Buchwald-Hartwig aryl amination

Aryl amination, also known as Buchwald-Hartwig coupling, is one of the most important transformations in organic synthesis, due to its versatility and the ubiquitous nature of nitrogen-containing molecules in therapeutically active compounds.There have been numerous studies on catalytic applications of NHC-Pd complexes to aryl amination, sometimes with impressive results. 

Although monoligated Pd° is the dominant paradigm in NHC-Pd catalyst development for Buchwald-Hartwig amination, other approaches were investigated with some success. - Shi reported the use of a (ferrocenyl)phosphine-functionalised NHC, but its activity was low, giving credit to the monodentate approach.  

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The powerful Buchwald-Hartwig aryl amination methodology [135-141] has been applied by Hartwig to the synthesis of N-arylpyrroles (196) [142,143]. 

Buchwald parlayed the powerful Buchwald-Hartwig aryl amination technology [439-447] into a simple and versatile indoline synthesis [448-452], For example, indole 368, which has been employed in total syntheses of the marine alkaloids makaluvamine C and damirones A and B, was readily forged via the Pd-mediated cyclization shown below [448], This intramolecular amination is applicable to the synthesis of -substituted optically active indolines [450], and o-bromobenzylic bromides can be utilized in this methodology, as illustrated for the preparation of 369 [451]. Furthermore, this Pd-catalyzed amination reaction has been applied to the synthesis of arylhydrazones, which are substrates for the Fischer indole synthesis [453,454],... 

The Buchwald-Hartwig aryl amination methodology cited above in this section was engaged by Hartwig and others to synthesize N-arylindoles 377 [469]. Carbazole can be N-arylaied under these same conditions with p-cyanobromobenzene (97% yield). Aryl chlorides also function in this reaction. The power of this amination method is seen by the facile synthesis of tris-carbazole 378 [469c]. 

The Buchwald-Hartwig aryl amination methodology cited above in this section was... 

Several tandem reaction sequences have been developed for indole synthesis featuring a Buchwald-Hartwig aryl amination as one component. For instance, a tandem double JV-arylation of substrate 5 leads to indoles 6 through a cascade of C-N bond forming reactions. A range of structurally and electronically diverse amines can be used successfully in this transformation. 

The first application of the single-step procedure to the synthesis of azacafix-arenes was laimched by Ito et al., who prepared a series of azacafix[ ]arenes 4a-f with different ring sizes (w = 3 to 8) [16,17]. As shown in Scheme 2, starting from the N-methylation of 3-bromoaniline (2) in 3 steps, the resultant 3-bromo-N-methylaniline (3) was subjected to the Buchwald-Hartwig aryl amination reaction to yield azacaUxarenes 4a-f, which were isolated by medium-pressure liquid chromatography. 

Scheme 1 Catalytic cyde of Buchwald-Hartwig aryl amination reaction of aromatic halide with N-alkylanUine [12]...

Another application of the single-step procedure tvas made by Miyazaki et al, who utilized the Buchwald-Hartwig aryl amination reaction for the preparation of azacalix[6]pyridine 7d [18], in which all the benzene rings of the above azacalix[6]arene 4d were replaced by pyridine rings (Scheme 3). Azacalix[ ]pyridines 8a-f (n = 3-8) with tolyl groups on the nitrogen bridges... 

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. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

Independently, Antane reported that arylisonipecotic acids were obtained from aryl bromides in a two-step process involving microwave-assisted palladium-catalyzed amination with ethyl isonipecotate followed by ester hydrolysis with KOH (Scheme 91) [96]. Interestingly, toluene, which is the standard solvent for Buchwald-Hartwig aminations under conventional heating, was used as the sole reaction medium, although it is a very weak... 

More challenging are the (hetero)aryl chlorides, since they are cheaper and more widely available than the corresponding bromides and iodides. Maes et al. published the first examples of microwave-assisted Buchwald-Hartwig aminations on (hetero)aryl chlorides in a commimication in 2003 [99]. The substrates 2- and 3-chloropyridine as well as 2-chloroquinoline were smoothly coupled with N-methylaniline and p-toluidine within only 10 min using a catalyst loading of only 1 mol% (Schemes 96 and 97). The diazine... 

The coupling between an aryl halide or triflate and an amine is known as the Buchwald-Hartwig amination [138]. Originally it was described using a tributyltin amine [139,140] and was thus considered to be a coupling reaction. Subsequently, tributyltin amine was replaced by a standard amine and a strong base. It is a reaction of great academic and industrial interest [11]. 

The first examples utilising A-heterocyclic carbenes as ligands in the Buchwald-Hartwig amination involved the in situ formation of the catalyst from the corresponding imidazolium salt and a Pd(0) source. Nolan reported IPr-HCl/PdjCdbalj as a catalytic system for the amination of aryl chlorides in excellent yields, using different types of amines, anilines, and also imines or indoles [142,143] (Scheme 6.46). Hartwig showed later that in some cases the reactions could be performed at room temperature and without anhydrous conditions even for aryl chlorides [ 144]. This was later shown for the less challenging bromides and iodides [145,146]. 

However, the Buchwald-Hartwig reaction with NHCs as hgands is not limited to palladium. Nickel has also been successfully employed in this catalytic amination. In situ procedures have been described for the coupling of aryl chlorides [163] and tosylates [164] and, more interestingly, anisoles [165]. The use of well-defined Ni(0) catalysts has also been studied [166] (Scheme 6.49). 

Scheme 6.61 Buchwald-Hartwig amination reactions of aryl chlorides.

Independent investigations by Maes and coworkers have involved the use of commercially available and air-stable 2-(dicydohexylphosphanyl)biphenyl (ligand B) as a ligand system for the successful and rapid coupling of (hetero)aryl chlorides with amines under microwave Buchwald-Hartwig conditions (0.5-2 mol% palladium catalyst) [129, 130]. Both methods provide very high yields of products within an irradiation time of 10 min. 

A more recent publication by Weigand and Pelka has disclosed a polymer-bound Buchwald-Hartwig amination [40], Activated, electron-deficient aryl halides were coupled with conventional PS Rink resin under microwave irradiation. Subsequent acidic cleavage afforded the desired aryl amines in moderate to good yields (Scheme 7.22). Commercially available Fmoc-protected Rink amide resin was suspended in 20% piperidine/N,N-dimethylformamide at room temperature for 30 min to achieve deprotection. After washing and drying, the resin was placed in a silylated microwave vessel and suspended in dimethoxyethane (DME)/tert-butanol... 

A synergistic combination of Pd-catalyzed amination and arylation was the central operation of Sakamoto s synthesis of carbolines [147]. Diarylamine 187 was first installed via the Buchwald-Hartwig amination protocol. Subsequent intramolecular Heck-like arylation of 187 provided a novel route to a-carboline 188. 

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