Mercury-lithium exchange

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. 

Enantiomerically pure organolithium compounds have been prepared by tin-lithium exchange [261], mercury-lithium exchange [549], or by lithiation in the presence of a chiral, enantiomerically pure or enriched amine, for example sparteine [271]... 

Mercury-lithium exchange, like other soft metal-lithium exchanges, is stereospecific and presumably retentive.11... 

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

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. 

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... 

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 >. 

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). 

Transmetalation using tin-lithium exchange is an excellent method — even better than mercury-lithium exchange — for the preparation of organolithium compounds, but it works only for compounds being more stable than n-butyllithium. This is also true for the synthesis of polylithiumorganic compounds. Examples are ( , )-1,5-dilithio-1,4-pentadiene 99the (Z,Z)-l,5-dilithio-l,4-pentadiene derivative 101 —... 

Therefore we used a stronger base than n-butyllithium, Zert-butyllithium, but in this case one has to switch over to mercury-lithium exchange (see Sect. 3.3), because it is not possible to place four terZ-butyl groups around the tin atom due to steric crowding. Thus Kuivila and coworkers recently have found that upon treatment of tetramethylstannane with tert-butyllithium only two methyl groups can be displaced by rerz-butyl groups. 

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... 

Alkynyllead triacetates can be prepared by tin-lead or by mercury-lead exchange.88,8811 However, more convenient methods were subsequently devised. In situ transmetallation can be directly realized by lithium-lead or zinc-lead exchange. This avoids the isolation of unstable alkynylmetal derivatives. Ikegami et al. found that, if a THF solution of... 

Anaiyze and Conciude Zinc-carbon, alkaline, mercury, lithium, and NiCad batteries all contain a separator between the anode and cathode that allows exchange of ions but keeps the anode and cathode reactants from mixing. No such separator is present (or needed) in a lead-acid battery. 

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)... 

Large scale enrichment of lithium for thermonuclear uses took place at the Oak Ridge National Laboratory in the 1950 s. The enrichment primarily employed ion exchange between aqueous/organic solutions and amalgam, commonly mercury-based (Palko et al. 1976). Electrochemical separation has also been employed for such operations (Umeda et al. 2001). These practices have not been taken up in academic laboratories in the intervening years, partly as they tend to be most effective only with relatively pure starting materials, partly because of the difference in scales involved. Enrichment factors of Li of 1-7% are typical for these techniques (Symons 1985). 

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. 

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