Organolithium with aryl halides

Pyrrolyl- and indolyl-stannanes and -boronic acids, which can be prepared from the corresponding organolithium derivatives, have received increasing use in palladium-catalyzed coupling reactions with aryl halides (Scheme 82) (91S613,92JOC1653). 

The equilibria mean that n-BuLi can be used to form organolithiums from aryl halides at low temperature (the subsequent reaction of ArLi with the BuX formed in the exchange is slow6) r-BuLi will form organolithiums from primary alkyl halides.7 The formation of secondary organolithiums by halogen-metal exchange is difficult,9 and reductive lithiation is usually preferable. 

The reaction of strong bases with aryl halides results in the generation of an aryne through loss of HX, Arynes can also be formed by reaction of aryl halides with lithium and organolithium compounds. 

B.iv.c. trans-a,pSubstituted Alkenylmetals. The accessibility of trani-a,/3-substituted alkenyhnetals is still rather limited, and only a few examples of this type of crosscoupling reactions have been reported (Table 6). Notably, trtmi-a,/3-substituted alkenylboronates can be prepared by hydroboration of 1-bromo-l-alkynes followed by subsequent treatments with organolithiums and bases, a procedure similar to that for the preparation of (Z)-/3-substituted alkenylboron compounds.t " Et The Pd-catalyzed crosscoupling reaction of the resultant organoboranes with aryl halides proceeds with complete retention of alkenyl stereochemistry to give trisubstituted alkenesP (Scheme 33). 

Organolithium reagents (Section 14 3) Lithi um metal reacts with organic halides to pro duce organolithium compounds The organic halide may be alkyl alkenyl or aryl Iodides react most and fluorides least readily bro mides are used most often Suitable solvents include hexane diethyl ether and tetrahy drofuran... 

Next to the formation of Grignard reagents, the most important application of this reaction is the conversion of alkyl and aryl halides to organolithium compounds, but it has also been carried out with many other metals, (e.g., Na, Be, Zn, Hg, As, Sb, and Sn). With sodium, the Wurtz reaction (10-93) is an important side reaction. In some cases, where the reaction between a halide and a metal is too slow, an alloy of the metal with potassium or sodium can be used instead. The most important example is the preparation of tetraethyl lead from ethyl bromide and a Pb—Na alloy. 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

Arylcopper intermediates can be generated from organolithium compounds, as in the preparation of cuprates.95 These compounds react with a second aryl halide to provide unsymmetrical biaryls in a reaction that is essentially a variant of the cuprate alkylation process discussed on p. 680. An alternative procedure involves generation of a mixed diarylcyanocuprate by sequential addition of two different aryllithium reagents to CuCN, which then undergo decomposition to biaryls on exposure to oxygen.96 The second addition must be carried out at very low temperature to prevent equilibration with the symmetrical diarylcyanocuprates. 

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