Arylstannanes—

Some examples of the application of the general methods for forming the Ar-Sn bond, which are described in Chapter 3, are given in equations 7-1,2 7-2,3 and 7-3.4 Lee has described the synthesis of arylstannanes in good yield by the Barbier reaction between aryl halides, magnesium, and bis(tributyltin) oxide, with ultrasonication (equation 7-4).5 

Some other methods are special to arylstannanes. The aromatic ring can be constructed by a Diels-Alder reaction 6-8 

Organotin Chemistry, Second Edition. Alwyn G. Davies Copyright 2004 Wiley-VCH Verlag GmbH Co. KGaA. ISBN 3-527-31023-1 

Metallic stannides can react with aryl halides,9 11 or phenols via the aryl dialkyl phosphates,12 13 or aryl ammonium salts,14 in a photostimulated SrnI reaction, for example  

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). 

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Aryl halides react with a wide variety of aryl-, alkenyl- and alkylstan-nanes[548-550]. Coupling of an aryl tritlate with an arylstannane is a good preparative method for diaryls such as 688. The coupling of alkenylstannanes with alkenyl halides proceeds stereospecifically to give conjugated dienes 689. The allylstannane 690 is used for allylation[397,546,551-553]. Aryl and enol triflates react with organostannanes smoothly in the presence of LiCl[554]. 

The intramolecular coupling of organostannanes is applied to macrolide synthesis. In the zearalenone synthesis, no cyclization was observed between arylstannane and alkenyl iodide. However, intramolecular coupling take.s place between the alkenylstannane and aryl iodide in 706. A similar cyclization is possible by the reaction of the alkenylstannane 707 with enol triflate[579]. The coupling was applied to the preparation of the bicyclic 1,3-diene system 708[580]. 

For coupling, the cheaper aryl fluorosulfonate 713 is used as an alternative to the expensive aryl triflates to give the same results[473]. The arenesulfonates 714 are active for the reaction with vinylstannanes when dppp and LiCI are used in DMSO[583], The bromide 715 attacks the arylstannane moiety selectively without reacting with the organoboron moiety in 716 in the absence of a base[584]. 

Ketones can be prepared by trapping (transmetallation) the acyl palladium intermediate 402 with organometallic reagents. The allylic chloride 400 is car-bonylated to give the mixed diallylic ketone 403 in the presence of allyltri-butylstannane (401) in moderate yields[256]. Alkenyl- and arylstannanes are also used for ketone synthesis from allylic chlorides[257,258]. Total syntheses of dendrolasin (404)f258] and manoalide[259] have been carried out employing this reaction. Similarly, formation of the ketone 406 takes place with the alkylzinc reagent 405[260],... 

The allylstannane 474 is prepared by the reaction of allylic acetates or phosphates with tributyltin chloride and Sml2[286,308] or electroreduction[309]. Bu-iSnAlEt2 prepared in situ is used for the preparation of the allylstannane 475. These reactions correspond to inversion of an allyl cation to an allyl anion[3l0. 311], The reaction has been applied to the reductive cyclization of the alkenyl bromide in 476 with the allylic acetate to yield 477[312]. Intramolecular coupling of the allylic acetate in 478 with aryl bromide proceeds using BuiSnAlEti (479) by in situ formation of the allylstannane 480 and its reaction with the aryl bromide via transmetallation. (Another mechanistic possibility is the formation of an arylstannane and its coupling with allylic... 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

Various phosphine ligands have been employed. Tm-(f-butyl)phosphine is an excellent ligand and is applicable to both vinyl and arylstannanes, including sterically hindered ones. Aryl chlorides are reactive under these conditions.195... 

Porco s pathway to complete the synthesis of (- )-kinamycin C (3) is shown in Scheme 3.8. The arylstannane 34 and the a-bromoenone 35 were efficiently coupled by a Stille reaction using tris(dibenzylideneacetone)dipalladium and triphenylarsine... 

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... 

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. 

Innumerable aryl- and heteroarylstannanes take part in Stille couplings with halopyridines. In one such example, the union of arylstannane 106 and 4-bromopyridine furnished arylpyridine 107 [90], Arylstannane 106 was prepared from the Pd-catalyzed reaction of hexabutylditin with iodoarene 105, which arose from aminobenzolactam 104 via a Sandmeyer reaction. 

Triorganostannyl anions are excellent nucleophiles in SrnI processes with aromatic substrates,86 and arylstannanes can be prepared from the photostimulated reactions of Ph3SnNa or Me3SnNa (from R3SnCl + Na) and aryl halides in DMSO, or of aryl amines, via their ammonium salts,88 in liquid ammonia, or of phenols, via their phosphates, in liquid ammonia.89 Examples are given in Equations (11)—(13). 

In Lambert s approach, the triarylstannylium ion is generated by the reaction of an electrophile with an allyltri-arylstannane. The bulky aryl groups sterically protect the tin center in the stannylium ion from attack by nucleophiles, yet the allyl ligand permits unhindered conjugate electrophilic displacement of the tin (Equation (42)).145... 

The classical Diels-Alder cycloaddition reaction is seldom used for the construction of the aromatic ring in arylstannanes, but a promising palladium-catalyzed cycloaddition reaction of two similar or different enynes (Equation (50)) has recently been published.175... 

The arylstannanes undergo electrophilic substitution more readily than do the parent protic compounds, through a similar Wheland intermediate (Equation (52)). 

Table 2 Reaction of arylstannanes with metal electrophiles...

The heteroarylstannanes bear the same relation to the arylstannanes as do the heteroarenes to the arenes, with the extra factor that the Lewis acidity of the stannyl group may cause the heteroarylstannanes to have self-associated structures. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

Disubstituted 4-chloro-2-cyclobutenones 75 undergo the palladium-catalyzed cross-coupling reaction with vinyl- and arylstannanes 76 or vinylzir-conium reagents to give the 4-R sa,-2-cyclobutenones 77. Without isolation, these cyclobutenones 77 are rearranged to the substituted phenols 78 on thermolysis [38], Application of this method to the stannylated heteroaromatics 79 provides a synthetic route to the aromatic benzoheterocycles 80 [39]. (Scheme 27 and 28)... 

Stille cross-coupling and reductive N-heteroannulation steps. lodination of 1033, followed by Stille cross-coupling of the corresponding iodo derivative 1034 with the arylstannane 1035, gave the annotation precursor 1036. Reductive N-heteroannula-tion of 1036 afforded the carbazolone 1001, which was previously (99) (see Scheme 5.143) used to prepare clausenalene (90) via a Wolff-Kishner reduction followed by aromatization (659) (Scheme 5.155). 

Another major route for synthesis of stannanes is reaction of an organometallic reagent with a trisubstituted halostannane. This is the normal route for preparation of arylstannanes. 

Carbonylative cross-coupling reaction between triarylbismuth(V) compounds and organostannanes is smoothly catalyzed by PdCl2 under 1 atm CO at room temperature (Scheme 48) [34]. Not only arylstannanes, but also heteroaryl-, alkynyl-, and vinyltributylstannanes were employed efficiently. 

Methylthio- and phenylthiopyrazines 121 undergo cross-coupling with arylstannanes in the presence of palladium catalyst and copper(l) 3-aminosalicylate to afford 92-93% yields of arylpyrazines 122 (Equation 23) <20030L801>. 

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