Vinylstannanes aryl chlorides

Coupling of aryl chlorides with vinylstannane can be carried out using Ni(0)-Ph3P as a catalyst [158a]. 

Scheme 13 Cross-coupling of aryl chlorides with Sn( Bu)3Ph or vinylstannanes mediated by in situ generated Pd/NHC system...

Alkenyl-aryl and alkenyl-alkenyl couplings are also widely used. Pd/P(r-Bu)3 is an efficient catalyst for the coupling of aryl chlorides with aryl-, alkenyl-, and alkylstannanes [21], Coupling of 4-chloroanisole with vinylstannane proceeded at 100 C with the use of CsF as a base. Addition of CuCl and LiCl in DMSO is recommended, which accelerates the transmetallation step in the coupling of sterically congested substrates. Coupling of the alkenylstannane 44 with naphthyl nonaflate proceeded smoothly to give 45 in 88% yield [18]. 

Stille Coupling. The combination of TTBP and Pd(dba)2 was the first catalyst system to promote the coupling of aryl chlorides with aryl- and vinylstannanes. The coupling is t)q)ically carried out at 100 °C. Under these conditions, the preformed Pd(TTBP)2 catalyst gave comparable yields to the catalyst derived from TTBP/Pd(dba)2 (2 1 TTBP/Pd). Catalyst loadings as... 

After having proposed a general solution to the Stille coupling of unactivated aryl chlorides, the Fu laboratories reported in 2003 a general method for the coupling of unactivated primary alkyl bromides with vinylstannanes (33 + 34 35). It is a real tour de force since these substrates harbor p-hydrogens... 

Subsequently, an in situ Pd(OAc)j-imidazolium salt mixture, developed by Nolan and Grasa, has demonstrated its efficiency for the coupling of aryl bromides and even chlorides with aryl and vinylstannanes. This improved reactivity is due to the TBAF additive, whereby F anions coordinate to Sn forming hypervalent fluorostannate anions that are more reactive towards transmetallation to Pd [121] (Scheme 6.37). 

Analogously, 5-tributylstannylimidazole 29 was easily obtained from the regioselective deprotonation of 1,2-disubstituted imidazole 28 at C(5) followed by treatment with tributyltin chloride [24]. In the presence of 2.6 equivalents of LiCl, the Stille reaction of 29 with aryl triflate 30 afforded the desired 1,2,5-trisubstituted imidazole 31 with 2,6-di-tert-butyl-4-methylphenol (BHT) as a radical scavenger. Reversal of the nucleophile and electrophile of the Stille reaction also provided satisfactory results. For example, the coupling reaction of 5-bromoimidazole 33, derived from imidazole 32 via a regioselective bromination at C(5), and vinylstannane 34 produced adduct 35 [24],... 

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