- Reactions of alkenylruthenium(II) complexes with hydrosilane: C-Si vs C-H bond formation
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Alkenylruthenium complexes, Ru{C(R1)=CH(R2)}Cl(CO)-(PPh3)2 (R1 = H, R2 = Ph; R1 = H, R2 = t-Bu; R1 = Ph, R2 = Ph; R1 = CH=CH(SiMe3), R2 = SiMe2Ph), react with HSiMe2Ph via two reaction courses (path A and path B), leading to C-Si and C-H bond formation, respectively. Relative ratio of the two courses is strongly dependent upon steric bulkiness of substituent(s) on the alkenyl ligands.
- Maruyama, Yooichiroh,Yamamura, Kunihiro,Ozawa, Fumiyuki
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Read Online
- Intermolecular chemistry of a cyclopropylcarbene and its mechanistic implications
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Trans-3-(2-tert-butylcyclopropyl)-3H-diazirine was decomposed both thermally (100°C) and photochemically (350 nm, -25 to 25°C) to give the anticipated ring-expanded 3-tert-butylcyclobutene product (50% photochemical, 64% thermal), along with azine and products of trapping by solvent. In the presence of tetramethylethylene (TME), a bicyclopropyl adduct was formed in yields as high as 37% (thermal) or 32% (photochemical). The yield of 3-tert-butylcyclobutene product, however, is only very slightly (0-7%) decreased upon increasing the concentratin of TME. Similar results were obtained with propylamine as the carbene trapping agent. The response of the product mixture to changes in the concentration of the trapping agent shows that there are two product-forming pathways. The mechanistic implications of these observations are discussed.
- Huang, Haiyong,Platz, Matthew S.
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Read Online
- The Application of 1,2-Oxazinanes as Chiral Cyclic Weinreb Amide-Type Auxiliaries Leading to a Three-Component, One-Pot Reaction
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1,2-Oxazines were synthesised via a copper-catalysed aerobic acyl nitroso Diels-Alder reaction from 1,4-disubstituted 1,3-dienes and N-Boc-hydroxylamine. From this, 1,2-oxazinanes were obtained in a novel follow-up reaction path. The stability of several 1,2-oxazines and 1,2-oxazinanes towards organometallic compounds was tested to rate their operability as cyclic chiral Weinreb amide auxiliaries. 3,6-Di-tertbutyl-1,2-oxazinane gave the best results and was introduced as a chiral Weinreb amide-type auxiliary to yield chiral α-substituted ketones in a diastereomeric ratio of up to 98:2. The removal of the auxiliary can be performed with BuLi to form unsymmetrical α-chiral ketones. Thereafter, the chiral auxiliary can be re-isolated and purified by sublimation under vacuum.
- F?hrmann, Jan,Hermann, Ludmila,Hilt, Gerhard
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- Deoxygenation of Epoxides with Carbon Monoxide
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The use of carbon monoxide as a direct reducing agent for the deoxygenation of terminal and internal epoxides to the respective olefins is presented. This reaction is homogeneously catalyzed by a carbonyl pincer-iridium(I) complex in combination with a Lewis acid co-catalyst to achieve a pre-activation of the epoxide substrate, as well as the elimination of CO2 from a γ-2-iridabutyrolactone intermediate. Especially terminal alkyl epoxides react smoothly and without significant isomerization to the internal olefins under CO atmosphere in benzene or toluene at 80–120 °C. Detailed investigations reveal a substrate-dependent change in the mechanism for the epoxide C?O bond activation between an oxidative addition under retention of the configuration and an SN2 reaction that leads to an inversion of the configuration.
- Maulbetsch, Theo,Jürgens, Eva,Kunz, Doris
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p. 10634 - 10640
(2020/07/30)
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- Carboxylate-Assisted β-(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by a Zwitterionic Bis-NHC Rhodium(III) Complex
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The zwitterionic compound [Cp*RhCl{(MeIm)2CHCOO}] is an efficient catalyst for the hydrosilylation of terminal alkynes with excellent regio- and stereoselectivity toward the less thermodynamically stable β-(Z)-vinylsilane isomer under mild reaction conditions. A broad range of linear 1-alkynes, cycloalkyl acetylenes, and aromatic alkynes undergo the hydrosilylation with HSiMe2Ph to afford the corresponding β-(Z)-vinylsilanes in quantitative yields in short reaction times. The reaction of aliphatic alkynes with HSiEt3 is slower, resulting in a slight decrease of selectivity toward the β-(Z)-vinylsilane product, which is still greater than 90%. However, a significant selectivity decrease is observed in the hydrosilylation of aromatic alkynes because of the β-(Z) → β-(E) vinylsilane isomerization. Moreover, the hydrosilylation of bulky alkynes, such as t-Bu-CCH or Et3SiCCH, is unselective. Experimental evidence suggests that the carboxylate function plays a key role in the reaction mechanism, which has been validated by means of density functional theory calculations, as well as by mass spectrometry and labeling studies. On the basis of previous results, we propose an ionic outer-sphere mechanism pathway in which the carboxylate fragment acts as a silyl carrier. Namely, the hydrosilylation mechanism entails the heterolytic activation of the hydrosilane assisted by the carboxylate function to give the hydrido intermediate [Cp*RhH{(MeIm)2CHCOO-SiR3}]+. The transference of the silylium moiety from the carboxylate to the alkyne results in the formation of a flat β-silyl carbocation intermediate that undergoes a hydride transfer from the Rh(III) center to generate the vinylsilane product. The outstanding β-(Z) selectivity results from the minimization of the steric interaction between the silyl moiety and the ligand system in the hydride transfer transition state.
- Puerta-Oteo, Raquel,Munarriz, Julen,Polo, Víctor,Jiménez, M. Victoria,Pérez-Torrente, Jesús J.
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p. 7367 - 7380
(2020/07/21)
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- A smarter approach to catalysts by design: Combining surface organometallic chemistry on oxide and metal gives selective catalysts for dehydrogenation of 2,3-dimethylbutane
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2,3-dimethylbutane is selectively converted into 2,3-dimethylbutenes at 500 °C under hydrogen or at 390 °C under nitrogen in the presence of bimetallic catalysts Pt-Sn/Li-Al2O3. The high stability of the catalyst along the reaction is obtained by selective modification of the Pt/Li-Al2O3 catalyst using Surface Organometallic Chemistry (SOMC).
