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Carbonyl-olefin metathesis reactions had been limited in scope, but this study demonstrates a catalyst and optimal reaction conditions that are generalizable for a variety of reactants opening the door for the synthesis of complex compounds. Iron(III)-catalysed carbonyl–olefin metathesis, Nature (2016). DOI: 10.1038/nature17432 Abstract The olefin metathesis reaction of two unsaturated substrates is one of the most powerful carbon–carbon-bond-forming reactions in organic chemistry.The reaction is practically useful as iron is an earth-abundant metal and is environmentally safe, and the reactions are scalable. Specifically, the catalytic olefin metathesis reaction has led to profound developments in the synthesis of molecules relevant to the petroleum, materials, agricultural and pharmaceutical industries1.
The carbonyl-olefin metathesis reaction was discovered around the same time as the olefin metathesis reaction, but has had limited applicability due to harsh reaction conditions and stringent reactant requirements. The olefin reaction mechanism involves a metal-catalyzed [2 2] cycloaddition of two allyls to form a four-member cyclic intermediate in which the metal is one of the members of the four-member ring.One involves a two-step photochemical cycloaddition followed by a thermolysis cycloreversion.Another involves a stoichiometric amount of transition-metal catalyst.These reactions provided insight into the carbonyl-olefin reaction mechanism.The reaction either proceeds via a carbocation intermediate or in a concerted fashion that involves metal coordination to Fe Cl.In general, they needed a catalyst that promotes [2 2] cycloaddition and [2 2] cyclorevrsion, does not promote side reactions such as polymerization or reforming reactants, and is chemoselective.They decided to investigate metal-derived Lewis acids.These reactions are characterized by their use of discrete metal alkylidene catalysts that operate via a well-established mechanism2.While the corresponding carbonyl–olefin metathesis reaction can also be used to construct carbon–carbon bonds, currently available methods are scarce and severely hampered by either harsh reaction conditions or the required use of stoichiometric transition metals as reagents.To test various Lewis acid catalysts, they reacted a β-ketoester with a pendant isoprenyl group with MCl in dicholorethane at room temperature for one-to-twelve hours. Their reaction conditions worked with electron withdrawing and donating groups producing products in good yield.Once a catalyst was determined and reaction conditions were optimized, the next step was to test the generalizability of the reaction. They then changed the β-substitution on the β-ketoesters and found the reaction worked with a variety of substituents.