Multiple metal-carbon bonds. 40. Reaction of Mo(CCMe3)[OCH(CF3)2]3(dimethoxyethane) with tert-butylacetylene, an aborted acetylene polymerization

Cross‐Metathesis of Terminal Alkynes

“…As in every metathesis of unsymmetrical substrates, the two possible orientations of the reactants in the first [2 + 2] cycloaddition open two competing pathways I and II which are interconnected by virtue of the fact that this initial step is reversible (Scheme 1). [9,10] The productive channel II is not free of serious complications either. Specifically, it was shown that a metallacyclobutadiene of type B is prone to transannular CÀH activation with concomitant loss of one of the ancillary ligands X that picks up the proton and hence formally acts as a base; this pathway is par-ticularly favorable in the presence of external donor ligands L. Although X-ray data show that the ligand in the resulting deprotiometallacycle C is h 3 -bound in the solid state, [4][5][6][7][8] a hapticity change suffices to reveal an alkynyl-alkylidene motif (D), which can trigger rapid substrate polymerization.…”

“…Cycloreversion of E forms the desired product RCCR and leads to the metal methylidyne F, which then reacts with a terminal alkyne substrate RCCH via metallacycle H to regenerate the loaded complex A, as necessary for sustained metathesis; at the same time, however, acetylene is produced which is well soluble in most organic media, despite its low boiling point. [4,10] Likewise, the Mortreux group found that treatment of various terminal alkynes with complex 1 b (R = tBu) resulted only in a short initial phase of metathesis, which was then quickly superseded by polymerization. An increasing concentration of the methylidyne F, favors the dimerization of this fairly unstable species [11] with formation of dimetallatetrahedranes G (Scheme 1).…”

“…Specifically, it was shown that a metallacyclobutadiene of type B is prone to transannular CÀH activation with concomitant loss of one of the ancillary ligands X that picks up the proton and hence formally acts as a base; this pathway is par-ticularly favorable in the presence of external donor ligands L. Although X-ray data show that the ligand in the resulting deprotiometallacycle C is h 3 -bound in the solid state, [4][5][6][7][8] a hapticity change suffices to reveal an alkynyl-alkylidene motif (D), which can trigger rapid substrate polymerization. [9,10] The productive channel II is not free of serious complications either. Cycloreversion of E forms the desired product RCCR and leads to the metal methylidyne F, which then reacts with a terminal alkyne substrate RCCH via metallacycle H to regenerate the loaded complex A, as necessary for sustained metathesis; at the same time, however, acetylene is produced which is well soluble in most organic media, despite its low boiling point.…”

“…Thus, Schrock and co-workers had already noticed that phenylacetylene is polymerized on contact with (tBuO) WCR (1 a, R = Ph). [4,10] Likewise, the Mortreux group found that treatment of various terminal alkynes with complex 1 b (R = tBu) resulted only in a short initial phase of metathesis, which was then quickly superseded by polymerization. [15] In a courageous endeavor, these authors tried to empirically optimize the system such that metathesis would endure.…”

Recent Developments in the Chemistry of Fluorinated Isopropoxides and Tertiary Butoxides

“…Metathesis of internal acetylenes via W(VT), Mo(VI), and Re(VTJ) alkylidyne complexes has also been reported (35)(36)(37)(38)(39). The two key requirements for a successful Mo(VI) or W(VI) acetylene metathesis catalyst are a crowded alkoxide coordination sphere to prevent further reaction of the metallacyclobutadiene intermediate with more of the acetylene (which forms polyacetylenes), and electronegative alkoxy groups to afford an electrophillic metal center.…”