ABSTRACT
Computational-experimental analysis has allowed determining that the stereochemistry of the Staudinger reaction between ketenes and imines is strongly associated with the nature of the imine, which affects the two steps of the reaction. The first step, namely the nucleophilic attack of the sp2-hybridized nitrogen atom of the imine on the sp-hybridized carbon atom of the ketene, is affected by the energetically accessible in situ isomerization patterns of the imine. The second step consists of a conrotatory electrocyclization of the zwitterionic intermediate formed in the previous step. This latter pericyclic step depends on the inward/outward torquoelectronic effects generated by the substituents of the imine. The impact of these factors on the stereochemistry of this reaction has been analyzed kinetically by numerical methods. The results of these simulations are compatible with the experimental results and support these conclusions.
ABSTRACT
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(CCâ³N)]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 â¼0.1 Å shorter than in the respective neutral species 2-4, whereas the hydride hard sphere radius increases by â¼10%, and the ν value decreases by â¼20%.
ABSTRACT
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.
ABSTRACT
The synthesis (and full spectroscopic and crystallographic characterization) of new classes of bioorganometallic Ir- and Rh-complexes having ß-lactam containing ligands has been achieved in three steps starting from simple precursors. The procedure for preparing these bioorganometallic compounds uses ß-lactams having a phenylpyridyl moiety attached to the C4, N1 or C4 and N1 positions simultaneously, and a directed C-H metal-insertion, in the presence of (MCp*Cl2)2 (M = Ir, Rh). Enantiomerically pure 2-azetidinones can be transformed into diastereomeric (at the metal) mixtures of enantiopure metalla-2-azetidinones. Bimetallic 2-azetidinones are also accessible by this approach. The insertion of electron-poor alkynes into the M-C bond of the bioorganometallic complex occurs regioselectively and in excellent yields. Overall, the sequence imine-ß-lactam-metalla-ß-lactam is a versatile and efficient full methodology to prepare and functionalize unprecedented, novel Ir- and Rh-complexes having ß-lactam containing ligands.
