Cation-benzene complex

The parent bis-benzene complex is hydrolytically sensitive [20] and must be used in its crude form. All the reactions with carbanions give ET products. First, the purple 19e mono-cation [Fe(C6Me6)2] + [25-28] is formed immediately at —90 °C. Then the black 20e complex Fe(C6Me6)2 is observed [25-29], However, recently, Zaworotko et al. have succeeded in making a C-C bond using AlEt3 as the carbanion source [30]. 

The nitrosonium cation bears a formal relationship to the well-studied halogens (i.e. X2 = I2, Br2, and Cl2), with both classes of structurally simple diatomic electron acceptors forming an extensive series of intermolecular electron donor-acceptor (EDA) complexes that show well-defined charge-transfer absorption bands in the UV-visible spectral region. Mulliken (1952a,b 1964 Mulliken and Person, 1969) originally identified the three possible nonbonded structures of the halogen complexes as in Chart 7, and the subsequent X-ray studies established the axial form II to be extant in the crystals of the benzene complexes with Cl2 and Br2 (Hassel and Stromme, 1958, 1959). In these 1 1 molecular complexes, the closest approach of the... 

FIGURE 15. (a) X-ray structure of the jr-complexed lithium in the monomeric benzene complex of 2,6-bis(2,4,6-tri-isopropylphenyl)phenylhthium . (b) X-ray structure of the jr-complexed lithium in the dimeric 2,6-bis(2,6-di-isopropylphenyl)phenyUithium . It is evident from the Figure that the local environments around the lithium cations are similar... 

Due to computational savings, the open and closed forms of the 6,6-dimethyl-6-sila-2-norbornyl cation, 42 and 43, the closed 5,6,6-trimethyl-6-sila-2-norbomyl cation (44) and their corresponding benzene complexes 45, 46 and 47, respectively, were optimized... 

FIGURE 20. Energetics (kcal mol 1) of benzene complexation to cations 42 and 43, forming complexes 45 and 46, respectively. Reproduced by permission of Wiley-VCH from Reference 110... 

A cationic rhodium complex, [(Me5Cp)Rh(j/6-benzene)]2+, catalyses direct aldol condensation of ketones.153... 

Mattes and Farid [42] have argued that the formation of phenylsuccinic anhydride need not be regarded as an indication that the ortho cycloadduct is formed via a zwitterionic intermediate. They considered the possibility that charge-transfer excitation of the maleic anhydride-benzene complex leads to the anhydride radical anion and the benzene radical cation. Protonation of the radical anion followed by addition to the benzene radical cation can be considered as an alternative route to phenylsuccinic anhydride. 

In the oxidative aromatic substitution of benzene with the nitrosonium cation (NO+), the benzene complex with symmetry 12 has been calculated as a local minimum at the B3LYP/6-31G(d) level of theory with an energy of 48 kcal mol-1 above that of the 7t-complex 13 <1999PCA4261> therefore, the former should not be relevant for the nitrosation mechanism as was previously proposed (Figure 3) <1985RJOC842>. 

The intermediate in both reactions is a cation but the first (from cyclohexene) adds an anion while the second (from benzene) loses a proton so that the aromatic system can be restored. Notice also that neutral bromine reacts with the alkene but the cationic AICI3 complex is needed for benzene. Another way to produce a more electrophilic source of bromine is to use a pyridine catalyst. Pyridine attacks the bromine molecule producing a cationic bromine compound. 

Clearly, both situations (silylium cation with strong delocalization of positive charge or solvent complexed silylium cation) would not justify to speak of a free silylium cation in benzene solution in the case of 61. To clarify the situation Cremer and co-workers [48] carried out an extensive DFT study for both 61 and a potential benzene complex of 61 (62, Scheme 10). Results of this investigation are summarized in Figures 19, 20, and 21 as well as in Table 9. 

The intermediate allyloxycarbonylplatinum complexes XCIV are isolated in fair yield. After reflux in benzene, the cationic / -allyl complexes XCV are afforded in good yield. The conversion can also be effected by adding AgCI04. Allylamines can also be used in a path analogous to Eq. (az) . 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

The interesting 1,3-cyclohexadiene metal hydride complex, (CflH8)W(CO)H(C5H5), has been obtained by reduction of the benzene complex cation [(C6Hg)W(CO)(C5H6)] with NaBH4 in aqueous methanol (201). On the basis of its infrared and NMR spectra structure (14) has been assigned. An infrared absorption band at 1851 cm and a... 

The reduction of the benzene complex cation [(C(5Hg)Mn(CO)s]+ with NaBH4 or LiAlH4 gives (612) the previously reported (220) rr-cyclohexadienyl complex (C6H7)Mn(CO)3 in addition to a low yield of the (l,3-cyclohexadiene)tricarbonylmanganese hydride (49). Simi-... 

Moreover, the selenol esters can acylate reactive arenes and heteroaromatic compounds when cop-per(I) triflate is employed as the selenophilic metal cation. - The acylation of aromatics by use of the benzene complex of copper(I) triflate (43 Scheme 12) was complete within an hour at room temperature, with benzene as solvent, and the acylation products were obtained in high yields. Intramolecular acylation was examined successfully, as shown in equation (20). 

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