- Rouge, Pascal,Garron, Anthony,Norsic, Sébastien,Larabi, Cherif,Merle, Nicolas,Delevoye, Laurent,Gauvin, Regis M.,Szeto, Kai C.,Taoufik, Mostafa
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- Stereoselectivity in a series of 7-alkylbicyclo[3.2.0]hept-2-enes: Experimental and computational perspectives
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Rate constants for overall decomposition (kd) for a series of exo-7-alkylbicyclo[3.2.0]hept-2-enes are relatively invariant. For the alkyl substituents ethyl, propyl, butyl, isopropyl, and t-butyl, the ratio of the rate constant for [1,3] sigmatropic rearrangement to the rate constant for fragmentation, k13/kf, is significantly lower than k13/kf?=?150 observed for exo-7-methylbicyclo[3.2.0]hept-2-ene. Regardless of the size and mass of the alkyl group, the stereoselectivity of the [1,3] carbon migration appears to be quite stable at 80% to 89% suprafacial inversion (si), an observation consistent with conservation of angular momentum but not conservation of orbital symmetry. This global result comports with the phenomenon of “dynamic matching” espoused by Carpenter and collaborators for [1,3] sigmatropic rearrangements in general.
- Leber, Phyllis,Kidder, Katherine,Viray, Don,Dietrich-Peterson, Eric,Fang, Yuan,Davis, Alexander
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- Bifunctional Catalysts Based on Tungsten Hydrides Supported on Silicated Alumina for the Direct Production of 2,3-Dimethylbutenes and Neohexene from Isobutene
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Well-defined bifunctional supported catalysts that comprise tungsten hydride moieties and Br?nsted acid sites were prepared successfully. The catalysts showed outstanding activities and selectivities toward the formation of high-value-added products, 2,3-dimethylbutenes and 3,3-dimethylbutene, through a combination of the metathesis and dimerization of isobutene. The relationship between the physicochemical properties of the catalysts and their activities and selectivities indicated that isobutene conversion increased from 4 to 95 % as a function of the silica content of the silicated alumina (obtained from Sasol). Nevertheless, the selectivity toward branched hexenes showed a volcano-shaped curve that presented a maximum for the catalyst with 5 wt % silica. Therefore, the control of the support acidity by the silica loading on alumina resulted in an increase of the selectivity toward neohexene.
- Larabi, Cherif,Garron, Anthony,Rouge, Pascal,Szeto, Kai C.,Norsic, Sébastien,De Mallmann, Aimery,Merle, Nicolas,Taoufik, Mostafa
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p. 2160 - 2166
(2017/06/27)
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- Hydrosilylation of Terminal Alkynes Catalyzed by a ONO-Pincer Iridium(III) Hydride Compound: Mechanistic Insights into the Hydrosilylation and Dehydrogenative Silylation Catalysis
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The catalytic activity in the hydrosilylation of terminal alkynes by the unsaturated hydrido iridium(III) compound [IrH(κ3-hqca)(coe)] (1), which contains the rigid asymmetrical dianionic ONO pincer ligand 8-oxidoquinoline-2-carboxylate, has been studied. A range of aliphatic and aromatic 1-alkynes has been efficiently reduced using various hydrosilanes. Hydrosilylation of the linear 1-alkynes hex-1-yne and oct-1-yne gives a good selectivity toward the β-(Z)-vinylsilane product, while for the bulkier t-Bu-C≡CH a reverse selectivity toward the β-(E)-vinylsilane and significant amounts of alkene, from a competitive dehydrogenative silylation, has been observed. Compound 1, unreactive toward silanes, reacts with a range of terminal alkynes RC≡CH, affording the unsaturated η1-alkenyl complexes [Ir(κ3-hqca)(E-CH=CHR)(coe)] in good yield. These species are able to coordinate monodentate neutral ligands such as PPh3 and pyridine, or CO in a reversible way, to yield octahedral derivatives. Further mechanistic aspects of the hydrosilylation process have been studied by DFT calculations. The catalytic cycle passes through Ir(III) species with an iridacyclopropene (η2-vinylsilane) complex as the key intermediate. It has been found that this species may lead both to the dehydrogenative silylation products, via a β-elimination process, and to a hydrosilylation cycle. The β-elimination path has a higher activation energy than hydrosilylation. On the other hand, the selectivity to the vinylsilane hydrosilylation products can be accounted for by the different activation energies involved in the attack of a silane molecule at two different faces of the iridacyclopropene ring to give η1-vinylsilane complexes with either an E or Z configuration. Finally, proton transfer from a η2-silane to a η1-vinylsilane ligand results in the formation of the corresponding β-(Z)- and β-(E)-vinylsilane isomers, respectively.
- Pérez-Torrente, Jesús J.,Nguyen, Duc Hanh,Jiménez, M. Victoria,Modrego, F. Javier,Puerta-Oteo, Raquel,Gómez-Bautista, Daniel,Iglesias, Manuel,Oro, Luis A.
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p. 2410 - 2422
(2016/08/02)
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- Selective Oligomerization and [2 + 2 + 2] Cycloaddition of Terminal Alkynes from Simple Actinide Precatalysts
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A catalyzed conversion of terminal alkynes into dimers, trimers, and trisubstituted benzenes has been developed using the actinide amides U[N(SiMe3)2]3 (1) and [(Me3Si)2N]2An[κ2-(N,C)-CH2Si(CH3)N(SiMe3)] (An = U (2), Th (3)) as precatalysts. These complexes allow for preferential product formation according to the identity of the metal and the catalyst loading. While these complexes are known as valuable precursors for the preparation of various actinide complexes, this is the first demonstration of their use as catalysts for C-C bond forming reactions. At high uranium catalyst loading, the cycloaddition of the terminal alkyne is generally preferred, whereas at low loadings, linear oligomerization to form enynes is favored. The thorium metallacycle produces only organic enynes, suggesting the importance of the ability of uranium to form stabilizing interactions with arenes and related π-electron-containing intermediates. Kinetic, spectroscopic, and mechanistic data that inform the nature of the activation and catalytic cycle of these reactions are presented. (Chemical Equation Presented).
- Batrice, Rami J.,McKinven, Jamie,Arnold, Polly L.,Eisen, Moris S.
