效应的理论研究

用密度泛函B3LYP方法研究了过渡金属钐类卡宾与乙烯的环丙烷化反应的机理. 对钐类卡宾试剂CH₃SmCH₂I和CH₂CH₂反应的反应物、中间体、过渡态和产物构型的全部结构几何参数进行了优化, 并计算了THF溶液的溶剂化效应, 用内禀反应坐标(IRC)计算和频率分析方法, 对过渡态进行了验证. 结果表明: CH₃SmCH₂I与CH₂CH₂环丙烷化反应按亚甲基转移机理(通道A)和卡宾金属化机理(通道B)都可以进行, 与锂类卡宾的反应机理相同, 通道A比通道B反应的势垒降低了14.65 kJ/mol. 溶剂化效应使通道B比通道A的反应势垒大幅度提高, 更有利于反应沿通道A进行, 而不利于通道B.     

Matrix isolation and theoretical study of the photochemical reaction of CH3CN with CrO2Cl2 and OVCl3

The matrix isolation technique has been combined with theoretical calculations to identify and characterize the photoproducts in the reactions of CH₃CN with CrCl₂O₂ and OVCl₃. Twin jet co-deposition of these reagents led to the formation of a 1:1 molecular complex which was observed using UV/visible spectroscopy. Irradiation of these matrices with light of λ>300 nm led to the observation of new bands in the infrared spectra, the most intense of which was seen at 1942 cm⁻¹ for the CrCl₂O₂/CH₃CN system. The product bands are assigned to the ²η complexes of acetonitrile n-oxide with CrCl₂O and VCl₃, respectively. Identification of these species was supported by extensive isotopic labeling (²H and ¹⁵N), as well as by B3LYP/6-311++G(d,2p) density functional calculations.