New Strategies for Carbonyl-Olefin Metathesis
Ludwig, Jacob
2019
Abstract
Organic molecules are primarily made up of carbon-carbon (C-C) bonds. As a result, methods that promote the formation of these bonds are of the utmost importance in synthetic chemistry. Of the many known strategies that achieve this transformation, transition metal-catalyzed olefin-metathesis is distinctively suited for the construction of carbon-carbon bonds – specifically C-C double bonds – that would be otherwise difficult to form. Therefore, olefin metathesis enables access to new molecules with extraordinary properties that are used in biology, medicinal chemistry, and materials science. Advantageous characteristics that contribute to its importance include: highly active, discrete metal catalysts that function through a thoroughly-characterized mechanism, an operationally simple protocol that often produces an inert, easily removable olefin byproduct, and remarkable functional group tolerance that leads to a broad substrate scope. The corresponding carbonyl-olefin metathesis has the potential to exhibit the same beneficial characteristics, however, this reaction is comparatively less developed. New strategies for carbonyl-olefin metathesis are reported herein. FeCl3 has been identified as an earth abundant, environmentally friendly Lewis acid catalyst for ring-closing carbonyl-olefin metathesis. This reaction fundamentally differs from the state of the art for carbonyl-olefin metathesis that relies on metal alkylidene reagents. Specifically, it uses Lewis acid activation to promote the formation and fragmentation of oxetane intermediates, thus avoiding catalytically inactive metal-oxo species and allowing for catalytic turnover. Subsequent efforts build upon a divergent reaction pathway discovered during the expansion of Lewis acid catalyzed carbonyl-olefin metathesis: the interrupted carbonyl-olefin metathesis reaction. This reaction uses TfOH as a cheap, readily available catalyst, allows for the synthesis of uniquely substituted fluorene scaffolds, and operates via oxygen atom transfer as a new reaction paradigm for carbonyls and olefins. While metal alkylidene-mediated carbonyl-olefin metathesis reactions form catalytically incompetent metal-oxo species, and thus suffer from the need for stoichiometric amounts of precious reagent, they are superior over other strategies for carbonyl-olefin metathesis in that they exhibit a broad substrate scope reminiscent of olefin metathesis. A ruthenium alkylidene-mediated hydrazone/oxime-olefin metathesis is developed as a method that stimulates metathesis between aldehyde derivatives and terminal olefins, which are two functional groups that are incompatible with the Lewis acid-catalyzed method. Unlike the inert metal-oxo byproducts of previously reported approaches, this reaction forms a highly reactive Ru nitride as the metal byproduct. The high reactivity of this species could be exploited to regenerate a Ru alkylidene, thus permitting Ru-catalyzed hydrazine/oxime-olefin metathesis as a new approach for catalytic carbonyl-olefin metathesis.Subjects
carbonyl-olefin metathesis
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