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p. 4039 - 4050
(2015/09/01)
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- Homogeneous model complexes for supported rhenia metathesis catalysts
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A series of rhenium trioxo complexes (L-ReO3) was synthesized, characterized, and demonstrated to be active in olefin metathesis. The relationship between perrhenate (ReO4-), perrhenyl (ReO3+), and metathesis-active rhenium complexes (L-ReO3) was elucidated. Their chemical behavior can be tuned through the Lewis acid-base interaction. DFT calculations were performed for the metathesis reaction of L-ReO3 with norbornene, which demonstrates that electron-withdrawing substituents or ligands are beneficial for olefin metathesis activity. Moreover, a rhenium-alkylidene complex was synthesized, which can be activated by B(C6F5)3 to afford an active metathesis catalyst. This system can be regarded as a homogeneous model of the hypothetical species present in the heterogeneous catalytic systems. The findings from the present study are consistent with our previous gas-phase studies and constitute the elucidation of the working principles for the metathesis-active rhenium species on the surface.
- Lai, Yu-Ying,Bornand, Marc,Chen, Peter
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p. 7558 - 7565
(2013/01/15)
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- Z-selective semihydrogenation of alkynes catalyzed by a cationic vanadium bisimido complex
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Early metal gets the H: Under 1 atm of H2, the vanadium complex 1 (PFTB=perfluoro-tert-butoxide) catalytically semihydrogenates alkynes to Z alkenes. Synthetic and DFT studies, in combination with H2/D 2 and NMR experiments, indicate that H2 is activated by 1,2-addition to 1. Upon insertion of an alkyne into the V-H bond of A, the product alkene and 1 are generated by the 1,2-α-NH-elimination of the alkenyl ligand.
- La Pierre, Henry S.,Arnold, John,Toste, F. Dean
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supporting information; experimental part
p. 3900 - 3903
(2011/06/24)
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- Two metals are better than one in the gold catalyzed oxidative heteroarylation of alkenes
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We present a detailed study of the mechanism for oxidative heteroarylation, based on DFT calculations and experimental observations. We propose binuclear Au(II)-Au(II) complexes to be key intermediates in the mechanism for gold catalyzed oxidative heteroarylation. The reaction is thought to proceed via a gold redox cycle involving initial oxidation of Au(I) to binuclear Au(II)-Au(II) complexes by Selectfluor, followed by heteroauration and reductive elimination. While it is tempting to invoke a transmetalation/reductive elimination mechanism similar to that proposed for other transition metal complexes, experimental and DFT studies suggest that the key C-C bond forming reaction occurs via a bimolecular reductive elimination process (devoid of transmetalation). In addition, the stereochemistry of the elimination step was determined experimentally to proceed with complete retention. Ligand and halide effects played an important role in the development and optimization of the catalyst; our data provides an explanation for the ligand effects observed experimentally, useful for future catalyst development. Cyclic voltammetry data is presented that supports redox synergy of the Au...Au aurophilic interaction. The monometallic reductive elimination from mononuclear Au(III) complexes is also studied from which we can predict a ～15 kcal/mol advantage for bimetallic reductive elimination.
- Tkatchouk, Ekaterina,Mankad, Neal P.,Benitez, Diego,Goddard III, William A.,Toste, F. Dean
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supporting information; experimental part
p. 14293 - 14300
(2011/11/05)
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- The β-silicon effect. 4: Substituent effects on the solvolysis of 1-alkyl-2-(aryldimethylsilyl)ethyl trifluoroacetates
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Solvolysis rates of 2-(aryldimethylsilyl)-1-methylethyl and 2-(aryldimethylsilyl)-1-tert-butylethyl trifluoroacetates were determined conductimetrically in 60% (v/v) aqueous ethanol. The effects of aryl substituents at the silicon atom on the solvolysis rates at 50 °C were correlated with σmacr; parameters of r+ = 0.15 with the Yukawa-Tsuno equation, giving ρ values of-1.5 for both secondary α-Me and α-tert-Bu systems. The ρ values for those secondary systems are less negative than-1.75 for the 2-(aryldimethylsilyl)ethyl system that proceeds by the Eaborn (non-vertical) mechanism, while they are distinctly more negative than-0.99 for 2-(aryldimethylsilyl)-1-phenylethyl system that should proceed by the Lambert (vertical) mechanism. There was a fairly linear relationship between the reaction constants (ρ) for the β-silyl substituent effects and the solvolysis reactivities for a series of β-silyl substrates. The solvolyses of the α-Me and tert-Bu substrates proceed through the transition state (TS) with an appreciable degree of the β-silyl participation, close to the Eaborn (non-vertical) TS rather than to the Lambert (vertical) TS. Copyright
- Fujiyama, Ryoji,Alam, Md. Ashadul,Shiiyama, Aiko,Munechika, Toshihiro,Fujio, Mizue,Tsuno, Yuho
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experimental part
p. 819 - 827
(2011/04/24)
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- Selective and unexpected transformations of 2-methylpropane to 2,3-dimethylbutane and 2-methylpropene to 2,3-dimethylbutene catalyzed by an alumina-supported tungsten hydride
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2-Methylpropane and 2-methylpropene, in the presence of the W(H) 3/Al2O3 catalyst, are unexpectedly transformed to 2,3-dimethylbutane and 2,3-dimethylbutenes, respectively, with high selectivity; in case of 2-methylpropane
- Merle, Nicolas,Stoffelbach, Francois,Taoufik, Mostafa,Le Roux, Erwan,Thivolle-Cazat, Jean,Basset, Jean-Marie
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body text
p. 2523 - 2525
(2009/09/30)
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- Hydrogen activation by high-valent oxo-molybdenum(vi) and -rhenium(vii) and -(v) compounds
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The high-valent oxo-molybdenum(vi) and -rhenium(vii) and -(v) derivatives MoO2Cl2, ReCH3O3 (MTO) and ReIO 2(PPh3)2 catalyze the selective hydrogenation of alkynes to alkenes at 80 °C under 40 atm of pressure. The reduction of sulfoxides to sulfides has also been performed by oxo-rhenium and -molybdenum complexes using hydrogen as a reducing agent. Activation of hydrogen by MoO 2Cl2 and MoO2(S2CNEt 2)2 was shown by means of DFT calculations to proceed by H-H addition to the Mo=O bond, followed by hydride migration to yield a water complex. The Royal Society of Chemistry.
