Alkynylstannanes, addition

The Pd(0)-catalyzed addition of trimethylsilyl iodide to an alkyne, followed by capture with alkynylstannane, affords the stereo-defined enyne 186. The reaction is explained by the oxidative addition of iodosilane, the insertion of an alkyne to generate the vinylpalladium 185, and the capture of 185 with the alkynylstannane 184[102]. 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

Cyclopentannelated tetrahydrofurans 169 [129] and substituted dihydro-furans 171 [130] can be synthesized by the treatment of functionalized al-kynyliodonium salts 168 and 170 with the appropriate nucleophile (Scheme 64). Alkynyliodonium salts 168 and 170, the key precursors in these reactions, are conveniently prepared from the appropriate alkynylstannanes and can be used without additional purification. 

Carbostannation of alkyne 474 with alkynylstannane 475, catalysed by the Pd iminophosphine complex 476 gives (Z)-stannylenyne 477 as the main product. The reaction is syn addition [184]. 

Indium trichloride promotes catalytically the addition of alkynylstannanes to aldehydes (Table 25).42 Metallic indium also mediates the same Barbier-type coupling between alkynyl halides and aldehydes or ketones to give secondary or tertiary propargyl alcohols (Table 26). Secondary alcohols can be oxidized in situ according an Oppenauer process.395 Thus, alkynyl ketones have been prepared from aldehydes via an indium-mediated alkynylation reaction followed by an indium-mediated Oppenauer oxidation. They are also obtained via an indium-mediated alkynylation of the relevant acyl chlorides (Table 27).396... 

The most common routes to alkenyltin compounds involve reaction between an alkenyl-metallic compound and a tin halide, or the hydrostannation of an alkyne (see Section 4.4). In recent years, the stannylmetallation of alkynes has become increasingly important. Direct hydrogenation of an alkynylstannane to a vinylstannane does not appear to be practicable, presumably because the catalyst is poisoned by the tin compound, but cis addition of dihydrogen can be achieved by hydrozirconation followed by hydrolysis,1 or in some cases, by cyclotitanation followed by hydrolysis2 (Section 8.2.2 below). 

As an alternative to this addition of an organostannane to an alkynylborane, vinylstannanes can also be formed by the complementary process of adding an organoborane to an alkynylstannane.64 Examples of these reactions are shown in equations 8-27 and 8-28, and a mechanistic rationalisation is included in equation 8-52. 

In a similar manner to alkynes, arynes accept the addition of alkynylstannanes in the presence of a catalytic amount of a palladium-iminophosphine complex (Scheme 5.7.15). Here, a characteristic feature is the participation of a vinylstannane. The usual catalytic cycle (Scheme 5.7.16, left), starting with oxidative addition of a carbon-tin bond, is possible, but rather improbable, because there are no signs of oxidative addition of alkenylstannanes that can participate in the present carbostannylation. Consequently, the palladium(O) complex is likely to undergo oxidative cyclization with an aryne to form a palladacyclopropene (Scheme 5.7.16, right), which then reacts with an organostannane. 

The proposed mechanism involves die carbometalation of an alkynylstannane with an a-stannylketone, protonation of fhe resultant gem-bisstannylated intermediate, and double bond shift to an allylmetal species. In contrast, the GaCl3-mediated addition of SEE to trimefhylsilylefhyne forms /i. j -unsaturated ketones, vinylation products, without isomerization to u,/I-urisaluraled ketones [272]. A similar mechanism via a gem-bismetalated intermediate formed by carbometalation has been postulated for fhe GaCh-mediated vinylation. 

Cross-couplings. The enynes are formed from allyl halides and alkynylstannanes. The Ni(acac)2-catalyzed cross-coupling of alkyl iodides and diaUcylzincs is particularly useful because many functionalities are tolerated. An intramolecular addition to an alkene can intervene in this coupling thereby creating functionalized cyclic compounds. ... 

The regiospecific Diels-Alder addition of alkynylstannanes 551 to 1-substituted buta-1,3-dienes 552 gives solely the 1,4-cyclohexadienes 553 ". The enediyne 554 does not function as a diene towards maleic anhydride or tetracyanoethylene with 2,3-dimethylbutadiene it reacts as a dienophile to give the cyclohexadiene 555. ... 

An in-depth study of the StiUe reaction of alkynylstannanes with aryl iodides reveals the high complexity of many cross-coupling processes, in which the often ignored isomerization reactions can compete with the oxidative addition or the transmetaUation as the slow steps in the overaU process [232]. The effect of ligands as weU as that of fluoride anion in the StiUe reaction has been studied in detail computationaUy within this overall mechanistic scenario [233]. 

The addition of boranes to acetylenes also generates functionalized allenes. Organoboration of the alkynylstannanes (90) offers a general route to the interesting functionalized allene (91), which shows promise as a precursor to other allenes, by selective reaction of either the carbon-tin or carbon-boron bonds (Scheme 34). Midland has described procedures for the conversion of... 

In contrast to C-Si bonds, transition-metal catalyzed insertions of unsaturated bonds into C-Sn bonds take place to give corresponding adducts. Relatively reactive C-Sn bond readily undergoes oxidative addition to palladium and nickel complexes and achieves carbostannylation of unsaturated bonds. Alkynylstannanes, ailylstannanes, acylstannanes, and vinylstannanes are available for the carbostannylation. 

Palladium-catalyzed addition of alkynylstannanes to alkynes takes place to give enyne products. For example, palladium/iminophosphine (Ll)-catalyzed reaction of... 

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