Transmetallation Hexamethylditin

A copper-mediated cyclization of metallated thiophenes has been utilized to prepare polycyclic thiophenes and thiophene cyclophanes. Treatment of dibromide 106 in succession with M-butyllithium (halogen-metal exchange), zinc chloride (transmetallation), and copper chloride gave 7//-cyclopcnta[ 1,2-fc 4,3-6 dithiophene (107) <00H(52)761>. This conversion has also been achieved using a palladium-mediated cyclization performed in the presence of hexamethylditin . Copper-mediated cyclizations have also been applied to the syntheses of cyclopenta[2,l-6 3,4-A ]dithiophen-4-one (108) (three steps from 73) <00S1253> and cyclophane 109 <00CC2329>. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

Oxidative addition of certain stannanes to Pd(0) complejKS is also possible. Thus, aUcynylstannanes have been shown to react with Pd(0) complexes [207, 208). In addition, the Pd(0)-catalyzed reaction of allylstannanes with alkynes has been found to afford aUylstannylation products 31 (Scheme 1.23) [209]. A likely mechanism involves oxidative addition of the allylstannanes to Pd(0) to give (ri -allyl) palladium complexes 32 (L = alkyne) (Scheme 1.23). In this transformation, the usually nucleophilic allylstannanes behave as electrophiles. Complexes of type 32 are probably formed by transmetallation of (T) -aUyl)palladium complexes with hexamethylditin [210]. An oxidative addition to form complexes 32 has been proposed in the Pd(0)-catalyzed carboxylation of allylstannanes with COj [211]. Although compleres 32 have not been isolated as stable species, work on the intramolecular reachon of allylstannanes with alkynes and theorehcal calculations give support to the formation of these complexes by the oxidahve addition of allylstarmanes to Pd(0) [212]. 

Dithieno[3,4-6 3, 4 -t7 thiophene (243) was first prepared in 1971 starting from 3,4-dibromothiophene (14) [95]. Transmetallation then reaction with (PhS02)2S formed 300, reaction of which with n-BuLi and then oxidative ring closure provided dithieno[3,4-h 3, 4 -df thiophene (243). Alternatively, 243 can be accessed from 300 via the use of Pd(PPh3)4 and hexamethylditin (Scheme 74) [118, 119]. 

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