Arene stepwise mechanism

A more complicated pH-rate profile is also observed for the hydrolysis reactions of benzo[a]pyrene diol epoxide epoxide 80, and is shown in Fig. 5.102 This profile shows Regions A-D that are similar to those for reaction of precocene I oxide 76 (Fig. 4), except that Region B reaches a full plateau that extends from pH 5 to 9 in water. The interpretation of this pH-rate profile is essentially the same as the interpretation of the profile for hydrolysis of precocene I oxide (Fig. 4). The pH-independent reaction of 80 in Region B (discussed in detail in Section Benzylic epoxides and arene oxides ) yields 60% tetrols in a stepwise mechanism involving a carbocation intermediate and 40% ketone from a completely separate pathway (Scheme 31). The negative inflection of the profile at pH 10-11.5 indicates that hydroxide ion reacts as a base with the intermediate carbocation to reform diol epoxide 80 and thus slow the reaction rate. There is a corresponding increase in the yield of ketone 107 at pH >11. 

A stepwise mechanism similar to that proposed for arene displacement reactions (see p. 178) cannot be eliminated. 

Alternatively, arene displacement can also be photo- rather than thermally-induced. In this respect, we studied the photoactivation of the dinuclear ruthenium-arene complex [ RuCl (rj6-indane) 2(p-2,3-dpp)]2+ (2,3-dpp, 2,3-bis(2-pyridyl)pyrazine) (21). The thermal reactivity of this compound is limited to the stepwise double aquation (which shows biexponential kinetics), but irradiation of the sample results in photoinduced loss of the arene. This photoactivation pathway produces ruthenium species that are more active than their ruthenium-arene precursors (Fig. 18). At the same time, free indane fluoresces 40 times more strongly than bound indane, opening up possibilities to use the arene as a fluorescent marker for imaging purposes. The photoactivation pathway is different from those previously discussed for photoactivated Pt(IV) diazido complexes, as it involves photosubstitution rather than photoreduction. Importantly, the photoactivation mechanism is independent of oxygen (see Section II on photoactivatable platinum drugs) (83). 

The study of mercuration and thallation provides a shaip focus on the experimental delineation of stepwise and concerted mechanisms for arene activation. Thus the unequivoc demonstration of arene radical cations as key intermediates in thallation, particularly of durene and pentamethylbenzene, is consistent with a stepwise (electron-transfer) mechanism for arene activation (compare Scheme 6 and equation 39). 

Phosphine and phosphite ligands react with arene tricarbonyl complexes of Cr, Mo, and W yielding complexes of the type /ac-(PRg)3M-(CO)3 32, 261, 341-343, 466). Kinetic studies reveal that the reaction proceeds by an 8 2 mechanism 341-343, 466) which is thought to involve the stepwise displacement of the arene via tetrahapto- and dihapto-coordinated arene intermediates 466) ... 

The pH-independent reaction of cyclopentadiene oxide 12 in water is clearly different from that of 13.97 This reaction yields 33% of 3-cyclopentenone, 35% of cis-2,4-pentadienal and 35% of a mixture of cis and trans 1,2- and 1,4-diols (Scheme 28). When the reaction is carried out in D20 instead of H20, no deuterium is incorporated into the ketone product. Thus, 1,2-hydrogen migration is required for this reaction just as it is in the rearrangement of arene oxides to phenols. The mechanisms of product formation in this reaction are not fully understood. The observation that the diol mixture is similar to that from the acid-catalyzed hydrolysis of 12 suggests that an allylic carbocation may be involved in the diol-forming reaction. Ketone (96) and dienal (97) products are potentially formed either by stepwise or concerted mechanisms, and there is insufficient evidence to rule out either one. There is a significant normal salt effect on this pH-independent... 

Tanaka D, Romeril ASP, Myers AG (2005) On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes. J Am Chem Soc 127 10323-10333... 

An inverse electron demand Alder-Ene reaction has been reported in which a nitrilium ion acts as the ene and an arene as enophile. The reaction occurred at room temperature and was proposed to be a double Alder-Ene reaction. The first step provided compound 133, which could undergo a stepwise addition of the vinyl sulphide to the azonia-allene followed by elimination or alternatively, a second concerted Alder-Ene type mechanism was suggested. The heterobicycle 134 was isolated in 90% yield. 

A schematic representation of the twelve pyridino homologues derivable from p-tert-butylcalix[4]arene 1 and possible reaction pathways for their formation are shown in Scheme 1. Of these compounds, only anf/-(l,2) and anh-(l,3)-di-0-alkylated regioisomers 3c and 3d have not been reported yet. The remainder have been obtained either by direct O-alkylation of the parent calix[4]arene 1 or by alkylation of an appropriate precursor. The mechanism of alkylation of calix[4]arene 1 with PicCl-HCl is based on product analysis of stepwise alkylation experiments, and MM2 calculations of the involved intermediates and their anions [18]. 

Reduction of the bulky terphenyl substituted aluminum iodide Dipp All2 with KCg in the presence of toluene [27] or Me3SiCCSiMe3 [84] afforded novel Al-Al metallacycle species, possibly formed through [2 -i- 4] or [2 + 2] cycloaddition reactions between the dialuminene intermediate Dipp Al=AlDipp and an arene or alkyne source (Scheme 6). However, the exact mechanism remains to be studied and may well involve stepwise ionic or radical processes. For that matter, theoretical calculations suggested the existence of a partial diradical character in dialuminene species [85]. 

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