Double Group Transfer Reactions of an Unsaturated Tantalum Methylidene Complex with Pyridine

Adam Capriola

“Double Group Transfer Reactions of an Unsaturated Tantalum Methylidene Complex with Pyridine N-Oxides”

It is widely known among inorganic chemists that multiply bonded metal-ligand species take part in a diverse set of atom and group transfer reactions. It is common to witness CR2 groups transferred to unsaturated organic substrates, but viewing the insertion of CH2 into C-H bonds to yield saturated product is quite unusual. In the case of [TolC(NSiMe3)2]2Ta(CH2)CH3, its electrophilic nature allows for an improbable double group transfer to occur when exposed to pyridine N-oxides. This reactions yields [TolC(NSiMe3)2]2Ta(O)CH3 due to simultaneous deoxygenation and regioselective methylation of the pyridine N-oxide.

The benzamidinate tantalum ethylidene complex is also able to react with nitrones, which are similar in structure for pyridine N-oxides. It is not however able to react with weak oxidants such as styrene oxide and triphenylphosphine oxide. Only one equivalent of the pyridine N-oxide was needed for the aforementioned reaction to take place. 2-Methylpryidine is produced as well, as confirmed by comparison using NMR integration versus a trimethoxybeneze internal standard. The trimethoxybenze reacts further with 2-methylpyridin N-oxide to ultimately yield 2,6-dimethylpyridine and oxo complex.

It should be noted that methylation occursregioselectively at the unsubstituted ortho position in each pyridine N-oxide. Also, the substituted pyridine N-oxide species react much slower than the unsubstituted variant, comparatively in minutes versus microseconds.

Proton and carbon 13 NMR, IR spectroscopy, and X-ray crystallography were all used to verify the tantalum oxo complex product. The IR spectrum shows a strong stretch at 922 cm-1, which is a feasible number to indicative of terminal Ta-O multiple bonds (typically 850-1000 cm-1). X-ray crystallography reveals a distorted-octahedral coordination geometry surrounding the tantalum and thus confirmed the presence of a terminal oxo character. The measured bond length of the tantalum atom to oxygen bond is reported to be 1.76 Å, which is in line with previously reported figures for Ta-O multiple bonds. Thus, all the statistics seem to confirm that a double group transfer does indeed take place.

The mechanism of this reaction is thought to take place via two possible schemes involving a total of three mechanisms, but it is not known which scheme or mechanism is correct. There is an absence of intermediates in the reaction as evidenced by UV, IR, and 1H NMR spectroscopy, so deuterium labeling is used to distinguish these potential routes of formation. GC-MS shows parent ion at m/z 95 and 111 corresponding to the methyl and dimethylpyridine products, respectively. At 2.40 ppm there is a 1:1:1 triplet indicative of the CH2D group. This group also appears in both the proton and carbon 13 NMR spectras, which in all suggests that the mechanism of reaction takes place via scheme one and a mechanism label B.

Finally, nitrones which is similar in structure to pyridine N-oxides are also reacted with the benzamidinate tantalum ethylidene complex to see if they have a comparable interaction. Only after heating the complex with N-tert-butyl-α-phenylnitrone at 45 °C for 40 hours did styrene and another new organometallic product come to fruition. The new product is suspected to be [TolC(NSiMe3)2]2TA(O)(NtBuMe) through 1H, 13C{1H} NMR, IR, and mass spectroscopic techniques.

In conclusion, it is the enhanced electrophilicity of the benzamidinate tantalum ethylidene which allows for the reaction pathway to occur. The atom transfer reactions allow for Ta-O double bonds and organic product with new C-C bonds to be formed. Further investigation into these matters is ongoing. I believe that following steps that could be taken would to delve into other metals complexes that could allow for double group transfers. Logically, I would think that the next metals to investigate would be other group 5 metals, possibly replacing Ta with Nb or Db. These metals should have the most similar properties in relationship to Ta. Reactants other than N-oxides and nitrones could also be analyzed to see if it is possible to replicate the double group transfer.

In a related study performed by ….