Mercury-Lithium Exchange Reactions

The procedure to replace mercury in organic compounds by lithium was developed already in 1917 by Schlenk and Holtz and it is still the method of choice for the preparation of halide-free organolithium compounds. Wittig and Bickelhaupt were the first who used a mercury-lithium exchange reaction for the synthesis of a dilithiocompound not available by halogen-lithium exchange, o-dilithiobenzene 75. 

A recent example for this reaction is the successful synthesis of heterosubstituted 

The mechanism proceeds via radicals and is obviously rather complicated passing even through dimers as intermediates. Thus during synthesis of 1,1-dilithio-l-alkenes 81 from 80 by direct mercury-lithium exchange, we often obtained up to 7% of the 2,3-dilithio-l,3-butadienes 83 which might have been formed from the corresponding mercury compounds 82. That 82 is in fact an intermediate in the 

A similar mechanism might become operative during synthesis of 1,1 -dilithioalkanes, because besides CHjLij traces of lithium carbide (CjLij) have been found upon treatment of CHjfHgl) with lithium dust in diethyl ether 

Starting with C(HgCl), finally the dimeric products hexalithioethane (CjLig) and tetralithioethylene (C Li ) (ratio 4 3) have been obtained predominantly, without any doubt attributable to dimerization of radical intermediates The direct mercury-lithium exchange reaction therefore is not very suitable for the synthesis of CLi. 5 . 

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The dilithiated vinyllithium compound 1,2-dilithioethylene (132) was synthesized by Maercker and coworkers, using the mercury-lithium exchange reaction . Bis(chloromercurio)ethylene (131) was reacted with four equivalents of f-butyllithium... 

Lithiotrisilanes, which are used as precursors to polysilane dendrimers, have been obtained by a mercury-lithium exchange reaction (equation 25)56,61. The terminally metalated trisilane Me3Si(Me2Si)2Li has been synthesized analogously56. 

A variety of linear and branched oligosilanyUithiums can also be obtained by mercury-lithium exchange reactions in good yields (Scheme 20) (70). 

Transmetalation with rert-butyllithium, however, is not always the better way to achieve mercury-lithium exchange reactions. Although we had no problems with the... 

Due to intramolecular coordination (see Sect. 2.7) 4,4-dilithio-l-butene 33 is more stable and can be prepared by a direct mercury-lithium exchange reaction although lithium hydride elimination yields a conjugated system 97 Rearrangement to a cyclopropylcarbinyl species 34, however, was not observed (see Sect. 2.7). 

Lithiation of dibenzofuran with butyllithium and mercuration both occur at the 4-position. Thallation occurs at the 2-position, however (57IZV1391). The mercury and thallium derivatives serve as a source of the iodo compounds by reaction with iodine. Bromodibenzofurans undergo bromine/lithium exchange with butyllithium and the derived lithio compounds may be converted into phenols by reaction with molecular oxygen in the presence of a Grignard reagent, into amines by reaction with O-methylhydroxylamine, into sulfinic acids by reaction with sulfur dioxide, into carboxylic acids by reaction with carbon dioxide and into methyl derivatives by reaction with methyl sulfate (Scheme 100). This last reaction... 

Such exchange reactions between mercurials and lithium alkyls and between mercurials and Grignard reagents had previously been demonstrated by Gilman and Jones 49> as well as by Salinger and Dessy 105>. In addition, Daniel and Paetsch 39> carried out a low temperature oxidation of the ylid with molecular oxygen in tetrahydrofuran, producing a soluble peroxide whose presence was detected both by iodometric titration with sodium thiosulfate and by the isolation of formaldehyde from the reaction mixture after hydrolysis of the formocholine intermediate 3 with acid. 

Lithium-Halogen Exchange. Reaction of (dibromomethyl)-trimethylsilane with n-butyllithium at — 110°C results in the formation of trimethylsilylbromomethyllithium. The silane and the -BuLi are added simultaneously in order to suppress side reactions. Treatment of the intermediate lithium reagent with chlorotrimethylsilane (eq 1) or mercury(II) bromide (eq 2) affords bis(trimethylsilyl)bromomethane or bis(trimethylsilyl-bromDmethyl)mercury, respectively. If the lithium reagent is... 

Mercury phthalocyanine (PcHg) is prepared by lithium-metal exchange between Li2Pc and mercury(II) chloride.59 A different kind of phthalocyanine, bis(methylmercury) phthalocyanine [Pc(HgX)2], can be obtained by the reaction of phthalocyanine and methylmercury bis(trimethylsilyl)azide in benzene293 or by heating phthalocyanine with methylmercury(II)... 

Suitable candidates for a-elimination reactions are silylmethyl halides (— base-induced elimination of H-Hal), silylmethyl dihalides (— halide/metal exchange followed by elimination of a metal halide) and stable carbenoid-type compounds such as (a-halo-a-silylalkyl)mercury compounds (— thermal elimination of mercury(II) halide). Bis(phenylthio)(trimethylsilyl)methyl lithium (— elimination of LiSPh) represents a borderline case (see Section III.E.8). 

An examination of Table 2 reveals that although mercuric acetate and mercuric nitrate have often been used as electrophilic reagents, there are but few instances in which independent evidence as to their mechanism of reaction has been put forward. Positive kinetic salt effects have been observed in the substitution of sec.-butylmercuric acetate by mercuric acetate (with lithium nitrate in solvent ethanol)2, the substitution of di-sec.-butyl mercury by sec.-butylmercuric nitrate (with lithium nitrate in solvent ethanol)11, and the substitution of tetraethyltin by mercuric acetate (with tetra-n-butylammonium perchlorate in methanol)7. In the latter case, it was suggested7 that the observed very large positive kinetic salt effect was possibly due to anion exchange between mercuric acetate and the perchlorate ion. 

Generally, the metal-metal exchange is more rapid than the halogen-metal exchange. The reaction of n-butyllithium with di-p-bromophenyl-mercury, for example, yields only di-n-butylmercury and -bromophenyl-lithium. 

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