Aromatic donor

Is there any evidence that this rule can be contravened To answer this question, the complexes of vinyl fluoride, furan and thiophene with HC1 and ClF will be considered. Vinyl fluoride, CH2CHF, is an example of a mixed n-pair/jt-pair donor in which, unlike CO, HCN, CH3CN or CH2O, the pairs of electrons (a Tt-pair shared between Ci and C2 and an n-pair on F) do not have an atom in common. In addition, its complexes with HC1 and ClF are important in the context of linear/non-linear hydrogen and halogen bonds. On the other hand, furan and thiophene are examples of mixed n-pair/n-pair aromatic donors in which the n-pair can be withdrawn into the ring. 

It is important to recognize that the intermolecular long-distance bonding with the participation of halogen derivatives represents a specific example of the broad general area of donor/acceptor interactions. Moreover, the complexes of molecular iodine, bromine and chlorine with aromatic donors represent classic examples of charge-transfer compounds [26-28] that are vital for the development of Mulliken theory of intermolecular association [29-31]. The latter thus provides the convenient framework for the... 

The additions of other (polycyclic) aromatic donors to solutions of dichlorine, dibromine or diiodine afford similar new bands, which show significant red shifts with increasing strength of the arene donor. For example, the absorption maximum of the dibromine complexes varies from 280 nm (with chlorobenzene) to 369 nm (with hexamethylbenzene) and similar variations of the new absorption maxima are observed with diiodine complexes (Fig. 2). 

Fig. 2 Electronic spectra of the carbon tetrachloride solutions of diiodine attendant upon addition of excess amounts of benzene (Act = 285 nm), mesitylene (Act = 327 nm) and hexamethylbenzene (Act = 369 nm) with the charge-transfer band showing significant red-shifts with increasing strength of the aromatic donors...

In contrast to the dihalogens, there are only a few spectral studies of complex formation of halocarbon acceptors in solution. Indeed, the appearance of new absorption bands is observed in the tetrabromomethane solutions with diazabicyclooctene [49,50] and with halide anions [5]. The formation of tetrachloromethane complexes with aromatic donors has been suggested without definitive spectral characterization [51,52]. Moreover, recent spectral measurements of the intermolecular interactions of CBr4 or CHBr3 with alkyl-, amino- and methoxy-substituted benzenes and polycyclic aromatic donors reveal the appearance of new absorption bands only in the case of the strongest donors, viz. Act = 380 nm with tetramethyl-p-phenylendiamine (TMPD) and Act = 300 nm with 9,10-dimethoxy-l,4 5,8-... 

No new absorption bands are observed in other cases, largely due to the fact that the strong absorptions of the aromatic donors obstruct the UV-spectral measurements. For the complex between CBr4 and TMPD, the quantitative analyses of the temperature and concentration-dependent absorptions of the new band at 380 nm afford the extinction coefficient of ct = 3.2 x 103 M 1 cm x, as well as the thermodynamic parameters for complex formation AH = - 4.5 kcalM x, AS = - 14 e.u., and Kda = 0.3 M x at 295 K. Such thermodynamic characteristics are similar to those of the dihalogen complexes of as well as those of other acceptors with aromatic donors. Similar results are also obtained for CBr4 associates with halide and thio-cyanide anions [5,53]. 

Tetrabromomethane shows two types of n-bonding with aromatic donors that are contrasted in Fig. 8a and b, showing over-the-rim coordination to the aromatic C-C bond and over-the-center coordination to the benzene ring. The over-the-rim coordination is generally similar to that observed in the dibromine complexes but the C - Br distance in the former is longer, in agreement with weaker acceptor abilities of tetrabromomethane. Note that picryl bromide shows similar bromine coordination to the outer pyrene C-C bond with Br - C distances of 3.35 and 3.39 A [83]. A second type of coordination was reported earlier in ihc p-xylcnc complex [80] and recently in the associate with dimethylnaphtalene [53]. 

Table 2 Overview of crystal structures of CBr4 complexes with aromatic donors ...

Fig. 8 Two modes over-the-rim (a) and over-the-center (b) coordination of tetrabro-momethane to aromatic donors (data from [53,80])...

X-ray structural analyses reveal that the jt-bonding of dihalogens, halocar-bons and halides to arene donors and acceptors are characterized mostly by over-the-rim coordination in which the dihalogen acceptor generally follows the position of highest electron density on the aromatic donor, and the arrangement of halide donor mostly follows the LUMO shape of the aromatic acceptor. 

Note that the competitive aromatic chlorination of the aromatic donors effected by the strongly oxidizing antimony pentachloride (in equation 37) is circumvented by the use of the mild Et30+SbCl[Pg.244]

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

Fig. 13 Charge-transfer absorption bands from dichloromethane solutions containing Os04 and various (a) benzene, (b) naphthalene, and (c) anthracene donors (as indicated) showing the progressive bathochromic shift with aromatic donor strength. Reproduced with permission from Ref. 96b.

