Cross-dehydrodimerization

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. 

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. 

Alkane functionalization on a preparative scale by mercury-photosensitized C-H bond activation has been recently developed by Crabtree [22], Mercury absorbs 254-nm light to generate a Pi excited state which homolyzes a C-H bond of the substrate with a 3° > 2° > 1° selectivity. Radical disproportionation gives an alkene, but this intermediate is recycled back into the radical pool via H-atom attack, which is beneficial in terms of yield and selectivity. The reaction gives alkane dimers and products of cross-dehydrodimerization of alkanes with various C-H compounds ... 

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. 

The observed selectivity is 3°>2°>1° as expected for a homolytic pathway and species with 4 °-4 ° C—C bonds are very efficiently assembled, especially in the presence of H2 which increases the selectivity because H atoms which are formed are somewhat more selective for the weakest C—H bonds in the molecule. H atoms are also very tolerant of functional groups, so a variety of functionalized molecules (esters, epoxides, ketones etc.) can also be dehydrodimerized. Methylcyclohexane only gives 12% of the 4°-4° dehydrodimer in the absence of H2, but in its presence enough of this dimer is formed to allow it to crystallize from the product mixture (equation 44). Alkanes can also be functionalized by cross-dimerization with other species. Equation 45 shows the results from cyclohexane and methanol. The three products are formed in approximately statistical amounts. The polarities of the three species are so different that the glycol can be removed with water and the bicyclohexyl separated by elution with pentane. 

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