Addition of arylstannane

The nucleophilic addition of organometallic reagents to imines provides an attractive route to amines [4]. Recendy, however, some completely different approaches to the synthesis of a-aryl amine were reported. Hayashi and Ishigeda-ni found a new catalytic system for the asymmetric addition of arylstannanes to imines derived from aromatic aldehydes (Scheme 11) [20]. 

Pioneering studies of the catalytic, enantioselective arylation of imines date back to 2000, when Hayashi disclosed a rhodium/phosphine-catalyzed addition of arylstannanes to N-tosylarylimines (31). Whereas, the method gave rise to highly enantioenriched diarylmethylamines, five equivalents of the stannane were required to obtain high yields. 

Recently, Li and co-workers also reported the Rh-catalyzed arylstannane addition to enones in water under reflux.139 The addition of KOH helps the reaction to proceed smoothly (Scheme 50). A tentative mechanism was proposed for the Rh-catalyzed conjugated addition of phenyltin trichloride under basic conditions (Scheme 51). Later, this kind of reaction was found to work even in water at 50 °C. 

The Stille coupling of 2-chloro-5-tributylstannylpyridine with an enantiopure 2-iodo-cyclohexenon (7.29.) derivative formed the basis of the total synthesis of (+)-epibatidine. The reaction is a nice example of the chemical inertness of arylstannanes and the mildness of the coupling conditions. Both the enone moiety and the chiral ally lie centre remained untouched in the process. The effective coupling required the use of a soft ligand, triphenylarsine and the addition of copper(I) iodide as co-catalyst.40... 

Support-bound stannanes have been prepared from phenyllithium bound to macro-porous polystyrene and chlorostannanes [14,41], by treatment of support-bound alkyl chlorides with lithiated stannanes [21,41], and by radical or palladium-mediated addition of stannanes to alkenes and alkynes (Figure 4.7 [42-47]). The chloride of poly-styrene-bound chlorostannanes can be displaced by treatment with arylzinc reagents, thereby yielding resin-bound arylstannanes [46]. Polystyrene-bound stannanes have also been prepared by copolymerization of 4-[2-(dibutylchlorostannyl)ethyl]styrene with styrene and divinylstyrene [48],... 

Arylstannanes can also be made by the reaction between aryl bromides and hexame-thylditin with a palladium catalyst,15-17 for example reaction 7-9.17 The mechanism might reasonably be assumed to involve oxidative addition of ArX, followed by trans-metallation and reductive elimination (equation 7-10). 

The stannane was introduced using the directed ort/io-metalation reaction followed by addition of trimethyltin chloride to give 168. Hydroboration of arylstannane 168 with 9-BBN and oxidative work-up with basic peroxide provided the primary alcohol 169. Mitsunobu reaction of this alcohol with the cyanomethyl ester of 4-hydroxyphenoxyacetic acid afforded preactivated ester derivative 170 in good yield (Scheme 4.2.1). 

Pyridylbenzenes are directly ortfio-arylated with tetra-arylstannanes in the presence of a rhodium(I)-phosphine complex as catalyst [140]. A mechanistic pathway was proposed based on the oxidative addition of a rhodium] I) complex to the ortho position of the phenyl ring directed by the pyridine nitrogen, followed by arylation by the tetra-arylstannane. A somewhat related reaction of arylboronic acids was achieved with a [RhCl(C2H4)2]2/P[p-(CF3)QH4]3 catalyst system [141]. In this instance, the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical was used as a stoichiometric oxidant. Arylboronic acids also arylate benzophenone imines in the presence of Rh(I) catalysts [142]. 

Insertion into Heteroatom-Heteroatom tr-Bonds. Kunai et al. have demonstrated the addition of stannyl sulfides, aminosilanes, and tributyltin fluoride to arynes leading to thiostannylation [73], aminosilylation [74], and fluorostannylation [75] of arynes, respectively. So, a variety of 2-(arylthio) arylstannanes 83, 2-silylanilines 84, and 2-fluoroarylstannanes 85 can be obtained in modest to high yields. As expected, with unsymmetrical arynes only good regioselectivities were obtained with 3-methoxy, 3-halo, and 3-phenylbenzyne (Scheme 12.44). 

