Agostic interactions reactions

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

The current scope of the controlled monocarbotitanation of alkenes and alkynes is still very limited at least in part due to competitive side-reactions arising via ft- and a-agostic interactions, as alluded to Scheme 6. On the other hand, polymerization, also shown in Scheme 6, may be largely avoided or minimized in most cases. T o overcome some of the difficulties mentioned above, cyclic version of monocarbotitanation have been explored,25-27 as shown in Scheme 12. None of these reactions has as yet been widely used, but their further development might lead to synthetically useful methods. 

Complex 6 reacts with HC=CPy in a similar manner as described above akin to the reaction of complex 4 with HC=CPh to form the complex Cp"2Zr[C(SiMe3)-=CH(SiMej) Kn -C=CPh) (52), giving a complex with an agostic interaction, rac-(ebthi)Zr[-C(SiMe3)=CH(SiMe3)](Ti1-C=CPy) [14]. 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

The single-site product of the reaction of the perhydrocarbyl Re(VII) complex [Re(-CH2- Bu)2(=CH- Bu)(=C- Bu)] with siUca Si02 7oo, [(=SiO)Re(=CH- Bu)(=C- Bu)], is very closely modeled by the reaction with the monosilanol silsesquioxane la [149] and by the reaction with triphenylsilanol [150], where, similarly to the surface data, the syn and anti isomers can be idenhfied, with the former giving rise to an H-agostic interaction [151]. An intereshng difference exists while the... 

The crystal structures of truTO-[RuCl2(dppp)2] " and [RuCl(dppp)2][PF6] " have been determined. The latter complex cation was formed electrochemically. The reaction of [RuF(dppp)2] with diphenylallyl bromide results in the formation of [RuBr(dppp)2]. " The unsaturated center in [RuH(dppp)2] is brought up to six-coordination by an agostic interaction involving a C-H group of one ligand. A structure determination confirms that the C-H—Ru interaction is cis to the Ru—bond. " ... 

For the industrially important Pd-catalyzed methoxycarbonylation of ethene to methyl propanoate, all the intermediates of the cycle have been identified. Starting from 34, CH2=CH2 and CO, the process has been shown to proceed via a hydride rather than a methoxycarbonyl cycle (Scheme 1.1) [25][26]. Figure 1.9 shows the P NMR spectrum at 193 K of a 1 1 mixture of the two isotopomers 35a and 35b, formed in the reaction of 34 with CH2= CH2- The presence of an agostic interaction is supported by the C chemical shifts ( (CHj) 31, and (CHj) 8), which are reversed with respect to classical Pd-ethyl complexes. 

Proton/deuterium isotope effects on reaction rates are useful mechanistic probes. In the zirconocene-catalyzed alkene polymerization, the observed values of k .iH/feo(.2H determined by NMR fall in the range of 1.2-1.3 and support a transition state in which there is an a-agostic interaction (see 88) [132]. 

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