Alkylidene-metals from protonation

Two different methodologies are available for the proton-mediated formation of dimetal complexes from the compounds 1 or 2. In the first, mixtures containing one of the reagents 1 or 2 together with one of the neutral alkylidyne(cyclopentadienyl)metal complexes 3 are protonated. Protonation of the salts 1 or 2 generates an electronically and coordinatively unsaturated alkylidene-metal species, which can bind a molecule of type 3 to generate a metal-metal bond. This process is formally similar to the... 

Dimetal complexes may equally well be obtained from the reagents 1,2 or 3 alone, by treatment with 0.5 molar equivalents of acid. With this stoicheiometry a mixtxue is effectively created upon protonation, containing alkylidyne- and alkylidene-metal species in equal proportions. The validity of this methodology was first demonstrated in 1985 during protonation studies... 

In this mechanism the destruction of the stereospecificity can only come about if the second protonation occurs at the carbon atom remote from the metal. This will result in the formation of an alkylidene and even if deprotonation can occur, the rapid rotation about the C-C single bond would result in the loss of stereospecificity. 

The rate of norbomene polymerization is first order in catalyst and first order in NBE over greater than three half lives when the catalyst concentration is less than 10 mM (correlation coefficients > 0.995). At higher catalyst concentrations we suspect that bimolecular decomposition reactions that destroy the alkylidene ligand compete with polymerization. We have found that kp = 0.027 (3) M ls l at 23 °C. Since kp/ki = 5 (0.5), ki = 0.0050 M ls l. PolyNBE can be cleaved from the metal by adding a large excess of benzaldehyde (e.g., 50 equiv). GPC analysis showed the samples to be essentially monodisperse, indicative of a well-behaved living polymerization. By proton NMR we can say that the double bonds in the polymer are -- 60% cis,... 

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