Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex

Abstract

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C₂OH₃]^- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C₂OH₃]^- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C₂O₂H₃]^3- fragment, which undergoes subsequent deoxygenation.

Keywords: Carbon Monoxide; Deoxygenation; Homologation; Hydride; Magnesium.

Figure 1

Examples of CO coupling and deoxygenation with metal hydrides. Non‐hydride and CO derived ligands have been omitted for clarity.

Figure 2

(a) Tetrameric magnesium hydride cluster 1 and its reaction with CO. (b) The crystal structure of 2.

Figure 3

Reactions of 2 with Ph₂CO, CNXyl (Xyl=2,6‐dimethylphenyl), PhCN, and t‐BuN=C=O. Ar=2,6‐di‐iso‐propylphenyl. Isolated yields in parentheses.

Figure 4

The crystal structure of 3b.

Figure 5

Proposed pathway for CO coupling and deoxygenation based on DFT calculations G09: ωB97X‐D/6‐311+G**/PCM (benzene)//ωB97X‐D/6‐31G**/6‐311+G*/SDDAll (Mg).

Figure 6

Electron‐flow in the deoxygenation step.