2-silicon-substituted 1,3-dienes: Participants in Diels-Alder chemistry and metal-mediated processes

Two of the major focuses in current synthetic organic chemistry are asymmetric carbon–carbon bond forming processes and catalytic one-pot reactions. Two processes that are of interest to our research are the Diels–Alder reaction and transition-metal-catalyzed cross-coupling reactions. Issues that arise in these processes include poor efficiency and poor stereoselectivity. My dissertation research has centered around the synthesis and use of 2-silicon-substituted 1,3-dienes to alleviate these problems. These dienes, whether utilized as reagents or intermediates, serve as synthons for less reactive/selective 2-aryl 1,3-dienes.;In our early work three 2-trialkoxysilyl-1,3-butadienes were synthesized on multigram scales. Two of these dienes proved to be highly reactive towards olefins of varied substitution in Diels–Alder chemistry. The resulting silicon-substituted cycloadducts were then utilized in palladium-catalyzed cross-coupling reactions with various aryl iodides to yield aryl-substituted cyclohexenes in moderate to excellent yield.;Progression of this research led us to dienes of higher substitution; specifically at the prochiral termini of the diene. A series of 4-aryl- and 4-alkyl-2-silyl-1,3-dienes were synthesized using the ene-yne metathesis reaction. These dienes participated in diastereoselective Diels–Alder reactions with several cyclic dienophiles. In addition, palladium-catalyzed cross-coupling with various aryl iodides afforded the aryl-substituted cycloadducts with full retention of the relative stereochemistry.;Literature precedent for the Lewis acidity of ruthenium benzylidene metathesis catalysts led us to develop an ene-yne metathesis/ Diels–Alder multicomponent reaction of silyl acetylenes. Near complete endo-selectivity and decreased reaction times suggested that Lewis-acid-catalysis was occurring in the cycloaddition. This reaction proved to be tolerant of a wide variety of olefins and dienophiles of varied substitution and resulted in the synthesis of seven silicon-substituted cyclohexenes.;In pursuit of another catalytic one-pot process, we attempted to utilize our 2-silicon-substituted 1,3-dienes in a transmetallation/ Diels–Alder/ cross-coupling catalytic cycle. A series of trials were conducted to test various transition metal catalysts, fluoride sources, and other reaction conditions to affect this process. Research in this area is continuing in our group to not only achieve this reaction but to explore asymmetric catalysis.