Metallocene protonation

The second aspect, predicting reaction dynamics, including the quantum behaviour of protons, still has some way to go There are really two separate problems the simulation of a slow activated event, and the quantum-dynamical aspects of a reactive transition. Only fast reactions, occurring on the pico- to nanosecond time scale, can be probed by direct simulation an interesting example is the simulation by ab initio MD of metallocene-catalysed ethylene polymerisation by Meier et al. [93]. 

Recently the ylide 16 or the corresponding protonated ligands allowed, in presence of metallated bases of groups I and II (Li, K, Ba), the synthesis of the first phosphonium bridged metallocene 17 (dicyclopentadienylide) (Scheme 12). Chiral kalocene and barocene, observed only in racemic forms, have thus been obtained [57]. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

Another interesting comparison of Group VIII metallocenes concerns their ease of protonation in boron trifluoride hydrate (14). It appears that the proton is attached directly to the metal atom, and that ruthenocene and osmocene are protonated to a lesser extent than ferrocene. 

The protonation studies are of interest in another connection. If protonation of metallocenes can be considered to be a simple form of electrophilic attack, it is possible that other types of electrophilic substitution reactions may proceed through initial coordination of the electrophile with the central metal atom (14, 93). The mechanism of acylation of metallocenes may therefore be more complex than might be expected by analogy to similar reactions of benzenoid compounds. Clearly more studies are needed along these lines, better to define specific metal effects on the properties and reactions of these remarkable compounds. 

The effectiveness of these anions depends to a large degree on the integrity of the jl-X linkage. The p,-NH2 compound is air and moisture stable and exhibits a particularly strong bridge, possibly by virtue of four N-H- F hydrogen bonds between ortho fluorine atoms and the amido NH protons. Activation of common metallocenes with the trityl salt... 

In Section IV,C,2 octa- and pentaphenyl-, as well as penta(isopropyl)-and penta(neopentyl)-Cp, were cited as examples for hindered rotations about substituent-C5 bonds. Half-sandwich metal complexes or sym-metallocenes with these ligands will be chiral when there is a barrrier for the interconversion of the enantiomers created by the phenyl canting or the isopropyl and neopentyl directionalities of the substituents (clockwise or counterclockwise) around the C5 ring. For penta(isopropyl)- and penta(neopentyl)-Cp this difference in directionalities is commonly sketched (24) by showing the methine proton only and omitting the methyl or ethyl groups for clarity (14). 

Re(jj -C6H6)Cp] is a sandwich complex and (147) is a bent metallocene like ReCp2H. Protonation of several of the reported complexes gives new Re hydrides, and complex [Re(jj -C6H6)Cp] has a remarkable chemistry in this respect. Compound (149) can be considered as a Re-Re bonded mixed valency d" -d species. Double-bond hydrogenation also occurs, for example, with (148). 

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