title: Reaction pathways leading to arylene ethynylene macrocycles via alkyne metathesis.
abstract: Mechanistic studies on the direct formation of arylene ethynylene macrocycles via alkyne metathesis catalyzed by a molybdenum complex are reported. Gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry on the products from metathesis of monomer 1 show 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 reveal the reversibility of macrocycle formation. Nearly identical product distributions are observed from the cross metathesis of hexacycle 2 with diphenylacetylene and from the metathesis of bis(phenylethynyl) substituted monomer 4, demonstrating that macrocycle formation is thermodynamically rather than kinetically controlled. The metathesis byproduct, 3-hexyne, is shown to inhibit the catalyst. It is suggested that the relative metathesis rates of dialkylalkynes versus diarylalkynes trap the catalyst in a nonproductive 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.
authors.affiliation: Roger Adams Laboratory, Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Moore
mesh_terms: D000480:Alkynes / Q000138:chemical synthesis* / Q000737:chemistry; D001713:Biphenyl Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D003500:Cyclization; D047028:Macrocyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods; D013696:Temperature
chemical_list: D000480:Alkynes; D001713:Biphenyl Compounds; D047028:Macrocyclic Compounds