Highly Reactive Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Catalysts for Alkyne Metathesis

Abstract

The tetracoordinated cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes [Mo(CC 6 H 4 -p-OMe)(IMes)(OCMewere synthesized from the pentacoordinated progenitor Mo(CC 6 H 4 -p-OMe)(IMes)-(OCMe(CF 3 ) 2 ) 2 (OTf) (Mo4). Complexes Mo4−Mo6 were evaluated for their ability to catalyze the self-metathesis of several internal alkynes. The presence of a triflate group facilitates formation of a cationic species while preformation of the cationic molybdenum center in molybdenum alkylidyne NHC complexes indeed results in a strong increase in catalyst productivity and activity, also in the presence of functional groups, compared to previously reported neutral congeners. The most striking feature of this class of tetracoordinate cationic complexes is the excellent catalytic activity in the alkyne metathesis of non-protic substrates, thereby supporting our previously published proposal of a tetracoordinate cationic active species in alkyne metathesis formed from the neutral, pentacoordinate molybdenum alkylidyne NHC progenitors. Catalyst productivity, expressed as turnover number, reached 20 000 in the self-metathesis of 1-phenyl-1-propyne (S1) using Mo(CC 6 H 4 -p-OMe)(1,3-dimesitylimidazol-2ylidene)(OCMe(CF 3 ) 2 ) 2 [B(Ar F ) 4 ] (Mo6) and 5-(benzyloxy)-2-pentyne (S2) at catalyst loadings as low as 0.005 mol %.

A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I

“…This aspect notwithstanding, further improvements of the catalysts are necessary and perhaps likely in the near future. 134,135 At the same time, the present study illustrates that the implementation of alkyne metathesis can be limited by the lack of appropriate triple bond surrogates and/or the inability to address and manipulate a given alkyne in the presence of other functionality: the difficulties in subjecting compounds 58 and 70 to seemingly trivial chemoselective hydrometalation or semihydrogenation illustrate this aspect. In the future, the significance of alkyne metathesis will also be defined by the upstream and/or downstream alkyne chemistry.…”

“…The conquests of the polycyclic alkaloids keramaphidin B, ingenamine, xestocyclamine A, and, not least, nominal njaoamine I bear witness of the notion that alkyne metathesis has reached strategy level. This aspect notwithstanding, further improvements of the catalysts are necessary and perhaps likely in the near future. , …”

The Ascent of Alkyne Metathesis to Strategy-Level Status

“…A “volcano-type” correlation between the degree of fluorination and catalytic activity was established, which has to do with the fact that, from a certain degree of peripheral fluorination onward, the central atoms are too Lewis acidic and the pertinent intermediates on the catalytic cycle overstabilized. ,− Yet other catalyst families are distinguished by heteroleptic ligand spheres, comprising, for example, imidazolin-2-iminato or N-heterocyclic carbenes. Some members of this series such as 38 show impressive turnover numbers and rates; − their relevance for advanced synthesis, however, is currently difficult to assess as the set of test substrates is (too) narrow and does not contain any of the truly challenging functional groups.…”

Ligand Variations in Neutral and Cationic Molybdenum Alkylidyne NHC Catalysts

“…In the neutral, pentacoordinate Mo and W alkylidyne N-heterocyclic carbene (NHC) alkoxide catalysts developed by our group, , one alkoxide ligand dissociates to provide a tetracoordinate cationic active species. We managed to isolate a cationic molybdenum alkylidyne NHC complex, [Mo­(CC 6 H 4 - p -OMe)­(DME)­(OCMe­(CF 3 ) 2 ) 2 ]­[B­(Ar F ) 4 ], which allowed for turnover numbers (TONs) up to 20 000 in alkyne metathesis while providing tolerance toward nonprotic functional groups such as ether, ester, thioether, and nitro groups . The overall objective of this work was to extend the existing catalyst scope and to introduce new ligands into already existing catalyst systems.…”