- Reis, Patricia M.,Costa, Paulo Jorge,Romao, Carlos C.,Fernandes, Jose A.,Calhorda, Maria Jose,Royo, Beatriz
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p. 1727 - 1733
(2008/09/20)
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- Process for manufacturing neohexene
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The present invention relates to a process for manufacturing neohexene, comprising contacting isobutene with a supported catalyst comprising a tungsten compound chosen from tungsten hydrides, organometallic tungsten compounds and organometallic tungsten hydrides, and a support comprising an oxide of aluminium, so as to form a reaction mixture comprising neohexene, and preferably separating neohexene from the reaction mixture, so as to isolate it. The contacting leads to the direct production of neohexene, in particular in a single (reaction) stage and with a high molar selectivity for neohexene. The contacting can be performed at a temperature of 50 to 600 °C, under a total absolute pressure of 0.01 to 100 MPa.
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Page/Page column 14
(2008/12/06)
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- Surface versus molecular siloxy ligands in well-defined olefin metathesis catalysts: [{(RO)3SiO}Mo(=NAr)(=CHtBu)(CH2tBu)]
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Surface organometallic chemistry: Similar electronic properties of molecular and surface siloxy ligands in 1 m and 1 were revealed by a structural investigation with solid-state NMR spectroscopy and a reactivity study in olefin metathesis. Despite similar initial turnover frequencies, the silica-supported catalyst 1 is more stable than 1 m and allows it to achieve higher turnovers, thus showing the advantage of site isolation on surfaces. (Figure Presented)
- Blanc, Frederic,Coperet, Christophe,Thivolle-Cazat, Jean,Basset, Jean-Marie,Lesage, Anne,Emsley, Lyndon,Sinha, Amritanshu,Schrock, Richard R.
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p. 1216 - 1220
(2007/10/03)
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- Mechanism and structure-reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phase
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The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323-435 °C and pressure range 28.5-242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z = substituent) give an approximate linear correlation when using the Taft-Topsom method, log(k z/kH)= -(0.57 ± 0.19)σα + (1.34 ± 0.49)σR (r = 0.9256; SD = 0.16) at 400 °C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the C3- H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH versus Esc for CH 3OCOOCH2CH2R at 400°C, R = alkyl, δ= -0.17 (r=0.9993, SD = 0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright
- Chuchani, Gabriel,Marquez, Edgar,Herize, Armando,Dominguez, Rosa Maria,Tosta, Maria,Brusco, Doris
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p. 839 - 848
(2007/10/03)
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- Et2SiH2 assisted the selective dimerization of terminal alkynes catalyzed by Cp*2UMe2
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A practical approach has been developed for the catalytic synthesis of short oligomers, dimers and/or trimers of terminal alkynes. The method allows control of the extent and, in some cases, the regiospecificity in the catalyzed oligomerization of terminal alkynes promoted by bis(pentamethylcyclopentadienyl)uranium dimethyl complex (Cp*2U(CH3)2, Cp= C5Me5). The metallocene precursor is known to promote the simultaneous production of a large number of differently sized oligomers in the presence of terminal alkynes. However, the addition of a specific secondary silane ensures the selective synthesis of short oligomers.
- Wang, Ji Quan,Eisen, Moris S.
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- Re-based heterogeneous catalysts for olefin metathesis prepared by surface organometallic chemistry: Reactivity and selectivity
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Herein we describe the catalytic activity of 1, a well-defined Re alkylidene complex supported silica, in the reaction of olefin metathesis. This system is highly active for terminal and internal olefins with initial rates up to 0.7 mol per mol Re per s. It also catalyses the self-metathesis of methyl oleate (MO) without the need of co-catalysts. The turnover numbers can reach up to 900 for MO, which is unprecedented for a heterogeneous Re-based catalyst. Moreover the use of silica as a support can bring major advantages, such as the possibility to use branched olefins like isobutene, which are usually incompatible with alumina-based supports; therefore, the formation of isoamylene from the cross-metathesis of propene and isobutene can be performed. All these results are in sharp contrast to what has been found for other silica- or alumina-supported rhenium oxide systems, which are either completely inactive (silica system) or typically need co-catalysts when functionalised olefins are used. Finally the initiation step corresponds to a cross-metathesis reaction to give a 3:1 mixture of 3,3-dimethylbutene and trans-4,4-dimethylpent-2-ene, and make this catalyst the first generation of well-defined Re-based heterogeneous catalysts.
- Chabanas, Mathieu,Coperet, Christophe,Basset, Jean-Marie
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p. 971 - 975
(2007/10/03)
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- Method for producing organo-alkali metal compounds
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The invention relates to a method for producing organo alkali-metal compounds by reacting metal lithium, sodium or potassium with organic compounds containing at least one acidic CH bond in a solvent. The inventive method is characterized in that the reaction is carried out in the presence of a hydrogen acceptor, wherein 0.5 to 5 moles of the hydrogen acceptor are used per mole of acid hydrogen, which hydrogen can be replaced by lithium, sodium or potassium, whereby 1 to 3, moles of lithium, sodium or potassium are used per mole of acid hydrogen and the acid CH bond has a pKavalue of 10 to 30. Cyclopentadiene, indene, fluorene and substitution products thereof, or monosubstituted alkynes or methane substitution products, are preferred CH acid organic compounds. Hydrocarbons used as hydrogen acceptors include these with at least one CC double bond in conjugation with either another CC double bond or with a monocyclic aryl radical.
