A-ferrocenyl carbocation

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). 

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]. 

The ferrocenyl group is a very good electron donor. The a-ferrocenyl P-silyl substituted carbocation 20 is accessible by protonation of ( )-1 -ferrocenyl-2-(triisopropylsilyl)alkene 21 with trifluoroacetic acid in SO2CIF at - 95 °C (13, 22). 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Si NMR spectroscopy is a suitable tool to monitor the electron demand in /3-silyl-substituted carbocations. The 29Si NMR chemical shifts in the a-phcnyl-, a-p-tolyl,a-p-anisyl- and a-ferrocenyl-substituted /J-silylethyl cations 331, 333, 323 and 339 are 8 = 66.3, 56.9, 38.9 and 23.5 ppm, respectively, decreasing regularly as expected on increasing the electron-donating capability of the a-substituent. 

Comparing p-silyl-substituted vinyl cations however with different a-aryl substituents shows that the stabilizing effect of a P-silyl groiq> is not constant but is dependent on the electron demand of the carbocation. Fig. 4 shows the pora-carbon C NMR chemical shift difference A8 between the p-silyl and P-unsubstituted vinyl cations (For the a-ferrocenyl substituted cation A8 C3,4 is used to probe the silyl effect). 

The Si NMR chemical shifts in the a-phenyl-, a-tolyl-, a-/7-anisyl-, and a-ferrocenyl-substituted P-silyl ethyl cations, shown in Fig. 7, are 66 34 ppm, 56.92 ppm, 38 88 ppm, and 23.48 ppm, respectively. Si NMR shifts are thus suitable for monitoring the electron demand in P-silyl substituted carbocations. 

An essential characteristic of ferrocene chemistry is the stabilization of ferrocenyl carbonium ions. These carbocations are mesomers of the corresponding hexahapto fulvene complexes [FeCp(r -fulvene)]+. They are even more stable than the trityl cation PhsC". The stabilization of the a-ferrocenyl carbonium ions explains the acetolysis of vinylferrocene, the hydrolysis of the acetate formed, the ease of nucleophilic substitution in a position, and the OH" abstraction from the a-ferrocenyl alcohols. This stabilization is still enhanced by going down in the iron column of the periodic table, because the size of the d orbital increases, which facilitates their insertion with the carbocation and accelerates the solvolysis of acetates (Os > Ru > Fe). 

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