Dehydrodimerization alkanes

This has recently been developed for use on a preparative scale for alkane dehydrodimerization and functionalization by Brown and Crabtree [20a], by adopting an experimental setup in which the substrate refluxes continuously at ambient pressure and temperatures from 35°-300° in the presence of a drop of liquid Hg. In this way the product, once formed, promptly condenses and is protected from further conversion because the condensed phase is entirely unreactive, probably because the dissolved Hg... 

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. 

In looking at some control reactions for transition-metal catalyzed alkane photodehydrogenation, we came across mercury-photosensitized dehydrodimerization of alkanes. The very high efficiency of the procedure, when performed under reflux conditions at ambient temperature and pressure was immediately obvious. 

In this section, not only alkane isomerization and dehydrodimerization (equatirm 13) ate consictered, but also the dehydrogenation of alkanes to alkenes, as in diis case, two adjacent C—bonds ate replaced by a ir-type C— bond. Other C—C bond-forming reactions are also mentioned. 

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 ... 

Alkanes are relatively stable species thermodynamically and so many reactions of alkanes (dehydrogenation, dehydrodimerization, carbonylation) are unfavorable under ambient conditions. This means we often need to couple some favorable process with the unfavorable alkane conversion in order to drive it. We look at the details in Section III.B, but only note here that the common appearance of photochemistry in alkane chemistry can be seen as a way to drive reactions thermodynamically and to access highly reactive transition metal fragments that are kinetically competent to react with alkanes. 

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. 

Photocatalyzed reactions have been very important in alkane activation chemistry[l-3] because, other than oxidation, most reactions of alkanes are thermodynamically unfavorable. Such is the case for alkane dehydrogenation (eq. 1), carbonylation (eq. 2) and dehydrodimerization (eq. 3), and these are three reactions that have been the best studied to date and are covered in this chapter. 

The triplet excited state from elemental mercury, accessible on excitation of the vapor at 254 nm is known to be an exceptionally powerful H atom abstractor and can abstract efficiently from 2° and 3° C-H bonds of alkanes. Mercury photosensitized dehydrodimerization (eq. 1) has been known since the work of Hill [19a] and of Steacie [19b] in the 1920s and 30s and was intensively studied in the period 1940-1973 very little work has been done since 1973, however. The majority of studies concerned themselves with the physico-chemical and mechanistic aspects of the dehydrodimerization of the light alkanes (eq. 29) ... 

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 ... 

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