1,3-Diene is useful in synthetic organic chemistry, and many synthetic procedures are known: 1,2 elimination from allyl bromide, allyl alcohol, 1,4-or 2,3-dihalogenated compound by a base or Zn, or ring opening of cyclobutene, and symmetrical or unsymmetrical coupling reactions of vinylic compounds using organometallic complexes. Here we report a novel 1,3-diene synthesis from alkyne and ethylene using a ruthenium-catalyzed metathesis reaction. Our plan is shown in Scheme 1. Namely, the double bond of ethylene is cleaved by the ruthenium catalyst, and each methylene part is introduced onto the alkyne carbon.Metathesis reaction is a powerful strategy in synthetic organic chemistry, 3 and it is generally accepted that this reaction is catalyzed by highly efficient transition metal alkylidenes. 4 Intermolecular-diene metathesis produces many olefins, 5 and it has usually been used as intramolecular-diene metathesis. 6 Intramolecular-enyne metathesis is a very unique reaction. It seems likely that the alkylidene part of the alkene migrates to the alkyne carbon. Thus, the resultant cyclized product has diene moiety. 7 However, intermolecular enyne metathesis is more complicated than intermolecular diene metathesis, because three kinds of metatheses, intermolecular diene metathesis, intermolecular alkyne metathesis, and intermolecular enyne metathesis, are included in this reaction and they produce many olefins, dienes, and polymers as shown in Scheme 3.Moreover, these products are further metathesized by carbene complex to give a complicated mixture. Thus, it seems that it is impossible to use this reaction in synthetic organic chemistry.We planned 1,3-diene synthesis by intermolecular rutheniumcatalyzed enyne metathesis. In this reaction, we used ethylene gas as the alkene. The possible pathway is shown in Scheme 4.The real catalyst 4 is generated from ruthenium carbene complex 3 and ethylene Via ruthenacyclobutane 5. 8 It reacts
