Arenes, reduction

The facility of arene reductive elimination underpins numerous C-C, C-O and C-N bond-forming reactions, which may be catalysed by late transition metals, in particular palladium (Figure 4.10). Although there are many variants, the general reaction scheme involves introduction of the aryl in electrophilic form via oxidative addition of an aryl halide (or sulfonate), substitution of the palladium halide by a nucleophile (which may also be carbon based) followed by reductive elimination. It is noteworthy that nucleophilic aromatic substitution in the absence of such catalysts can be difficult. 

Reductive elimination of ethane from five-coordinate Pt(IV) alkyl complexes has also led to the generation of three-coordinate complexes that have shown catalytic activity in the hydroarylation of olefins. In contrast to the f-Bu or Ph substituted pypyr ligands which underwent facile cyclometalation and trapping with ethylene (Scheme 7), when the Me-substituted ( pypyr)PtMe3 (4c) was heated in benzene solvent under a pressure of ethylene, ethyl benzene product was produced with a TON of 26 [94]. Other combinations of arenes and olefins were also observed to yield hydroarylation products when ( pypyr)PtMe3 complex 4c was used as a catalyst precursor. Presumably C-C reductive elimination of ethane is followed by C-H activation of the arene, reductive elimination of methane, and then... 

Recent investigation of the effect of substituents in the para position of the phenylalanine ligand on the stability of the ternary complexes has revealed the secpience Br > OH > Q NH2 > H > F". Interestingly, analysis of CD spectra indicates a reduction of the arene-arene interaction" upon addition of 1,4-dioxane to aqueous solutions of the mixed-ligand complexes, in disagreement with previous observations by Sigel" . 

The literature on arene - arene interactions in ternary metal-ion complexes, as reviewed in Section 3.2.3, indicates that these interactions are generally enthalpy-driven and counteracted by a reduction... 

Also the arene-arene interactions, as encountered in Chapter 3, are partly due to hydrophobic effects, which can be ranked among enforced hydrophobic interactions. Simultaneous coordination of an aromatic oc amino acid ligand and the dienophile to the central copper(II) ion offers the possibility of a reduction of the number of water molecules involved in hydrophobic hydration, leading to a strengthening of the arene-arene interaction. Hence, hydrophobic effects can have a beneficial influence on the enantioselectivity of organic reactions. This effect is anticipated to extend well beyond the Diels-Alder reaction. 

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

The Birch reduction not only provides a method to prepare dienes from arenes which cannot be accomplished by catalytic hydrogenation but also gives a nonconju gated diene system rather than the more stable conjugated one... 

Section 1111 An example of a reaction m which the ring itself reacts is the Birch reduction The ring of an arene is reduced to a nonconjugated diene by treatment with a Group I metal (usually sodium) m liquid ammonia m the presence of an alcohol... 

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... 

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. 

Sulfonic acids are prone to reduction with iodine [7553-56-2] in the presence of triphenylphosphine [603-35-0] to produce the corresponding iodides. This type of reduction is also facile with alkyl sulfonates (16). Aromatic sulfonic acids may also be reduced electrochemicaHy to give the parent arene. However, sulfonic acids, when reduced with iodine and phosphoms [7723-14-0] produce thiols (qv). Amination of sulfonates has also been reported, in which the carbon—sulfur bond is cleaved (17). Ortho-Hthiation of sulfonic acid lithium salts has proven to be a useful technique for organic syntheses, but has Httie commercial importance. Optically active sulfonates have been used in asymmetric syntheses to selectively O-alkylate alcohols and phenols, typically on a laboratory scale. Aromatic sulfonates are cleaved, ie, desulfonated, by uv radiation to give the parent aromatic compound and a coupling product of the aromatic compound, as shown, where Ar represents an aryl group (18). 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

The synthetic procedure described is based on that reported earlier for the synthesis on a smaller scale of anthracene, benz[a]anthracene, chrysene, dibenz[a,c]anthracene, and phenanthrene in excellent yields from the corresponding quinones. Although reduction of quinones with HI and phosphorus was described in the older literature, relatively drastic conditions were employed and mixtures of polyhydrogenated derivatives were the principal products. The relatively milder experimental procedure employed herein appears generally applicable to the reduction of both ortho- and para-quinones directly to the fully aromatic polycyclic arenes. The method is apparently inapplicable to quinones having an olefinic bond, such as o-naphthoquinone, since an analogous reaction of the latter provides a product of undetermined structure (unpublished result). As shown previously, phenols and hydro-quinones, implicated as intermediates in the reduction of quinones by HI, can also be smoothly deoxygenated to fully aromatic polycyclic arenes under conditions similar to those described herein. 

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 ... 

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