Mercury-photosensitized dehydrodimerization reactions

The mercury-photosensitized dehydrodimerization reaction has been known for nuuiy years, but it has only been made preparatively useful very recently. The key feature of the process is that the system is only active in the vapor phase, so that after condensation the product is protected from further conversion. This implies that the reaction can be run to essentially quantitadve conversion without a fall-off in yield. In order to run on a gram scale to tens of grams, all diat is needed is a quartz flask and a low pressure mercury lamp. Heating the substrate or substrates in the quartz flask with a small drop of mercury leads to smooth formation of the products. Aspects of the process are shown in equations (IS) to (18). 

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. 

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 ... 

Mercury photosensitized ( Pi-excited state) dehydrodimerization of hydrocarbons [103] has been developed into a useful organic synthetic method by using a simple reflux apparatus in which the radical reaction products are protected from further transformation simply by condensation (vapor-pressure selectivity) [104]. The selectivity of C-H cleavage increases from primary to tertiary carbons (350 1) and the method permits the formation of highly substituted C-C bonds (eq. (13)). One limitation for product formation is the appearance of four sets of obligatory 1,3-syn methyl-methyl steric repulsions (e. g., 2,3,4,4,5,5,6,7-octa-methyloctane). 

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 ... 

Dehydrodimerization of cyclohexane to bicyclohexyl was also achieved by mercury photosensitization". However, Hg-, Cd- and Zn-photosensitized reactions of acyclic... 

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See also in sourсe #XX -- [ ]

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