Correspondence ethylene, stepwise

Reduction of dibenzothiophene with sodium in liquid ammonia has been shown to be sensitive to the experimental methods employed however, the major product is usually 1,4-dihydrodibenzothiophene. 27 -28i The electrochemical reduction of dibenzothiophene in ethylene-diamine-lithium chloride solution has been shown to proceed via stepwise reduction of the aromatic nucleus followed by sulfur elimination. In contrast to the reduction of dibenzothiophene with sodium in liquid ammonia, lithium in ethylenediamine, or calcium hexamine in ether, electrolytic reduction produced no detectable thiophenol intermediates. Reduction of dibenzothiophene with calcium hexamine furnished o-cyclohexylthiophenol as the major product (77%). Polaro-graphic reduction of dibenzothiophene 5,5-dioxide has shown a four-electron transfer to occur corresponding to reduction of the sulfone group and a further site. ... 

Noble metals such as Pt, Pd or Rh act also as catalysts in hydrogenation processes of olefin or acetylenes. Hydrogenation of ethylene by molecular hydrogen on a metal surface involves a stepwise mechanism outlined in Eq. (34-37). In fact, dissociation of molecular hydrogen and formation of surface-adsorbed hydrogen, Eq. (34), corresponds to the intermediate catalytic species formed in the photo-... 

Formation of the propionyl compound occurs stepwise. The iridium(I) derivative IrH(CO) 3 (PPr-13) with ethylene under pressure (14 atm) at 50 C was converted into the corresponding ethyl derivative ... 

Described in this paper is a model system - one in which well-characterized lanthanide complexes exhibit high catalyst activities for ethylene polymerization but where the corresponding oligomerization of propene is sufficiently slowed so that stepwise insertion of the olefin can be studied quantitatively and all important intermediates observed or isolated. Emphasized in this paper is the effect of added Lewis acids and bases on the rate of olefin insertions, and comparison between ethylene and propene reactions. The catalysts, of general structure M(ri -Cp )2CH3 L (M = Yb, Lu ... 

This result suggests a stepwise mechanism. The first step is the formation of a transoid tetramethylene biradical. Then, this intermediate rotates, thereby permitting closure of the cyclobutane ring in a second step. Recent high-quality ab initio calculations [7-37] support this mechanism. The reverse of ethylene dimerization, the pyrolysis of cyclobutane, is experimentally observed [7-38]. Both quantum-chemical calculations [7-39] and thermochemical considerations [7-40] suggest that the pyrolysis proceeds through a 1,4-biradical intermediate. This shows the value of the additional information yielded by the orbital correspondence approach. 

When a similar reaction of 1,1-diphenylethene with ethylene di(3-oxobu-tanoate) was stopped for 2.5 h, an intermediate monodioxane was obtained in 60% yield. The reaction that continued for 20 h resulted in the corresponding bis(l,2-dioxan-3-ol) in 52% yield together with the monodioxane (2%). Furthermore, the isolated monodioxane was allowed to react with the alkene under the same reaction conditions to produce the bis(l,2-dioxan-3-ol). Accordingly, it was suggested that the formation of the bis(l,2-dioxan-3-ol)s apparently occurred during the stepwise mechanism. 

The principal component of the reaction coordinate is the approach of the two ethylene molecules towards one another with retention of the full symmetry assumed in the construction of the correspondence diagram as Fig. 6.2 Wcts set up in D2/1, this least motion approach has the irrep ag. The diagram then tells us that the reaction coordinate for concerted conversion of the two tt bonds into the two (T bonds of cyclobutane also has to include a 625 component. Several symmetry coordinates, and the subgroups of D2/1 to which they desymmetrize the reaction path, are shown in Fig. 6.3. If the correspondence diagram had called for an displacement, the relatively facile formation of cyclobutane in its stable puckered D2 conformation would have been expected. If a b u component were required to induce the neccesary correspondence, the favored pathway would generate a cisoid biradical, which would immediately collapse to cyclobutane. The nominally stepwise reaction would then be kinetically indistinguishable from one in which the formation of both bonds is synchronous. 

Figure6.4. Correspondence diagram for stepwise dimerization of ethylene...

The formation of 1,3-oxathiolane with chloral and 1,3-dithio-lanes with cycloaliphatic thioketones occurs regioselectively to yield the sterically less hindered products. On the other hand, aromatic thioketones, e.g., thiobenzophenone or 9/ fluorene-9-thione, intercept 8 to give comparable amounts of both regio-isomeric adducts. Usually, stereoisomeric dipolarophiles such as fumaronitrile and maleonitrile as well as dimethyl fumarate and maleate form 18 in a stereoselective manner. However, in the case of extremely electron-poor dipolarophiles, e.g., dimethyl 1,2-dicyanofumarate or ( )-l,2-bis(trifluoromethyl)ethylene-l,2-dicarbonitrile, non-stereospecific formations of the corresponding tetrahydrothiophenes are described. i- s This result is interpreted in terms of a stepwise reaction mechanism with a zwitterion as the key intermediate. Alternatively, this intermediate can cyclize to form seven-membered ketenimines of type 20. With R = Cp3, this product can be isolated in a crystalline form, whereas in the case of R = CN, stable lactam 21 is obtained only after addition of water (eq 10). 

These quadruply BCOD-fused porphyrins were quantitatively converted to the corresponding TBPs at 200-230 °C for 10-60 min under a reduced pressure. In the case of 44a-H2, extrusion of ethylene moiety occurred stepwise... 

A combined experimental and density functional theory (DFT) study of the thermal decomposition of 2-methyl-l,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, and cyclopen-tanone ethylene ketal, in the gas phase, has established that acetaldehyde and the corresponding ketone are formed by a unimolecular stepwise mechanism concerted nonsynchronous formation of a four-centred cyclic transition state is rate determining and leads to unstable intermediates that then decompose rapidly through a concerted cyclic six-centred transition state. ... 

In the Ziegler process, alcohols of a distribution spectrum which corresponds to a Poisson distribution are obtained in a growth reaction from trialkylaluminium to which ethylene is added stepwise to give higher-molecular compounds [23]. These alcohols are exclusively straight-chain and are comparable to natural alcohols. 

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