Zwitterionic zirconocene

Employment of the less sterically hindered yttrocene catalyst [(Cp )2YMe]2 or the more reactive zwitterionic zirconocene catalyst Cp 2ZrMe(/x-Me)B(C6E5)3 allowed cascade cyclization/hydrosilylation of trienes that possessed one or more 1,1-disubstituted alkene. As examples, reaction of 2-(3-butenyl)-l,6-hexadiene and phenylsilane catalyzed by [(Gp )2YMe]2 gave silylated spirocycle 74 in 88% yield. Likewise, the reaction of the dialkenyl alkylidene cyclopentane 75 gave silylated propellane 76 in good yield (Equations (50) and (51)). 

Organozirconates. Zwitterionic zirconocene-ate see Ate Complexes) intermediates have been evoked to explain some reaction mechanisms (see Section 2.3.3). In fact, electron transfer from an organic ligand to 16-electron metal center permits the generation of a carbocationic center which can, then, undergoes isomerization and rearrangement characteristic of carbocations. It is now possible to isolate and characterize stable zwitterionic phosphonium zirconate complexes (eqnation 18). ... 

Similar reactions were conducted with 16a and various aldehydes [28]. It can be emphasized that the [3+2] cycloaddition reactions take place selectively on the carbonyl group allowing therefore the preparation of diversely functionalized zwitterionic zirconocene complexes such as compounds 21 a-e possessing either a free phosphino group (21 b) or free alkenyl group (21 c- e) (Eq. 4) ... 

FIGURE 5.1 Hypothetical boron-bridged zwitterionic zirconocene complex. 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

The use of )j2-benzyne and )j2-phosphabenzyne zirconocene in phosphorus chemistry is reviewed. The regiospecific reactions of these complexes with small unsaturated organic molecules incorporating phosphorus or not led to a variety of cyclic neutral or zwitterionic complexes showing a versatile behavior. New methodologies of synthesis of phosphorus heterocycles and of phosphorus ligands are described, as well as strategies of C-H and M-H (M=N, O, P, S) bond activations. 

Keywords. tf-Benzyne-zirconocene, )j2-Phosphabenzyne-zirconocene, a-Zirconated phosphines, Phosphorus heterocycles, Zwitterionic [phosphonium anionic zirconocene (IV)] complexes... 

Other zwitterionic 18-electron d°-anionic metallocene complexes 26-29 with one Zr-heteroelement cr-bond can be prepared following the same strategy. This implies the treatment of 16a with an equimolecular amount of heterocumulenes (C02, CS2, Cy-N=C=N-Cy, R-N=C=S, R-N=C=0) (Scheme 7) [29]. It is also reasonable to propose here that the formation of all these new systems is due first to a nucleophilic attack of the free phosphine of 16a at the electrophilic center of the organic substrate. In the second step, cyclization reaction on the coordi-natively unsaturated metal center occurs to form the stable pentavalent anionic zirconocene products. With isocyanates and isothiocyanates, the anionic charge... 

Direct (3-Mo elimination was observed when activating zirconocene methyl neopentyl complexes with B(C6F5)3.555 With sterically bulky Cp ligands, instantaneous isobutylene elimination is observed at — 75 °G however, for the bis-(Cp) compound, the zwitterionic neopentyl complex species is stable at 0 °C but undergoes clean and reversible f3-Me elimination at 25 °C (Scheme 177). This finding is consistent with (3-Me elimination as the major chain-transfer pathway in propylene polymerizations using a sterically encumbered metallocene catalyst. 

The phosphine-stabilized ethylene complex of zirconocene(n) reacts with 1 equiv. of B(G6F5)3 to form the girdle-type zwitterionic complex 754 (Equation (49)).574 Both the solution and solid-state structures of 754 feature a strong f3-CH agostic interaction. The zwitterion 754 is a single-component catalyst for the polymerization of ethylene under ambient conditions, although for optimal activity an additional equivalent of B(G6F5)3 is needed. 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

The zwitterionic tris(cyclopentadienyl)zirconocene complex 1213 is obtained by treatment of Cp2ZrCl2 with the lithium, sodium salt of the B(C6F5)3-substituted borato-Gp dianion according to Scheme 286.930 The X-ray crystal structure of this complex reveals that the three 75-cyclopentadienyl ligands are in a nearly trigonal-planar coordination around zirconium with a pronounced Zr-F-G(aryl) coordination perpendicular to it this Zr-F-G(aryl) coordination is persistent in solution based on solution NMR studies. 

It is worth mentioning, on passing, that charge separation within a cationic complex is not a common situation. Another example of cationic-zwitterionic organometallic complex has been reported in the case of the zirconocene system [Zr(G5H4GMe2G6H4Me-/>)(MeB(G6F5)3]+ [MeBICftFslj]. ... 

Activation of ry. ry -Cp tuck-in zirconocene com-plexes results in formation of zwitterionic single-component olefin polymerization catalysts. Reaction of tuck-in zirconocene 37 with 1 equiv of FAB in hexane initially forms a yellow kinetic product which under mild conditions subsequently undergoes conversion to an orange thermodynamic product 38. in which the Zr center is stabilized by interactions with the methylene carbon and the ortho-hydrogen of the phenyl group.Hydrogenoly-sis of 38 affords the corresponding hydride derivative... 

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