A-Metallocenyl carbonium

Richards and Hill have recently obtained quantitative evidence of the stabilization of a -metallocenyl carbonium ions (38, 95). They have shown that sol-volyses of methylmetallocenylcarbinyl acetates proceed via a carbonium ion mechanism, and that these acetates solvolyze with rates greater than even tri-phenylmethyl (trityl) acetate. Further, the relative rates of solvolysis and therefore the order of carbonium ion stabilities increas.e, proceeding from the iron to the osmium acetate. A portion of these data is summarized in Table II. 

The solvolytic reactions of a-metallocenylcarbinyl acetates, which presumably involve a-metallocenyl carbonium ions, provide further examples of reactions in which metal orbital electrons may be involved. Although the evidence for neighboring group participation by the metal atom is fairly compelling, the precise mode by which the metal atom may contribute to charge stabilization is still a matter of some dispute 6, 15),... 

In order to understand better what happens to the cations 2, 4 and 6 produced in the first step of the solvolysis reaction it is necessary to consider a recently investigated aspect of metallocene chemistry, namely the role of the metal in the formation of a-metallocenyl carbonium ions. Several investigators (5, 6) have discussed the unusual stability possessed by carbonium ions adjacent to a metallocene nucleus. 

Methylmetallocenylcarbinyl acetates are readily solvolyzed. In fact, the rates are greater than that of tritylacetate (T able 6-3). These rates of solvolysis also result from the stability of the corresponding a-metallocenyl carbonium ions. 

Replacement of the phenyl nucleus of acetophenone with a ferro-cenyl group enormously increases basicity (8,291) in accordance with numerous other indications that metallocenyl nuclei are potent stabilizers oi carbonium ions. 

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