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- A structure-reactivity correlation with three slopes in the elimination kinetics of 2-substituted ethyl N,N-dimethyl-carbamates in the gas phase
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The elimination kinetics of 17 2-substituted ethyl N,N-dimethylcarbamates in the gas phase were determined in the temperature range of 269.5-420.2°C and the pressure range of 24-186 Torr. The reactions in a static system and in the presence of a free radical inhibitor are homogeneous and unimolecular and follow a first-order rate law. The kinetics and thermodynamic parameters are described. The use of several structure-reactivity relationship methods meaningless results, except for Taft σ* values. Three good slopes are originated at σ*(CH3) = 0.00. Slope a: the 2-substituted alkyl groups gave a good straight line when log (k/kCH3) vs σ* values (ρ* = - 1.94 ± 0.30, r = 0.977 at 360°C) were plotted. Slope b: Polar2 substituents gave an approximate straight line with ρ* = - 0.12 ± 0.02, r = 0.936 at 360°C. Slope c:the correlation of multiple bonded and electron-withdrawing substituents interposed by a methylene group at the 2-position of ethyl N,N-dimethylcarbamate was found to give a very good straight line wirh ρ* = 0.49 ± 0.02, r = 0.991 at 360°C. Mechanisms are suggested on the basis of these relationships. The point position of the substituents phenyl (C6H5) and isopropenyl [CH2=C(CH3)] at the 2-position was found to fall far above the three slopes of the lines. These results are interpreted in terms of neighboring group participation of these substituents in the elimination process of the carbamates. However, the acidity of the benzylic and allylic Cβ-H bond for a six-membered cyclic transition state may not be ignored. Copyright
- Chuchani, Gabriel,Nunnez, Oswaldo,Marcano, Norka,Napolitano, Suvighey,Rodriguez, Henry,Dominguez, Marianella,Ascanio, Judany,Rotinov, Alexandra,Dominguez, Rosa M.,Herize, Armando
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p. 146 - 158
(2007/10/03)
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- Diverse catalytic activity of the cationic actinide complex [(Et2N)3U][BPh4] in the dimerization and hydrosilylation of terminal alkynes. Characterization of the first f-element alkyne π-complex [(Et2N)2U(CCtBu)(η2-HCC tBu)][BPh4]
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The cationic actinide complex [(Et2N)3U][BPh4] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis-dimer is also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex [(Et2N)2UCCR][BPh4] with the concomitant reversible elimination of Et2NH, followed by the formation of the alkyne π-complex [(Et2N)2UCCR(RCCH)][BPh4]. This latter complex (R=tBu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the CC bond insertion of the terminal alkyne into the actinide-acetylide bond to give the corresponding alkenyl complex with ΔH?=15.6(3) kcal mol-1 and ΔS?=-11.4(6) eu. The following step, protonolysis of the uranium-carbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH3CCD, allowing the formation of trimers. The unexpected cis-isomer is presumably obtained by the isomerization of the trans-alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et2N)3U][BPh4] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH3 at room temperature produced a myriad of products among which the cis- and trans-vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)CCH and PhSiH3 with [(Et2N)3U][BPh4] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium-hydride complex [(Et2N)2U-H][BPh4], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.
- Dash, Aswini K.,Wang, Jia Xi,Berthet, Jean Claude,Ephritikhine, Michel,Eisen, Moris S.
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- Investigating the kinetics of homogeneous hydrogenation reactions using PHIP NMR spectroscopy
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The combination of parahydrogen induced polarization (PHIP), kinetics and NMR spectroscopy yields a powerful analytical tool: quantitative in situ NMR spectroscopy. Two versions of PHIP NMR experiments are presented to investigate the kinetics of homogeneously catalyzed hydrogenations. The first method, an experimental variation of the ROCHESTER experiment (ROCHESTER = rates of catalytic hydrogenation estimated spectroscopically through enhanced resonances), allows one to determine the hydrogenation rate independently of relaxation and other sources of decay, e.g., subsequent chemical reaction steps. The second method named DYPAS (dynamic PASADENA spectroscopy) uses a variable delay between the end of the hydrogen-addition period and the detection pulse. In principle, all processes during this delay can be described by a set of coupled differential equations. Their solutions can be fitted to the experimental data by a least-squares optimization of the involved kinetic parameters. The DYPAS method can be used to determine the rates of formation as well as the rates of decomposition of stable intermediates and has been applied to the case of freshly hydrogenated and still catalyst-attached product molecules. We provide kinetic data for the formation and decomposition of these unusual product-catalyst complexes during the hydrogenation of different styrene derivatives with a cationic Rh1 catalyst containing a chelating diphosphine ligand. The kinetic measurements indicate that the rate of formation of the catalyst-attached product increases whereas the rate constant of its decomposition diminishes if the para position of the arene ring of styrene carries an electron- donating substituent. In the case of p-aminostyrene as the substrate, the detachment step turned out to be rate limiting for the catalytic cycle. With certain substituted styrenes and cationic Rh1 complexes containing chiral chelating diphosphine ligands, two geometrically different (diastereomeric) product-catalyst adducts can be discriminated via PHIP NMR spectroscopy. The associated alternative reaction pathways have been analyzed by applying the DYPAS method, which can also be used to investigate the mechanism of an asymmetric hydrogenation.
- Hübler, Patrick,Giernoth, Ralf,Kümmerle, Günther,Bargon, Joachim
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p. 5311 - 5318
(2007/10/03)
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- Study of trans-2-tert-butylcyclopropylcarbene by laser flash photolysis and chemical analysis
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Laser flash photolysis (LFP) of trans-2-tert-butylcyclopropyldiazirine (4) produces trans-2-tert-butylcyclopropylcarbene (5). Carbene 5 can be trapped with pyridine to form ylide 6. The rate of formation of ylide 6 was resolved and found to be linearly dependent on the concentration of pyridine. The lifetime of carbene 5 was determined to be 22 ns (pentane), 18 ns (acetonitrile and CF2ClCFCl2). 15 ns (cyclohexane), and 27 ns (cyclohexane-d12) at ambient temperature. The lifetime of carbene 5 in solution, at ambient temperature, is controlled substantially by reaction with solvent and only to a minor extent by rearrangement to cyclobutene 7. Photolysis of 4 (350 nm) in Freon-113 produces 3-tert-butylcyclobutene (7) along with products derived from attack of the carbene on solvent. Photolysis of 4 in the presence of trapping agents (tetramethylethylene, cyclopentene, propylamine, trifluoroethanol) produces adducts with little or no corresponding decrease in the yield of 7. The preferred interpretation is that cyclobutene 7 is formed by a rearrangement of the excited state of the diazirine (diazirinyl biradical), and not by a carbenic process at 0°C and lower temperatures. This mechanism is supported by product analysis of a non-nitrogenous cyclopropylcarbene precursor. At ambient temperature, cyclobutene 7 is formed by both diazirine excited state (diazirinyl biradical) and carbene rearrangement.
- Huang, Haiyong,Platz, Matthew S.
