Hydrostannylation, of an alkyne

Direct hydrostannylation of an alkyne with a tin hydride can be radical-initiated in the way we saw in Chapter 39. The product of kinetic control is the Z-isomer but, if there is excess tin hydride or enough radicals are present, isomerization into the more stable E-isomer occurs. The regiocontrol of this process is good with terminal alkynes. 

The Stille coupling, which also applies to dienes, is typically catalyzed by Pd(PPh3)4. Since the preliminary hydrostannylation of an alkyne such as 61 uses the same catalyst, a one-pot procedure was recently designed to access (ZJS)-2-arylsulfonyl-1,3-dienes 62 in good yields (Scheme 35) [55]. 

Only a few radical reactions have been applied to the functionalization of 1,3-dioxanes or 1,3-dithianes bearing one exo- or mdo-Ao ih c bond. In all cases, ring formation was the goal. The common reagent system, BusSnH/AIBN, was used to achieve a stereoselective ring closing hydrostannylation of an alkenyl alkyne subunit (AIBN = 2,2 -azobisiso-butyronitrile Equation 36) <1998JOC9626>. 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Vinyl cuprates.17 Vinyl cuprates can be prepared conveniently by in situ trans-metallation of vinylstannanes, available by hydrostannylation of alkynes, with a cuprate such as (CH3)2Cu(CN)Li2 (equation I). Reagents prepared in this way effect conjugate addition of the vinyl group to an enone with essentially no transfer of the methyl group. 

Mechanistically, this sequence can be rationalized as an interdigitating hydrostannylation-Stille coupling-tin recycling process (Scheme 65). The Pd(0)-catalyzed hydrostannylation of the terminal alkyne with tributyl-tinhydride furnishes the vinyl stannane, which is transmetallated to the... 

Palladium-catalyzed hydrostannylation is stereoselective, giving syn-adduct. Regioselectivity is not always perfect, however. Exclusive formation of (-B)-l-stannylalkene is obtained only when 1-alkyne with a bulky substituent is employed (Scheme 34) [147-150]. Nicolaou also reported a stereo- and regio-selective hydrostannylation (Scheme 35) [151]. Use of a combination of tri-butyltin chloride and poly(methylhydro)siloxane in the presence of TBAF is also an impressive method in this class [152]. 

Starting from (+)-diethyl tartrate (2), bromobutenolide 18 was obtained in nine steps. Three of the four C=C double bonds were built up using a Wittig reaction (11—>12), an Ando- y Q Horner-Wadsworth-Emmons reaction (13— 15) and (3-elimination (16 18). From (-)-actinol (3) stannane 23 and sulfone 24 were synthesized in 9 and 13 steps, respectively. Their common intermediate, alkyne 22, was synthesized using methoxycarbonylation. Sharpless asymmetric epoxidation and Ci-elongation with lithio trimethylsilyldiazomethane. Stannane 23 was obtained upon hydrostannylation and TBS deprotection. Sulfone 24 was obtained after addition to methyl tetrolate, reduction, Mukaiyama redox condensation, acetylation and catalytic oxidation. 

Direct hydrostannylation ofthe alkyne, on the other hand, gave an 85 15 (E/Z)-mixture, which was not suitable for an industrial synthesis. However, hydrozir-conation with Cp2Zr(H)Cl proceeded imder cis-addition and resulted in 100% of the ( )-vinylzirconate. TransmetaUation with complex lithium cuprates proceeded analogously to the vinylstannanes. Along this route, misoprostol is obtainable in 73 % yield. 

Stannanes have become prominent members in the field of multifunctional anchoring groups. Nicolaou et al. [358] used polymer-bound tin hydride 693 to hydrostannylate alkynes 694 under palladium-catalysis to give polymer-bound alkenyl stannanes. Alternatively, the latter ones can be prepared from a polymer-bound tin chloride and an alkenyl-Hthium or -magnesium halide reagent. These alkenyl stannanes were employed in intermolecular as well... 

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