Arenes formation

The discussion has focused so far on activation of alkanes, where formation of the a-complex seems to precede oxidative addition. For arenes, formation of the analogous a(cH)-arene complex is thought to occur before oxidative addition to form an aryl hydride. These a-com-plexes have never been observed, presumably because they are unstable with respect to the 71-complexes. Both types of arene complexes are, for the case of benzene, shown in Scheme 25 the a(CH)-arene complex as A and... 

The Pd-catalyzed alkynylation is one of the most widely used —C bond formation reactions. It has been thoroughly and extensively reviewed recently15-17. So only a brief overview and a discussion of some of the most recent advances will be presented in this section. As in the case of alkenyl-alkenyl coupling, Ni catalysts have rarely been used for alkynylation, even though Ni catalysts do affect the desired alkynylation in some cases. One main reason for disfavoring Ni is that Ni can readily react with alkynes to undergo alkyne cyclooligomerization represented by arene formation. The Pd-catalyzed alkynylation can, in principle, be performed in three discrete manners, as shown in Scheme 40. 

Clearly, there is a delicate balance between metal-arene formation and dehalogenation of the arene. Among the aforementioned isolated compounds, those of chromium show greater resistance to air oxidation than those of vanadium for example, Cr[C8H4(CF3)2]2 is completely stable in air (32, 59, 79, 115). 

The 1,3-dioxoles and 1,4-dioxene are the only alkenes with which exciplex formation and decay can be studied in conjunction with photoaddition. The dioxoles give ortho as well as meta photocycloadducts with benzene [13,14,122], Mattay et al. [15,134,183] have pictured the exciplex as a common precursor to both types. The exciplex is supposed to possess charge-transfer character, from the dioxole to the arene. Formation of the ortho adduct is thought to proceed stepwise as shown in Scheme 36 for anisole and 1,3-dioxole... 

The use of PPF-OMe (3) also allowed the coupling of secondary alkylary-lamines that possess alkyl groups other than methyl. The reaction of AT-ethyl-aniline and 5-bromo-meta-xylene proceeds in excellent yield with no reduced arene formation, Eq. (26). It should be noted that the 3/Pd-catalyst system tolerates the use of CS2CO3 as base. 

Figure 15. Encapsulation complexes of NOx gases with calix[4]arenes. Formation of calixarene-NO complexes (71-73). [Adapted from (72).]...

This explains why, at equal conversions and propane concentrations, the propene fraction on Ga-silicate is much lower than on Pt/Ga-silicate. This route of arene formation is catalyzed by acidic centers and it competes with cracking. At higher temperatures, gallium became more active in alicyclic hydrocarbon dehydrogenation [7,8] and selectivities for the both catalysts became similar. 

To study the electron transfer from [60]fiillerene to the arene radical cation, deoxygenated DCM solutions of, for example, m-terphenyl were irradiated in the presence of variable [60]fullerene concentrations (0.7 - 6.0) x 10 M. The short life-time of some of the (arene)", in combination with the unfavorably high ionization potential of [60]fiillerene limited the ability to measure the electron transfer process to, however, only a few (arene) . Formation of the electron transfer product, namely, was confirmed spectroscopically by measuring the NIR fingerprint at > max= 980 nm, which resembles that found upon direct oxidation of [60]fullerene. 

An extension of Hashmi s Au(III)-catalyzed phenol synthesis [81] to furan substrates 9 bearing an additional alkyne moiety allowed the preparation of C6-C7-heterofused benzofuran 11 (Scheme 9.3) [82]. According to the proposed mechanism, the Au(III)-catalyzed arene formation reaction generates o-alkynylphenol 10. A subsequent Au(III)-catalyzed cycloisomerization of the latter, following the general mechanism for an intramolecular nucleophilic addition of heteroatom to transition metal-activated carbon-carbon multiple bonds, gives 11 (Scheme 9.3). 

Asar[ ]arene rings with more than six phenylene units n = 6-15) can be accessed when conducting the synthesis of asar[n]arenes at lower temperatures ( 36°C). Over time, this reaction mixture starts slowly to become enriched in asar[6]arene (12.32a). This finding suggests that, as in the synthesis of pillar[ ]arenes, formation of the asar[n]arene macrocycles... 

For calix[5]arenes, similarly to calix[4]arenes, formation of inclusion dimers is observed for unsubstituted at upper rim or substituted with small e.g. methyl groups calix[5]arenes. As an example para-methylated calix[5]arene is shown in Fig. 38.17a [37]. 

When one of the N—H bonds of primary amines is used for arene formation, the other N—H bond can serve as another reaction site for further elaboration. For example, IV-unsubstituted 2-alkynylaniline derivatives 309 undergo a copper-catalyzed direct C—H/N—H coupling with azoles 310 followed by cycloisomerization to form A -azoylindoles 311 in good yields (Scheme 19.83) [153]. Oxadiazoles, benzoxa-zoles, and benzothiazoles can be used as the azole 310 for the reaction. Double hydroamination reaction of the anilines of type 309 with terminal alkynes leading to A -vinylindoles was reported by Zhang et al. [154]. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

A rational classification of reactions based on mechanistic considerations is essential for the better understanding of such a broad research field as that of the organic chemistry of Pd. Therefore, as was done in my previous book, the organic reactions of Pd are classified into stoichiometric and catalytic reactions. It is essential to form a Pd—C cr-bond for a synthetic reaction. The Pd— C (T-bond is formed in two ways depending on the substrates. ir-Bond formation from "unoxidized forms [1] of alkenes and arenes (simple alkenes and arenes) leads to stoichiometric reactions, and that from oxidized forms of alkenes and arenes (typically halides) leads to catalytic reactions. We first consider how these two reactions differ. 

