Arenes with Alkenes

In the palladium-catalyzed coupling reactions of arenes with alkenes, the cr-arylpalladium complexes react with CO to give aromatic acids in AcOH, as shown in Scheme u 97>97a 97c This carboxylation reaction of arenes with CO proceeds catalytically with respect to Pd at room temperature under atmospheric pressure of CO, when K2S2O8 is added as an oxidant and TFA is employed as a solvent. 

In our attempt to extend the coupling reaction of arenes with alkenes to the coupling with alkynes, as shown in Scheme 4, it was found that the reaction of arenes with ethyl propiolate in TFA (trifluoroacetic acid) gave addition products instead of a coupling product [3]. This addition reaction has been extended to various alkynes and various arenes and also to intramolecular reactions for synthesis of heterocycles such as coumarins, quinolines, and thiocoumarins. 

Scheme 1. Pd-catalyzed reactions of arenes with alkenes or CO.

Barrelene, benzobarrelene and dibenzobarrelene derivatives are readily synthesized by Diels-Alder reactions of arenes with alkenes. For barrelene and related derivatives, the DPM rearrangement proceeds solely from the triplet state, giving semibullva-lenes (Scheme 4.1) [3, 4]. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

Several reviews have appeared during the year. The photoaddition reactions of aromatic compounds have been discussed with special attention to the mechanistic and structural features governing the outcome of the interaction of photoexcited arenes with alkenes and other arenes. Morrison has published an account of the photoactivation of distal functional groups in polyfunctionai... 

The alkylation of arenes with alkenes such as ethylene and propene are of great commercial interest. Ethylbenzene and isopropylbenzene (cumene), products of the Friedel-Crafts alkylation of benzene with ethylene and propene, respectively, are two of the most important petrochemical raw materials. Roberts and Khalaf have follow the developments made in this vast field up to the early part of this decade. This is evident from the large number of references quoted, most of which describe efforts to evaluate conditions for optimal production in the presence of various catalyst systems. 

Arylation of alkenes. Coupling reaction of arenes with alkenes by PdfOAc), is carried out in the presence of benzoquinone and /-BuOOH as a reoxidant. Note, this is not a Heck reaction (involving ArH instead of ArX). 

Reactions of [bis(acyloxy)iodo]arenes with alkenes in some cases can give products of an oxidative, carbocationic rearrangement. Such a rearrangement of pentaalkoxychalcones 376 upon treatment with [bis(trifluoroacetoxy)iodo]benzene has been applied toward the preparation of the rearranged chalcones... 

Oxidative coupling of arenes with alkenes consumes a stoichiometric amount of Pd(II). Efficient catalytic reaction of cinnamate (165) with benzene to afford 166 was carried out using BQ and f-butyl hydroperoxide as oxidant in AcOH [70]. The coupling proceeds smoothly in the presence of catalytic amounts of Pd(OAc)2 and molybdovanadophosphoric acid (HPMoV) under oxygen (1 atm) in AcOH [71]. 

A theoretical study has been reported of the Heck reaction of electron-deficient arenes with alkenes catalysed by palladium(n) carrying a pyridine-type ligand. 

The direct catalytic alkylation of C(sp )-H bonds with alkenes to create C-C bonds offers an atom-economical alternative to other C-H alkylation methods, as it does not require a sacrificial functional group. In a seminal report, Yamazaki and coworkers [72] described that a dehydrogenative alkenylation of arenes with alkenes yielded a regioisomeric mixture of alkenylation and alkylation products in the presence of Rh4(CO)i2 (Scheme 19.46). 

Scheme 19.46 Dehydrogenative alkenylation of arenes with alkenes using Rh4(CO),2.

It has been determined that impurities such as moisture can accelerate this reaction, as AlCb-promoted reactions conducted under strictly anhydrous conditions can occur at a significantly lower rate of reaction. Due to this observation, it was subsequently discovered that the addition of cocatalysts can accelerate the rate of this reaction, such as oxygen, proton-releasing substances (ROH), Bronsted acids, and cation/carbocation producing substances. For example, the F-C alkylation of arenes with alkenes and alkynes promoted by AICI3 was found to be cocatalyzed by trace amounts of moisture effecting an increase in both the rate and yield of the reaction whereas the opposite was observed with use of FeCb as the cocatalyst. 

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