Mercury alkyls hydride

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). 

The Addition of Tin and Mercury Hydrides to Activated Double Bonds Hydro-alkyl-addition... 

Fig. 1.14. NaBH4, reduction of (/Thydroxyalkyl)mercury(II) acetates to alcohols and radical fragmentation of (/f hydroxy-alkyl)mercury (II) hydrides. According to the terminology used in Figure 1.2 it is a "substitution by fragmentation."...

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. 

The key propagation step in the mechanism is abstraction of hydride from the starting alkyl mercury. In the propagation step anything will do to cleave the weak Hg-H bond but once the chain is running it is an alkyl radical that does this job, just as in tin hydride chemistry. 

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 alkylmercury-derived radicals efficiently because the radicals react more rapidly with the mercury hydride (which has an even weaker metal-H bond than Bi SnH) than with the styrene. 

Cyclopentyl radicals substituted in the /1-position relative to the radical center are formed during the solvomercuration/reductive alkylation reaction of cyclopentene34. The organomer-curial produced in the first solvomercuration step is reduced by sodium borohydride and yields free cyclopentyl radicals in a radical chain mechanism. Addition of alkenes can then occur tram or cis to the / -alkoxy substituent introduced during the solvomercuration step. The adduct radical is finally trapped by hydrogen transfer from mercury hydrides to yield the tram- and ris-addition products, The transicis ratio depends markedly on the alkene employed and it appears that the addition of less reactive alkenes occurs with higher trans selectivity. In reactions of highly substituted alkenes, this reactivity control is compensated for by steric effects. Therefore, only the fnms-addition product is observed in reactions of tetraethyl ethenetetracarboxylate. The choice of alcohol employed in the solvomercuration step has, however, only a small influence on the stereoselectivity. 

It is a colourless gas which decomposes on heating above 420 K to give metallic tin, often deposited as a mirror, and hydrogen. It is a reducing agent and will reduce silver ions to silver and mercury(II) ions to mercury. SnSn bonding is unknown in hydrides but does exist in alkyl and aryl compounds, for example (CH3)3Sn-Sn(CH3)3. 

In the presence of proton and/or Lewis acid and strong nucleophiles bicyclo[3.2.0]heptan-6-ones are converted to 3-substituted cycloheptanones (Table 15). Bicyclo[3.2.0]heptan-6-ones rearrange to give 3-iodocycloheptanones on treatment with iodotrimethylsilane. Zinc(II) iodide or mercury(II) halides as catalysts enhance the rate and the selectivity of the reaction.31 If a second, enolizable carbonyl group is present, an intramolecular alkylation may follow the ring enlargement under these reaction conditions.32 Consecutive treatment with tributyltin hydride/ 2,2 -azobisisobutyronitrile affords reduced, iodo-free cycloheptanones, whilst treatment with l,8-diazabicyclo[5.4.0]undecene yields cycloheptenones.33 Similarly, benzenethiol adds to the central bond of bicyclo[3.2.0]heptan-6-ones in the presence of zinc(II) chloride and hydrochloric acid under anhydrous conditions to form 3-(phenylsulfanyl)cycloheptanones.34... 

When (/3-hydroxyalkyl)mercury(II) acetates are treated with NaBH4 and no additional reagent, they first form (( - h y d roxy a Iky 1) me rcu ry (II) hydrides. These decompose via the chain reaction shown in Figure 1.12 to give a mercury-free alcohol. Overall, a substitution reaction R—Hg(OAc) — R—H takes place. The initiation step for the chain reaction participating in this transformation is a homolysis of the C—Hg bond. This takes place rapidly at room temperature and produces the radical Hg—H and a /3-hydroxylated alkyl radical. As the initiating radical, it starts the first of the two... 

Dimethylbismuth chloride Dimethylcadmium Dimethylmagnesium Dimethyl mercury Dimethyl-phenylethynylthallium Dimethyl-l-propnylthallium Dimethylzinc Ethoxydiethylaluminium Methylbismuth oxide Methylcopper Methyllithium Methyl potassium Cobalt Hafnium Iridium Iron Lead Lithium Manganese Nickel Palladium Platinum Plutonium Potassium Pyrophoric alkyl non-metals Hydrides Dietbylarsine Diethylphosphine Dimethylarsine 1,1 -Dimethyldiborane 1,2-Dimethyldiborane Dimethylphosphine Ethylphosphine Methylphosphine Methylsilane... 

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