Sulfur-lithium exchange reactions

Sulfur-lithium exchange is easier and has much greater potential (much of it still unrealised) when the sulfur is at the sulfoxide oxidation level. It has long been known that organolithiums, like Grignard reagents, will attack a sulfoxide, displacing with inversion at sulfur the substituent best able to support an anion. The reaction has been commonly used to form sulfoxides with defined stereochemistry 152157... 

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... 

Procedures which utilize selenides are similar, but a-lithio selenides are not generally preparable via simple deprotonation chemistry, due to facile selenium-lithium exchange. - Selenium-stabilized anions are available, however, by transmetalladon reactions of selenium acetals and add readily to carbonyl compounds. The use of branched selenium-stabilized anions has been shown to result exclusively in 1,2-addidon to unhindered cyclohexenones, in contrast to the analogous sulfur ylides. The resulting 3-hydroxy selenides undergo elimination by treatment with base after activation by alkylation or oxidation (Scheme 10). An alternative method of activating either p-hydroxy selenides or sulfides toward elimination involves treatment of a chloroform solution of the adduct with thallium ethoxide (Scheme 11). A mechanism involving the intermediacy of a selenium ylide is proposed. 

It is known that on treatment of alkyl aryl sulfoxide 1 with alkylmetal (alkyl-lithium or Grignard reagent) sulfur-aryl (path a) or sulfur-alkyl (path b) bond cleavage occurs to give arylmetal 2 or alkylmetal 3 (Scheme 3.9). This reaction is commonly called the hgand exchange reaction of sulfoxides. Although the predominant path of this reaction depends on the structure of the sulfoxide 1, this dependence remains somewhat obscure at present. 

An enantioselective synthesis of chiral QUINAP 234 was reported by Knochel et al. (07SL2655). The organolithium species obtained from l-(2-bromo-l-naphthyl)isoquinoline by treatment with f-BuLi reacted with (—)-menthyl (S)-p-toluene-sulfinate at-78 °C. The resulting diastereomers were separated via column chromatography. One pot sulfoxide lithium exchange at low temperature, Ph2PCl reaction, sulfur protection with Ss and a Raney-Ni desulfurization step afforded optically pure QUINAP (99% ee) in 60% yield. The s)mthetic route avoided the use of Pd complexes for the resolution. The ees were determined after resulfurization on Chiralcel OD-H. 

Enantiomerically pure methyl phenyl sulfoxide and methyl p-tolyl sulfoxide in diethyl ether or dimethoxyethane were reacted with alkyllithium reagents. The exchange reactions were found to proceed with inversion of configuration at sulfur. The yields are somewhat variable, but Johnson s work did indicate that special procedures are necessary for the generation of hthium a-sulfinyl carbanions for use in other synthetic purposes for example, Johnson recommended the use of lithium dialkylamides as bases, since nucleophilic substitution can occur either on the neutral molecule or on the carbanion with alkyllithium reagents. 

A further special area of propulsion systems is Chemical Thermal Propulsion (CTP). CTP is defined in contrast to STP (solar thermal propulsion) and NTP (nuclear thermal propulsion). In CTP, in a very exothermic chemical reaction in a closed system, heat but no pressure is generated since the products of the reaction are solid or liquid. The heat energy is then transferred to a liquid medium (the propellant) using a heat exchanger, which is responsible for the propulsion of for example, the torpedo. Suitable propellants are e.g. water (the torpedo can suck it in directly from its surroundsings) or H2 or He, due to their very low molecular or atomic masses. The basic principles of CTP can also be used in special heat generators. A good example for a chemical reaction which is suitable for CTP is the reaction of (non-toxic) SF6 (sulfur hexafluoride) with easily liquified lithium (m.p. 180 °C) ... 

---