MOLECULAR MECHANICAL STUDIES ON THE OLEFIN METATHESIS REACTION .3. MODELING OF THE WELL-DEFINED CARBENES

A new force field termed METHOD has been originally constructed to simulate the carbene complexes that initiate the metathesis of cyclic and acyclic olefins with or without the application of a Lewis acid cocatalyst. The largest and most studied group of them are the so-called Osborn carbenes. They can be regarded as derivatives of the WX4CR2 (X = Cl, Br or 1; R = H or alkyl group) carbenes in which one or more W-X bonds are substituted to W-0 bonds, e.g. WX4-n(OY)nCR2, where n = 1-4, Y = R, SO2CF3 or SiPh3. In the present part of the work the force field is extended to all the mononuclear Osborn carbenes and some other structures related to them. Trigonal bipyramidal (TBP) structures are proposed for these compounds in which the carbene ligand occupies an equatorial site with its two substituents and with the two neopentoxo groups lying in the trigonal plane of the TBP. Parameter development was based on experimental NMR, X-ray and IR data as well as on ab-initio calculations. METMOD1 is composed of 16 new atom types and 571 new data in addition to the MMX parameters, and it is used within the well-documented computational software of PCMODEL-PI V4.0. The general transition metal force field of the original software is abandoned while all the seven MMX interactions are turned on and parametrized. Simulated structures fit ab initio and X-ray structures with high accuracy. Experimental (NMR) rotational barriers for W=Cc bonds are simulated with an average error of 0.563 kcal mol-1.