Synthesis and Activity of Vanadium Oxo NHC Alkylidenes. Remarkable Preference for Degenerate Metathesis and Application for Carbon Isotope Exchange

Abstract

V imido NHC phosphine alkylidenes are the most efficient V catalysts for ring-closing olefin metathesis of various terminal dienes. The presence of imido and phosphine ligands is responsible for catalyst decomposition. Therefore, the development of phosphine-free V oxo NHC alkylidenes is a logical next step to further improve V-based olefin metathesis. Herein we report V oxo NHC chloride and alkoxide alkylidenes and their reactivity in olefin metathesis. V oxo NHC chloride is readily involved in cycloaddition/cycloreversion steps with olefins. However, a remarkable preference for the formation of a 1,3-metallacyclobutane (MCB) leads to exclusive methylene group exchange (degenerate metathesis) utilized for carbon isotope exchange. DFT studies further support the preference for the 1,3-MCB formation.

Metallacyclobuta‐(2,3)‐diene: A Bidentate Ligand for Stream‐line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene

“…Multidimensional and multinuclear NMR spectral studies were performed on complex 3 with the most salient feature being the broad alkylidene carbon resonating at 348 ppm in the 1 H‐ 13 C{ 1 H} HMBC NMR spectrum (Figure S10). The latter feature is in agreement with the few characterized [V V ] alkylidene‐oxo complexes, (PNP)V=CH t Bu(=O) (335 ppm), [61] and (L) 2 V(=O)(=CHSiMe 3 )(Cl) (L=IMes, 332; PEt 3 , 337–343 ppm) [63] . Another salient feature is the 51 V NMR chemical shift of +809 ppm (Δν 1/2 =368 Hz, Figure S11), which is far downfield when compared to the vanadium oxo alkylidene, (PNP)V=CH t Bu(=O) (−58 ppm, Δν 1/2 =1304 Hz) [61] .…”

“…The pendant alkynyl motif was observed by IR at ν C≡C =2012 cm −1 (Figure S17), which is significantly red shifted in comparison to a previously reported chromium carbene‐alkynyl complex, (OC) 5 Cr(=C(NMe 2 )CC(AuPPh 3 ) (ν 1/2 =2321 cm −1 ) [60] . The V=C as well as V=O distance (1.601(1) Å) are comparable to those of the few known examples of [V V ] alkylidene‐oxo derivatives [61–64] . It should be noted that [V V ] alkylidene‐imido complexes are also active catalysts for ring‐opening metathesis polymerization reactions so this strategy provides an independent route to vanadium alkylidenes [65–67] .…”

“…[60] The V=C as well as V=O distance (1.601(1) Å) are comparable to those of the few known examples of [V V ] alkylidene-oxo derivatives. [61][62][63][64] It should be noted that [V V ] alkylidene-imido complexes are also active catalysts for ring-opening metathesis polymerization reactions so this strategy provides an independent route to vanadium alkylidenes. [65][66][67] Multidimensional and multinuclear NMR spectral studies were performed on complex 3 with the most salient feature being the broad alkylidene carbon resonating at 348 ppm in the 1 H-13 C{ 1 H} HMBC NMR spectrum (Figure S10).…”

“…The latter feature is in agreement with the few characterized [V V ] alkylidene-oxo complexes, (PNP)V=CH t Bu(= O) (335 ppm), [61] and (L) 2 V(= O)(= CHSiMe 3 )(Cl) (L = IMes, 332; PEt 3 , 337-343 ppm). [63] Another salient feature is the 51 V NMR chemical shift of + 809 ppm (Δν 1/2 = 368 Hz, Figure S11), which is far downfield when compared to the vanadium oxo alkylidene, (PNP)V=CH t Bu(= O) (À 58 ppm, Δν 1/2 = 1304 Hz). [61] The formation of complex 3 from oxidation of 2 provides strong credence that an unsaturated and strained metallacycle such as C could spring-open (e.g, retro-cyclo eliminate) to E to allow for an open coordination site for alkyne to bind and propagate the chain presumably via REMP.…”

Metallacyclobuta‐(2,3)‐diene: A Bidentate Ligand for Stream‐line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene

“…The pendant alkynyl motif was observed by IR at ν C≡C =2012 cm −1 (Figure S17), which is significantly red shifted in comparison to a previously reported chromium carbene‐alkynyl complex, (OC) 5 Cr(=C(NMe 2 )CC(AuPPh 3 ) (ν 1/2 =2321 cm −1 ) [60] . The V=C as well as V=O distance (1.601(1) Å) are comparable to those of the few known examples of [V V ] alkylidene‐oxo derivatives [61–64] . It should be noted that [V V ] alkylidene‐imido complexes are also active catalysts for ring‐opening metathesis polymerization reactions so this strategy provides an independent route to vanadium alkylidenes [65–67] .…”