Mechanistic Insight into the Facilitation of ß-Lactam Fragmentation through Metal Assistance.

The mechanism of OsH6(PiPr3)2-mediated fragmentation of a 4-(2 pyridyl)-2-azetidinone has been investigated by DFT calculations. The addition of the C4-H bond of the substrate to OsH2(PiPr3)2 allows the active participation of an osmium lone pair in the B-type ß-lactam fragmentation process. This new mechanism makes the N1-C4/C2-C3 fragmentation of the lactamic core thermally accessible through a stepwise process.

2-Azetidinones as Precursors of Pincer Ligands: Preparation, Structure, and Spectroscopic Properties of CC'N-Osmium Complexes.

A metal-promoted degradation of 2-azetidinones to afford CC'N-pincer ligands is reported. The hexahydride complex OsH6(P(i)Pr3)2 (1) reacts with (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(pyridin-2-yl)azetidin-2-one (I), (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(isoquinolin-2-yl)azetidin-2-one (II), and (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(quinolin-2-yl)azetidin-2-one (III) to give the respective OsH2(P(i)Pr3)2(CC'N) (2-4) complexes, which add HBF4·OEt2 to yield [OsH2(P(i)Pr3)2(CC″N)]BF4 (5-7). These salts are the result of the addition of the proton of the acid to the dianionic CC'N-pincer ligand. The hydride ligands of these compounds undergo quantum mechanical exchange coupling, which has been experimentally quantified according to a two-dimensional harmonic oscillator model, where Jex is determined by the separation between the hydrides, their hard sphere radius, and a ν parameter describing the H-M-H vibrational wag mode allowing the movement along the H-H vector. The comparison of the results reveals that the phenomenon is particularly intense for 5-7. Furthermore, in these compounds, the separation between the hydrides is ∼0.1 Å shorter than in the respective neutral species 2-4, whereas the hydride hard sphere radius increases by ∼10%, and the ν value decreases by ∼20%.

Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes.

The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH2Cl2(PiPr3)2 (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl2{=C(CH3)(CH2) n NH(CO)Ph}(PiPr3)2 (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl2{=C(CH3)(CH2)3NHC(O)Ph}(PiPr3)2 (4). The new compounds are intermediate species in the cleavage of the C-C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the Cα-CH3 bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride-alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl2{≡C(CH2)3C(O)NHPh}(PiPr3)2 (5) and OsHCl2{≡CCH(CH3)(CH2)2C(O)NHPh}(PiPr3)2 (6). The first complex contains a linear separation between the alkylidyne Cα atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl2{≡C(CH2)3NHC(O)Ph}(PiPr3)2 (7). In spite of their instability, these compounds give the alkylidene-allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl2(=C=CMe2)(PiPr3)2 (8) and the monosubstituted OsCl2(=C=CHCy)(PiPr3)2 (9).