A series of complexes of the form TpW(NO)(PMe3)(η2-arene) (where arene = benzene, anisole, dimethoxybenzene, trifluorotoluene, and naphthalene) are evaluated as practical synthons for the versatile dearomatization agent {TpW(NO)(PMe3)}, considering air stability, synthetic scalability, and substitution lability. Large-scale syntheses (9−30 g) are reported for several of these complexes as well as their precursors (TpW(NO)(CO2), TpW(NO)Br2, and TpW(NO)(PMe3)Br), and methods for spectroscopic characterization of composition and stereochemistry for the arene complexes (including J PW correlations) are summarized.

Several π-allyl complexes of {TpW(NO)(PMe 3 )} are investigated as possible sources of η 2 -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 WÀC 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 PMe 3 group, tends toward an sp 2 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)(PMe 3 )(C 5 H 7 )] þ fails to form either isomer of the corresponding η 2 -diene complex.

The isoquinuclidine (2-azabicyclo[2.2.2]octane) core is found in numerous molecules of biological and medicinal importance, including the widely investigated Iboga alkaloids and their related bisindole Cantharanthus alkaloids (Sundberg, R. J.; Smith, S. Q. Alkaloids (San Diego, CA, United States) 2002, 59, 281-386). A diverse range of synthetic methods for the stereoselective construction of this architecture is required for the efficient development of related pharmaceuticals. Here, we report a fundamentally new methodology that constructs the isoquinuclidine core directly from pyridines, using a pi-basic tungsten complex to disrupt the aromatic stabilization of these otherwise inert heterocycles. By this approach, common pyridines are found to undergo stereoselective Diels-Alder reactions with electron-deficient alkenes under mild reaction conditions, thus providing access to a broad range of functionalized isoquinuclidines. Further, by using the common terpene alpha-pinene, a single enantiomer of the tungsten fragment can be isolated and used to provide access to enantio-enriched isoquinuclidines from pyridines.

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-η(2)-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.