Hydrostannylation Of terminal alkynes

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

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

Palladium-catalyzed cyclization was used by Grigg et al. [95] to construct complex heterocycles 47 (Scheme 32). Due to the excellent regioselectivity of the hydrostannylation of terminal alkynes, only one major isomer was formed in most cases. The strategy was used for preparation of combinatorial libraries. 

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. 

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. 

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). 

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... 

Hydrostannylation of a terminal alkyne [71] Alkyne 97 (2equiv.) and AIBN (0.08 equiv.) were added to the tin hydride resin 95 in dry toluene under argon. The mixture was slowly stirred for 20 h at 60 °C. At intervals of 2.5 h, further portions of AIBN (4 x 0.08 equiv.) were added. The polymer was subsequently separated from the solvent, washed several times with dry toluene and Et20, and dried. 

They react with terminal alkynes by electrophilic addition of the empty p-orbital to the unsubstituted end of the triple bond 83. The intermediate would then be the more substituted vinyl cation 84. It is easier to draw this mechanism with R2BH than with the full structure for 9-BBN. The intermediate 84 is not fully formed before hydride transfer begins so that the reaction is semi-concerted and the transition state is something like 86. The result is a regioselective and stereospecific cis hydroboration of the triple bond to give the A-vinyl borane 85. The intermediate 84 is quite like the radical intermediate in hydrostannylation but the difference is that hydrogen transfer is intramolecular and stereospecific in hydroboration. 

The orientation and steric course of the hydrostannylation of alkynes to yield alkenylstannanes is determined by the nature of the reagent and the catalyst used. The trimethylstannylcopper reagent Me3SnCu Me2S adds regioselectively to terminal acetylenes 293 (n = 1,2,3,4 or 6, X = OH, Cl, OSiMe2Bu or 2-tetrahydropyranyl) to give, after treatment with methanol, the stannanes 294. ... 

Hydrostannylations. Hydrostannanes add to alkynes in uncatalyzed reactions at 60 °C. Phenylacetylene, for instance gives a mixture of ( )- and (Z)-vinylstannanes, wherein the tin atom has added to the terminal carbons. In the presence of Wilkinson s catalyst, however, the hydrostannylation proceeds at 0 °C to give mostly the regioisomeric vinylstannanes (eq 23). Terminal stannanes in the latter process seem to result from competing free radical additions. This may not be a complication with some other catalysts the complexes PdCl2(PPh3)2 and Mo( j -allyl) (CO)2(NCMe)2 also mediate hydrostannylations of alkynes, and they are reported to be 100% cis selective. Hydrostannanes and thiols react in a similar way to silanes and alcohols (eq 24). ... 

Scheme 6.17 One-pot hydrostannylation/Stille coupling of a terminal alkyne and a bro-moalkene to 1,3-dienes [74].

Radical-Initiated and Transition Metal-Catalyzed Additions. Some radical and transition metal-catalyzed additions to TMSA are unique when compared with additions to other terminals alkynes, because they show remarkable regioselectivity and/or stereoselectivity. The regioselectivity of a metal-catalyzed addition may be complementary to that of a radical-initiated process (eq 12). For example, rhodium and molybdenum complex-catalyzed additions of trialkyltin or triaryltin hydrides to TMSA give mainly the 1,1-disubstituted ethylenes, whereas radical hydrostannylation through sonication or triethylhorane initiation gives the 1,2-adducts with the (E)-isomers predominating. Other terminal alkynes undergo radical or metal-catalyzed hydrostannylation with either poorer or reverse selectivity. 

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... 

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