Vanadium‐based catalysts have shown activity and selectivity in ring‐opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring‐closing olefin metathesis catalyzed by well‐defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron‐donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both β‐hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.
Catalytic olefin metathesis based on the second-and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to firstrow metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known.
Vanadium bis-phosphine imido and
oxo chloride alkylidenes have
been extensively applied in the ring-closing metathesis of various
acyclic olefins. However, their reactions involving ethylene have
shown limited productivity due to rapid decomposition. The primary
degradation pathway involving V bis-phosphine imido complexes is β-H
elimination at an unsubstituted metallacyclobutane. In contrast, β-H
elimination is disfavored for V oxo species, but bimolecular decomposition
precludes its high productivity. Herein, we present the synthesis
of V imido NHC complexes that are the most productive V catalysts
toward various terminal olefins in ring-closing metathesis reactions.
Experimental and computational studies suggest that β-H elimination
and bimolecular decomposition are disfavored for V imido NHC complexes.
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