Cross Buchwald-Hartwig aminations

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

The generally accepted catalytic cycle for the Buchwald-Hartwig amination mirrors that of other palladium catalyzed cross-coupling reactions.10 11 irThere is an oxidative addition (A to B), followed by an exchange on palladium (B to C), and finally a reductive elimination (C to D and A). The main difference involves the exchange step. In a Suzuki, or Stille, reaction this step proceeds through a discrete transmetallation event, whereas... 

Indole is one of the most important heterocyclic scaffolds for drug discovery. Approaches for indole synthesis using hydroamination as a key step in their syntheses has been an ongoing area of investigation over the past several years [329, 330]. Early examples focused on one-pot intermolecular hydrohydrazination followed by the Lewis-acid-catalyzed Fischer indole synthesis [331, 332] (Scheme 15.102) or one-pot intermolecular hydroamination followed by cross-coupling to either form a C-N bond (via the Buchwald-Hartwig amination) [333] (Scheme 15.103) or a C-C bond (via a Heck reaction) [334, 335] (Scheme 15.104). Alternatively, o-alkynylanilines can be used directly as substrates for intramolecular hydroamination (Scheme 15.105) [198, 200, 336-340]. These approaches have been thoroughly reviewed [10]. 

Preprepared (ADC)Pd complexes H2 (Fig. 11.6) was employed [29] as catalyst for the Buchwald-Hartwig amination of aryl halides (for the recent surveys on the Buchwald-Hartwig cross-coupling, see References [62-65]). The obtained results (Scheme 11.13) were compared with those for the structurally related oxyamino- (H4), thioaminocarbene (H5), and the related NHC complex, H14. Catalysts H2, H4, and H5 were efficient in the amination of bromobenzene (yields range from 82% to 92%), demonstrating activities similar to those of the NHC complex H14 (yield 84%). With 2-chloropyridine as the substrate, H2, H4, and H14, allowed the preparation of the target product in a quantitative yield. Catalyst H5 demonstrated a moderate efficiency (yield 47%). 

Pd-catalyzed Buchwald-Hartwig cross-coupling Direct Pd-catalyzed C-N and C-O bond formation between aryl halides and amines or alcohols. 70... 

The transition-metal catalyzed cross-coupling reaction of (hetero)aryl hahdes and triflates with primary and secondary amines or (hetero)aryl amines is know as the Buchwald-Hartwig reaction [144]. Mechanistically, this reaction is related to the crosscoupling reactions outlined thus far (Fig. 4.6). The modification arises at the point of transmetalation. This step in the process is substituted with the coordination of the amine reactant. Deprotonation of the amine nitrogen now precedes the reductive elimination step to generate the aryl amine product. This reaction has foimd utility in the academic setting, for use in natural product total synthesis, and in industry, for the preparation of materials up to the multi-hundred kilogram scale. 

The Buchwald-Hartwig reaction has been widely used in organic chemistry for the palladium-catalyzed cross-coupling of amines with aryl halides. In particular, it has been used in conjunction with benzothiophenes for arylamination of the thiophene. The final diarylamine product below demonstrated anti-fungal activity against dermatophytes, yeasts, and the Aspergillus species. ... 

Arylation of a wide range of NH/OH/SH substrates by oxidative cross-coupling with boronic acids in the presence of catalytic cupric acetate and either triethyl-amine or pyridine at room temperature in air. The reaction works for amides, amines, anilines, azides, hydantoins, hydrazines, imides, imines, nitroso, pyrazi-nones, pyridones, purines, pyrimidines, sulfonamides, sulfinates, sulfoximines, ureas, alcohols, phenols, and thiols. It is also the mildest method for NIO-vinylation. The boronic acids can be replaced with siloxanes or starmanes. The mild condition of this reaction is an advantage over Buchwald-Hartwig s Pd-catalyzed cross-coupling. The Chan-Lam C-X bond cross-coupling reaction is complementary to Suzuki-Miyaura s C-C bond cross-coupling reaction. 

Nucleophilic aromatic substitution reactions were employed to place amines on pyrimidine and pyrimidinone rings. As a more economical substitute for Buchwald-Hartwig cross-coupling reaction conditions GeUis and collaborators developed a DMAP-catalyzed, microwave-assisted method to form 4-hetero-arylamino-substituted quinazolines (Scheme 33) (14T8257). Treatment of... 

Solid-supported versions of the Buchwald-Hartwig reaction are known too. In 2012, Al-Amin et al. [42] reported a stable heterogeneous sulfur-modified gold-supported palladium material (SAPd), which was used for the amination of aryl bromides and chlorides. The catalyst was employed at a loading of about 0.2 mol%, and can be used for a minimum of 10 reaction cycles without loss of catalytic activity, affording very good yields for a large cross-section of substrates. 

In another example, the Buchwald-Hartwig A)-arylation conditions were applied to the cross-coupling reaction of 3,7-dibromo-10-hexyl-10f/-phenothiazine with a variety of primary and secondary anilines and amines, including carbazole, leading to 3,7-diaminophenothiazine derivatives in excellent yields (eq 17). ... 

Although one of the more recent additions to the armamentarium of these palladium-catalysed reactions, the Buchwald-Hartwig cross-coupling of amines with aiyl halides already has found applications in macrocycle synthesis. In the first example, Iqbal et al used this coupling as the cyclization step in the synthesis of constrained tri- or tetra-peptidomimetics with a biaiyl linker (Scheme 11.13 illustrates a tripeptidomimetic (112)). Fair to moderate yields were obtained for products containing 16- to 22-membered rings. 

The palladium-catalyzed amination of aryl halides and sulfonates has emerged as a valuable method for the preparation of aromatic amines [2], Numerous ligands and catalysts have been reported to effect this type of cross-coupling. This reaction, known as the Hartwig-Buchwald amination, is shown in Eq. (1). 

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