Hydrostannylations, 1-alkynes

Stannaries have become prominent in multifunctional anchoring groups. A polymer-bound tin hydride 41 has been used to hydrostannylate alkynes under the action of palladium-catalysis to give polymer-bound alkenylstannanes 42. These alkenyl stannanes have been employed in intermolecular [45] and intramolecular Stille reactions [46]. Alkenylstannanes can also undergo protonation to give alkenes 44 in a traceless fashion. This linker is therefore multifunctional (Scheme 6.1.12). 

C.ii.b. Stille Couplings. A polymer-bound tin hydride has been used to hydrostannylate alkynes under palladium catalysis hydrostaimylation to give polymer-bound alkenyl-stannanes.t Alternatively, the latter could be prepared from a polymer-bound tin chloride and an alkenyllithium or -magnesium halide reagent. These alkenylstannanes were employed in intermolecular as well intramolecular Stille reactions. The inter-molecular reactions provided the coupling products in good yields. In addition, the... 

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). 

Hydrostannylation of terminal alkynes can also be achieved by reaction with stannyl-cyanocuprates. 

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. 

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>. 

The final stages of the synthesis illustrate both the power and the current limitations of transition-metal mediated C-C bond formation. Coupling of 2 and 3 led to the ene-yne 7. Pd-mediated hydrostannylation of the alkyne proceeded with high geometric control, but tended to... 

Rhodium21 and molybdenum1813 complexes are also effective catalysts for the hydrostannylation of terminal alkynes, interestingly resulting in preferential formation of the a-regioisomer. 

The palladium-catalyzed coupling of aryl iodides with vinylstannanes (Stille coupling) leads to the formation of styrenes. With resin-bound vinylstannanes, this reaction can be conducted in such a way that simultaneous detachment from the support of the newly formed styrenes occurs. This has been realized intramolecularly in the preparation of macrocyclic lactones (Entry 4, Table 3.43). The required resin-bound vinylstannanes were prepared either by hydrostannylation of alkynes with a resin-... 

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. 

Hydrostannylation of a P-chloro phosphaalkene and subsequent R3SnCl elimination is a low-yield route to specific lA3,3A3-diphosphetanes <1998HAC453>. Hydrostannylation of the P-alkynes 44 generates the related stannylphosphaalkenes which react with the starting phosphaalkynes in a [2+2] cycloaddition to form... 

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. 

A particularly nice application of the hydrostannylation is the Stille coupling (for reviews on the Stille coupling see [119-123]) that becomes catalytic in tin, as introduced by Maleczka and coworkers [124,125]. Terminal alkynes and organobromides can be coupled in the presence of catalytic amounts of tributyltinchloride and Pd(II) and Pd(0) catalysts and stoichiometric amounts of PMHS (polymethylhydrosiloxane) to give the alkene derivatives 166-173 in good to excellent yields (Scheme 64). 

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]. 

A synthetically useful and convenient method for the B(C6F5)3-catalyzed hydro-stannylation of alkynes with tributyltin hydride, prepared in situ from easily handled and inexpensive chlorostannane and hydrosilane, has been developed by Yamamoto and his colleagues [155]. The hydrostannylation of monosubstituted alkynes proceeds in a regiospecific manner to give the /3-hydrostannylation products exclusively (Eq. 100). The reaction is trans stereoselective. This method can also be applied to the hydrostannylation of allenes and alkenes (Eqs 101 and 102). 

Examples of Sb—H bond formation by reactions of Sb compounds with either covalent or ionic binary hydrides are rare. The alkynlstibine, n-BUjSbC CH, reacts with PhjSnH, where instead of hydrostannylation of the alkyne bond, tin hydride cleavage of the Sb—C bond ... 

The alkyne is transformed into the corresponding organotin reagent by radical hydrostannylation. First under kinetic control the Z-organo-stannane is formed. But in the presence of excess tributyltinhydride, reversible addition of the tributyltinradical to either carbon atom of the double bond equilibrates the Z-isomer to the more stable -organo-stannane, which is obtained in 53 % (2 steps). TBS- deprotection furnishes 23 (building block II) in 63 % yield. When hydrostannylation and deprotection are carried out the other way round, the yield of this sequence from 20 to 23 drops from 33 % to 29 %. 

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. 

---