Synergistic effects in the trimerization of isocyanates by organometallic catalysts

Use was made of combinations of organometallic catalysts, having different modes of interacting with the isocyanate group, to obtain strong synergistic effects in catalysing the trimerization of aromatic, aliphatic and strongly hindered isocyanates. The induction period observed in the trimerization of isocyanates with Group 2 catalysts (Pb, Zr and Co-naphthenates) was eliminated in the presence of Group 1 catalysts (tributyltin oxide and Zr(OBu)₄). Group I catalysts in combination with nucleophiles such as KI also showed strong synergistic effects.     

Novel pentacoordinated bridged silicon anions

Both ab initio and semiempirical electronic structure calculations are used to investigate the molecular and electronic structures and eneregetic stabilities of an unusual bridged compound with the general formula [Y? SiH₃? X? SiH₃? Y]^?, with Y = H or F and X = H, CH₃, NH₂, OH, F, or Cl. Most of these bridged anions are quite stable relative to YSiH₃ + XSiH₃Y^?, and the stability is predicted to increase considerably when Y = H is replaced with Y = F.     

Effect of polar aprotic solvents on the trimerization of aliphatic isocyanates by organometallic catalysts

The polymerization of aliphatic isocyanates by organometallic catalysts such as tributyltin oxide, Ti(OBu)₄ or Zr(OBu)₄ was slow and no enhancement in rate was found on addition of DMF or DMSO, contrary to what was found with PhNCO. However, similar to the latter, fast reactions were observed with catalysts such as Pb- or Co-naphthenate in the presence of DMSO. The results were explained, on the basis of spectral evidence, as due to the weak interaction of DMSO or DMF with the aliphatic isocyanate and formation of a charge transfer complex between them in very low concentration.     

Catalytic synthesis of phosphoryl isocyanates

A general efficient method for the synthesis of phosphoryl isocyanates was developed. The method involves reactions of phosphoryl chlorides with sodium cyanate in the presence of anhydrous magnesium chloride as a catalyst. Dedicated to the Corresponding Member of the Russian Academy of Sciences T. A. Mastryukova. For the preliminary report, see Ref. 1. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2543–2545, November, 2005.     

Investigations providing a plausible mechanism in the hexamethyldisilazane-catalyzed trimerization of alkyl isocyanates

Using HMDS as catalyst for the trimerization of isocyanates presents many advantages as the expected isocyanurate is not contaminated by the catalyst or other side-products resulting from its degradation. In addition, HMDS presents a low toxicity, and is compatible with industrial applications. This article describes the hexamethyldisilazane (HMDS)-catalyzed trimerization of octylisocyanate. Experimental investigations and mechanistic considerations indicate that the true catalyst of the trimerization is trimethylsilyloctylamine, which results from the preliminary condensation of HMDS with octylisocyanate.     

Reactions of silicon hydrides catalyzed by rhodium(III) sulfoxide complexes

Russian Journal of General Chemistry - Dehydrocondensation reactions of silicon hydrides catalyzed by the rhodium(III) complex [RhCl3(Me2SO)3] in the absence of the second substrate were studied....     

Reaction of silicon hydrides with primary aliphatic amines

Conclusions The direction of the reaction of the silicon hydrides: R₂SiH₂, RSiH₃, and with primary aliphatic amines, in the presence of catalysts based on Group VIII compounds, as a function of the structure of the silicon hydride and the nature of the employed catalyst, was established.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2330–2333, October, 1972.     

Ab-initio SCF study of the nature of bonding in neutral and cationic silicon hydrides and analogous carbon compounds

The nature of bonding in a number of neutral and cationic silicon hydrides has been studied on the basis of atomic charge (q_A), valency (V_A), molecular valency (V_M) and localised molecular orbitals (LMO). Several structural isomers of each species have been considered. It is found that the most stable isomer is generally associated with maximum q_si and minimum V_si and V_M. Protonation of the neutral hydrides leads to an increase in the positive charge of Si, and in molecular valency. The bonding characteristics of the most stable isomers of the silicon hydrides have been compared with that of analogous hydrocarbons. It has been observed that the difference in bonding stems mainly from the highly electro-positive nature of the Si atom which prefers to form three-centre rather than multiple bonds. The molecular hardness parameters (η) have also been calculated. The most stable isomers of silicon hydrides are generally associated with maximum η. The HSAB principle has been tested by considering some hydrogenation and proton transfer reactions. It is qualitatively satisfied in both cases.     

