Lithium-metal traps

Preparation of pure lithium-metal traps and ingots (molten lithium chloride electrolysis). Even if in some older processes the metal was prepared by direct metallothermic reduction of the lithium oxide with magnesium or aluminum, today lithium metal is essentially obtained directly by molten-salt electrolysis of LiCl-KCl according to a process... 

Lithium metal or alkyllithium derivatives react with dihalocyclopropanes to provide the corresponding lithiohalocyclopropanes I which are stable at temperatures around —100 °C. These metalated species are easily trapped with electrophiles (R—X) like methyl or ethyl iodide, trimethylstannyl chloride, tri-methylsilyl chloride etc. In the case of the unsaturated bicyclic substrate II a double bond migration is observed, which in the presence of excess starting bromide is accompanied by isomerization of the axo-lithio intermediate III to its endo-isomer IV [58],... 

Various examples of the use of dissolving metal reduction on exocyclic double bonds, conjugated either with another C —C double bond53 or a carbonyl group,54 have been reported. For example, reaction of 2,6,6-trimethyltricyclo[5-4.0.0I,5]undec-7-en-9-one with lithium metal in liquid ammonia gave, after trapping of the intermediate enolate, 2,6,6-trimethyl-9-tri-fyloxy[5.4.0.015]undec-8-enc (3).53... 

The same hypercoordinate silylene (79) was generated by the Corriu group in dehalogenation reactions the best results were with the difluoride Ar2SiF2 (Ar = o-Me2NCH2Ph) and lithium metal or lithium naphthalene97. The silylene was trapped by 1,4-addition to 2,3-dimethylbutadiene. Similar defluorination reactions were used to obtain silylenes 87-89 shown in Scheme 25, all trapped with dimethylbutadiene. 

Treatment of substituted phthalans 1172 with lithium metal in the presence of catalytic quantities of naphthalene leads to reductive cleavage of the arylmethyl carbon-oxygen bond to form a stable dilithium compound 1173, which upon trapping with carbon dioxide furnishes isochroman-3-ones 1174 (Scheme 289) <1996JOC4913>. 

Benzyne is a good dienophile. For example, reaction of 2-bromofluorobenzene with lithium metal or decomposition of benzenedi-azonium-2-carboxylate both generate benzyne, which can be trapped with furan to give a naphthalene endoxide (Scheme 4.10). 

We started our investigation with the reduction of (Z)-styryltrimethylsilane, (Z)-13, R = Ph [21] When brought to reaction with lithium metal in diethyl ether the usual product of reduction 23 is found, trapped as the dimethyl derivative 24 after work-up with dimethyl sulfate 24 is isolated in 87% yield as a 1 3 mixture of erythro and threo compounds. On the other hand, upon heating (Z)-13, R = Ph, for 10 hours in toluene 28 is obtained derived from the 1,4-dilithium intermediate 27 (Scheme 5). 

The reaction of l-chloro-3,3-dimethyl-2-phenylcyclopropene with lithium metal resulted in lithium-halogen exchange and the derived cyclopropene 1 was trapped by a range of electrophiles giving substituted cyclopropenes 2. 

A major limitation of the Lagow procedure for preparative purposes is the trapping of the lithiocarbon species in the lithium metal matrix, but techniques have been developed to minimize this problem The reaction products have been purified by grinding under argon and the resulting powder sieved resulting in removal of substantial amounts of lithium metal and approximately 85 % pure lithiocarbon product. Subsequently the products were extracted with cold THF to remove LiCl and were then separated from lower density lithium rich particles by flotation on cold THF. Unfortunately, however, methods have not yet been developed to separate one lithium substituted hydrocarbon from a mixture of others. 

Our approach for starting material suitable as a silyl dianion synthon (D) was via the bis(diphenylmethyl)-substituted silane 15. It was selectively cleaved to give the silyllithium compound 16, which resulted, after a trapping reaction with chlorotrimethylsilane, in disilane 17 (Scheme 3 yield of 17 80 %). The isolated and purified disilane 17 then was cleaved at the Si-C bond with lithium metal, resulting in the lithiated silane 18. Another trapping reaction with chlorotrimethylsilane gave the trisilane 19 (yield of 19 78 %). 

In the experiments described the enantiomerically pure disilane (/ )-l was cleaved with lithium metal in THF at -70 °C (Scheme 1). The completeness of the cleavage reaction was proven by GC-MS and H NMR spectroscopy after a trapping reaction with MesSiCl. The resulting disilane 3 could be isolated with ee > 98 %. In solution lithiomethylphenyl(l-piperidinylmethyl)silane (2) racemizes within a few hours at temperatures between 0 and 20 °C, but a very slow decomposition of 2 was also observed. Due to this decomposition no exact determination of the reaction rate law was possible. 

A report by the Corriu group, however, shows that this is not necessarily the case (equation 49) . When difluorosilane was treated with lithium metal in the presence of dimethylbutadiene, the silacyclopentene was obtained in good yield but no silylene was intercepted if EtsSiH was present as the trapping agent. The product silacyclopentene could result either from reaction of the diene with a silylenoid, or from reaction of the diene with lithium and trapping of the lithium compound by the difluorosilane. In earlier studies of the alkali metal induced reactions of dihalosilanes with 1,3-dienes, it was found that the diene, rather than the dihalosilane, reacts initially with the metal. ... 

The synthesis of S-phosphonothiazolin-2-one 133 started with 2-bromothiazole 129. Nucleophilic displacement of the 2-bromide proceeded cleanly with hot anhydrous sodium methoxide to give 2-methoxythiazole 130. Low-temperature metalation of 130 with n-butyl lithium occurred selectively at the 5-position (76), and subsequent electrophilic trapping with diethyl chlorophosphate produced the 5-phosphonate 131. Deprotection of 131 was accomplished either stepwise with mild acid to pn uce the thiazolin-2-one intermediate 132, or directly with trimethylsilyl bromide to give the free phosphonic acid 133, which was isolated as its cyclohexylammonium salt. 

In recent years, a variety of aryl boronic acids are commercially available, albeit in some cases they may be expensive for large scale purposes. During our work in the mid-1990 s boronic acid (II) was not commercially available and so two different protocols were used to prepare this acid. The first approach involved the transmetallation with n-butyl lithium of aryl bromide (I) and trapping the lithio species generated with trialkyl borate followed by an acid quench. Aryl bromide (I) is easily prepared by reaction of o-bromobenzenesulfonyl chloride with 2-propanol in the presence of pyridine as a base. The second approach was a directed metallation of isopropyl ester of benzene sulfonic acid (VII), to generate the same lithio species and reaction with trialkyl borate. The sulfonyl ester is prepared by reaction of 2-propanol with benzenesulfonyl chloride. From a long-term strategy the latter approach is... 

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