| A reductive recycle strategy for convenient synthesis of trisamidomolybdenum(VI) alkylidyne complexes has been developed. Alcoholysis of molybdenum(VI) propylidyne complex with 3 equiv of phenol or alcohol generated catalytically active species for alkyne metathesis at room temperature, among which the catalyst with p-nitrophenol as ligand shows the highest catalytic activity and is compatible with a variety of functional groups and solvents.;Catalyzed by the p-nitrophenol molybdenum catalyst, the alkyne metathesis of butynyl substituted compounds proceeds well in 1,2,4-trichlorobenzene at 30°C under reduced pressure, providing high yields (83%-97%) of dimers. Ring expansion mechanism was proposed to explain the catalyst poisoning by butyne byproduct. It was also suggested that the relative metathesis rates of dialkylalkynes versus diarylalkynes could trap the catalyst in a non-productive manifold, rendering it unavailable for the productive metathesis of aryl alkylalkyne substrates. This finding indicates that dialkyl-substituted alkyne byproducts should be avoided (or efficiently removed) if the metatheses of aryl substrates, especially those with electron withdrawing groups, are to proceed to high conversion.;The highly active metathesis catalyst has been successfully applied to syntheses of conjugated, defect-free poly(phenyleneethynylene)s (PPEs) and poly(thienyleneethynylene)s (PTEs) with high molecular weight (M n = 35000, Pn = 128) at room temperature. A convenient, multigram synthesis of arylene ethynylene macrocycles through reversible alkyne metathesis is also accomplished. Driven by the precipitation of a diarylacetylene byproduct, the desired macrocycles are obtained in one step from monomers in high yields at room temperature.;Mechanistic studies on the direct formation of arylene ethynylene macrocycles via alkyne metathesis showed the initial formation of linear oligomers and large macrocycles (n > 6), followed by their transformation into the thermodynamically most stable product distribution---mainly the cyclic hexamer. Variable temperature and scrambling experiments as well as observed pathway-independent product distribution reveal the reversibility and thermodynamically controlled nature of macrocycle formation.;Calculations on thermodynamic stabilities of several shape-persistent macrocycles indicated that the high yields of those macrocycles are due to their own stabilities, which serve as the driving force for their amplification under reversible metathesis conditions. These successful cases demonstrate the great importance of energy gap principle in design and synthesis of shape-persistent macrocycles. |
