Alkenes from transfer hydrogenation

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

It is also possible to hydrogenate alkenes by transferring hydrogen from another organic compound (transfer hydrogenation). Many transition metal complexes, particularly those... 

In this method of hydrogenation, the hydrogen molecule is transferred to an alkene from a hydrogen donor in the presence of catalysts like platinum, palladium or Raney Nickel (equation 4). The methodology and mechanism involved in this reaction have been reviewed recently with extensive coverage Reduction of hindered double bonds... 

FIGURE 6 6 Electron flow and orbital interactions in the transfer of a proton from a hydrogen halide to an alkene of the type H2C=CHR... 

The published quantification of the rate of hydrogenation of the dienes COD and NBD of a large number of cationic rhodium(I) chelate complexes allows a good estimation of expected effects on the rate of enantioselective hydrogenation of prochiral alkenes. From the first-order pseudo-rate constants the time needed for complete hydrogenation of the diene introduced as part of the rhodium precursor can be easily calculated as six- to seven-fold the half life. It is recommended that the transfer into the solvent complex be followed by NMR spectroscopy. 

The loss of alkenes from aliphatic onium ions via onium reaction comprises scission of the C-X bond and concomitant transfer of a hydrogen from the leaving alkyl moiety to the heteroatom, and a merely phenomenological description of this reaction has already been included in the preceding schemes. 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Esters of long-chain alcohols show a diagnostic peak at mlz 61, 75, or 89. ... from elimination of the alkyl moiety as an alkene and transfer of two hydrogen atoms to the fragment containing the oxygen atoms, which in essence is the protonated carboxylic acid. 

Dihydrogen is not used as a reactant in transfer-hydrogenation reactions. Instead, the hydrogen required for alkene reduction is abstracted from an organic molecule. Secondary alcohols have proved to be the most generally useful hydrogen sources. 

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