Preparation and tandem reactions of 2-trifluoroborate substituted dienes

Our present study was initiated to perform both enantioselective and exo-selective Diels-Alder(DA) reactions using main group element substituted dienes and catalytic amounts of transition metals. There are certain types of dienes (specifically, substituted Z-dienes), which do not react well in thermal or Lewis acid-catalyzed reactions. Due to steric reasons, these dienes are not suitable in adapting the required s-cis conformation, crucial for a concerted cycloaddition reaction. Nevertheless, an E-diene that reacts through an exo transition state gives the same relative stereochemistry in DA cycloadducts as that of Z-diene, reacting through the traditionally favored endo transition state. Based on these facts, our group developed a synthetic methodology using transition metal substituted dienes (mostly cobalt substituted dienes) to access these cycloadduct's stereochemistries that were not readily available using Z-dienes. But the methodology involved stoichiometric amounts of transition metal dienyl complexes. We anticipated that the use of main group element substituted dienes (for e.g. boron, silicon and aluminium) would offer practical advantages over stoichiometric use of transitionmetal-substituted dienes in terms of cost of preparation, disposal and their participation in catalytic chemistry.; In recent years organotrifluoroborate salts have emerged as a new class of boron derivatives, and are being widely used in different synthetic processes (e.g., Suzuki reactions, rhodium-catalyzed 1,4 additions and allylation of aldehydes). The beauty of this class of compounds lies in their monomeric nature, high stability towards air and moisture, easy preparation in high yields and they can be stored for long periods of time. To date only a few reports are available on 2-boron substituted 1,3-dienyl derivatives and corresponding DA cycloaddition chemistry presumably due to their high propensity towards dimerization. We for the first time have prepared 1,3-dienyl-2-trifluoroborates as a stable, monomeric salts and demonstrated their ability to participate in tandem Diels-Alder/cross-coupling reactions under mild conditions.; To extend our methodology representative model alkyl and aryl 4-substituted dienyl 2-trifluoroborates have also been prepared. The first step of their synthesis was the preparation of corresponding 2-halo-substituted dienes via allene intermediates using a standard literature protocol. The halo-substituted dienes were then subjected to halogen magnesium exchange reactions at room temperature followed by quenching with boron electrophile. In spite of its simplicity, the protocol has a major drawback, as undesired alkyl organoborates are also formed as by-products during the course of reaction. Unexpectedly, the usual Grignard chemistry was not successful to make the 4-substituted 2-trifluoroborates dienes series. Probable reason may be the presence of trace amount of impurities in conjunction with instability of halo-dienes at high temperature. Finally, a one-pot tandem Diels-Alder/cross-coupling reaction was performed with 4-phenyl-substituted 2-BF3 diene, which resulted exclusively the endo adduct.; A preliminary study as proof of principle has also been performed on transmetallation/Diels-Alder/hydrolysis sequences using different rhodium catalyst for enantioselective and exo selective DA reactions. The rationale behind this approach is to do transmetallation from boron to rhodium and perform DA reaction under conditions where the boron diene itself is unreactive while the rhodium dime generated in situ is expected to react via exo transition states. Then rhodium-carbon bond in that rhodium bonded cycloadduct eventually gets hydrolyzed to generate a metal free cycloadduct and rhodium catalyst. To prove our hypothesis, first we have done some preliminary work with unsubstituted boron diene along with either maleimide or N-phenyl maleimide as a dienophile. The results indicate that these reactions cannot be...