Alkaline-Earth-Promoted CO Homologation and Reductive Catalysis

Reaction between a β-diketiminato magnesium hydride and carbon monoxide results in the isolation of a dimeric cis-enediolate species through the reductive coupling of two CO molecules. Under catalytic conditions with PhSiH3, an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.


General experimental procedures
All reactions dealing with air-and moisture-sensitive compounds were carried out under an argon atmosphere using standard Schlenk line and glovebox techniques in an MBraun Labmaster glovebox at O 2 , H 2 O < 0.1 ppm. NMR experiments using air-sensitive compounds were conducted in J. Youngs tap NMR tubes prepared and sealed in a glovebox under argon. All NMR data were acquired on a Bruker 300 Ultrashield TM for 1 H (300 MHz) and 13 C{ 1 H} (75.48 MHz) spectra at room temperature or a Bruker 400 Ultrashield TM for 1 H (400 MHz) and 13 C{ 1 H} (125.76 MHz) spectra. 1 H/ 13 C NMR spectra were referenced using residual solvent resonances. Elemental analyses of all moisture-and air-sensitive compounds were performed by Stephen Boyer of London Metropolitan Enterprises.
Solvents for air-and moisture-sensitive reactions were provided by an Innovative Technology Solvent Purification System. C 6 D 6 and toluene-d 8 were purchased from Fluorochem and dried over molten potassium prior to vacuum transfer into a sealed ampoule and storage in the glovebox under argon.
CO was purchased from BOC. Phenylsilane (PhSiH 3 ) and 13 CO were purchased from Sigma-Aldrich Ltd. Compound IV was synthesized using literature procedures. [1]

Synthesis of compound 1
A solution of compound IV (70 mgs, 0.15 mmol) in 0.5 ml toluene was freeze-thaw degassed and exposed to ~1 atm of CO. The reaction flask was resealed and left at room temperature for two hours before the solution was interrogated by NMR spectroscopic analysis, which revealed stoichiometric conversion to compound 1. The solvent was then removed in vacuo and the resultant pale yellow solid was redissolved in n-pentane. Storage of this solution at 30ºC for two days afforded crystals of compound 1 suitable for single crystal X-ray diffraction analysis. 1 Figure S4: Formyl signal observed in the 1 H NMR spectrum of a reaction performed between compound IV and 13 CO at 40ºC. Figure S5: Formyl signal observed in the gated 13 C-1 H NMR spectrum of a reaction performed between compound IV and 13 CO 40ºC.

X-ray analysis of compound 1
Single crystals of C 60 H 84 Mg 2 N 4 O 2 , compound 1, were obtained from n-pentane solution at 30ºC. A suitable crystal was selected and data were collected on a SuperNova, Dual, Cu at zero, EosS2 diffractometer. The crystal was kept at 150(2) K during data collection. Using Olex2, [2] the structure was solved with the olex2.solve [3] structure solution program using Charge Flipping and refined with the ShelXL 4 refinement package using Least Squares minimization.

Catalytic reduction of CO (representative procedure)
In a glovebox, to a vial containing compound IV (0.02 mmol, 10 mg), was added toluene-d 8 (0.5 mL) followed by PhSiH 3 (0.2 mmol, 27.8µl). The resultant solution was transferred to a NMR tube equipped with a J. Youngs tap which was sealed and removed from the glovebox. The solution was then freeze pumped thaw degassed to remove argon, exposed to 1 atmosphere of CO, resealed and warmed to 60 o C. NMR analysis was performed at regular intervals and conversions were analyzed by ratios of starting material to product, with products identified by comparison to literature values. Figure S6: 13 C{ 1 H} NMR spectra recorded after (a) one day, (b) three days, (c) one week during the reaction of PhSiH 3 and 13 CO catalyzed by 10 mol% IV at 60 o C. Figure S7: Expansions of the signals assigned to (a) PhH 2 Si-O-13 CH 2 -SiH 2 Ph and (b) PhH 2 Si-13 CH 3 products in the gated 13 C-1 H NMR spectra during the reaction of PhSiH 3 and 13 CO catalyzed by 10 mol% IV at 60 o C.