Direct arylations silver® oxide

Direct arylation of methyl imidazole with 2,7-dichloronaphthyridine leads to a potentially tetradentate and practically tridentate bis-carbene ligand on a naphthyridine scaffold [363]. Reaction of the bis-imidazolium salt with silver(l) oxide in the usual way yields a linear trinuclear silver carbene complex with this tris-bridging hgand (see Figure 3.115). 

Sulfonic esters are most frequently prepared by treatment of the corresponding sulfonyl halides with alcohols in the presence of a base. This procedure is the most common method for the conversion of alcohols to tosylates, brosylates, and similar sulfonic esters. Both R and R may be alkyl or aryl. The base is often pyridine, which functions as a nucleophilic catalyst, as in the similar alcoholysis of carboxylic acyl halides (16-61). Propylenediamines have also been used to facilitate tosylation of an alcohol. Silver oxide has been used, in conjunction with KI. Primary alcohols react the most rapidly, and it is often possible to sulfonate selectively a primary OH group in a molecule that also contains secondary or tertiary OH groups. The reaction with sulfonamides has been much less frequently used and is limited to A,A-disubstituted sulfonamides that is, R— may not be hydrogen. However, within these limits it is a useful reaction. The nucleophile in this case is actually RO . However, R may be hydrogen (as well as alkyl) if the nucleophile is a phenol, so that the product is RS020Ar. Acidic catalysts are used in this case. Sulfonic acids have been converted directly to sulfonates by treatment with triethyl or trimethyl orthoformate, HC(OR)3, without catalyst or solvent and with a trialkyl phosphite, P(OR)3. ... 

A new approach for direct arylation of pyridine W-oxides with arylboronic acids through C-H functionalization has been developed (Scheme 45) [99]. This reaction can be performed at room temperature using catalytic silver (I) nitrate in the presence of potassium persulfate, thus giving 2-aryl derivatives of pyridine W-oxides. 

A variety of 2- or 3-substituted thiophenes, as well as benzothiophenes, have been subjected to the catalytic direct arylation [3, 9, 10]. As expected, 2,2 -bithio-phene can be diarylated at the 5,5 -positions (Equation 10.47) [72], although the use of a bulky phosphine is of key importance for this reaction. 2,2 -Bithiophene protected by benzophenone at the 5-position reacts with aryl bromides, initially with liberation of the ketone (see Scheme 10.6, to give 5-aryl-2,2 -bithiophene, which is then arylated at the 5 -position (Equation 10.48) [72]. 5-Bromo-2,2 -bithio-phenes undergo oxidative homocoupling in the presence of a palladium complex and a silver salt (Equation 10.49) [73]. 

Silver(I) carbonate was the best choice for oxidant, outperforming AgOAc. Pivalic acid was utilized as an additive. Different arenes could be employed, providing separable mixtures of arylated and diarylated products. The resulting products were converted to phenanthroline derivatives and tested in transition metal-free direct arylation and Mizoroki-Heck reactions. 

Direct Arylation of Heteroarenes, Aryl silicon reagents were employed to directly arylate heteroarenes via a Pd-catalyzed, ligand-free process. Silver(I) fluoride was the most effective source of fluoride (93%) compared with FeFs, KF, wBu4NF-H20, and CsF, although CuF2 gave a comparable performance (82%). In addition to activating the arylsilanes, silver(I) fluoride acts as a co-oxidant with Cu(0Ac)2-H20 to oxidize Pd° to Pd. The electron-rich and electron-deficient aryl (trimethoxy)silanes could be coupled to access arylated benzothiazoles in excellent yields (83-93%) (eq 19). 

Using a 2-pyrimidyl directing group, indoles can be directly arylated with arylsilanes in aqueous media via a mild Rh-catalyzed process. The catalyst system consisted of [Cp RhCl2]2 with Cu(OAc)2 as an oxidant. Silver(I) fluoride was selected a fluoride source to activate the arylsilanes. Other fluoride sources, such as CsF and TBAF, showed no reactivity. Additionally, the reaction could be performed in water using THF as a cosolvent. Other arylsilanes could be employed, although... 

