Dehydrodimerization hydrogenation

It is noteworthy that acetone reacts with the organoaluminum in a manner quite different from aldehyde to form mesityloxide and acid amide. This dehydrodimerization reaction of acetone is identical with that observed between A1R3 and ketone. That a hydrogen atom bonded to a nitrogen atom of the product originates from that of the methyl group in acetone was proved by using acetone-d6 as a reactant. 

Hydrodimerization of olefinsIn addition to dehydrodimerization of alkanes 15. 198), hydrodimerization of alkenes can be effected by mercury-photosensitiza- jon, and has the advantage that it is applicable to a wide range of unsaturated wbstrates alcohols and derivatives, ketones, and others. Since the hydrogen adds to ae alkene to give the most stable intermediate (tert > sec > primary), this dimeriza-son can be regioselective. The last example shows that cross-dimerization is possible In this case the hydrodimer of both components is also formed, but in lower ld. 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

Some of the most successful applications of LSV and CV are concerned with the study of the kinetics and mechanisms of the reactions of electrode generated intermediates and a large share of the electrochemical literature deals with this aspect of voltammetry [8,9,13-38,72]. The majority of electrochemical reactions include radical ions as the primary intermediates, and the reaction schemes describing the conversion of a substrate A to products are typically composed of one or two one-electron transfers and one or two chemical steps. The examples include cathodic hydrogenations, (-l-2e , +2H ") (see Chapter 6) and hydrodimerizations (-l-e , -t-H ) (see Chapter 21), and anodic additions (—2e , - -2Nu ) (see Chapter 24), dehydrodimerizations (—e , —H ) (see Chapter 22), and substitutions (—2e , +Nu , —H ) (see Chapter 24), where Nu is nucleophile)... 

The reaction may proceed as homo- or cross-dehydrodimerization [105] and takes place with a wide range of substituted substrates such as higher alcohols, ethers, silanes, and partially fluorinated alcohols and ethers, but also with ketones, carboxylic acids, esters, amides, and amines [106]. Besides the formation of 1,2-diols from saturated alcohols, unsaturated substrates are also dimerized under hydrogen to form l,n-diols other than the 1,2-isomers [107]. The regio-selectivity of the diols is controlled by the formation of the most stable radical, which then dimerizes. 

A photoprocess that may involve the formation rather than the cleavage of metal-hydrogen or metal-carbon bonds is the mercury-photosensitized dehydrodimerization or the alkane functionalization reaction. The reaction involves the coupling of an alkane to give the alkane dimer and hydrogen ... 

---