Title : Metathesis of functionalized alkanes towards the solution to the unsolved problem
Abstract:
etathesis method is widely used in fundamental research and industrial processes. It allows for the efficient and environmentally-friendly production of a variety of products, such as fuels, pharmaceuticals and organic building blocks. After the breakthrough discovery of the olefin metathesis, which showed the conversion of propene to a mixture of ethylene with 2-butenes and ring-opening metathesis polymerization (ROMP) of norbornene, organometallic chemists became very interested in this new method due to its high potential in synthetic organic and polymer chemistry. In the following years, the metathesis of non-functionalized as well as functionalized alkynes and olefins was reported in the literature. In 1997 for the first time metathesis of linear alkanes was reported and subsequently, this field emerges as an alternative route for the preparation of diesel range alkanes for transportation. To date, a plethora of catalysts has been synthesized and used for alkane metathesis reaction. However, metathesis of functionalized alkanes remains elusive in the scientific community. In this presentation, we will show the strategies that we have been using to develop a new reaction – metathesis of functionalized alkanes and our work towards the design of catalyst/substrate compatibility. The first strategy to overcome the challenge was based on protection of the catalyst active sites by modification of the support. We have thus grafted octyltriethoxysilane as well as 1H,1H,2H,2H-perfluorooctyltriethoxysilane (protective chains) on dehydroxylated conventional silica and KCC-1 supports followed by grafting the alkane metathesis catalyst precursor – W(CH3 )6 . The obtained materials with different morphology and ratios between tungsten and protective chains (varying from 15 to 90%) have been thoroughly characterized and subjected to catalysis. Along with that strategy, we designed substrates bearing sterically hindered functional groups to avoid interaction with the catalyst. However, even though the catalysts were active in olefin and alkane metathesis, no metathesis of functionalized alkanes was achieved. The second strategy was aimed to design a compatible substrate for the metathesis of functionalized alkanes. We investigated the pyrrole-based family of substrates bearing in mind that the lone pair of electrons on the nitrogen atom is involved in the formation of an aromatic system and thus is poorly available for coordination. However, metathesis of 1-hexyl-2,5-dimethyl-1H-pyrrole gave only traces of metathesis and isomerization products. These results pushed us to synthesize 9-hexyl-9H-carbazole – a rigid aromatic molecule. While utilizing it on silica-supported W(CH3 )6 , we reached 5% conversion to lower and higher homologs. This result proved to be the first ever known example of metathesis of a functionalized alkane.
