Ethers 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. 

Tl(III) trifluoroacetate, sometimes with the requirement that boron trifluoride etherate should be present, causes fast dehydrodimerization of many aromatic compounds to give biaryls and/or diphenylmethanes in competition with thallation (McKillop et al., 1980). This system thus seems to be a typical exponent of (96). Table 16, entry no. 16, presents a crude estimate of the possibility of non-bonded electron transfer between Tl(III) trifluoroacetate and naphthalene, showing that it indeed appears to be feasible. Note however that both observed and estimated rate constants are based upon rather uncertain... 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

Dehydrodimerization of Cyclic Enol/Allyl Ethers and Olefins... 

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>. 

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. 

B. Dehydrodimerization of cyclic enol/allyl ethers and olefins... 

Because the dehydrodimers 12a-12c were not known before, this reaction is the first example of the preparation of a novel compound through semiconductor photocatalysis. The analogous products 13, 14 are obtained from 3,4-dihydro-pyran (3,4-DHP), 3-methyl-2,3-dihydropyran (3-MeDHP), and cyclohexene in isolated yields of 30%-60% (Eq. 29, Figs. 3.14, 3.15) [97]. The saturated ether THF is also dehydrodimerized while 1,4-dioxane does not react. 

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