A Divergent Reactivity of a Bis(carbonyl) ether towards Low-valent Titanium Reductive Coupling Reaction

In the past few decades, numerous reducing metal reagents such as aluminum, cerium, iron, magnesium, manganese, niobium, samarium, silicon, titanium, vanadium, ytterbium and zirconium in their low valence state were developed extensively to operate as reductive coupling agents to afford vicinal diols or olefins [1].An unparalleled representative of this category is low-valent titanium reagents which have expanded greatly in terms of number and variety of applications for many years [2]. Hitherto we have reportedthe application of the convenient, high throughput titaniuminduced reductive coupling cyclization reaction to synthesizea series of monomeric and dimeric stilbenophanes stereoisomers and their counterpart pinacolaphanes, bearing variant length alkyl chains. We persistently utilized Mcmurry coupling reaction by preparation of low valent titanium from TiCl4 and Zn in ethereal solvents as crucial step in ring- closing of the stilbenophanes and their counterpart pinacolaphanesfrom their correspondingbis(carbonyl) ethers[3]. In this context, we discovered under the typical Mcmurry coupling reaction,bis(carbonyl) ether (5) containing single etheric methylene bridge,unlike larger analogues showed a divergent behavior in respect of the products profile. Isolation and characterization of crude reaction mixture led us to compounds (1), (2) and geometric isomers (3). While it was anticipated to obtain monomeric stilbenophanes (4) besides dimers (3), instead compound 1 and 2 were unpredictably detected. These findings reflected the role of alkyl bridge length play in outcome of the reaction, eluding intramolecular coupling whatsoever and preferring less strained structures. Thus herein, in continuation of our investigation concerning the intramolecular and intermolecular coupling of bis(carbonyl) ether series (5) with various size of alkyl etheric linkage under Mcmurry reaction, we have reported the synthesis, isolation and characterization of the compound 1, 2 and 3 by NMR, Mass spectroscopy and X-ray analyses and postulated on the mechanism of the reaction leading to 1 and 2.