Synthesis of 1-Azatriene Complexes of Tungsten: Metal-Promoted Ring-Opening of Dihydropyridine

For nearly a century, chemists have explored how transition-metal complexes can affect the physical and chemical properties of linear conjugated polyenes and heteropolyenes. While much has been written about higher hapticity complexes (η4–η6), less is known about the chemistry of their η2 analogues. Herein, we describe a general method for synthesizing 5,6-η2-(1-azatriene) tungsten complexes via a 6π-azaelectrocyclic dihydropyridine ring-opening that is promoted by the π-basic nature of {WTp(NO)(PMe3)}. This study includes detailed spectroscopic and crystallographic data for the η2-dihydropyridine and η2-1-azatriene complexes, both of which were prepared as single regio- and stereoisomers.

and Table S6) Computational Methods (S40) (Table S7) General Methods: NMR spectra were obtained on an 800 MHz spectrometer.Chemical shifts are referenced to tertramethylsilane (TMS) utilizing residual 1 H or 13 C signals of the deuterated solvents as internal standards.Chemical shifts are reported in ppm and coupling constants (J) are reported in hertz (Hz).Infrared Spectra (IR) were recorded on a spectrometer as a glaze on a diamond anvil ATR assembly, with peaks reported in cm -1 .Electrochemical experiments were performed under a nitrogen atmosphere.Most cyclic voltammetric (CV) data were recorded at ambient temperature at 100 mV/s, unless otherwise noted, with a standard three electrode cell from +1.25 V to -1.25 V with a platinum working electrode, N,N-dimethylacetamide (DMA) or acetonitrile solvent, and tetrabutylammonium hexafluorophosphate (TBAH) electrolyte (~1.0 M).All potentials are reported versus the normal hydrogen electrode (NHE) using cobaltocenium hexafluorophosphate (E1/2 = -0.78V, -1.75 V) or ferrocene (E1/2 = 0.55 V) as an internal standard.Peak separation of all reversible couples was less than 100 mV.All synthetic reactions were performed in a glovebox under a dry nitrogen atmosphere unless otherwise noted.All solvents were purged with nitrogen prior to use.Deuterated solvents were used as received from Cambridge Isotopes.
The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back extracted with 50 mL of DCM to prevent product loss.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to 100 mL of stirring pentane.A tan precipitate formed immediately and was allowed to stir for ~10 minutes to ensure total precipitation.This powder was collected on a 60 mL medium porosity frit and washed 2x with 30 mL of pentane.This powder was dried in a desiccator under vacuum for ~30 minutes (78 mg, 43% yield).Methyl bromoacetate (224 mg, 1.46 mmol), 5D (956 mg, 1.07 mmol), and THF (12 mL) were charged to a flame-dried 50 mL round-bottom flask with a 1-inch stir bar.Zinc powder (281 mg, 4.30 mmol) was added to initiate the reaction, and the heterogenous solution was stirred for 1 hour.The zinc was removed by filtering the solution through a celite plug set up in a 60 mL coarse porosity frit with 1 inch of celite, which was then washed with residual DCM to prevent loss of product.The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back extracted with 50 mL of DCM to prevent product loss.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to 200mL of stirring pentane.A tan precipitate formed immediately and was allowed to stir for ~10 minutes to ensure total precipitation.This powder was collected on a 60 mL medium porosity frit and washed 2x with 30 mL of pentane.This powder was dried in a desiccator under vacuum for ~30 minutes (690 mg, 80% yield).

