Mercury dehydrodimerization

Similar mercury-photosensitized alcohol dehydrodimerizations leading to glycols have been documented (Equation (17)).26... 

Several fluorinated diols 6 have been synthesized from highly fluorinated alkanols 5 via mercury-photosensitized dehydrodimerization in the vapor phase163 in a special reactor.164 In almost all cases, the corresponding dl- and meso-diols are obtained in a 50 50 ratio.163... 

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. 

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 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). 

Mercury photosensitization has been used to synthesize 1,3-dioxolane dimers by dehydrodimerization. For 1,3-dioxolane itself or analogues with substitution at C-2 or C-2 and C-4, radical formation occurs at the C-2 position, whereas when substitution is at the C-4 position radical formation occurs at the 4-position. In most cases, a small amount of product from ring opening was also reported <1996TL6853>. For 2,2-dimethyl-l,3-dioxolane, radical formation occurs at the 4-position, and one common way to achieve this is the use of dimethylzinc in air. Addition of various alkenes and related groups can then be performed, such as addition to tosyl imines <2004JOC1531>, and addition to perfluorinated alkenes and alkenyl ethers <1999JFC(94)141>. 

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... 

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) ... 

S.H. Brown, R.H. Crabtree - Chem. Commun., 970,1997 Making mercury-photosensitized dehydrodimerization into an organic synthetic method. Vapor-pressure selectivity and the behavior of functionalized substrates. J. Am. Chem. Soc. Ill, 2935,1989 and 2946 Chemtech. 21, 634,1991. 

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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