Acetylenic stannanes

A second consequence of the cyclic transition state of these reactions is that (Z)-alkenyl-stannanes are less reactive than their (E)-isomers and can in some cases be recovered unchanged from reactions in which mixtures of isomers are used109. a-Alkoxy-2-butenylstannanes fail to react with 0,/J-dialkylated a./J-unsaturated aldehydes, on heating, and give low syn/anli stereoselectivity with ,/i-acetylenic aldehydes, perhaps because of reduced steric effects110. 

The allylic germanes and stannanes obtained are useful synthons. The hydrogermylation of acetylenes can also be performed with dimethyl(2-thiophenyl)germane58. This compound can be prepared quantitatively by the Grignard reaction of Cl2GeMe2 with 2-bromothiophene (Scheme 8). 

Interestingly, the alkyne-oxazole Diels-Alder cycloaddition strategy provides a unique entry to some furyl stannanes [52]. Thus, thermolysis of bis(tributylstannyl)acetylene (50) and 4-phenyloxazole (51) led to a separable mixture of 3,4-bis(tributylstannyl)furan (52, 19% yield) and 3-tributylstannylfuran (53, 23% yield). 

The fact that the cyclization is directed to an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide would be expected to lead to a more complex product mixture because the cyclized product could undergo iodine abstraction and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive, and the reaction product is stable to iodine-atom abstraction. Because of the potential for competition from reduction by the stannane, other reaction conditions have been developed to promote cyclization. Hexabutylditin is frequently used.238... 

In Pd(0) mediated addition of trialkyl[dimethyl(phenyl)silyl]stannane to acetylenes <89S218> the product obtained from l-hydroxybut-3-yne after normal workup contained the substituted 1,2,5-oxasilastannolene (354) in addition to the expected 2-tributylstannylalk-l-ene (Equation (32)). 

Dioxa-, oxathia-, and dithiaboroles undergo efficient transmetallation with alkynyl stannanes, furnishing the 5-alkynyl boroles. Thus the reaction of bis(trimethylstannyl) acetylene 273 with 2 equiv of bromoboranes 272 in toluene at room temperature furnishes air-sensitive boroles 274 as colorless solids in good yield (Equation 10) <2004EJI4223>. 

Both syntheses made the bond between the two alkenes by a Stille coupling. One put the tin on the amide part by a Cu(I) catalysed conjugate addition of Bu3SnLi to the acetylenic ester 212 and Weinreb amide formation. Coupling this vinyl stannane with a single enantiomer of the iodide derived from the rest of the molecule gave crocacin C in good yield. The synthesis of the iodide uses an asymmetric aldol reaction and is described in the workbook.30... 

Several other types of polymer-bound stannanes have since been reported, including a polymer-bound distannane used in radical-mediated cyclizations and radical addition to acetylenes [87], and stannanes with longer tethers to the polymer [88, 89] applied in the Barton-McCombie deoxygenation of alcohols [90, 91] and... 

Functionalised vinyl stannanes have been prepared by the Pd(0)- catalysed addition of (Me Sn)2 to a,3-acetylenic esters and amides and some interesting observations have been made concerning the distribution/stability of the possible products (Scheme 65). Piers has also explored some of the chemistry of these adducts and, for... 

The great majority of o-acetylide transition metal complexes are prepared by interaction of a metal halide with acetylide, RC C", or the formal oxidative addition of terminal alkynes or alkynyl stannanes to the metal center. As amply demonstrated in the previous section, alkynyliodonium salts may serve as electrophilic acetylene equivalents. In other words, transition metal complexes may act as nucleophiles in reactions with alkynyliodonium species. Indeed, the reaction [81] of the square planar Vaska s complex, 106, and its Rh analog, 107, with a variety of alkynyliodonium triflates in toluene results in 89-96% isolated yield of the hexa-coordinate o-acetylide complexes, 108 and 109 [Eq. (58)]. Reaction is essentially instantaneous and occurs with retention of stereochemistry around the metal center. 

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. ... 

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