Part I. Palladium-catalyzed silylstannylations of diynes: Dynamic behavior and functionalization of helically chiral dienes. Part II. Palladium-catalyzed silylstannane additions to epoxyalkynes and their titanium(III)-mediated cyclizations

The formation of silylstannyl dienes from 1,6-diynes was first reported by our former research group members. These dienes are fascinating molecules as they exhibit helical chirality. However, monocyclic dienes are known to undergo fast helical isomerization. For our main goal of making a unidirectional helical polyolefin, a diene in which this helical isomerization is completely frozen is necessary. A bicylic motif was designed to obtain such a system by slowing down this isomerization. The chemistry of these silylstannyl dienes was unexplored. We probed the Stille coupling reactions of these compounds with various electrophiles. A new protocol for the Stille coupling which used 2 mol% Pd(MeCN)2Cl2, 4 mol% AsPh3 and 8 mol% CuI in DMF at room temperature was found to be effective for several difficult substrates. Using these conditions, iodobenzene, bromotrimethylsilyl acetylene, benzyl bromide and 1,4-diiodobenzene were efficiently coupled. Synthesis of a polyolefin was attempted by coupling of a silylstannyl diene with its iodide derivative under the same conditions. However, instead of the expected tetraene, a rearranged product was observed which presumably was formed via a 1,7-hydrogen shift.;The Palladium-catalyzed addition of silylstannane reagents to alkynes is known to be a synthetically useful reaction. The tolerance of a sensitive functionality like epoxide had not been reported before. We examined the addition of Bu3SnSiMe3 to epoxyalkynes and found that the desired addition product was formed in excellent yields. Such products can be subjected to the well-known Ti(III)-mediated epoxide opening-radical cyclization. The reaction presumably occurs via radical formation followed by an addition-elimination sequence to give the expected product. This reaction affords five-membered carbo- and heterocyclic compounds. The presence of the vinyl-TMS group at the terminal provides a potentially useful alcohol. Similarly epoxyalkynes undergo Pd-catalyzed distannation followed by Ti(III) mediated reaction to give cyclic alcohols with vinylstannane, which is more reactive than the corresponding vinyl vinylsilane. The application of this addition-elimination reaction was expected for the formation of four-membered carbocyclic compounds. However, the radical formed by epoxide opening abstracts the vinyl hydrogen via a 6-membered transition state and an internal alkyne was obtained.