Ferrocenyl carbocations

Due to the pronounced electron donating character of ferrocene, ot-ferrocenyl carbocations 3 possess a remarkable stability and can therefore be isolated as salts [16]. They can also be described by a fulvene-type resonance structure 3 (Fig. 4) in which the Fe center and the ot-center are significantly shifted toward each other as revealed by crystal stmcture analysis, indicating a bonding interaction [17]. 

While the ambiguity of the catalysis of the Diels-Alder reaction needs to be carefully elucidated, the application of the ferrocenyl carbocations in the Mukaiyama aldolization turned out evidently to be unrealisable due to their interaction with the TMS enol ether that produces TMSOTf, which proved readily to catalyze the aldolization [154]. 

Copolymerization of isopropenylferrocene with styrene, initiated by BF3 OEt2 in CH2CI2 at 0 °C, resulted in the incorporation of styrene when the styrene content of the comonomer feed exceeded 90 wt.% [18]. This result also points to the stable ferrocenyl carbocation acting as an inhibitor in the polymerization. The low level of incorporation of styrene could be attributed to the greater reactivity of the isopropenylferrocene, as also determined for vinylferrocene by Aso and Kunitake, who showed that r for vinylferrocene was greater than that of styrene [20]. 

Cyclic voltamograms of 4 and 8 show the reversible oxidation event to be shifted 0.30 V and 0.32 V, respectively, more positive than the ferrocene/ferrocenium redox couple. These data would suggest that the iron centers in 4 and 8 are much less capable of stabilizing a positive charge. It is logical to argue they would also form a much less stable a-ferrocenyl carbocation intermediate hence, we tend to rule out such a species in the homopolymerization of complex 4. 

The synthesis of the stable a-ferrocenyl carbocations 40 led to further investigation ofcarbenium salts catalyzed reactions (Scheme 16.37). Kagan et al. [108-110] designed the o-substituted ferrocenyl scaffold that allowed them to avoid the placement of two aryl groups on the carbocation and provided the stabilization and asymmetry, preventing isomerizahon by facile rotation about the carbenium center. Catalyst 40 was applied in the Diels-Alder reaction of cyclopentadiene with methacrolein and resulted an excellent exo/endo diastereoselectivity of up to 99 1 in the presence of 4A MS in nearly quantitative yield (Scheme 16.37). 

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