Chiral Carbocations as Lewis Acid Catalysts in Diels-Alder Reactions

Sammanfattning: Lewis acids can be used as catalysts in different reactions, but the term Lewis acid catalysts often refers to metal salts. Metal complexes have been widely used for asymmetric catalysis. Asymmetric synthesis can however be performed in a metal-free way by using organocatalysis. New Lewis acid catalysts that are more effective, enantioselective and environmental friendly is of interest. This new type of Lewis acid catalysts could for example be of carbocation based character. The aim of this project was to synthesize chiral carbocations with different degree of sterical hindrance and investigate their catalytic ability in Diels-Alder reactions. It was presumed that the Diels-Alder reactions were going to be performed in an asymmetric way since the carbocation catalysts were achiral. Two chiral carbocations were synthesized successfully. The first synthesized carbocation, the less sterical hindered compound 8, was formed as a racemic mixture. The second carbocation, compound 16, could be formed as an enatiomeric pure compound. Both carbocations showed catalytic ability in Diels-Alder reactions and compound 8 was comparable with some common Lewis acid catalysts. In general, when using compound 8 as catalyst, higher catalyst amount gave higher conversions. Higher concentrations also gave higher conversions, but up to a certain level. No trend between polarity of different solvents and conversions could be seen. However, an increased temperature leads to faster reactions. The more rigid and sterical hindered compound 16 catalyzed the reactions slower than compound 8. The longer reaction time may indicate that the reaction occurs with higher selectivity, but no method to measure the ee of the product was found. An attempt to synthesize a third even more sterical hindered chiral carbocation, compound 19, resulted in a product contaminated by impurities that showed a catalytic ability lower than compound 8 and compound 16 in Diels-Alder reactions. The synthesis and the use of carbocations as Lewis acid catalysts in Diels-Alder reactions seem promising as a new type of catalysts even though there are questions that are still unanswered, e.g. counter ions effects, possible side reactions, selectivity etc.