Arenium complexes, stabilization

Substituents Relative Arenium Ion stability Relative n-Complex Stability Rate of Chlorination" Rate of Nitration ... 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

Harman and co-workers738 reported the synthesis of arenium cations stabilized by Os complexation (for example, 415) via protonation with triflic acid of [Os(NH3)5(Ar)](OTf)2 complexes (Ar = benzene, toluene, xylenes, naphthalene, anthracene). 

The stability of cationic Re(I) arenium complexes (see above) renders the arenium ligand susceptible to a second controlled transformation with a nucleophile. Thus, an electrophile can be added to a dihapto-coordinated aromatic ligand followed by a nucleophile in a stepwise tandem addition. For example, TpRe(CO)(MeIm)... 

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

TABLE 11.1 Relative Stabilities of Arenium Ions and ir Complexes and Relative Rates of Chlorination and Nitration ... 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

Vigalok, A. Shimon, L. J. W. Milstein, D. Methylene arenium cations via quinone methides and xylylenes stabilized by metal complexation. J. Am. Chem. Soc. 1998, 120, 477 183. 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Wheland intermediates, a complexes, or arenium ions In the case of benzenoid systems they are cyclohexadienyl cations. It is easily seen that the great stability associated with an aromatic sextet is no longer present in 1, though the ion is stabilized by resonance of its own. The arenium ion is generally a highly reactive intermediate and must stabilize itself by a further reaction, although it has been isolated (see p. 504). 

TABLE 11.1 Relative stabilities of arenium ions and ir complexes and relative rates of chlorination and nitration... 

The predominance of meta-diethyl benzene in the isomerization of diethylbenzenes is easily rationalized. Since isomerization reaction proceeds via arenium ion intermediates, the cr-complex derived from mefa-diethylbenzene is the most stable one. Moreover, mefa-diethylbenzene is also the most basic of the isomeric diethylben-zenes. Therefore, more acidic catalysts increase the amount of the meta isomer at the expense of the para (and ortho) isomers, due to the increased stability of the substrate-catalyst complex. 

It corresponds to addition followed by elimination, and is symbolised by AE + De. The departing X+ is often a proton, while Z is a general substituent. The key step in this scheme is the formation of an intermediate arenium ion (Wheland intermediate, a complex), and the relative stability of this species is crucial to the outcome of the reaction. Isolable arenium ions are known, and the benzenonium ion itself C6Hy has been inferred from NMR of strongly acidic solutions [255],... 

The synthetic methodology outlined in this chapter is fundamentally different than that described elsewhere in this book. It is based on a transition metal acting as an electron-donor (i.e., a 71 base) for the aromatic ligand. Thus, reactions with electrophiles are promoted, and the resulting arenium systems are stabilized such that a nucleophile may then be added. In this regard, the reactions outlined in this chapter complement the more established q -arene methodology. Whereas the chromium q -arene complexes have been known for over 40 years, viable q -arene complexes have been known for the past 15, and only recently has an alternative to osmium emerged. Ultimately the impact that 71 bases... 

In the early 1960 s it was described 20,24,55-5 ) salt-like compounds of aromatic hydrocarbons are o-complexes, i.e. their cations AH possess the structure of arenium ions. This conclusion was first based on indirect arguments ensuing from the analysis of the AH -cation electronic absorption spectra (in particular, from the similarity of the spectra of anthracene and 1,1-diphenylethylene solutions in cone. HjSO j. It also results from the linear dependence of the logarithms of the. relative stability constants of A HF BF3 complexes on tho% of the rate constants of electrophilic substitution reaction of the hydrocarbons A Direct proof of this point of view was obtained from studies into the A HY mMY complexes and the solutions of aromatic hydrocarbons or their derivatives in various acids (HF, HF + BFj, HSO3F and others) by the nuclear magnetic resonance nKasurements of Dutch investigators... 

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