Dienes, Stille coupling

Hydrostannylation and Stille coupling. In situ reduction of triorganochlorostan-nanes with PMHS during Pd-catalyzed hydrostannylation is convenient. Modification of reaction conditions enables an efficient synthesis of dienes (Stille coupling)." ... 

Wong reported that stoichiometric amounts of copper(I) chloride alone can promote the intramolecular Stille coupling (equation 138)243. In fact, copper(I)-mediated reaction was cleaner and faster compared with that catalysed by Pd(0) species. Selected examples of intermolecular Stille coupling reactions leading to dienes (Table 25)236a,242b 244, polyenes (Table 26)245 and macrocyclizations (Table 27)246 are given in the respective tables. 

TABLE 25. Synthesis of dienes through a Stille coupling reaction... 

Vinylic triflates such as 244 also allow the introduction of C-bonded substituents reaction with dilithium diallylcopper cyanide gives the 1,4-diene 245 in 76% yield,232 and Stille coupling with tributylvinyltin and carbon monoxide in the presence of palladium(O) produces the dienone 246 in 71% yield.233 234... 

One possible explanation is that each isoprene unit adds to the titanium (and we will drop the pretence at this point that we have any idea which other ligands are on the Ti atom) to form ant]4 diene complex. This must necessarily have the s-cis conformation. Addition of R to one end of this complex gives an T]3 allyl complex still maintaining the cis configuration. The next diene then adds to form a newt 4 diene complex, couples to the allyl complex, and so on. As the chain grows, each diene is added as an r)4 complex and an all-cis polymer results. 

The synthesis of electron-deficient diene 285 was achieved by the Stille coupling of P-trifluoromethanesulfonyl-a,P-unsaturated sulfone (284) with a 3-stannyl-a,P-unsaturated ester (283) (Scheme 74).148 Similarly, the preparation of a diverse range of enantiomerically pure 1- and 2-sulfinyl dienes has been achieved via Stille coupling of halovinyl sulfoxides and vinyl stannanes.149,150 Enantiomerically pure 1- and 2-sulfinyl dienes have been used extensively in asymmetric Diels-Alder reactions.111... 

The same authors also presented a completely selective three-component cascade reaction but of a slightly different type. By reacting 26 with a vinyl-stannane 27 (R = SnBus) in the presence of methyl acrylate or diethyl maleate, a sequential Stille-type [31] cross-coupling-Diels-Alder reaction took place, which only gave the desired products [30]. Since the Stille coupling proceeds without a base present, the dienophile does not participate in a Heck reaction, but only in a [4 + 2] cycloaddition with the initially formed electron-rich diene. 

Metallated 1-ethoxy-1,3-dienes 697 and 712, obtained from the corresponding acetals by means of the LICKOR base, have been treated with alkyl halides, epoxides, carbonyl compounds, carbon dioxide and carboxylic esters affording ( )-l-substituted 1-ethoxy-1,3-dienes and, after hydrolysis, a,P-unsaturated carbonyl compounds1007-1010 (Scheme 186). Intermediates 697 and 712 have been transformed into the corresponding vinyl stan-nanes, which were submitted to Stille couplings with iodobenzene and benzoyl chloride823. 

Later, the dimethylphenylsilyl adduct 75 d was not only converted to the vinyl-ketene acetal 81 by a Wittig-Horner reaction via the ester 80, but also by reduction of 80 to the aldehyde 84 followed by silylation of the corresponding anion to the silylenol ether 85. Vinylcyclohexenes without terminal substitution (e. g., 83) was prepared via an alcohol derived from 80 or 84 [73,74]. The least substituted diene 83 was alternatively prepared from the triflate 82 in a Stille coupling with trialkylvinyl stannane [75] (Scheme 24), a reaction also used by Toshima etal.[76]. 

Vinyl triflates (C=C-0S02CF3) react with vinyl tin derivatives in the presence of palladium catalysts to form dienes, in what is known as the Stille coupling. Vinyl triflates can be prepared from the enolate by reaction with Al-phenyl trifli-... 

A route to difluorosugar 426 has also been developed. It includes a Stille coupling [538,539] of alkenetin compound 421 with ( )-3-iodoprop-2-en-l-yl 4-methoxybenzoate that generates diene 422. Sharpless asymmetric dihydroxylation of 422 was chemoselective and provided diol 423 in 54% yield. The corresponding acetonide 424 was then saponified with H202/Li0H. After treatment with 12-M HCl in THF, the semi-protected l-deoxy-l,l-difluoro-D-xylulose 426 was obtained (O Scheme 104) [540]. 

Stille coupling. Vinyl-vinyl coupling and S 2 displacement of allylic carbonates with vinylstannanes, the latter in an aqueous medium, give rise to 1,3- and 1,4-dienes, respectively. 

Alkenylstannanes. Hydrostannylation of 1-bromoalkynes by employing BUjSnH, (dba)jPd2, and PhjP provides ( )-alkenylstannanes. It is convenient to effect a Stille coupling immediately because the proper Pd catalyst is already present. From 1 -alkynes and 1-bromoalkenes the synthesis of 1,3-dienes requires only catalytic amounts of the tin reagent. 

The stereospecific Stille coupling of alkenyl bromides or iodides with alkenylstannanes is one of the most widely applied methods for the synthesis of dienes. Thus, a slight modification of the original method was applied in the coupling of alkenyl iodide 1 with (Z)-alkenylstannane 2, stereospecifically building a polyene as the last step in the synthesis of (—)-stipiamide (Scheme 5.4.1).25... 

Subsequently, the bromo-function can be vinylated in a Stille coupling reaction to afford compounds (279). The resulting diene can undergo Diels-Alder reactions with electron-poor dienophiles (280) such as maleic anhydride, maleic amides, and para-quinones. 

In attempting to prepare sugar-hased radical acceptors, Martin and Xie found that treatment of 282 with base to generate enone 283 gave instead the dimerized product 284 the result of a [4+2] cycloaddition reaction. Compoimd 283 was never isolated from the reaction mixture (Scheme 51) [76]. Quayle and coworkers used a Stille coupling approach to prepare a number of sugar-based dienes that then under-... 

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