Coupling reactions electrophilic site activation

Synthetic reactions involving activation of alkenyl- and arylsilanes by copper salts have also been reported. A copper-mediated system is effective in the cross-coupling reaction of aryl- and heteroarylsilanes with aryl halides under Pd-free conditions (Equation (13)).66 Alkenyl- and arylsilanes as well as alkynylsilanes undergo homocoupling in the presence of Cul and air.65,65a Functionalized alkenyl- and arylsilanes bearing a coordination site in the vicinity of silicon (e.g., 7) are easily activated by copper salts even at room temperature (Scheme 5).67,67l 67e 68 q-jle organocoppers such as 8 are useful for carbon-carbon bond-forming reaction with carbon electrophiles. 

SYNTHETIC STRATEGY SOLID PHASE APPROACH 229 TABLE 8.6 Activation of electrophilic site for coupling reaction... 

The classical example of the selective activation of a reaction site in a substrate with more than one reactive center is acetoacetic ester 168 (Scheme 2.80). Its reactive form is the enolate 169, which reacts with a variety of electrophiles selectively at the central carbon atom. A subsequent hydrolysis and decarboxylation of the product 170 leads to the formation of ketone 171. The structure of 171 corresponds to the coupling of the electrophile with the carbanion 172, or, in other words, with deprotonated acetone. Thus acetoacetic ester is actually employed in this sequence as a synthetic equivalent to 172. 

Enzyme-catalyzed Claisen-type condensations, and metal ion-independent decarboxylases. Carbon-carbon lyases generally involve inversion of configuration in cases where there is no readily identifiable electrophilic component to catalysis. This implies that catalysis of these reactions requires either the concerted or tightly coupled action of two or more active site functional groups, in order to trap (or avoid) unstable intermediates formed along the reaction pathway. 

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