Alkynylstannanes, oxidation

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... 

The acidity of terminal alkynes makes available routes to alkynylstannanes that do not apply to alkenylstannanes. Thus, alkynylstannanes can be made by the acidolysis of aminostannanes with alkynes, and the iV-stannylpyr-roles are recommended for this purpose as they can be readily prepared by azeotropic dehydration of the trialkyltin oxide and pyrrole, and the amine that is liberated is non-basic.242... 

Shirakawa, E. Nakao, Y. Murota, Y. Hiyama, T. Palladium-iminophosphine-catalyzed homocoupling of alkynylstannanes and other organostannanes using allyl acetate or air as an oxidant./. Organomet. Chem. 2003, 670, 132-136. 

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... 

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. 

A general procedure for Pd-catalyzed homocoupling of different kinds of organostan-nanes (aryl-, heteroaryl-, alkenyl-, and alkynylstannanes) affording high yields of products was conducted in the presence of triphenylarsine in dichloroethane (Table 8). In this instance dichloroethane acts as an oxidant of the Pd° species to catalytically generate Pd° species. tran -Diiodoethene as an oxidant in Pd-catalyzed homocoupling alkynylstannanes was used for the preparation of polyethyne and diynes. ... 

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]. 

As illustrated with the formation of 379, oxidative coupling between primary alkylzincs and alkynylstannanes occurs under similar conditions as those discussed above for the reaction between arylzincs and alkylindium reagents [165,166]. Desyl chloride is used as oxidant in the presence of Pd(dba)2 (2.5 mol%) at 60 °C in THF (Scheme 4.87) [273]. The same catalyst is also competent to promote the same coupling but starting from terminal alkynes. In this case, air can be used as oxidant and secondary alkylzincs are also suitable. The presence of CO is necessary to achieve good chemical yields (Scheme 4.87) [274]. 

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. 

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