Mercury alkyl halides

Alkyl mercury halides participate in a photo-sdmidated rathcM chain reacdon of the anion of nitroalkanes Isee Eq. 5.38 in which a 375-W sun lamp Is used." Primary, secondary, and ternary alkyl rathcMs generated from alkyl mercury halides react with the anion of ttitroalkanes to form new C-C bonds. 

The alkyl mercury halides are irritating to the eyes, mucous membranes, and skin and may cause severe dermatitis and burns skin sensitization has occasionally occurred. ... 

Alkyl mercury halides and alkyl mercury acetates are quite stable, but reduction with sodium borohydride leads to highly unstable alkyl mercury hydrides, which collapse at room temperature or in the presence of light to yield alkyl radicals. One other product is mercury metal and you might think you would get H as well but this is too unstable to be formed and is captured by something else (X)—you will see what X is in a moment. This initial decomposition of RHgH initiates the chain but its propagation is by the different mechanism shown below. 

Some organometallic compounds, for example, organomercury or organocobalt compounds, have very weak carbon-metal bonds, and are easily homolyzed to give carbon-centered radicals. Alkyl mercury hydrides are formed by reducing alkyl mercury halides, but they are unstable at room temperature because the Hg-H bond is very weak. Bonds to hydrogen never break to give radicals spontaneously because H" is too unstable to exist, but interaction with almost any radical removes the H atom and breaks the Hg-H bond (Scheme 4.9). 

Initiation by light, retardation by radical scavengers, and the cyclization of intermediates indicate that the reaction of primary and secondary alkyl-mercury halides with the salts of secondary nitroalkanes [equation (53)]... 

Similarly, several examples of reactions of perliuoroalkyl halides have been demonstrated to follow an SrnI -type mechanism (Section 3, p. 75). Since, here too, dissociative electron transfer is likely under the conditions of the reactions [Section 2, pp. 54-56 (Andrieux et ai, 1990b)], the substitution process most probably also follows mechanism (134) rather than (103). The same is also likely to be true with alkyl mercurials (Russell, 1989). 

Grignard reagents, prepared from alkyl halides, react with a mercury(n) halide that contains the same halogen as the reagent to form alkyl mercury(n) halides ... 

Although the tin hydride + alkyl halide method is probably the most important way of making alkyl radicals, we should mention some other methods that are useful. We said at the beginning of the chapter that carbon-metal bonds, particularly carbon-transition metal bonds, are weak and can homolyse to form radicals. Alkyl mercuries are useful sources of alkyl radicals for this reason. They can be made by a number of routes, for example, from Grignard reagents by transmetailation. 

Table 1. Examples of Intra- and [ntermolecular Radical Addition to Alkcncs. Halogen Abstraction from Alkyl Halides and Reductive Cleavage of Alkyl Mercury Compounds H R1...

Halides of zinc, cadmium, and mercury are readily alkylated by aluminum alkyls (117, 303). All of the alkyl groups of the aluminum participate in the reaction with HgCl2. But the alkylation does not proceed beyond the formation of alkyl mercury chlorides, RHgCl, except in the presence of a complexing agent (e.g., NaCl). Then complete alkylation to the mercury dialkyl occurs. 

Unfortunately, radicals derived from alkylmercuries are even more limited in what they will react with than radicals made from alkyl halides by the tin hydride method. Styrene, for example, cannot be used to trap alkyl mercury-derived radicals efficiently because the radicals react more rapidly with the mercury hydride (which has an even weaker metal-H bond than Bu3SnH) than with the styrene. 

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