Exchange sulfur-lithium

V. FUNCTIONALIZED ORGANOLITHIUM COMPOUNDS BY HALOGEN-LITHIUM OR SULFUR-LITHIUM EXCHANGE... 

These compounds, also called carbenoids for their tendency to decompose via an a-elimination into carbenes, have been prepared by a chlorine-lithium or sulfur-lithium exchange catalyzed by an arene. 

There are many examples of /3-amido and /3-oxido functionalized organolithium compounds, which were prepared mainly using the stoichiometric version of the arene-promoted lithiation ". In this section, the preparation of the same type of intermediates by halogen-or sulfur-lithium exchange will be considered, using the catalytic version of the mentioned lithiation. 

Oxido functionalized organolithium compounds of type 128 or 131 are also accessible by sulfur-lithium exchange using the arene-catalyzed lithiation technology. 

A different approach to 1,1-dilithium synthons takes advantage of both the activation of a sulfur atom at the a-position and a sulfur-lithium exchange, so making it possible... 

The sulfur-lithium exchange catalyzed by an arene (DTBB, 3.5%) generates in THF at —78°C, the 1,3-dilithium synthons, namely the l,3-(phenyhnercapto)propane 477, which is then treated with a carbonyl compound R R CO at the same temperature. After final hydrolysis 1,5-diols 478 were obtained (Scheme 133) °. 

Also in this case, the use of the chloro thioether 479 allowed the introduction of two different electrophiles in a sequential process. Using lithium naphthalene (the stoichiometric version of the arene-promoted lithiation) in THF at — 78°C, only a chlorine-lithium exchange occurred, so the first electrophile R R CO was introduced (—78 to —50°C). Then the second lithiation (sulfur-lithium exchange) takes place under catalytic conditions (naphthalene) and the second electrophile R R CO was introduced. After final hydrolysis, differently substituted 1,5-diols 476 were isolated (Scheme 134) °. 

Sulfones and sulfonamides - the S+O class are similarly powerful directors,141 and do not suffer electrophilic attack at sulfur (though occasionally suffer nucleophilic attack on the aromatic ring)134143 but are less useful because of their more limited synthetic applications.144 143 Aryl J-butylsulfoxides are also powerful ortho-directors,145 but less hindered diaryl sulfoxides are susceptible to attack by organolithiums at sulfur146 (see section 3.3.3 for discussion of this sulfur-lithium exchange ). 

There are no more than three or four examples of successful sulfur-lithium exchange in sulfides, and, as with alkyl chlorides, the conversion of sulfides to organolithiums is much better achieved reductively (see section 4.4.1). Nucleophilic attack on sulfide S displaces an organolithium only when that organolithium is highly stabilised, and was first noted by Brandsma,148 who found that PhLi displaced the alkynyllithium 185 from 184. 

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... 

The corresponding organolithium species Me3SiCH2Li, prepared from the chloride and lithium metal, or by bromine- or iodine-lithium exchange with butyl-lithium or sulfur-lithium exchange with lithium naphthalenide, can also prove effective. Addition of CeCls to this organolithium species provides a softer reagent that allows its use with sensitive carbonyl compounds. ... 

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