Alkene elimination dimerization

The yield of unimolecularly formed cyclohexene, and by that the yield of unimolecular H2 elimination (quantum yield in photolysis or G value in radiolysis), XiWj), may be calculated from the yields of alkene and dimer dehydrogenation products by the relation [91,92] ... 

The quantum yields and G values of H2 elimination for a larger group of alkanes are eolleeted in Table 4 the values were mostly determined by using Eq. (10). Because in the photolysis and radiolysis of the -alkanes shown in the table various kinds of radicals are produced simultaneously (e.g., -propyl and ec-propyl from propane), the weighted averages of several k /kc values were used in Eq. (10). The ratios can be determined by suppressing the radiolytic alkene and dimer yields in the presence of radical scavenger (e.g., I2) ... 

By way of contrast, 2-germoxetanes can be prepared by the catalyzed dehydrogenation of jS-hydroxyethylgermanes (62), but at room temperature (63) is in equilibrium with its dimer. Though this mixture readily inserts chloral and hexafluoroacetone, heating leads to alkene elimination and germanone formation (Schemes 93 and 94) (73RTC321). 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

The reaction rate is half-order in palladium and dimeric hydroxides of the type shown are very common for palladium. The reaction is first order in alcohol and a kinetic isotope effect was found for CH2 versus CD2 containing alcohols at 100 °C (1.4-2.1) showing that probably the (3-hydride elimination is rate-determining. Thus, fast pre-equilibria are involved with the dimer as the resting state. When terminal alkenes are present, Wacker oxidation of the alkene is the fastest reaction. Aldehydes are prone to autoxidation and it was found that radical scavengers such as TEMPO suppressed the side reactions and led to an increase of the selectivity [18],... 

Catalysis of hydrosilylation by dimeric or by monomeric rhodium (and iridium) siloxide complexes occurs via preliminary oxidative addition of siUcon hydride followed by elimination of disiloxane (detected by GC/MS) to generate the square planar 16e hydride complex with an already coordinated molecule of alkene (Scheme 7.7). 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

In Section 5.3.7.3 the formation of a-halogen carbanions and their alkylation was discussed. If these or related intermediates are left to warm, a-elimination will usually occur to yield carbenes, which either react with the solvent, dimerize, or undergo inter- or intramolecular C-H or C-C bond insertion [291, 292, 309, 436], Because of the electron deficit at the carbene carbon atom (six valence electrons only), these intermediates are highly energetic, and their formation by a-elimination is therefore much slower than the formation of alkenes by /3-elimination. 

The dimerization of propylene carried out by IFP is called the DIMEROSOL process and involves the use of nickel catalysts. This is shown in Fig. 7.7. Complexes 7.20 and 7.21 are the anti-Markovnikov and Markovnikov insertion products into the Ni-H bond. Structures 7.23(A) and (B) are intermediates derived from 7.21 by inserting the second propylene molecule in a Markovnikov and anti-Markovnikov manner, respectively. Similarly 7.22(A) and (B) are intermediates from 7.20 by the insertion of the second propylene molecule. These lour nickel-alkyl intermediates by /3-elimination give six alkenes. Under the process conditions these alkenes may undergo further isomerization. 

Polymerization reactions follow an insertion mechanism, that is, alkene coordination to a vacant site on the active metal species, followed by a migratory alkyl transfer step. The addition of donor molecules which can compete with the alkene for coordination sites is therefore a means of reducing the rate of propagation and allows /3-H elimination to take place, so that a polymerization reaction might be converted to oligomerization or dimerization. On the other hand, metals which... 

The mixed dimerization of perfluoro(2-methylpropene) (12) with fluorinated alkenes F2C = CFX (X = Cl, Br, CF3, Ph) in the presence of cesium fluoride leads to adducts 13 by nucleophilic addition of the (Cp3)3C anion and /1-elimination of fluoride anion. 

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