Tin amide

In contrast to the borylation of alkane C-H bonds, the coupling of aryl halides with amines was based on a literature precedent from another group published about a decade before our initial studies. Kosugi, Kameyama and Migita published the coupling of aryl halides with tin amides." Mechanistic studies we conducted on this process led us to the perhaps obvious realization that the reaction" could be conducted with amines and a silylamide base instead of tin amides (equation 4)." Surveys of bases with similar p a values led Janis Louie to conduct reactions with alkoxide bases. Similar studies were conducted at nearly the same time by Steve Buchwald and coworkers."... 

Cobalt hydrocarbonyl, diborane, and aluminum hydrides add, I think, to all of these carbonyl compounds. Of course, there is the well known Grignard reagent and the alkyllithium additions to carbonyl compounds. Aluminum alkyls add, and we could have listed all the other alkali metal alkyls. Recent work has shown that the tin alkoxides add readily to all these derivatives, and similarly, a tin amide adds to most of these carbonyl compounds. 

The photochemical synthesis and characterisation (by EPR solution spectra at 20 °C in C6H14) of the first stable trivalent germanium and tin amides M [N(SiMe3)2]3 was first reported in 1974 in solution they were shown to be persistent for more than three months at ambient temperature. An alternative synthesis from a tetravalent precursor for the Ge amide is shown in Equation (9.1) (X = Cl or Br). ... 

Bacteria, antimicrobials against, 12, 456 Baeyer-Villiger oxidation, via tin amides, 9, 370 Barbier-Grignard-type reactions, and sonochemical metal insertions, 1, 315 Barbier-type reactions allenyl and propargyl tins, 9, 358 with allylic tins, 9, 357 with antimony(III) compounds, 9, 426 with bismuth(III) compounds, 9, 433 with cerium reagents, 10, 409 with indium compounds, 9, 685... 

Guram and Buchwald showed that the chemistry could be extended beyond just electron-neutral aryl halides [78]. Using in situ-derived tin amides, this chemistry encompassed aryl halides bearing esters, amino, and alkoxo groups (Eq. (4)). However, reactions that gave 80% yield or greater were still limited to tin amides derived from secondary amines. 

In 1995, Hartwig and Buchwald published concurrently their two groups results on tin-free amination of aryl halides [79,80], Instead of isolating or generating a tin amide in situ, the amination reactions were conducted by reacting an aryl halide with the combination of an amine and either an alkoxide or silylamide base (Eq. (5)). These reactions were typically conducted between 80 and 100 °C in toluene solvent. The catalysts used initially were 1, 3, or a combination of [Pd2(dba)3] (Sa) (dba = trans, trans-dibenzylidene acetone) and P(o-C6H4Me)3. Catalysts used subsequently will be described below. As shown in Table 7.1, secondary amines were viable substrates, but primary amines gave substantial yields with... 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

The scope of this reaction appeared to be limited to dialkylamides and electron-neutral aryl halides. For example, nitro-, acyl-, methoxy-, and dimethylamino-substituted aryl halides gave poor yields upon palladium-catalyzed reaction with tributyltin diethylamide. Further, aryl bromides were the only aryl halides to give any reaction product. Vinyl bromides gave modest yields of enamines in some cases. Only unhindered dialkyl tin amides gave substantial amounts of amination product. The mechanism did not appear to involve radicals or benzyne intermediates. 

In 1994, Paul, Patt, and Hartwig showed that the Pd(0) catalyst in Kosugi s process was Pd[P(o-C6H4Me)312 (3), which underwent oxidative addition of aryl halides to give dimeric aryl halide complexes (4) [91]. These aryl halide complexes reacted directly with tin amides to form arylamine products (Eq. (3)). Thus, this chemistry could formally be viewed as being roughly parallel to Stille coupling. 

In 1995, Hartwig and Buchwald published concurrently their respective groups results on tin-free amination of aryl halides [98, 99]. Instead of isolating or generating a tin amide in situ, the amination reactions were conducted by allowing an aryl halide to react with a combination of an amine and either an alkoxide or silylamide base (Eq. (5)). 

In tin amide complexes including Shiff base ligand, unprecedented attack by the amide moiety to the imine carbon atoms of tridentate Schiff base ligands gives unusual tetradentate ligand systems. The so-formed complexes act as latent single-site initiators for the controlled polymerization of rac-lactide (Equation (101)).267... 

Tin(rv) bis(perfluorooctanesulfonyl)amide is a practical catalyst for transesterification and direct esterification using an equimolar ratio of the reactants in a fluorous biphase system (Equation (103)). The tin amide is completely recovered and reused in the immobilized fluorous phase without loss of its catalytic activity.269... 

The tin amide, formed in situ by treating an N-H substrate with -butyllithium, followed by addition of trimethyltin chloride, for one-pot bromination showed great selectivity for the reaction of indole and carbazole (Equations 76 and 77) <20020L2321>. In both cases, the bromination gave a single product in good yield, 80% and 69% for indole and carbazole derivatives, respectively. 

A tantalizing observation made by Kosugi was that triphenylphosphine-based catalysts were far inferior to PdCl2(o-tolyl3P)2 for the coupling of aryl halides with tin amides. In 1994, the groups of Buchwald and Hartwig... 

Paul, F., Patt, J., Hartwig, J. F. Palladium-catalyzed formation of carbon-nitrogen bonds. Reaction intermediates and catalyst improvements in the hetero cross-coupling of aryl halides and tin amides. J. Am. Chem. Soc. 1994,116, 5969-5970. 

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