Several π-allyl complexes of {TpW(NO)(PMe3)} are investigated as possible sources of η2-diene complexes. In order to prepare the diene complex as a single diastereomer, the allyl complex must undergo deprotonation stereoselectively. Allyl complexes of this tungsten system are highly distorted, with the difference between the W−C bond lengths for the two allyl termini being as much as 0.69 Å. DFT calculations and several crystal structures are presented that collectively suggest that one terminus, C1, distal to the PMe3 group, tends toward an sp2 carbocation. Consistent with this interpretation, deprotonation preferentially occurs at a carbon adjacent to this allyl terminus for six-membered rings. However, in the presence of base the five-membered cyclic analogue [TpW(NO)(PMe3)(C5H7)]+ fails to form either isomer of the corresponding η2-diene complex.
Publications
2011
A new method for the preparation of functionalized piperidines is described in which various dihydropyridine (DHP) complexes of {TpW(NO)(PMe3)} that are derived from pyridine–borane undergo [4 + 2] cyclocondensation with enones, enals, nitrosobenzene, and several isocyanates to form [2.2.2] bicyclic species. In several cases the diazabicyclooctene products derived from DHP complexes and isocyanates can be further elaborated into novel syn-2,5-disubstituted and 2,3,6-trisubstituted piperidinamides.
2010
Phenols, when coordinated to tungsten, undergo conversion to their 2H tautomers. In this form, these phenols undergo a variety of reactions with electrophiles including cyclopropanation, epoxidation, and electrophilic addition to the meta carbon, eventually leading to highly functionalized cyclohexenones.
An aniline coordinated to tungsten undergoes alkylation at C4 or protonation at C2. In the latter case, a second electrophilic addition is possible at C3, followed by nucleophilic addition at C4.
The N-acetylpyridinium complex of {TpW(NO)(PMe3)} undergoes regio- and stereoselective reactions with a broad range of common organic nucleophiles, providing a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(PMe3)(3,4-η2-DHP). The present study explores the elaboration of these systems into novel piperidines. The addition of an acid to the DHP complexes generates highly asymmetric π-allyl complexes that in turn react with a second nucleophile at either C3 or C5. The subsequent oxidative decomplexation of these materials yields several piperidinamides with unconventional substitution patterns.
Nucleophiles capable of four-electron donation (e.g., anilines, enolates) add to C2 of the N-acetylpyridinium (NAPy) ligand of TpW(NO)(PMe3)(NAPy)(OTf) and induce a Zincke−König-like ring scission to generate 3,4-coordinated acetamidotrienes or 1-azatrienes with 3Z,5E stereochemistry. This ring opening is driven by the enhanced π-acidity of the cyanine, relative to its purported dihydropyridine predecessor, which stabilizes the π-base {TpW(NO)(PMe3)}. X-ray data of the indoline-derived analogue (2) indicates that metal coordination disrupts the donor−acceptor (i.e., amide−iminium) conjugation in the cyanine ligand. Photolysis of 2 in the presence of indoline liberates a further derivatized cyanine.