Dimetallation and Related Reactions

Functionalized alkenes are prepared by the syn addition of dimetallic reagents and related compounds. a,j6-Dimetallated compounds 112, easily prepared by the Pd-catalyzed addition of dimetallic reagents (R M-MR 0 of main group metals to alkynes, are reactive, and can be converted to functionalized alkenes 113. Also addition of dichalcogen compounds produces the functionalized alkenes 114. Many examples are known, and some recent examples are cited. 

The first step of the Pd-catalyzed silylstaimation-cyclization of the 1,6-enyne 115 is silylpalladation of the triple bond to generate alkenylpalladium 117, and the cyclopentane bearing silylvinyl group 116 was obtained via 118. The carbene-Pd complex 119 was the most effective catalyst [35]. 

Borylsilylation of 1-octyne with 120 provided the alkenylboronate 121 in high yield using Pd(OAc)2 and bulky isocyanide (1,1,3,3-tetramethylbutyl isocyanide, XVll-6) as a ligand. The product 121 was subjected to Suzuki coupling catalyzed by PdCBCdppf) [36], Under similar conditions, no intermolecular bis-silylation of internal alkynes took place. However, intramolecular bis-silylation of disilanyl ether of homopropargylic alcohol proceeded smoothly with 5-exo-dig cyclization [36a]. 

Although mechanistically different, functionalized alkenylsilanes are prepared stereoselectively by the reaction of 1-alkynes with iodotrimethylsilane (123) and diethylzinc. At hrst oxidative addition of 123 to Pd(0) generates 125. Then insertion of 1-octyne to 125 affords the alkenylpalladium 126. Transmetallation with Et2Zn gives 127 and reductive elimination provides the alkenylsilane 124. The reaction can be regarded as a Heck-type reaction of alkyne with MesSi-I, followed by Negishi coupling [37]. 

Various 1,2-bifunctionalized alkenes containing heteroatoms are prepared by addition to alkynes. Thiophosphorylation of 1-octyne with phosphorothiolate 131 provided (Z)-l-(diphenoxyphosphinyl)-2-(phenylthio)-l-octene in good yield [40]. A useful synthetic method for (Z)-3-phenylthioacrylate derivative is the Pd-catalyzed thioesterification of 1-alkynes with 6)-methyl S-phenyl thiocarbonate (132). Addition of 132 to 1-octyne using Pd(PCy3)2 as a catalyst afforded methyl (Z)-3-phenylthio-2-nonenoate in 86% yield [41]. (Z)-l,2-Bis(phenylthio)alkene was prepared in good yield by stereoselective Pd-catalyzed addition of (PhS)2 (3) to 1-alkyne [2]. 

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