Tropylium complexes addition reactions

In Chapter 8 we described how metal carbides (first mentioned in Section 6-1-4) could serve as precursors to carbyne complexes by way of electrophilic addition. Scheme 10.8 revisits a portion of Scheme 8.12, showing Os-carbide complex 72—with its nucleophilic Ccarbide atom—reacting with methyl triflate or tropylium ion to give alkylidynes 73 and 74, respectively. Comparable reactions occur with the corresponding Ru-carbide complex.87 This method may become more general after the synthesis of additional carbide complexes occurs. 

A plethora of electron-deficient arenes can enter the VNS reaction carbocyclic and heterocyclic aromatic compounds activated by the nitro group and arenes that are active electrophiles due to their electronic configuration, such as azulene [35, 36], electron-deficient annulenes [37], tropylium cation [38], and particularly azines and azinium cations. Interestingly, q -transition metal complexes of arenes, such as benzene tricarbonylchromium, do not enter the VNS reactions. Although the addition of carbanions to these electron-deficient rings proceeds efficiently, and these adducts can be oxidized to form the products of ONSH, the p-elimination of HCl from the o adducts of a-halocarbanions does not occur [39, 40]. 

Not all organic salts are able to initiate the polymerization of NVK. While stable ammonium salts such as benzyltrimethylammonium hexafluoroantimonate do not initiate the polymerization, stable ammonium and tropylium salts will. A kinetic study of NVK polymerization initiated by stable tropylium hexachloroantimonate and perchlorate salts provided the first propagation rate constants for that monomer [440]. The authors showed that initiation involves direct addition of the tropylium ion to NVK via formation of a charge transfer complex [441]. This mechanism was proved by isolation of the corresponding methanol adduct. In this particular system the initiation reaction was almost instantaneous and quantitative, and no chain termination was observed. Under these conditions the propagation rate constants can be determined from conversion-time... 

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