Rhodium catalysts have been widely used for C-C bond formation processes [71], Particularly noteworthy are the Rh(I)-catalyzed additions of boronic acids and their derivatives to a.p-unsaturated carbonyl compounds [72-78] and aldehydes [75, 79] (Chapter 4). The groups of Miyaura and Hayashi have shown that Rh(I) catalyzes the addition of sodium tetraphenylborate and arylstannanes to N-sulfonylimines [80-82]. Miyaura and co-workers have also reported the first example of a Rh(I)-cat-alyzed addition of an arylboronic acid to an N-sulfonylimine (77), to give sulfonamide 78 (Equation 13) [83]. Reactions proceeded with 2 equivalents of arylboronic acids using either a cationic Rh(I) catalyst alone, or in combination with appropriate phosphine ligands such as bis(diphenylphosphino)propane or P(i-Pr)3. Boronic esters will also react, particularly in the presence of triethylamine. The reaction does not proceed with simple aldimines, such as PhCH NPh. 

In this section, we refer to some Stille reactions that serve as representative examples. As noted before, the Stille reaction is compatible with boronic esters. " Therefore, the Stille cross-coupling provides a simple method for homologation of aryl-and 1-alkenylboronic esters (Scheme 5-60). Since the carbon-boron bond is inert to transmetalation in the absence of a base and oxidative addition of the carbon iodide bond is faster than that of the carbon bromine bond, the arylation of iodoboranes with arylstannanes selectively occurs at the carbon iodo bond without affecting the other onej l In addition. Scheme 5-60 also shows a Stille reaction having a multitude of functional groups such as being present in a modified nucleotide. ... 

This catalytic reaction was believed to proceed analogously to those with phenylboronic acids (Scheme 49) 137 137a Transmetallation of the arylstannane with the cationic rhodium complex generated the rhodium aryl species a and trimethyltin tetrafluoroborate. Conjugate addition generated rhodium enolate b, which subsequently reacted with... 

All three elements are susceptible to ipso replacement by electrophiles - such reactions have been studied extensively for arylsilanes and arylstannanes, where they occur via an electrophilic addition/silicon elimination mechanism analogous to other aromatic substitutions, but at a much faster rate than the corresponding replacement of hydrogen." Ipso substitutions also take place on heterocycles and, in the case of electron-rich systems, probably via the same type of mechanism. 

Aryl halides (Br, I) have been converted in good yields into the corresponding arylstannanes or -silanes by treatment with RgSn2 (R = Bu, Me) or Hexamethyldisilane, respectively, in the presence of catalytic Pd(Ph3P)4 (eq 29). Aryl and vinyl triflates produce aryl- and vinylstannanes under similar conditions, provided Hexamethyldistannane and LiCl are used as co-reactants. In some cases, the presence of additional PhsP has been observed to improve yields. By using the (Ph3Sn)2Zn-TMEDA complex, vinyl halides can also be converted into vinylstannanes. ... 

During an investigation into the addition reactions of element-element o-bonds to the C—C triple bonds of arynes, Yoshida et al. observed aryne insertion into the S—Sn o-bond of stannyl sulfides (Equation 12.16) [22]. The reaction is initiated by nudeophihc attack of the sulfur atom of the stannyl sulfide on the aryne triple bond, resulhng in the formation of a zwitterion, which undergoes intramolecular nudeophihc substitution at the stannyl moiety. This affords the corresponding 2-(arylthio)arylstannane 19. 

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

In 2003, when the second edition appeared, it was clear that the Stille chemistr/ had really come of age, as shown by the increasing number of applications of palladium-catalyzed cross-coupling reactions of vinyl- and arylstannanes in natural product synthesis and in advanced organic synthesis. In the past decade, these types of applications, of course, remained very important and, in addition, the use of allylstannanes in palladium-catalyzed coupling reactions received much attention. Cross-coupling of allylstannanes with imines also forwarded the area of asymmetric catalysis. 

Tributyltin hydride-mediated hydrodebromination of bromoarenes is selectively carried out. However, removal of the tin residue should be carefully carried out. The 10% w/w anhydrous potassium carbonate-silica repeatedly reduces organotin impurity levels below 15 ppm in this system (Table 3-12). In addition, organotin reagents such as tributyltin hydride, allyl stannanes, vinyl stannanes, arylstannanes, and distannanes can be purified by the K2CO3 and silica column chromatography. On the contraiy, halostannanes and sulfonated stannanes are trapped through this process (Table 3-13). 

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