The preparation of amines by the methods described m this section involves the prior synthesis and isolation of some reducible material that has a carbon-nitrogen bond an azide a nitrile a nitro substituted arene or an amide The following section describes a method that combines the two steps of carbon-nitrogen bond formation and reduction into a single operation Like the reduction of amides it offers the possibility of prepar mg primary secondary or tertiary amines by proper choice of starting materials... 

Azo coupling (Section 22 18) Formation of a compound of the type ArN=NAr by reaction of an aryl diazonium salt with an arene The arene must be strongly activated toward... 

DCHA is normally obtained in low yields as a coproduct of aniline hydrogenation. The proposed mechanism of secondary amine formation in either reductive amination of cyclohexanone or arene hydrogenation iHurninates specific steps (Fig. 1) on which catalyst, solvents, and additives moderating catalyst supports all have effects. 

Tnfluoroacetic anhydnde in a mixture with sulfuric acid is an efficient reagent for the sulfonylation of aromatic compounds [44] The reaction of benzene with this system in nitromethane at room temperature gives diphenyl sulfone in 61% yield Alkyl and alkoxy benzenes under similar conditions form the corresponding diaryl sulfones in almost quantitative yield, whereas yields of sulfones from deactivated arenes such as chlorobenzene are substantially lower [44] The same reagent (tnfluoroacetic anhydride-sulfunc acid) reacts with adamantane and its derivatives with formation of isomeric adamantanols, adamantanones, and cyclic sultones [45]... 

The photolysis of 10.3a in pentane or the reaction of 10.3a with PPhs generate the corresponding sulfur diimide ArN=S=NAr (Ar = 2,4- Bu2-6-MeC6H2) as the major product, presumably via the intermediate formation of the thionitrosoarene." By contrast, thermolysis in arene solvents results in ring closure to give the 2,1-benzisothiazole 10.4 and the corresponding aniline."... 

Bis ( -arene) metal complexes have been made for many transition metals by the AI/AICI3 reduction method and cationic species [M( j -Ar)2]"" " are also well established for n = 1, 2, and 3. Numerous arenas besides benzene have been used, the next most common being l,3,5-Mc3C6H3 (mesitylene) and CeMce. Reaction of arenas with metal carbonyls in high-boiling solvents or under the influence of ultraviolet light results in the displacement of 3CO and the formation of arena-metal carbonyls ... 

In some instances the attack of the arene on the nitrilium salt occurs at the ipso carbon rather than the ortho carbon. For example, the Bischler-Napieralski cyclization of phenethyl amide 10 affords a 2 1 mixture of regioisomeric products 11 and 12. The formation of 12 presumably results from attack of the ipso aromatic carbon on the nitrilium salt 13 followed by rearrangement of the spirocyclic carbocation 14 to afford 15, which upon loss of a proton vields product 12. ... 

Thermal arene exchange of tetramethylthiophene with [(/ -cymene)RuCl2]2 affords 130 (89JA8828), which on reaction with AgBE4 and excess tetramethylthiophene yields 131. The Ru—S thiophenic cluster, 132, was synthesized by reaction of 130 with (Mc3Si)2S followed by anionic metathesis and formation of the PFg salt. The coordination geometry around each ruthenium atom is pseudooctahedral. 

Numerous reactions have been described in which the oxygen of the oxepin system is removed to give benzene derivatives. The formation of the aromatic products can be rationalized by an arene oxide as intermediate. A suitable reagent for the elimination of an oxygen atom from this heterocycle is triphenylphosphane, e.g. formation of l,24 2a,12 and 2b.1,9... 

Some degree of regioselectivity can be imposed on l//-azepine formation if the arene has substituents of high steric demand.63 For example, the thermolysis of ethyl azidoformate in a tenfold molar excess of 1,4-di-fert-butylbenzene yields a 95 5 mixture of the di-zerr-butyl-l//-azcpincs 3 and 4, crystallization of which yields the pure 3,6-di-/er/-butyl isomer 3. 

We examined the possibility of a direct formation of two C-C bonds by reaction of a carbanion with [Fe(arene)2]2+ in which the arene bears methyl groups. We could indeed repeat Hellings s experiments but found that mesitylene was the only aromatic allowing this possibility in reasonable yields. With p-xylene, a low yield of an unstable complex was obtained corresponding to double nucleophilic attack of phenyllithium on the same ring in spite of the bulk of the methyl groups [23]. Eq. (4) ... 

Fe+ + has been deprotonated, but the reaction is complicated by further nucleophilic attack of the methylene unit with the starting material [17]. Enhanced acidity of the ring hydrogens in arene-metal complexes is shown [21] by the formation of complexes of alkyllithium by proton abstraction. 

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