Stannyl reactions

The exocyclic C(2)-S bond of a 2-thiopenem 50 can undergo radical reactions. As depicted in Scheme 28, its desulfurative stannylation reaction with azobis(isobutyronitrile) (AIBN) and tributyltin hydride leads to a penem stannane 51 <1995TL775>. The latter allows subsequent palladium(0)-mediated cross-coupling reactions with... 

Discovered by Thorn and first communicated only a few years ago [43], electrophilic aromatic stannylation is the newest of all metalation reactions described in this chapter. The reactive Sn electrophile can be prepared by dearylation of Ph4Sn with TFAH or, preferably, by reaction of SnO with trifluoroperacetic acid generated from H202, TFAH, and trifluoroacetic anhydride. Because the structure of the Sn(IV) species (or mixtures of species) produced this way is unknown, the reactive electrophile is depicted as Sn(TFA)4 , for simplicity. The stannylation reactions of benzene and p-xylene were shown to produce tetratin clusters containing two inorganic and two organometallic Sn centers (Scheme 7). 

The structures of both the phenyl and 2,5-dimethylphenyl tin clusters were established by single-crystal X-ray diffraction. Under similar conditions, toluene was stannylated to produce a mixture of organotin compounds. Bromination of this mixture yielded 3-bromotoluene and 4-bromotoluene in a 2 1 ratio, with trace amounts of 2-bromotoluene. Similarly, PhBr was produced upon treatment of the phenyltin product with aqueous Br2. No applications of the stannylation reactions have yet been reported. 

Withasomnine Pyrazole, 1-phosphoryl-reactions, 5, 271 Pyrazole, 1-silyl-synthesis, 5, 236 Pyrazole, 1-stannyl-synthesis, 5, 236 Pyrazole, 1-styryl-synthesis, 5, 233 Pyrazole, 1-thienyl-reactions, 5, 268 Pyrazole, 4-(2 -thienyl)-nitration, 5, 238 Pyrazole, 4-(3 -thienyl)-nitration, 5, 238 Pyrazole, trifluoromethyl-synthesis, 5, 284... 

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

Stannyl esters have been prepared to protect a -COOH group in the presence of an -NH2 group [( -Bu3Sn)20 or -Bu3SnOH, CgH, reflux, 88%]. Stannyl esters of A -acylamino acids are stable to reaction with anhydrous amines and to water and alcohols aqueous amines convert them to ammonium salts. Stannyl esters of amino acids are cleaved in quantitative yield by water or alcohols (PhSK, DMF, 25°, 15 min, 63% yield or HOAc, EtOH, 25°, 30 min, 77% yield). ... 

The Boekelheide reaction has been applied to the synthesis of non-natural products with the preparation of quaterpyridines serving as an example. The sequence began with the 2,4-linked bipyridyl-N-oxide 25. Execution under the typical reaction conditions produced the expected bis-pyridone 26. Treatment with POCI3 afforded the corresponding dichloride that was submitted to a palladium-catalyzed coupling with 2-stannyl pyridine to produce the desired quaterpyridine 27. 

This catalyst was successfully applied to the Diels-Alder reaction of propargyl aldehydes as dienophUes [12] (Scheme 1.21, Table 1.8). Though 2-hutyn-l-al and 2-oc-tyn-l-al are unreactive dienophUes, silyl- and stannyl-suhstituted a,/ -acetylenic aldehydes react with cydopentadiene readily in the presence of 20 mol% of the catalyst at low temperature to give hicyclo[2.2.1]heptadiene derivatives in high optical purity these derivatives are synthetically useful chiral building blocks. 

On the other hand, the corresponding tin precursor (63) undergoes smooth cycloaddition with a wide variety of aldehydes to produce the desired methylene-tetrahydrofnran in good yields [32, 33]. Thus prenylaldehyde reacts with (63) to give cleanly the cycloadduct (64), whereas the reaction with the silyl precursor (1) yields only decomposition products (Scheme 2.20) [31]. This smooth cycloaddition is attributed to the improved reactivity of the stannyl ether (65) towards the 7t-allyl ligand. Although the reactions of (63) with aldehydes are quite robust, the use of a tin reagent as precursor for TMM presents drawbacks such as cost, stability, toxicity, and difficult purification of products. 

Remarkably, the addition of only 5-10 mol% of Me3SnOAc to the reactions of the sOyl precursor (1) with aldehydes also cleanly produce the cycloadducts in excellent yields [31]. It then appears that the capping of the alkoxide (61) to form the stannyl ether in situ is efficient enough that only a catalytic amount of Me3SnOAc is sufficient to facilitate reaction. This tin-ejfecC greatly enhances the... 

Schlenk equilibrium 93 s-cis 7, 9, 26, 31, 35 sdyl-substituted 16 Simmons-Smith reaction 87 SnClj 309 SnCU 309 solid-phase 198 square bipyramidal 255 rr-stacking 8 stannyl-substituted 16 s-trans 7, 26 - acrolein 307 succinimide 227 sulfonamides 122 synchronicity 306... 

Stannyl cuprates couple widi vinyl halides or trlGales [16c-d, 85], and a vinyl stannane produced diis way has been used in die syntliesis of 7-[f )-alkylidetie]-cepbalosporlns [117]. Vinyl substitution reactions slariing Grom ddiydrofciraiis are... 

Fully acetylated hexopyranoses react with thiols in the presence of Lewis acids, such as BF3-Et20.32,33 The reaction is faster with 1,2-trans acetates than with the corresponding 1,2-cij ones and 1,2-trans products predominate. Alkyl, alkenyl, and aryl thioglycosides are produced by this method. Variations on this method include the use of trimethylsilyl34 or stannyl derivatives35 of the thiols. 

For example, the sensitive imidoyl chloride moiety at the C-3 position of the pyrazinone scaffold is known to vmdergo Stille reactions with a variety of tetraaryltin reagents, generating the corresponding 3-substituted pyrazi-nones (Scheme 10) [26]. Furthermore, the transition-metal-catalyzed stannyl-ation at the C-3 position is also documented in the hterature, in view of cross-couphng with a variety of alkyl and (hetero)aryl hahdes [26]. However, this strategy is completely restricted to the C-3 position, while the Cl atom of C-5 position was found to be inert under these conditions. 

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