A Fine-Tuned Molybdenum Hexacarbonyl/Phenol Initiator for Alkyne Metathesis

Abstract

A new, highly potent activator for molybdenum hexacarbonyl and 2-fluorophenol is described. An "instant"catalyst formed in situ from molybdenum hexacarbonyl and 2-fluorophenol shows high activity for cross- and ring-closing alkyne metathesis reaction. The use of 2-fluorophenol can be combined with other activation methods to allow alkyne metathesis at relatively low temperature (80 degrees C).

Alkyne Metathesis: Catalysts and Synthetic Applications

“…Such a catalyst system was initially discovered by Mortreux, [7,52] and later improved and widely used by Bunz, [4,53,54] Mori, [55] Fürstner [3] and more recently Grela. [56,57] This "instant" method is attractive because all the ingredients are inexpensive, commercially available and stable "off the shelf" reagents can be used directly without the rigorous purifications and an inert atmosphere. [58] However, this catalytic system shows poor compatibility with some heteroatom functional groups, such as esters, aldehydes, pyridines etc.…”

“…More recently, Grela and co-workers surveyed a variety of phenols as catalyst additives and identified 2-fluorophenol and 2-fluoro-5-methylphenol as optimal. [57] Despite considerable optimization on this catalyst system, its relatively harsh operative conditions as well as the low activity towards highly functionalized substrates present limitations in its wide application in organic synthesis and polymer chemistry. Moreover, the exact nature of the catalytically active species is still not clear.…”

In SituGeneration of Molybdenum-Based Catalyst for Alkyne Metathesis: Further Developments and Mechanistic Insights

“…more acidic halogeno-substituted phenols are much better than phenol, as found earlier on Mo(CO) 6 /ArOH-based catalysts [17,18], although no direct relationship between the activity and the pKa value of the co-catalysts could be found. This is similar to what was observed in several cases where a systematic study of the reactivity of in situ generated catalysts was conducted.…”

“…Phenol itself and chlorophenol, as well as silanols and fluoro alcohols were rapidly recognized as suitable to activate the reaction, and revealed that such hydroxylcontaining compounds were essential for the reaction to proceed [17]. Following this paper, several reports aimed at the search for the best co-catalyst to be used for these in situ systems have appeared in the literature, for classical metathesis reactions conducted on disubstituted alkynes [18], and also for comparisons between the MoO 2 (acac) 2 /AlEt 3 /ArOH and the Mo(CO) 6 /phenol catalytic systems applied to polymerization reaction of aromatic diynes [19]. In this paper were also reported data where different fluoro-substituted phenols were used, confirming that the use of different co-catalysts results in different reaction rates.…”

“…This is similar to what was observed in several cases where a systematic study of the reactivity of in situ generated catalysts was conducted. These include the thermally activated Mo(CO) 6 /ArOH catalysts during ring-closing metathesis of a functionalized diyne [18], as well the screening of alcohols and phenols for the in situ synthesis of aryloxymolybdenum carbyne complex from trisamido molybdenum alkylidynes, where again the authors have found that no correlation could be found between the phenol/alcohol pKa values and the performance in ring-opening polymerization of ring-strained dibenzo[a,e]-8-annulene [20]. It appears from these results that the high oxidation state molybdenum/AlEt 3 /ArOH-based catalytic systems are by far much more reactive than thermally activated Mo(CO) 6 /PhOH, for which, using phenol as activator, a turnover frequency of 120 h À1 was found (versus 7000 h À1 ) [8].…”