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p. 5990 - 5999
(2007/10/03)
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- Sterically crowded monomeric neutral bis(benzamidinato) compounds of aluminium, [PhC(NSiMe3)2]2AlX (X = Cl, H); X-ray crystal structure of [PhC(NSiMe3)2]2AlH
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AlCl3 reacts with [PhC(NSiMe3)2]Li(OEt2) to afford the bis(N,N′-bis(trimethylsilyl)benzamidinato)aluminium chloro compound which, on treatment with KBEt3H, yields the structurally characterized monomeric hydrido derivative, [PhC(NSiMe3)2]2AlH, whose reactivity towards unsaturated hydrocarbons illustrates the low electrophilicity of the aluminium and high basic character of the hydrido group.
- Duchateau, Robbert,Meetsma, Auke,Teuben, Jan H.
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p. 223 - 224
(2007/10/03)
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- Dehydration of secondary alcohols via thermolysis of in situ generated alkyl diphenyl phosphates: An inexpensive and environmentally compatible method for the preparation of alkenes
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Secondary alcohols are converted into diphenyl phosphate esters by the action of triphenyl phosphate or diphenyl phosphorochloridate in high-boiling, water-miscible solvents in the presence of base. The alkyl diphenyl phosphates undergo thermolysis to afford high yields of alkenes which distill from the reaction mixtures. Purification of the products is achieved by extraction with dilute sulfuric acid which removes traces of solvent and base that may have codistilled. The ratios of 2- (14) and 3-menthene (15), obtained from menthol (13) and neomenthol (16), and the formation of rearranged alkenes by 1,2-shifts from 3, 6, and 3,3-dimethylbutan-2-ol are consistent with ionic intermediates of the elimination reaction. The novel dehydration method offers distinct and important advantages over the existing methods.
- Quast,Dietz
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p. 1300 - 1304
(2007/10/02)
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- Stereoselective Reactions of Lithio-vinylsulfoxides with Aldehydes
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A series of homochiral vinyl sulfoxides-synthesised by treating vinyl Grignard reagents with homochiral menthyl toluene-p-sulfinate or sulfinyl oxazolidinones 8a and 9a- were deprotonated with LDS and allowed to react with acetaldehyde, isobutyraldehyde, and trimethylacetaldehyde to give β-hydroxy sulfoxides with moderate diastereoselectivity.The sulfoxide 1 gave the best selectivity with the larger aldehydes.The same diastereoselectivity, within experimental error, is observed in the reactions with trimethylacetaldehyde of both E-1 and Z-1 giving 85:15 and 84:16 mixtures of 2c and 3c respectively; evidently, the geometry of the vinyl group does not affect the selectivity.
- Fawcett, John,House, Stuart,Jenkins, Paul R.,Lawrence, Nicholas J.,Russell, David R.
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- Evidence for Electron Transfer, Radical and Ionic Pathways in the Decomposition of Diacyl Peroxide
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The thermal decomposition mechanism of 4,4-dimethylpentanoyl m-chlorobenzoyl peroxide and its α- and β-dideuteriated analogues is described.Product analyses and CIDNP studies suggest that all three pathways, electron transfer, radical and ionic, are operative in decomposition of these peroxides.Two pulsed-NMR techniques have been employed to eliminate distortions of CIDNP intensities arising from spin-lattice relaxation.These quantitative CIDNP studies have revealed an additional pure ionic pathway which competes with the radical pair electron transfer pathway to form rearranged reaction products.
- Lee, Sueg-Geun
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p. 1361 - 1372
(2007/10/02)
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- Rate Constants and Arrhenius Functions for Rearrangements of the 2,3-Dimethyl-3-butenyl and (2,2-Dimethylcyclopropyl)methyl Radicals
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Rate constants for the rearangement of the 2,2-dimethyl-3-butenyl radical (1) to the 1,1-dimethyl-3-butenyl radical (3) via the intermediate (2,2-dimethylcyclopropyl)methyl radical (2) were measured over the temperature range 74 to -78 deg C by the competition method using the reaction of radical 1 with Bu3SnH as the basis reaction.Rate constants for ring opening of radical 2 to both 1 and 3 were measured over the temperature range 50 to -78 deg C by competition against reaction of 2 with PhSH.Arrhenius functions for the 1 to 3, 1 to 2, 2 to 1, and 2 to 3 conversions were calculated; at 25 deg C, the rate constants for these conversions are 4.8*106, 5.6*106, 1.2*108, and 7.7*108 s-1, respectively.At room temperature, cyclization of gem-dimethyl-substituted radical 1 is accelerated by about 3 orders of magnitude over its parent, 3-butenyl radical.Ring opening of 2 is about 1 order of magnitude faster than ring opening of its parent, cyclopropylcarbinyl radical.For the 1 to 2 conversion, K12 varies from 0.05 at 25 deg C to 0.02 at -78 deg C, ΔH12 is 1.0 kcal/mol, ΔS12 is -2.7 eu, and ΔG12 at 25 deg C is 1.8 kcal/mol.
- Newcomb, Martin,Glenn, Anne G.,Williams, William G.
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p. 2675 - 2681
(2007/10/02)
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- Five-coordinate olefin complexes of platinum(II) containing σ-bonded carbon ligands. Coordination environment and stability
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Using three different procedures, trigonal-bipyramidal complexes of the type PtClR(N-N′)(η2-olefin) have been prepared containing a variety of alkyl or aryl ligands (R), bidentate nitrogen (N-N′), and olefinic ligands. In contrast to other previously known five-coordinate complexes of platinum (II), the dissociation of the olefin from these species is reversible. The equilibria PtClR(N-N′) (olefin) ?K PtClR(N-N′) + olefin have been investigated through qualitative observations and through the determination of the dissociation constants by 1H NMR analysis of equilibrium mixtures. The main ligand effects may imply a variation of Kdiss of several orders of magnitude and are the following: (a) A dominating factor is the steric hindrance present on both sides of the N-N′ ligand. If properly oriented in the coordination plane, this stabilizes the bipyramidal complexes through a corresponding destabilization of the square-planar species. (b) Electron-releasing substituents on the olefinic double bond destabilize the five-coordinate complexes while electron-withdrawing groups stabilize them. This trend is opposite to that observed for square-planar species and provides direct evidence for the relative importance of π-back-donation in five-coordinate Pt(II) olefin complexes.