Synthesis and characterization of WTp(NO)(PMe3)(𝜂 2 -(N-tosyl)-2-(allyl)-1,2-dihydropyridine) (10D)
Allyl bromide (350 mg, 2.89 mmol), 5D (999 mg, 1.11 mmol), and THF (8 mL) were charged to a flamedried 50 mL round-bottom flask with a 1-inch stir bar.Zinc powder (323 mg, 4.93 mmol) was added to initiate the reaction, and the heterogenous solution was stirred for 2 hours.The zinc was removed by filtering the solution through a celite plug set up in a 60 mL coarse porosity frit with 1 inch of celite, which was then washed with residual DCM to prevent loss of product.The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back extracted with 50 mL of DCM to prevent product loss.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to 200mL of stirring pentane.A tan precipitate formed immediately and was allowed to stir for ~10 minutes to ensure total precipitation.This powder was collected on a 60 mL medium porosity frit and washed 2x with 30 mL of pentane.This powder was dried in a desiccator under vacuum for ~30 minutes (730 mg, 84% yield).
1 H NMR (CD2Cl2, δ, 25 °C): 8.14 (d, J = 1.9 Hz, 1H, PzA3), 8.05 (d, J = 1.9 Hz, 1H, PzB3  Propargyl bromide (220 mg, 1.85 mmol), 5D (935 mg, 1.04 mmol), and THF (10 mL) were charged to a flame-dried 50 mL round-bottom flask with a 1-inch stir bar.Zinc powder (391 mg, 5.98 mmol) was added to initiate the reaction, and the heterogenous solution was stirred for 2 hours.The zinc was removed by filtering the solution through a celite plug set up in a 60 mL coarse porosity frit with 1 inch of celite, which was then washed with residual DCM to prevent loss of product.The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back extracted with 50 mL of DCM to prevent product loss.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to 200mL of stirring pentane.A tan precipitate formed immediately and was allowed to stir for ~10 minutes to ensure total precipitation.This powder was collected on a 60 mL medium porosity frit.(666 mg, 82% yield).
Cleanup Procedure: extra clean material obtained by dissolving the obtained powder was then added to 100 mL of stirring diethyl ether and allowed to stir until homogeneous (~20 minutes).150 mL of pentanes were then added, causing a fine powder to precipitate out.This powder was then collected on a frit, and has a slight beige color.This powder is highly pure.

Synthesis and characterization of WTp(NO)(PMe3)(𝜂 2 -(N-tosyl)-2-(carbonitrile)-1,2dihydropyridine) (12D)
An oven dried 4-dram vial and stir pea were set up on a stir plate and charged with 5D (963 mg, 1.07 mmol).This was then diluted with MeOH (12 mL), and NaCN was added to the solution (189 mg, 3.86 mmol).This solution was then stirred for 2.5 hours.The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back extracted with 50 mL of DCM to prevent product loss.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to 200mL of stirring pentane.A tan precipitate formed immediately and was allowed to stir for ~10 minutes to ensure total precipitation.This powder was collected on a 60 mL medium porosity frit and washed 2x with 30 mL of pentane.This powder was dried in a desiccator under vacuum for ~30 minutes (534 mg, 65% yield).

Synthesis and characterization of WTp(NO)(PMe3)(𝜂 2 -(N-tosyl)-1,2-dihydropyridine) (13D)
Two separate solutions were prepared in two 30 mL glass screw-cap test tubes.Tube A was comprised of 5D (1.052 g, 1.17 mmol) dissolved in MeOH and Tube B was a heterogeneous slurry of NaBH4 (125 mg, 3.30 mmol) in dry THF (10 mL).The tube B slurry was added to tube A, and the tube sat for 10 minutes (bubbling occurred).The solution was diluted with 150 mL of DCM and added to a separatory funnel.This solution was washed 3x with 200 mL of saturated aqueous NaHCO3.The organic layer was isolated and set aside.The combined aqueous layers were combined and back-extracted with 50 mL of DCM to prevent loss of product.The organic layers were combined in a single flask and dried with anhydrous MgSO4.This powder was then filtered off into a 60 mL coarse porosity fritted funnel and washed with DCM.The dried organic layers were then reduced in vacuo to dryness.The residue in the flask was redissolved in minimal DCM (approximately 5 mL).The solution was then slowly added to a stirring 100mL solution of pentane.