Vinylic couplings

Oleno synthesis via Pd catalyzed vinyl coupling o( vinyl borates with vinyl halides... 

Sequential cross-coupling can be used also for synthesis of 1,1-disubstituted alkenes from 2,3-dibromopropene (Aldrich). The allylic halogen couples directly with a Grignard reagent vinylic coupling is slower and requires a catalyst (equation I). 

Aryl-vinyl coupling fi-aryl-fi-amino acids.2 A novel route to P-amino acids is based on the formal conjugate addition of an aryl iodide to an enantiomerically pure dihydropyrimidinone such as 1, prepared by pivaldehyde acetalization of (R)-aspara-gine (c/., 14, 69-70). Thus 4-iodoanisole couples with 1 under Heck conditions, Pd(OAc)2, Ar3P, triethylamine, and DMF to give 2 in 78% yield. Reduction of 2... 

Furthermore, coupling to form C-aryl glucals, as precursors to C-aryl glycosides, was carried out both from the C-metallated glucal154 and onto the C-iodo glucal (equations 90 and 91)155. A synthesis of vineomycinone B2 methyl ester has resulted from this chemistry. A vinyl-vinyl coupling reaction has also formed the cornerstone of aryl steroid synthesis (equation 92)156. 

Scheme 11. Effect of bases on vinyl-vinyl coupling...

Recently, the catalytic use of Cu(I) salts was applied to the aryl-aryl and aryl-vinyl couplings and to the allylation of furans and thiophenes, even without the benefit of such activation. In parallel with the use of Cu(I) salts, the reactivity of stoichiometric amounts of Cu(II) was estabhshed for the homocoupling of vinyltins" °°. Copper nitrate was found to be particularly efficient, leading to the cyclotrimerization of l-bromo-2-stannylaUcenes . Lately, the use of catalytic amounts of CuCla was shown to be superior to the use of Cu(I) salts for achieving these homocoupling reactions . ... 

The intramolecular version of vinyl-vinyl coupling has been less often applied, but the potential shonld be clear from one example of macrolide formation. This example also demonstrates the use of formic acid as an in situ reducing agent and the high reactivity of vinyl iodides 25 °C is an unusually low temperature for the Heck reaction (equation 18). ... 

SUZUKI Vinyl coupling 378 SWARTS Halogen exchange 377 SWERN Oxidation 378... 

The H NMR spectra of 120-122 in C6D6 show that signals for the protons / to boron [H-4] are always downfield from those a to boron [H-3]. The vicinal vinyl coupling constants [V3 4] range from 7.7 to 8.6 Hz, which is rather large for an unsaturated five-membered ring. On deprotonation to form 123-125, the V34 values are substantially reduced (range 3.7. 5 Hz). 

While the Wessling-Zimmermann route is a typical method of polymer synthesis, both PPVs and their corresponding oligomers can be synthesized by the extension of methods used for the synthesis of the easiest building block, stilbene 70 (scheme 13). Conceptionally, this is possible by (i) carbon-carbon double bond formation, for whieh synthetic organic chemistry provides a great number of methods [116], and (ii) by aryl-vinyl coupling [117] some examples of both methods are outlined in scheme 13. 

Instead of the amide (350), the corresponding methyl imidate also could be used for the photoreaction. In this case, the labeling studies uniquely identified the process as a di-7t-methane rearrangement with initial vinyl-to-vinyl coupling " ... 

The mechanism originally proposed for the vinylic coupling reactions involved decomposition of a diolefin tr-complex 281, 282). However, the information developed since then on reactions of vinylic Pd(II) compounds suggests that these compounds are intermediates in the coupling reaction. The vinyl Pd(II) compounds could either oxidatively couple, since this is known to occur, or the vinyl Pd(II) intermediate could add across the olefinic double bond ... 

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