Nucleophilic and Electron-Transfer Processes in Ion-Pair Annihilation

NUCLEOPHILIC AND ELECTRON-TRANSFER PROCESSES IN ION-PAIR ANNIHILATION 

Nucleophilic addition to unsaturated ligands, such as olefins, acetylenes, and arenes, coordinated to various metal centers is a useful strategy in organic synthesis (98). The ion-pair interaction of organometallic cations and anions is facile and can successfully lead to the nucleophilic activation of ethylene, acetylene, and benzene (99), for example, 

This mechanistic possibility is supported by sporadic reports of the isolation of dimeric by-products and other types of evidence of the radical intermediates m,L- and m2 during the annihilation of organometallic ion pairs (101). Indeed, the detailed comparison of a series of the structurally related alkenyliron cations (102), 

Radicals are formed spontaneously on the addition of CpMo(CO)3 to the cyclohexadienyliron cation 3, as observed spectroscopically at 25 °C by the rapid disappearance of both carbonylmetal ions and the simultaneous formation of the corresponding pair of homodimers, namely, 

The formation of 6a is particularly diagnostic, since this unique carbon-carbon bonded reductive dimer was demonstrated by Wrighton and coworkers to arise via the transient 19-electron radical ( -cyclohexa-dienyl)Fe(CO)3 by regiospecific coupling at a ligand center (105). Furthermore the 17-electron radical CpMo(CO)3- is the precursor to the accompanying oxidative dimer [CpMo(CO)3]2 (7), as described in the earlier anodic studies of CpMo(CO)3 (106). Accordingly these products (6 and 7) of ion-pair annihilation are referred to hereafter as radical (homo) dimers. 

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V. Nucleophilic and Electron-Transfer Processes in Ion-Pair Annihilation.. 96... 

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