Stannyl formation

The formation of a hexaalkyldistannane by heating together the hexaalkyl-distannoxane and formic acid (equation 18-20, R = Bu, Bu1, Oct) similarly involves the reduction of SnX by SnH.20 The stannyl formate is formed, and decarboxylates to give the tin hydride, which then reduces the distannoxane. Residual formate can be extracted into methanol, in which the distannanes (ca. 65-70% yields) are insoluble. 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

It is believed that this reaction involves the formation of the a-stannyl ester. Metals such as lithium that form ionic enolates would be more likely to reverse the addition step. 

Oxidation of (3-silyl and (3-stannyl acids leads to loss of the substituent and alkene formation.283... 

It is considered that the stannyl or silyl radical and the alkyl radical are reactive intermediates in these reactions. In contrast to the selective formation of the arylchalcogenosilanes in the above radical reactions, the cross-coupling reaction of a hydrosilane with alkyl(aryl)sulfides catalyzed by palladium nanoparticles results in the selective formation of the corresponding alkylthiosilanes.42... 

A new entry to exocyclic dienes was reported by Sha who uncovered that a radical cyclization of the vinyl iodide 100 can lead to the formation of an exocyclic dienes fused with a tetrahydrofuran ring. The cyclization is proposed to proceed in a 5-(n-exo)-exo-dig fashion <00OL2011>. 3,4-Disubstituted tetrahydrofurans can also be constructed via the cyclization of O-stannyl ketyls and allylic 0-stannyI ketyls onto electron-rich or electron-poor alkenes <00TL8941>. 

Dialkynylstannanes were synthesized and characterized by their NMR spectra. These compounds react with boranes yielding l-stanna-4-bora-2,5-cyclohexadienes (97), stan-noles (98), as shown in reaction 24, and stannolines (99), as shown in reaction 25. When R 4 H it is possible to isolate at low temperature the tautomeric intermediates 95 and 96 of reaction 23, one of which is a TT-stabilised stannyl cation complex279. Application of the same borane derivatization scheme to tetraalkynylstannanes leads to formation of 1-stannaspiranes where the rings are combinations of 97, 98, and 99 structures116,280. 

The electrolysis of unsaturated a-tributylstannyl ethers 12 yields cyclic compounds by a similar reaction, involving in this case carbon-carbon bond formation. Here, too, the evidence points to a facile ET with formation of an a-stannyl ether radical cation. This in turn cleaves to the tributylstannyl radical and an unsaturated ene-oxonium residue. 

Sn—C bond formation is achieved by electrochemical alkylation of stannyl chlorides. The polarogram of dichlorodiphenyltin Ph2SnCl2, on mercury cathode, shows four waves having 1/2 values which are dependent on the electrolyte (see Table 26). Of these four,... 

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

The steric course of this two-step process was examined with several chiral secondary propargylic phosphates (Eq. 9.35) [40], The derived propargylic stannanes were found to be formed with net inversion of configuration. Based on previous evidence that the initial formation of the allenyltitanium intermediate occurs with inversion, it can be deduced that stannylation proceeds by a syn pathway. This surprising result was attributed to coordination between the chlorine substituent of the Bu3SnCl and the electropositive titanium center (Scheme 9.11). 

In 1994, Badone et al. reported that the Stille coupling of allenylstannane 77 and aryl triflates 78 resulted in formation of various aryl-substituted allenes 79 in moderate to good yield (Scheme 14.18) [39]. The choice of catalyst was certainly a crucial issue in this process for optimizing yield and rate. The best results could be obtained employing a catalyst cocktail of Pd2(dba)3-TFP-LiCl-CuI. Similar Stille coupling reactions with stannylated allenes and aromatic iodides as substrates were described by Aidhen and Braslau [40a] and Huang et al. [40b],... 

The reactions did prove to be compatible with the use of a-stannyl amides. In these cases, the cyclization reaction involved the trapping of an iV-acyliminium ion by the olefin and led to the formation of substituted piperidines. 

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