- Cucciolito, Maria Elena,De Felice, Vincenzo,Panunzi, Achille,Vitagliano, Aldo
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p. 1180 - 1187
(2008/10/08)
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- Kinetics and Mechanism of Elimination of Primary Alkyl Methanesulfonates in the Gas Phase. Correlation of Alkyl Substituents
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The gas-phase elimination kinetics of eight primary alkyl methanesulfonates were studied in a seasoned, static reaction vessel over the temperature range of 280.6-350.2 deg C and the pressure range of 32-178 Torr.The reactions are homogenous and unimolecular, follow a first-order rate law, and are invariant to the presence of equal or excess amount of the radical chain suppressor cyclohexene and/or propene.The observed overall rate coefficients are represented by the following Arrhenius equations: For ethyl methanesulfonate, log k1(s-1) = (12.18+/-0.12)-(171.7+/-1.3) kJ mol-1 (2.303RT)-1; for n-propyl methanesulfonate, log k1(s-1) = (12.36+/-0.28)-(171.6+/-3.3) kJmol-1 (2.303RT)-1; for n-butyl methanesulfonate, log k1(s-1) = (12.16+/-0.20)-(168.7+/-2.3) kJmol-1 (2.303RT)-1; for n-pentyl methanesulfonate, log k1(s-1) = (12.25+/-0.09)-(169.4+/-1.0)kJmol-1 (2.302RT)-1; for n-hexyl methanesulfonate, log k1(s-1) = (12.21+/-0.19)-(168.9+/-3.3)kJmol-1 (2.303RT)-1; for 3-methyl-1-butyl methanesulfonate, log k1(s-1) = (12.74+/-0.19)-(174.7+/-2.2)kJmol-1(2.303RT)-1; for 3-methyl-1-pentyl methanesulfonate, log k1(s-1) = (12.28+/-0.17)-(167.9+/-1.9)kJmol-1(2.303RT)-1; for 3,3-dimethyl-1-butyl methanesulfonate, log k1(s-1) = (12.14+/-0.11)-(165.2+/-1.3) kJmol-1 (2.303RT)-1.The present data give good correlation lines when log k/k0 are plotted against several steric parameters Es and Esc and $v values.Apparently, steric factors seem to be operating in these elimination reactions.The primary alkyl methanesulfonates are found to be faster in rates of olefin formation when compared to other primary organic esters.The reaction of this work is interpreted in terms of an intimate ion pair type of mechanism.
- Chuchani, Gabriel,Pekerar, Sarah,Dominguez, Rosa M.,Rotinov, Alexandra,Martin, Ignacio
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p. 201 - 202
(2007/10/02)
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- Rate Constants and Arrhenius Functions for Hydrogen Atom Transfer from tert-Butyl Thiol to Primary Alkyl Radicals
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The radical clock method was used to determine rate constants for hydrogen atom transfer from t-BuSH to primary alkyl radicals over the temperature range -45 to 50 deg C.Three clocks, cyclization of the 5-hexenyl radical (3a), ring opening of the cyclopropylcarbinyl radical (3b), and rearrangement of the 2,2-dimethyl-3-butenyl radical (3c), were used in THF, and 3a was used in toluene.Arrhenius functions for the two studies with 3a and the one with 3b were quite similar, and the Arrhenius function for 3c, when adjusted for direct comparison to reactions with simple primary radicals, was also similar.Combining the results from 3a and 3b in THF gave an Arrhenius function for hydrogen atom transfer from t-BuSH to primary radicals of log(k/M-1 s-1) = (8.37 +/- 0.08) - (2.00 +/- 0.09)/2.3RT; the calculated rate constant at 25 deg C is 8.0 * 106 M-1 s-1.This study ultimately relates the t-BuSH rate constants to those for hydrogen atom transfer to primary radicals from both Bu3SnH and PhSH and serves to compare the latter two sets of kinetic data; it also demonstrates that the fast radical clock reactions of 3a and 3b are calibrated against equivalent standards.
- Newcomb, Martin,Glenn, Anne G.,Manek, M. Beata
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p. 4603 - 4606
(2007/10/02)
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- Electron-transfer processes. 43. Attack of alkyl radicals upon 1-alkenyl and 1-alkynyl derivatives of tin and mercury
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Alkyl radicals, obtained by reaction of Bu3Sn? or ClHg? with alkylmercury halides, will undergo regioselective and in some cases stereospecific substitution by a free radical chain addition-elimination mechanism with 1-alkenylstannanes or -mercurials. The chain reaction is also observed for 1-alkynyl derivatives and in the photostimulated demercuration of mixed alkyl and 1-alkenyl- or 1-alkynylmercurials. Chain propagation with alkyl radical formation is also observed to occur in the reactions of β-eliminated ClHg? with Grignard reagents in PhH-THF solution. In competitive reactions of Bu3Sn? or ClHg? with pairs of alkylmercury chlorides, it is observed that a tert-butylmercurial is >1000 times more reactive than a n-butylmercurial, suggesting a concerted dissociate electron-transfer process not involving the intermediacy of RHg? species.
- Russell, Glen A.,Ngoviwatchai, Preecha,Tashtoush, Hasan I.
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p. 696 - 702
(2008/10/08)
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- Synthesis and structures of titanaoxacyclobutanes
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Titanaoxacyclobutanes can be prepared by the addition of a ketene to a titanium methylene complex or by the addition of a methylene fragment to a titanium ketene complex. For example, the addition of 2 equiv of dimethylsulfoxonium methylide to titanocene chloro-acyl complexes yields titanaoxacyclobutanes. The first equivalent deprotonates the acyl to form the titanocene ketene complex which is trapped by the second equivalent of the methylide to yield the metallaoxacyclobutane. Both routes yield complexes that are stable to temperatures above 60°C. Variable-temperature NMR studies show that the metallaoxacyclobutane ring is puckered with a barrier to inversion of 13-19 kcal/mol.
- Ho, Suzzy C.,Hentges,Grubbs, Robert H.