In a process developed by Myers et al., aromatic carboxylic acids were directly employed as substrates for Heck olefinations, albeit in the presence of a large excess of silver carbonate [38]. This base both facilitates the decarboxylation step and acts as an oxidant, generating arylpalladium(II) intermediates. In related processes, arylphosphonic [39] and arylboronic acids [40] were used as aryl sources in the presence of an oxidant. 

O/t/20-arylation of benzoic acids is often preferable to ortho-arylation of benzamides if conversion of the amide moiety to other functional groups is desired. However, only a few reports have dealt with the orf/io-functionalization of free benzoic acids due to challenges that involve such transformations. The reactions can be complicated by decarboxylation of the product and the starting material. Despite those difficulties, several methods for direct o/t/io-arylation of benzoic acids have been developed. Yu has shown that arylboronates are effective in arylation of benzoic acids under palladium catalysis [59], The reactions require the presence of palladium acetate catalyst, silver carbonate oxidant, and benzoquinone. Even more interestingly, the procedure is applicable to the arylation of unactivated sp3 C-H bonds in tertiary carboxylic acids such as pivalic acid (Scheme 13) if aryl iodide coupling partner is used. Aryl trifluoroborates can also be used [60],... 

Based on studies directed towards chelation-assisted oxidative homocouplings of 2-arylpyridines, a protocol was developed for intermolecular coupling reactions [132]. Thus, benzo h quinoline (186) was efficiently arylated with a variety of arenes using silver salts and benzoquinone as additives (Scheme 9.53). 

Fused thiophene-cyclopentanes 39 can be synthesized by intramolecular dual C-H activation of 2-arylthiophenes 38 (Scheme 17, Table 10) [67]. The cross-coupling proceeds moderately well using palladium(ll) acetate as catalyst and silver (1) carbonate as oxidant. When the thiophene moiety is not substituted at positimi 2, homocoupling occurs easily. However, in contrast to this direct route, a two-step sequence consisting of a prior bromination and a subsequent palladium-catalyzed arylation is much more effective forming the fused thiophene-cyclopentane 39 in a yield of 83%. 

As a follow-up work, Yu et al. succeeded to develop O-methyl hydroxamic acids-directed P-C(sp )-H arylation with aryl boronic acids imder mild reaction conditions in 2008 (Scheme 1.6) [23]. Remarkably, they identified that air could be employed as a sustainable external oxidant to replace silver salts (Scheme 1.6b). Since the O-methyl hydroxamic acids are readily to undergo a series of organic transformations, this protocol provides a facile access to a class of bioactive target molecules. 

Abstract The selective catalytic activation/functionalization of sp C-H bonds is expected to improve synthesis methods by better step number and atom economy. This chapter describes the recent achievements of ruthenium(II) catalysed transformations of sp C-H bonds for cross-coupled C-C bond formation. First arylation and heteroarylation with aromatic halides of a variety of (hetero)arenes, that are directed at ortho position by heterocycle or imine groups, are presented. The role of carboxylate partners is shown for Ru(II) catalysts that are able to operate profitably in water and to selectively produce diarylated or monoarylated products. The alkylation of (hetero)arenes with primary and secondary alkylhalides, and by hydroarylation of alkene C=C bonds is presented. The recent access to functional alkenes via oxidative dehydrogenative functionalization of C-H bonds with alkenes first, and then with alkynes, is shown to be catalysed by a Ru(ll) species associated with a silver salt in the presence of an oxidant such as Cu(OAc)2. Finally the catalytic oxidative annulations with alkynes to rapidly form a variety of heterocycles are described by initial activation of C-H followed by that of N-H or O-H bonds and by formation of a second C-C bond on reaction with C=0, C=N, and sp C-H bonds. Most catalytic cycles leading from C-H to C-C bond are discussed. 

The transformation from arenes into phenol acetates can be achieved with hypervalent iodine compounds (such as phenyliodonium acetate, PhI(OAc)2), with chromates, or under aerobic conditions. Ligands, like picolinic acids, stabilize the intermediate palladium(IV) salts. In the presence of Lewis acids or silver salts, biaryl formation takes place. The influence of different directing groups has recently been reviewed. For example, diaryl sulfones or sulfoxides having at least one heteroaryl attached can be oxidized to the corresponding aryl acetates (Scheme 5-194, Experimental Procedure below). ... 

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