Zync vs precious metals: towards more sustainable synthetic transformations

Abstract

In this dissertation a comparative study of the activity of zinc versus noble metals has been carried out. Thus, different types of precious transition-metal catalysts and reagents have been studied. In Chapter 1, the isomerization of previously unexplored tetrasubstituted alkynyloxiranes in the presence of catalytic amounts of AuCl3 and Zn(OTf)2 has been accomplished. This study shows the strong influence of the structure of substrates in the reaction outcome compared with previously reported di- and trisubstituted epoxides. Indeed, a Meinwald-type rearrangement takes place leading to the formation of synthetically relevant α-alkynylketones, retaining the alkyne moiety. Among the two catalysts, Zn(OTf)2 showed similar or superior efficient. In Chapter 2, the feasibility of performing zinc(II) or rhodium(II)-catalyzed alkyne cyclopropenations using enynones as the carbene sources has been demonstrated. It is worth mentioning that both catalysts can control the selectivity of the process giving rise to different reaction outcomes. Thus, while the zinc catalyst affords cyclopropene frameworks, an unprecedented pattern in zinc carbenoid chemistry, the rhodium catalyst triggers cyclopropene cleavage to afford furan derivatives in an auto-tandem catalysis. In Chapter 3, a novel strategy for the catalytic generation of zinc vinyl carbenoids employing cyclopropenes as carbene source has been described. The zinc carbenoids generated can be efficiently trapped with alkenes. The overall transformation showed a remarkably broad scope and functional group tolerance, which makes this transformation a useful tool in organic synthesis. This is one of the few methodologies that allow the catalytic generation of vinyl zinc carbene intermediates. Noteworthy, the potential of this approach is far better when compared with gold catalysts employed, among others. Reactions accomplished in Chapters 1 and 3, employing inexpensive and low-toxic zinc catalysts, showed a better efficiency when compared with a prevalent noble metal such as gold. Besides, Chapter 2 represents an example of catalyst-controlled selectivity between zinc and rhodium catalysts. Based on these results, zinc catalyzed transformations could be considered as an efficient and sustainable alternative or complement to traditional precious transition-metal catalysts.