Heteroaryl-substituted lithium organics

Charge delocalization can also be promoted by multiple aryl substitution at the metalated carbon atom. A lucid example is the Ph3C trityl anion. The central carbon atoms in [(TMEDA)LiCPh3]2 (192) and [(Et20)2LiCPh3] 37 (193) show almost ideal planar environment. The three phenyl groups cannot all be arranged in a coplanar fashion due to 

FIGURE 32. Solid-state stmctures of Uthium organics with heteroaryl-substituted lithium organics 

The Uthiate anion from the SSIP KTHFlgNa] [Li(CHPy2)2l (197) 

The lithiate anion from the SSIP [(TMEDA)2Li] [LilCfSiMesljlJ (201) 

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The reaction of thienyllithium with organic halides to form C—C bonds has been discussed in Section 2.10.4.7.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable thienyl metal derivative. The metal is usually zinc, magnesium, boron or tin occassionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated thiophenes with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. Besides these, a few other miscellaneous Pd-catalyzed substitution reactions on thiophene will also be referred to. 

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