Acetals metal-arene complexes

An example of a more structured aryl acetate is offered by the monoacetyl derivative 2 ofp-tert-butylcalix[4]arene-crown-5 (1). The half-life for deacetylation in the presence of 1 mM MeONMe4 in MeOH at 25 °C is 34 weeks, but drops to 8 s upon addition of 1 mM BaBr2 [17]. Also, SrBr2 accelerates the reaction, albeit to a somewhat smaller extent. Kinetic and UV spectroscopic data show that in the absence of metal ions MeO reacts with the unionized form 2, but in the presence of metal ions the reactive species is the metal complex of the ionized form 3, which is present in significant amounts by virtue of the acidity enhancing effect of the metal ion complexed by the polyether chain. 

The addition-protonation procedure maintains the arene-chromium bond and allows further application of the activating effect of the metal. In an approach to the synthesis of anthraquinone antibiotics, the dihydronaphthalene complex (79) was allowed to react with a cyanohydrin acetal anion and then quenched with acid.129 The resulting tetralin complex (80) could be metallated effectively and carried on to a key intermediate (81) in anthraquinone construction (equation 54)... 

Perchloric acid increased the electrophilicity of Pd(II). A strong parallel between palladation and mercuration of aromatic hydrocarbons was drawn.568 (The effect of strong acids in increasing the electrophilicity of metal acetates has been discussed earlier.) Arylmercury complexes are relatively stable and do not afford biaryl readily. Analogous arylthallium(III) complexes only afford biaryls on photolysis in the presence of arenes via the following sequence ... 

In the presence of Lewis acids, N-substituted aniline complexes of [Os] also add electrophiles at C4, again at the arene face opposite to that involved in metal coordination. This reaction has been shown to be general for a broader range of Michael acceptors than may be utilized with anisole complexes of [Os]. The N-ethyl aniline complex, for example, adds Michael acceptors and acetals in yields ranging from 53-95 % (Table 13, entries 1-6) [27]. The N,N-dimethyl aniline complex (entries 7-9) also adds Michael acceptors to C4 in moderate to high yields (Table 13) and adds to the <5-carbon of an a,/ ,y,<5-un saturated ester (entry 3). 

A variety of arenes and heteroarenes react with alkenes in the presence of palladium(II) derivatives to produce alkenyl substitution products. Three methods are commonly employed for the in situ preparation of palladium derivatives (i) direct metallation of an arene or heteroarene with a Pd(II) salt (ii) exchange of the organic group from a main-group organometallic to a Pd(II) compound (iii) oxidative addition of an organic halide, an acetate, or triflate salt to Pd(0) or a Pd(0) complex. For catalytic reactions Cu(II) chloride or p-benzoquinone is usually used to reoxidize Pd(0) to Pd(II). 

The stability to oxidative degradation of palladium/NHC complexes has made possible the synthesis of complexes previously unknown or much less explored. (NHC)Pd(K-OAc)(OAc) shows a unique coordination of the acetate units that stabilizes a distorted square planar geometry around the metal center. These complexes are stable in acidic media and can activate C-H bonds of simple arenes by an electrophilic metalation mechanism (Scheme 32). 

A lower rim conjugate in which the p-bu -calix[4]arene was covalently linked at 1,3-distal positions to tetraphenylporphyrin through -CH2-CO-NH- at the para position of one of the phenyl rings was metallated using zinc acetate/triethyl amine to form 192. It reacts with the bidentate ligand l,4-diazabicyclo[2.2.2]octane (DABCO) to form a 1 2 complex in which both porphyrin units are ligated separately (2003T2409). 

The L -iodane PhI(OAc)2 has been used extensively, most notably by Sanford, to oxygenate the C-H bonds of arenes and even alkanes in the presence of palladium catalysts." Interestingly, this reaction was likely mediated by a binuclear complex featuring two Pd(III) atoms joined by a palladium-palladium (Pd-Pd) bond however, this complex can disproportionate into a Pd(IV)-Pd(II) complex without a formal Pd-Pd bond, and this intricate interplay is often controlled by the presence and nature of hypervalent iodine oxidants." We discovered that this reaction manifold could be extended to include the amination of arene substrates heating 2-phenylindole 51 in the presence of the -iodane 49 and palladium acetate provided the ortho-aminated product 52 in a 19% yield however, switching the metal from palladium acetate... 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Reactions involving electrophilic substitution of hydrogen in arenes are known for both nontransition [Hg(II), Tl(III), Pb(IV)] and transition metals [Au(III), Pd(II), Pt(IV)] [49]. Pd(II)-catalyzed acetoxylation involves arene activation via formation of an organometallic aryl-Pd c-complex followed by oxidative addition of oxidant and reductive elimination to restore Pd(II) and release the product [11, 50]. Without oxidant, coupling reactions predominate, suggesting arylpalladium(IV) and arylpalladium(II) intermediates in the routes leading to aryl acetates and biaryls, respectively (Scheme 14.10). 

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