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Concerning the Reaction Pathway of the Metathesis Reaction involving WW and CN Triple Bonds: A Theoretical Study

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journal contribution
posted on 02.11.2009, 00:00 by Shentan Chen, Malcolm H. Chisholm
The original “chop−chop” reaction reported by Schrock [J. Am. Chem. Soc. 1982, 104, 4291] involving W2(OtBu)6 and organic nitriles, RCN to give the metal alkylidyne and nitride products (tBuO)3WCR and (tBuO)3WCN, has been examined by a density functional theory based calculation where the bulky tBuO ligands have been substituted by MeO. The reaction between W2(OMe)6 and MeCN proceeds via a ditungstaazacyclobutadiene intermediate having a planar W2CN core, I, with a structure related to that seen for Mo2(OCH2tBu)6(μ-NCNMe2). Another possible intermediate having a pseudo tetrahedral W2CN core, II, a ditungstaazatetrahedrane was examined and shown to have a higher energy. The interconversion of I and II was found to be energetically unfavorable with respect to their formation of metathesis products. The highest energy transition state involving the conversion of I to products was comparable to that for the conversion of II to products but the initial formation of I from the reaction between W2(OMe)6 and MeCN was favored over the formation of II. The related reaction between Mo2(OMe)6 and MeCN was shown to be thermodynamically unfavorable with respect to either adduct formation or metathesis products. However the reaction between Mo2(OMe)6 and Me2NCN did yield a thermodynamically favored 1:1 adduct with a structure related to I.