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p. 780 - 782
(2008/10/08)
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- Catalytic synthesis of olefins from paraffins
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Saturated hydrocarbon is transformed catalytically into olefinic hydrocarbon of corresponding skeletal configuration by reacting the saturated hydrocarbon with a suitable alkene cyclopentadienyl or alkene arene transition metal molecular complex, such as bis(ethylene)pentamethylcyclopentadienyliridium, bis(ethylene)pentamethylcyclopentadienylrhodium and bis(ethylene)hexamethylbenzene osmium in the presence of free alkene as hydrogen acceptor. The reaction may be performed photochemically under irradiation with ultraviolet light or it may be performed thermolytically under application of heat. The catalyst may be charged to the reaction as a preformed alkene cyclopentadienyl or alkene arene transition metal molecular complex or the catalyst may be formed in situ in the reaction mixture via displacement of ligand from a suitable transition metal complex containing the displaceable ligand, such as dicarbonylpentamethylcyclopentadienyliridium or cyclooctadienepentamethylcyclopentadienyliridium.
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- A Gas-phase Anionic Analog of the Wittig Reaction. An Ion Cyclotron Resonance Study of the Gas-phase Ion Chemistry of Silyl Carbanions
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Gas-phase ion-molecule reactions of a variety of fluorosilyl carbanions with compounds containing double bonds to oxygen, X=O, have been examined using pulsed ion cyclotron resonance spectroscopy.The predominant reaction channel observed for species not containing acidic hydrogen is a Wittig-like process involving Si-O bond formation and elimination of X=CH2 species.The gas-phase acidity of F3Si(CH3) has been determined and those of F2Si(CH3)2 and FSi(CH3)3 have been estimated.From the fluoride transfer reactions of F3SiCH2- the fluoride affinity of F2Si=CH2 has been estimated and limits on the ? bond strength in this silaethene obtained.Potential analytical applications of the Wittig reactivity have been discussed.
- Campanaro, A.,Marvin, C. H.,Morehouse, S. P.,McMahon, T. B.
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p. 663 - 668
(2007/10/02)
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- Competing Nucleophilic Displacement and Radical Chain Reduction in Reactions of Transition-Metal Hydride Anions with Alkyl Bromides
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The reactions of group 6 metal hydrides, PPN+HM(CO)4L- (M=Cr, L=CO, M=W, L=CO, P(OMe)3), with two mechanistic probes, 6-bromo-1-hexene (1) and 4-bromo-3,3-dimethyl-1-butene (3), in THF at 25 deg C were studied.Neopentyl-like probe 3 was reduced (presumably) exclusively by a radical chain process, and the second-order rate constants (kH) for hydrogen atom abstraction from HM(CO)4L- by the intermediate radical, 2,2-dimethyl-3-butenyl, were determinated.Unhindered probe 1 was reduced by both an SN2 pathway and a radical chain process.The second-order rate constants for hydrogen atom abstraction from HM(CO)4L- by 5-hexenyl were estimated, and the percentages of reduction of 1 by the SN2 pathway and the radical chain process were calculated; the percentage of reduction by the SN2 pathway increased in the order HCr(CO)5- - -.The combination of a hindered and an unhindered probe as used in this study has expanded the utility of mechanistic probes by permitting quantitation of competing pathways.
- Ash, Carlton E.,Hurd, Philip W.,Darensbourg, Marcetta Y.,Newcomb, Martin
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p. 3313 - 3317
(2007/10/02)
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- Reversal of electronic Substituent Effects in the Retro-Diels-Alder Reaction. A Charge Neutral Analogue of Oxyanion-Accelerated Cycloreversion
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The retro-Diels-Alder reaction of anthracene cycloadducts is influenced by the dienophile substituents in the following ways: (1) electron-withdrawing groups increase the rate of the reaction; (2) strongly conjugating substituents make the reaction much faster than predicted by classical electron-withdrawing or -donating ability, in the best case by a factor of 3 x 106, and (3) there is no observable steric effect, in contrast to literature statements to the contrary.
- Nanjappan, Palaniappan,Czarnik, Anthony W.
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p. 2851 - 2853
(2007/10/02)
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- Infrared Multiphoton Decomposition of cis-2-pentene and 3-Methyl-cis-2-pentene
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The infrared multiphoton (IRMP) decomposition of cis-2-pentene and 3-methyl-cis-2-pentene has been studied in order to compare the fragmentation pattern of the photoexcited molecules with that observed in the vacuum ultraviolet photolysis.The decomposition were studied at pressures from 0.2 to 5 Torr, using a pulsed CO2 laser weakly focusedto give fluences from about 5 to 20 J/cm2 in the focal region.
- Back, R. A.,Collin, G. J.
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p. 5684 - 5689
(2007/10/02)
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- Acid catalyzed reactions of monovinyl aromatic compounds
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The self condensation of monovinyl aromatic compounds to acyclic dimers, the cross-reaction of monovinyl aromatic compounds with olefins in the presence of acid catalysts to produce cyclialkylated aromatic compounds, and the production of cyclialkylated aromatic compounds by reaction of olefins with acyclic dimers of monovinyl aromatic compounds in the presence of acid catalysts is improved by employing a tetrahydrothiophene 1,1-dioxide solvent.
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- The high pressure photochemistry of alkenes. III. The 184.9 nm photoisomerization processes in acyclic alkenes
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We have made a systematic study of the 184.9 nm photoisomerization of the gaseous acyclic alkenes.Apart from the cis-trans isomerization (geometric isomerization), we have also observed the formation of products arising from the 1,3-hydrogen and methylene shifts (structural isomerization). 1-Alkenes do not show evidence of structural isomerization.This kind of isomerization increases with an increase in the number of alkyl substituents around the double bound.These observations, combined with those from the literature, may be explained on the basis of the following: (a) the 1?,?* state is involved in cis-trans isomerization process; (b) the 1?,R(3s) state is responsible for the methylene shifts; (c) another singlet state is required for the 1,3-hydrogen shift; (d) this last state is either at an energy level higher than that of the Rydberg state or the hot ground state.Finally, the photoexcited molecules, through internal conversion, may convert from one state to another, and their lifetime is long enough to be stabilized by collision.
- Collin, Guy J.,Deslauriers, Helene
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p. 1424 - 1430
(2007/10/02)
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- 4-BROMO-3,3-DIMETHYL-1-BUTENE: A NEW PROBE FOR RADICAL INTERMEDIATES IN REACTIONS IN STRONGLY BASIC MEDIA
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The preparation, isolation and purification of the title bromide (1) are described, and the application of 1 as a mechanistic probe is demonstrated in the metal-halogen interchange reaction with tert-butyllithium.
- Newcomb, Martin,Williams, William G.,Crumpacker, Elizabeth L.
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p. 1183 - 1184
(2007/10/02)
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