Olefin Metathesis by First‐Row Transition Metals

Abstract

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.

Isolation and Reactivity of an Iron Azametallacyclobutene Complex

“…52 The 1 H NMR spectrum of the reaction of 6 and benzonitrile contains 10 paramagnetically shifted peaks, consistent with the formation of the benzonitrile complex, tBu LFe(NCPh) (7a, Scheme 4). A similar compound is observed by 1 H NMR spectroscopy in the reaction of 6 and acetonitrile (7b, Scheme ). Characterization by X-ray crystallography of 7a prepared by an alternate route (see the Supporting Information) confirmed its molecular structure as illustrated in Scheme 4 (Figure 3).…”

“…Given the common migratory insertion behavior of group 4 azametallacyclobutene complexes, we were interested to learn if 3 followed the same pattern in the presence of a substrate without the potential for protonation, such as an aldehyde. Reaction of 3 and a slight excess of benzaldehyde leads to a new organometallic compound by 1 H NMR spectroscopy after 30 min (eq 3). Molecular structure determination by X-ray crystallography revealed the formation of insertion product 6 (Figure 2) as illustrated in eq 3.…”

“…[2 + 2] cycloaddition of a metal–ligand multiple bond and a π-bond in an organic substrate represents a key mechanistic step in myriad synthetically valuable transformations mediated by transition metals. , In the case of a metal imide complex and an alkyne, cycloaddition of the MN bond and substrate π-bond results in an azametallacyclobutene complex . This type of reactivity is proposed in many important transition metal-catalyzed C–N bond-forming reactions, notably, alkyne hydroamination − and carboamination − as well as the formal [2 + 2 + 1] synthesis of pyrroles. ,, …”

Synthesis of Vanadium Oxo Alkylidene Complex and Its Reactivity in Ring-Closing Olefin Metathesis Reactions

“…In addition, first-row metal alkylidenes can offer a unique reactivity, such as C–H bond activation, , which has the potential to be coupled with OM. Although first-row metal alkylidenes can perform the critical steps of OM (cycloaddition and cycloreversion), catalytic systems based on those metals remain underdeveloped. , …”

“…Although first-row metal alkylidenes can perform the critical steps of OM (cycloaddition and cycloreversion), catalytic systems based on those metals remain underdeveloped. 10,11 V alkylidenes are a promising class of compounds that have shown reactivity in OM, 12 especially in ring-opening metathesis polymerization (ROMP) of cyclic olefins. 13,14 Recently, our group reported a method of promoting α-hydrogen abstraction from V trialkyl complexes 4 by using phosphonium hydrochlorides to obtain complexes 5a−e in good yields (Scheme 1).…”