Stille coupling catalytic in tin

In order to overcome the reluctance of using organotins, despite their great synthetic potential, Maleczka et al. proposed a vinyl-vinyl Stille coupling catalytic in tin by using a terminal alkyne and an alkenyl halide as substrates [143]. 

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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). 

Scheme 64 Sequential hydrostannylation-Stille coupling that is catalytic in tin [124,125] (bonds formed by Stille coupling are drawn in bold lines)...

Scheme 3 One-pot tandem hydrostannylation/Kosugi-Stille coupling procedure that is catalytic in tin...

Maleczka proposed an ingenious approach (the Sn-F route), catalytic in tin, for the Stille coupling involving alkenylstarmanes. The alkenylstannane is formed in situ from the corresponding terminal alkyne and a catalytic amount of trialkyltinhydride as depicted on the catalytic loop below. 

The synthesis of the key intermediate aldehyde 68 is outlined in Schemes 19-21. The two hydroxyls of butyne-l,4-diol (74, Scheme 19), a cheap intermediate in the industrial synthesis of THF, can be protected as 4-methoxybenzyl (PMB) ethers in 94% yield. The triple bond is then m-hydrostannylated with tri-n-butyl-tin hydride and a catalytic amount of Pd(PPh3)2Cl238 to give the vinylstannane 76 in 98 % yield. Note that the stereospecific nature of the m-hydrostannylation absolutely guarantees the correct relative stereochemistry of C-3 and C-4 in the natural product. The other partner for the Stille coupling, vinyl iodide 78, is prepared by... 

The pyrene-tethered dimethyltin hydride is made in six steps and 29% overall yield from commercially available pyrene carboxyaldehyde, but can be used catalytically with a stoichiometric source of hydride. Workup with activated carbon provides product in yields comparable to tri-n-butyltin hydride and with organotin contamination below 2 mol% and undetectable by NMR spectroscopy. Unfortunately, reagent recovery from the activated carbon is low-yielding. Allyl and phenyl pyrene-tethered tin reagents have also proved effective for use in allylation and Stille coupling respectively and, again, tin contamination of the product are undetectable by and NMR spectroscopy. 

An interesting development in Stille coupling by using only catalytic amounts of tin was reported [56]. 

One major drawback of the Stille coupling in general is the presence of tin impurities in the final product. To solve this problem, polymer-bound tin reagents can be used. Her nan et al. [88] synthesized both dimethyl- and dibutyltin chloride resins, and used them in a model reaction shown in Scheme 26. Substoichiometric rather than catalytic amounts of resin 41 were needed in order to obtain acceptable yields, and the usual Stille conditions had to be optimized to fit this specific reaction. The catalytic system consisted... 

The coupling of terminal alkynes with aryl or vinyl halides under palladium catalysis is known as the Sonogashira reaction. This catalytic process requires the use of a palladium(0) complex, is performed in the presence of base, and generally uses copper iodide as a co-catalyst. One partner, the aryl or vinyl halide, is the same as in the Stille and Suzuki couplings but the other has hydrogen instead of tin or boron as the metal to be exchanged for palladium. 

Using this principle, Kibayashi and coworkers [147] have introduced a sequential cyclic carbopalladation-Stille vinylation of enyne compounds. Upon treating the enyne 196 and vinyl tributylstannane with catalytic amounts of Pd2(dba)3 CHCI3 in the presence of AcOH the allyl-substituted methylene cyclopentane 197 was formed in 53% yield (Scheme 80). The subsequent cross-coupling occurs with complete suppression of /M I-climinalion and the Alder-ene product 198 was not detected. Likewise, this sequence was extended to heteroatom-linked enynes and further vinyl tin compounds to provide the heterocyclic analogs 199 in moderate to excellent yields (Scheme 81). 

In his review of 1986, Stille proposed a mechanism based primarily on data obtained from the coupling of benzoyl chloride with tri-n-butyl(phenyl)tin. This proposal already clearly stated four main steps of the catalytic cycle oxidative addition, transmetalation, isomerization, and reductive elimination. 

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