Alkanes mercury-photosensitized

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) ... 

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. 

So far, only the mercury-photosensitized chemistry, discussed above, allows direct functionalization of alkanes with a Si substituent under mild conditions (equation 36). ... 

The saturated and unsaturated hydrocarbons behave differently as far as photosensitization is concerned. The mercury-photosensitized reactions of the alkanes proceed by an initial reaction of the type... 

Relatively few rate constants are available for the alkyl homolysis reactions mainly because clean sources of the alkyl radical have proved difficult to find. Consequently, the data are not always reliable, but some check is available [64, 65] from thermochemical and kinetic data for the reverse reaction. Direct photolysis of azo-compounds and mercury-photosensitized decomposition of alkanes have so far provided the most reliable (although old) data [64]. For good results, the method depended on precise product analysis in the early stages of reaction, with equation (1.9) used to determine where Rabs and Rr r are the initial rates of formation... 

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 ... 

Mercury photosensitization has been shown to go via interaction between the Pi excited state, Hg, and the alkane, RH, ultimately to liberate free radicals, R... 

Mercury photosensitization has long been known to lead to alkane C—H bond cleavage. The excited state of mercury (Hg ) formed by irradiation at 254 nm follows... 

Product isolation from a catal3d ic reaction may be continuous, or a batch process may be used. If the products are volatile, they can often be separated by distillation from the mixture, assuming the catalysts are involatile, as is often the case. There can also be unexpected selectivity advantages in vapor phase reactions. In mercury photosensitized reactions, alkanes can be converted to liquid functionalization products by a radical pathway. Normally, a radical route would... 

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 ... 

To perform the dissociation of the hydrocarbon to alkyl radicals with C—C bond scission, a hydrocarbon molecule should absorb light with the wavelength 270-370 nm. However, alkanes do not absorb light with such wavelength. Therefore, photosensitizers are used for free radical initiation in hydrocarbons. Mercury vapor has been used as a sensitizer for the generation of free radicals in the oxidized hydrocarbon [206-212], Nalbandyan [212-214] was the first to study the photooxidation of methane, ethane, and propane using Hg vapor as photosensitizer. Hydroperoxide was isolated as the product of propane oxidation at room temperature. The quantum yield of hydroperoxide was found to be >2, that is, oxidation occurs with short chains. The following scheme of propane photoxidation was proposed [117] ... 

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