Alkynylstannane

The alkenylpalladium intermediate 364, formed by the intramolecular insertion of 363, is terminated by hydrogenolysis with formic acid to give the terminal alkene 365[266]. The intramolecular insertion of 366 is terminated by the reaction of the alkynylstannane 367 to afford the conjugated dienyne system 368[267j. 

Organotin compounds such as aryl-, alkenyl-, and alkynylstannanes are useful for the ketone synthesis by transmetallation of acylpalladium 529 and reductive elimination of 530 as shown[389-393]. Acetophenone (531) is obtained by the carbonylation of iodobenzene with Me4Sn. Diaryl ketones... 

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]. 

Modest enantiomeric excesses were obtained with the analogous alkynylstannane formed from 3-bromopropyne97. 

The reaction of triflates 6 (available via Diels-Alder reaction of 5 with furan or cyclopentadiene) with alkynylstannanes proceeds smoothty and with selectivity to afford good yields ofbicychc ene-diynes <96JOC6162>. 

Stannanes bearing aliphatic groups with unsaturation in the carbon chain are grouped in Table 3, including both open-chain and cyclic vinyl-, allyl- and alkynylstannanes. 

In many respects the reactions of alkynylstannanes are similar to those of vinylstan-nanes. 

Alkynylstannanes yield Z-vinylstannanes stereoselectively, when treated with zirconocene hydrochoride. This can be easily followed by other substitution processes, such a-iodination of the vinylstannane, as shown in reaction 22278. 

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... 

The reactivity of alkynylstannanes toward electrophiles is one element in the oxygen-to-carbon rearrangement of alkynylstannane derivatives of furanyl and pyranyl lactols (e.g., Equation (85)). The cleavage of the anomeric C-O bond is assisted by the Lewis acid to give an oxonium ion, which is trapped in situ by the nucleophilic stannylalkyne. The utility of this process has been demonstrated in the synthesis of the natural product muricatetrocin C, and the drug substance CMI-977.246... 

New routes to the alkoxides have been established, starting from the hydrido-, allyl-, or alkynylstannanes. Allyltributyltin and tributyltin hydride react with alcohols in the presence of triflic acid to give the tributyltin alkoxides in good yields,379 and the alkyltrialkynylstannanes are cleaved by alcohols under neutral conditions to give the trialkoxides (Equations (136)—(138)).247... 

A convenient route for the preparation of yne-allenes was recently described by Saalfrank et al. [19]. Products 29a/b were formed by Stille cross-coupling of allenyl bromides 27a/b with alkynylstannanes such as 28 (Scheme 14.9). Allenyl phospho-nates such as 30 were also suitable substrates in palladium-catalyzed couplings with propargylstannane 31 (Eq. 14.1). Bisstannylated acetylene 33 as alkyne component furnished the expected yne-bisallene 34 in reasonable yield, but without any diaster-eoselectivity (meso-34 (R,R)-/(S,S)-34 =50 50) (Eq. 14.2). 

SCHEME 16. Synthesis and iodonium salt cyclization chemistry of the alkynylstannane 115. 

Conversion of the alkynylstannane within 149 into the corresponding alkynyliodonium salt 150 proceeded as expected, and this fragile intermediate was treated immediately with base at low temperature (Scheme 23). TLC analysis of the crude reaction mixture indicated only a single off-baseline product, quite visible as a bright purple spot. Isolation of this compound by chromatography and characterization by standard spectral techniques led to the realization that the desired cycloheptatrienylidene product 151 had been formed in good yield. Careful examination of the crude reaction mixture s H NMR spectrum did not provide any indication that a 1,6 C-H insertion-derived... 

Alkynyl(trimethyl)silanes, germanes, and stannanes produce alkynyl-(phenyl)-A3-iodanes via ligand exchange on iodine under similar conditions [36]. Stang and co-workers developed a useful procedure for the preparation of diverse /3-functionalized alkynyl-A3-iodanes, which involves a ligand exchange of cyano-A3-iodane 19 with alkynylstannanes [37]. 

Feldman and co-workers developed a nicely designed intramolecular version of the MCI reaction [Eq. (108)] [190]. Treatment of the tosylamide-bearing alkynylstannane 118 with PhI(CN)OTf 19 gave via ligand exchange a labile alkynyliodane 119, which on exposure to a base undergoes an intramolecular tandem MCI reaction to afford the amide 120. Hydroxyalkynyl-A3-iodanes similarly undergo the intramolecular MCI reaction [191]. 

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