Mechanistic comparison of the nickel(0)-catalyzed homo-oligomerization andco-oligomerization of alkynes and nitriles

A comparative mechanistic study of the nickel(0)-catalyzed homo-oligomeriza tion and co-oligomerization of alkynes and of nitriles has been undertaken, with diphenylacetylene and benzonitrile, towards an array of nickel(0) rea gents, such as finely divided nickel, (COD)(2)Ni, (Bpy)(COD)Ni, (Et3P)(4)Ni , (Bpy)(PhC=CPh)Ni and (COD)(2)Ni-MeAlCl2 combinations in donor (THF) and n ondonor (PhMe or neat substrate) solvents. Special attention has been given to the detailed molecular structures of the initial 1:1 adducts, (Bpy)(PhC =CPh)Ni, (Ph3P)(3)(PhC=CPh)Ni and [(Ph3P)(PhC=N)Ni](4) by a consideration o f XRD and IR data. Data from the single crystal XRD analysis of (Bpy)(PhC=C Ph)Ni, reported here for the first time, are shown to be in excellent accor d with the presence of a 2,3-diphenylnickelacyclopropene ring for the (PhC= CPh)Ni moiety with almost coplanar chelating coordination of the bipyridyl ligand, rather than with the presence of simple "side-on" coordination of t he alkyne with the metal center. A parallel analysis of XRD and IR data for the two benzonitrile-nickel(0) complexes, which were drawn from previous p ublications, has concluded that the nickel in (Ph3P)(3)(PhC=N)Ni is coordin ated in an "end-on" fashion and the nickel centers in [(Ph3P)(PhC=N)Ni](4) are coordinated as bridges between nitrile units in both an "end-on" and "s ide-on" manner. The stereochemistry of the acid hydrolysis of the nickelacy clopropene complex to (E)- or (Z)-alkene was shown to depend on the structu re of the cleaving acid; parallel hydrolysis of nitrile-nickel complexes ha s shown that "end-on" complexes regenerate the nitrile, while "side-on" com plexes lead to the aldehyde. In homo-oligomerization of diphenylacetylene o r other alkyne the clean cyclotrimerization to the benzene derivative was s hown to proceed by way of a nickelacyclopentadiene intermediate, as was evi dent by chemical trapping. The homo-oligomerization of benzonitrile by nick el(0) was found not to lead to 2,4,6-triphenyl-1,3,5-triazine, as claimed i n the literature, but rather solely to benzyl phenyl ketone, the dimeric hy drolysis product. The attempted co-oligomerization of diphenylacetylene and benzonitrile with ordinary nickel(0) complexes led only to the homocyclotr imer of the alkyne, Only when the alkyne was prebonded to the nickel, as in (Bpy)(PhC=N)Ni, could significant amounts of a codimerization product with the nitrile be observed. The origin of the triazine, claimed in a previous report to form from benzonitrile and Raney nickel, has been traced to the presence of adventitious moisture and air. Other unexpected products formed from nickel(0) complexes and benzonitrile have been shown to arise from ox idative addition reactions of nickel(0) with various sigma C-E bonds.