Dimerization mercury-photosensitized

The term Mercat reaction refers to the use of mercury photosensitization to perform reactions on a preparatively useful, multigram scale. Typically these reactions involve CH activation, for example the conversion of methanol to ethylene glycol or indeed almost any organic compound to its dehydro dimer. 

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>. 

Recent work on the mercury photosensitized decomposition of aziridine has shown that initial rupture of the N-H bond leading to the ethylenimino radical is followed by dimerization, viz. 

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 ... 

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 ... 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

The antidepressant protriptyline (116) causes skin photosensitization in man. Jones and Sharpies irradiated an aqueous solution of the hydrochloride with a medium-pressure mercury lamp for 16 h and separated the products by preparative TLC. First formed was the epoxide (117) which photohydrated to the diol (118). Also isolated was the enol (119) [84], Earlier, Gasparro and Kochevar had shown that only the hydrochloride was photodegraded under nitrogen in water or ethanol. Three products were isolated and all lysed erythrocytes, but the structure of only one was suggested. This was a cyclobutyl dimer as shown by its mass spectrum and its photolysis back to protriptyline by light of 254 nm. Presumably, a [2 + 2] cycloaddition of the olefine bonds had occurred [85]. 

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