Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts

Deoxygenative Reduction of 1°, 2° and 3° Amides via In Situ Generated Tungsten Cluster Acting as H-Bonding Donor

A protocol of amide-to-amine transformation that is convenient, mild, and easy to perform is a long-sought goal from the viewpoint of catalysis. Herein, we have utilized easily accessible W(CO)4(NCMe)2 as a precatalyst for the deoxygenative reduction of 1°, 2° and 3° amides under mild conditions. Mechanism studies unraveled that the in situ generated W4-cluster is potentially a vital species in activating C═O of amides by providing the H-bonding network during the catalytic cycle. Significantly, this tactic offers a complementary pattern to conventional metal-catalyzed amide deoxygenative reactions with activation of the C═O bond via direct coordination with amide.

Thermal Formation of Metathesis-Active Tungsten Alkylidene Complexes from Cyclohexene

A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh3)2(C2H4), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh3)2(C2H4), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WCα position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

Ring Contraction of a Tungstacyclopentane Supported on Silica: Direct Conversion of Ethylene to Propylene

The reaction of W(NAr)(¹³C₄H₈)(OSiPh₃)₂ (1) (NAr = 2,6-diisopropylphenylimido) with silica partially dehydroxylated at 700 °C (SiO_2-700) is highly dependent on the reaction conditions. The primary product of this reaction is W(NAr)(¹³C₄H₈)(OSiPh₃)(OSi(O-)₃) (2) when the reaction is carried out in the dark. Grafting 1 onto SiO_2-700 in ambient lab light results in the formation of 2, W(NAr)(¹³CH₂¹³CH₂)(OSiPh₃)(OSi(O-)₃) (4), and one isomer of square-pyramidal W(NAr)(¹³CH₂¹³CH(¹³Me)¹³CH₂)(OSiPh₃)(OSi(O-)₃) (3). Heating 2 to 85 °C for 6 h results in the formation of 3, 4, W(NAr)(¹³CH(¹³Me)¹³CH₂¹³CH₂)(OSiPh₃)(OSi(O-)₃) (5), and W(NAr)((¹³CH₂)₂¹³CH(¹³Me)(¹³CH₂)₂)(OSiPh₃)(OSi(O-)₃) (6). Photolysis of 2 with blue LEDs (λ_max = 450 nm) produces 4, both isomers of 3, 5, and free ethylene. In the presence of excess ethylene and blue LED irradiation at 85 °C, 1/SiO_2-700 catalyzes the direct conversion of ethylene to propylene.

Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Herein we disclose the synthesis of sterically encumbered dialkylnickel(II) complexes bearing 2,9-dimethyl-1,10-phenanthroline ligands. A comparison with their unsubstituted analogues by both X-ray crystallography and theoretical calculations revealed significant distortions in their molecular structures. Eyring plots along with stoichiometric and photoexcitation studies revealed that sterically encumbered dialkylnickel(II) complexes enable facile C(sp³)–C(sp³) reductive elimination, thus offering an improved understanding of Ni catalysis.