Such a charge-transfer osmylation of aromatic donors constitutes the critical... 

A. Nitropyridinium cations. The spontaneous formation of vividly colored charge-transfer (CT) complexes occurs upon exposure of jV-nitropyridinium (PyNO ) cation to various aromatic donors,235 i.e.,... 

Figure 16a shows the progressive bathochromic shift in the CT absorption bands (hvct) obtained from PyN02+ with aromatic donors with increasing donor strength (or decreasing ionization potential). A similar red shift is observed in the CT absorption bands (hvCj) of hexamethylbenzene complexes with various para-substituted JV-nitropyridinium cations (X-PyNO ) as shown in Fig. 16b. Such a trend in the hvct is in accord with the increasing acceptor strength of X-PyNO in the order X = OMe < Me < H < Cl < C02Me < CN.

Fig. 16 Charge-transfer spectra of (a) MeOPyN02+ with various aromatic donors (as indicated) and (b) hexamethylbenzene with various TV-nitropyridinium acceptors (as indicated) in acetonitrile. Reproduced with permission from Ref. 235a.

Moreover, the thermal nitration of various aromatic substrates with different X-PyNO cations shows the strong rate dependence on the acceptor strength of X-PyNO and the aromatic donor strength. This identifies the influence of the HOMO-LUMO gap in the EDA complexes (see Chart 3), and thus provides electron-transfer activation as the viable mechanistic basis for the aromatic nitration. Indeed, the graphic summary in Fig. 18 for toluene nitration depicts the isomeric composition of o-, m- and p-nitrotoluene to be singularly invariant over a wide range of substrate selectivities (k/kQ based on the benzene... 

Aromatic nitrosation with nitrosonium (NO + ) cation - unlike electrophilic nitration with nitronium (NO ) cation - is restricted to very reactive (electron-rich) substrates such as phenols and anilines.241 Electrophilic nitrosation with NO+ is estimated to be about 14 orders of magnitude less effective than nitration with N02+. 242 Such an unusually low reactivity of NO+ toward aromatic donors (as compared to that of NO ) is not a result of the different electron-acceptor strengths of these cationic acceptors since their (reversible) electrochemical reduction potentials are comparable. In order to pinpoint the origin of such a reactivity difference, let us examine the nitrosation reaction in the light of the donor-acceptor association and the electron-transfer paradigm as follows. 

Thermal electron transfer. The high degree of charge-transfer character in [ArH, NO+] complexes is consistent with the fact that a variety of electron-rich aromatic donors undergo reversible electron transfer (in the dark) to form the corresponding cation-radical pair239 (equation 86). 

Although a detailed kinetic analysis of the nitrosation reaction with NO+BFJ is not available, the time/conversions with various aromatic donors suggest that the reactivity does not follow the variation of the ionization potentials of the... 

In a study of long range ET between aromatic donor (biphenyl) and acceptor molecules separated by steroid spacers [39], pulse radiolysis and electron beam techniques have been used for the injection of electrons (Closs and Miller, 1988 Closs et al., 1989 Liang et al, 1990). Here, the reaction rates (observed by changes in the absorption spectra) pass through a... 

Charge transfer versus electron transfer in the interaction of aromatic donors with the nitrosonium cation 230... 

Charge-transfer nitration of aromatic donors with tetranitromethane 237 Simultaneous electrophilic and charge-transfer nitration of aromatic donors with A-nitropyridinium ion 241... 

CHARGE TRANSFER VERSUS ELECTRON TRANSFER IN THE INTERACTION OF AROMATIC DONORS WITH THE NITROSONIUM CATION... 

The oxidative conversions of the aromatic donors hexamethylbenzene, anthracene, dianthracene, bicumene and methoxytoluene by the nitrosonium cation, as described above, are rather unequivocal examples in which the establishment of an electron-transfer equilibrium is a clear prerequisite for the further (follow-up) reactions. There are other donors, including... 

CHARGE-TRANSFER NITRATION OF AROMATIC DONORS WITH TETRANITROMETHANE... 

As useful as tetranitromethane is as a charge-transfer nitrating agent, it is generally too unreactive to effect the comparable electrophilic nitration of most aromatic donors, except the most electron-rich ones. Thus in order to make the direct comparison between the photochemical and thermal nitration of the same ArH, we now turn to the A-nitropyridinium acceptor (PyNOj) as the alternative nitrating agent (Olah et al., 1965, 1980). For example, PyNO can be readily prepared as a colourless crystalline salt, free of any adventitious nitrosonium impurity, and used under essentially neutral conditions. Most importantly, the electrophilic reactivity of this... 

Charge-transfer nitration of aromatic donors by the direct irradiation of EDA complexes with N-nitropyridinium ion... 

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