Cyclometalation of 1-Aza 1,3-Dienes in reactions with Ru3(CO)12 with formation of four- and five-membered azaruthenacycles

Article abstract of DOI:10.1021/om00053a030

The dinuclear compounds Ru₂(CO)₆[R¹CH₂CC(H)NR₂] (2a-e) and Ru₂(CO)₆[R¹C=C(H)CH₂NR₂] (3a,b,d) are the first isolable products during thermal reactions of Ru₃(CO)₁₂ with 1-aza 1,3-dienes R¹C(H)=C-(H)C(H)=NR²(R¹,R²-MAD; R¹, R² = CH₃, i-Pr (1a), CH₃, c-Hex (1b), CH₃, t-Bu (1c), C₆H₅, i-Pr (1d), C₆H₅, T-Bu (1e)). Both primary products 2 and 3 contain an isomerized MAD ligand that is part of a four-(2) or five-membered (3) azaruthenacycle. The molecular structure of 3a has been solved by X-ray crystallography: space group P1?, with a = 22.511 (4) ?, b = 13.740 (3) ?, c = 12.443 (2) ?, α = 103.21 (1)?, β = 113.00 (1)?, γ = 77.71 (1)?, V = 851.3 ?, Z = 2, and R = 0.027 (R_w = 0.042), for 2540 observed reflections. Compound 3a contains a formally 6-electron donating enyl-amido ligand and is μ-N,-σ-C_βη²-C=C-coordinated to a sawhorse Ru₂(CO)₆ core. In solution both 2 and 3 are stereochemically nonrigid, due to windshield wiper motions of the asymmetrically bridged amido ligands. This process is more facile for 2 (ΔG? ≈ 35 kJ mol^-1) than for 3 (ΔG? 62 kJ mol^-1), and the ΔG? value for 3 was shown to be independent of the R¹ and R² substituents. In refluxing heptane 2 and 3 are converted into the linear 66-electron cluster Ru₄(CO)₁₀[R¹C=C(H)C(H)=NR²] ₂ (5a-e), albeit conversion of 2 into 5 proceeded very slowly and incompletely. Conversion of 3 into 5 proceeds via the intermediacy of (μ-H)Ru₂(CO)₅[R¹C= C(H)C(H)=NR²] (4), which air-sensitive compound could be observed and isolated for R¹, R² = C₆H₅, i-Pr (4d). Reaction of 3a,b with C_H,R²-MAD or crotonaldehyde in heptane at 90°C results in the formation Of Ru₂(CO)₆[CH₂CC(H)C(H)=NR₂] (6a,b). During this conversion the coordinated ligand is dehydrogenated whereas the added substrate acts as a hydrogen acceptor. For crotonaldehyde it was shown that the olefin moiety was hydrogenated chemoselectively. The formation of 2 and 3 out of Ru₃(CO)₁₂ and R¹,R²-MAD and their conversion into 4-6 are discussed.