Complexes of N-derivatives of silicon hydrides with aluminum chloride

Conclusions A study was made of the reaction of the N-derivatives of silicon hydrides with aluminum chloride, and it was shown that aluminum-containing organosilicon complexes can be obtained from them.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2149–2150, September, 1973.     

The synthesis of alkenylsilanes by the high-temperature condensation of unsaturated compounds with silicon hydrides

Summary The general character of the new high-temperature reaction of unsaturated hydrocarbons with silicon hydrides, leading to the formation of unsaturated organosilicon compounds, has been shown on the basis of new examples.     

Density functional study of small neutral and charged silver cluster hydrides

Small neutral, anionic, and cationic silver cluster hydrides AgnH and anionic HAgnH (n=1-7) have been studied using the PW91PW91 density functional method. It was found that the most stable structure of the AgnH complex (neutral or charged) does not always come from that of the lowest energy bare silver cluster plus an attached H atom. Among various possible adsorption sites, the bridge site is energetically preferred for the cationic and most cases of neutral Agn. For anionic Agn, the top site is preferred for smaller Agn within n相似文献   

Gas-phase structures of neutral silicon clusters

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).     

Catalytic conversion of isocyanates to carbodiimides by cyclopentadienyl manganese tricarbonyl and cyclopentadienyl iron dicarbonyl dimer

Isocyanates are catalytically converted to the corresponding carbodiimides in the presence of low valent transition metal complexes. The best conversions and yields were accomplished using [(C₅R₅)Fe(CO)₂]₂ (R = H or Me) or MeCpMn(CO)₃ as catalysts.     

Reactions of trimethylgallium with silicon hydrides,germane and diborane

Methylation of SiH₄, MeSiH₃, Si₂H₆, GeH₄ and B₂H₆, but not of PH₃ or AsH₃, was observed during reaction (230–324°C) with GaMe₃. The products from the SiH₄ and Si₂H₆ reactions were MeSiH₃, Me₂SiH₂ and Me₃SiH. The GeH₄-derived products were similar, with Me₄Ge also being formed. The only methylated products from B₂H₆ was BMe₃. The silane reactions were surface-catalyzed (presumably by surface hydroxyl groups), while those of GeH₄ and B₂H₆ may have occurred via gas-phase free radical processes.     

Utility of trichloroisocyanuric acid in the efficient chlorination of silicon hydrides

The potential of trichloroisocyanuric acid (TCCA) as a chlorination agent for efficient conversion of Si-H functional silanes and siloxanes to the corresponding Si-Cl functional moieties was explored. In comparison to methods using other chlorinating agents, TCCA is inexpensive, results in a much faster reaction and produces a high purity product with a conversion that is essentially quantitative. A variety of chloro derivatives of linear and cyclic structures have been synthesized from silicon hydrides using this reagent with impressive yields that typically exceed 90%: PhSiCl₃ (97.5%); PhMeSiCl₂ (95.5%); Ph₃SiCl (97.5%); Vi₃SiCl (98.7%); (EtO)₃SiCl (99.7%); t-Bu₃SiCl (∼100%); (MeClSiO)₄ (86.5%); (MeClSiO)₅ (95%); (MeClSiO)₇ (96.5%); Ph(OEt)₂SiCl (98%); ClMe₂SiOSiMe₂Cl (98.6%); ClMe₂SiOSiMeClOSiMe₂Cl (94.6%); ClMe₂Si(OSiMeCl)₂OSiMe₂C l (92.3%); (Me₃SiO)₃SiCl (97%); Me₃SiOSiClPhOSiMe₃ (99%); Me₃SiO(SiMeClO)₃SiMe₃ (95.7%); ClSi(OSiMe₃)₂OSi(OSiMe₃) ₂Cl (93.6%).For monohydridosilanes, dichloromethane (CH₂Cl₂) was a suitable solvent in which nearly quantitative conversion was observed within several minutes following the addition of the silanes to TCCA. For certain cyclic and linear siloxanes, and especially silanes containing multiple hydrogen atoms on the same silicon for which the reaction is sluggish in CH₂Cl₂, tetrahydrofuran (THF) was the preferred solvent. For a sterically demanding silane that did not undergo chlorination even in THF viz., HSi(OSiMe₃)₂O-Si(OSiMe₃)₂H, 1,2-dichloroethane was the best solvent.