Synthesis of Organostannanes

The readily available organotin compounds include trisubstituted tin hydrides (stannanes) and chlorides. The tin hydrides can be added to carbon-carbon double and triple bonds. The reaction is normally carried out by a radical chain process. Addition is facilitated by the presence of radical-stabilizing substituents. 

With terminal alkynes, the stannyl group is added at the terminal carbon and the Z-stereoisomer is initially formed but is thermally isomerized to the -isomer.  

The other 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. 

Deprotonated trialkylstannanes are potent nucleophiles. Addition to carbonyl groups or iminium intermediates provides routes to a-alkoxy- and a-aminoalkylstan-nanes. 

RCH=0 + (C4H9)3SnLi — RCHSn(C4H9)3 RCHSn(C4H9 3 

The reaction with internal alkynes leads to a mixture of regioisomers and stereoisomers. Lewis acid-catalyzed hydrostannylation has also been observed using ZrC. With terminal 

Palladium-catalyzed procedures have also been developed for addition of stannanes to alkynes.  

CHAPTER 9 CARBON-CARBON BOND-FORMING REACTIONS OF COMPOUNDS OF BORON, SILICON, AND TIN 

Guibe andG. Balavoine, Tetrahedron Lett. 29 619 (1988) M. Benechie, T. Skrydstrup, andF. Khuong-Huu, Tetrahedron Lett. 32 7535 (1991). 

The reaction with internal acetylenes leads to a mixture of both regioisomers and stereoisomers.141 

Lewis acid-catalyzed hydrostannylation has been observed using ZrCl4. With terminal alkynes the Z-alkenylstannane is formed.142 These reactions are probably similar in mechanism to Lewis acid-catalyzed additions of silanes (see p. 811). 

Bond-Forming Reactions of Compounds of Boron, Silicon, and Tin 

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Matyjaszewski and coworkers reported the Wurtz-type synthesis of organostannane-organosilane copolymers under the influence of ultrasound starting from PhMeSiCl2 and Ph2SnCl2. This approach resulted in the formation of low molecular weight copolymers with a broad molecular weight distribution (M = 4 x 10, = 3.7). The NMR... 

Synthesis of Organostannanes. Air-sensitive 2-triorgano-starmyl imidazoles can be synthesized from Al-methyUmidazole via lithiation at the C2-position, followed by transmetalation with triorganostannyl chlorides (eq 15). V-Methylimidazole derivatives were less stable relative to their benzoxazole and benzothia-zole analogs. 

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]. 

The Stille reaction has developed as a popular protocol for the formation of C-C bonds due to the air- and moisture-stability as well as functional group compatibility of organotin compounds. Together with the Suzuki-Miyaura coupling it is one of the most powerful methods for the synthesis of molecules containing unsymmetrical biaryl moieties. However, despite its efficiency, this versatile reaction has slowly been displaced by other procedures that avoid the use of highly toxic organostannanes. 

Procedures for the synthesis of ketones based on coupling of organostannanes with acyl halides have also been developed.211 The catalytic cycle is similar to that involved in coupling with aryl halides. The scope of compounds to which the reaction is applicable includes tetra-u-butylstannane. This example indicates that the reductive elimination step competes successfully with (3-elimination. 

Two additional syntheses of 12 employed organostannane and organobo-rane chemistry. Marshall s synthesis of 12 was based on chiral organostannane A as shown in Scheme 20 [30], while Oehlschlager s one was based on chiral organoborane A (Scheme 21) [31]. 

Acetylenic compounds and acetylenic organotin compounds are widely used in the synthesis of many organostannane derivatives of importance in further synthetic procedures. 

Since its introduction by Allred and Liebsekind in 1996 [114], copper thiophene-2-carboxylate (CuTC) has emerged as a mild and useful reagent for mediating the cross-coupling of organostannanes with vinyl iodides at room temperature. CuTC is especially effective for substrates that are not stable at high temperature. In Paterson s total synthesis of elaiolide, he enlisted a CuTC-promoted Stille cyclodimerization of vinyl iodide 131 to afford the 16-membered macrocycle 132 under very mild conditions [115]. 

In the model studies toward the total synthesis of dimethyl sulfomycinamate, Kelly et al. successfully carried out the Stille couplings of oxazolyl triflate 18 with an array of organostannanes [19, 20]. Thus, 2-aryl-4-oxalone 17 was transformed into the corresponding triflate 18, which was then coupled with 2-trimethylstannylpyridine under the agency of Pd(Ph3P)4 and LiCl to provide adduct 19. The couplings of triflate 18 with phenyl-, vinyl- and phenylethynyl trimethyltin all proceeded in excellent yields. Unfortunately, application to the more delicate system in the natural product failed and the oxazole moiety was installed from acyclic precursors. 

We note that while tin reagents have often been employed for the organoboron halides/ the use of organostannanes as starting materials can also be applied to the synthesis of heavier group 13 derivatives. In the context of polyfunc-tional Lewis acid chemistry, this type of reaction has been employed for the preparation of ort/ o-phenylene aluminum derivatives. Thus, the reaction of 1,2-bis(trimethylstannyl)benzene 7 with dimethylaluminum chloride, methylaluminum dichloride or aluminum trichloride affords l,2-bis(dimethylaluminum)phenylene 37, l,2-bis(chloro(methyl)aluminum)phenylene 38 and 1,2-bis(dichloroalumi-num)phenylene 39, respectively (Scheme 16). Unfortunately, these compounds could not be crystallized and their identities have been inferred from NMR data only. In the case of 39, the aluminum derivative could not be separated from trimethyltin chloride with which it reportedly forms a polymeric ion pair consisting of trimethylstannyl cations and bis(trichloroaluminate) anions 40. 

T. Forngren, Y. Andersson, B. Lamm, B. Langstrom, Synthesis of [4-F-18]-1-bromo-4-fluorobenzene and its use in palladium-promoted cross-coupling reactions with organostannanes, Acta Chem. Scand. 52 (1998) 475-479. 

Synthesis of a-fluoro-ot, -unsaturated ketones via palladium-catalyzed crosscoupling reaction of 1-fluorovinyl halides (79) with organostannanes (80)... 

C. Chen, K. Wilcoxen, C.Q. Fluang, N. Strack, J.R. McCarthy, New methods for the synthesis of fluoro olefins via the palladium catalyzed cross-coupling reaction of 1-fluorovinyl halides with organoboranes and organostannanes, J. Fluor. Chem. 101 (2000) 285-290. 

In 2003, Fairlamb and co-workers reported on the synthesis of complex 57 as a novel catalyst for Stille reactions. The complex is prepared in one step from Pd2dba3-GHGl3, PPh3, and A-bromosuccinimide, and catalyzes the coupling of allylic and benzylic bromides with a variety of organostannanes in toluene at 60 °G. 

Sulfones. Pd-catalyzed coupling of organostannanes with sulfonyl chlorides provides a direct synthesis of sulfones, particularly vinyl and allyl sulfones. 

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