Grignard reagents with enynes

Terminal enynes Reaction of aryl Grignard reagents with pyridazine 1-oxide leads exclusively to terminal (E)-enynes in 50-75% yield. Alkyl Grignard reagents lead to 1,3-dienes in rather low yield. 

The 1,5-substitution of l-chloro-2-en-4-ynes with Grignard reagents has been described by Dulcere and co-workers [41] but lacks generality with regard to the nucleophile (see Section 2.3). In contrast, the regioselective reaction of enyne acetates 47 with various lithium cuprates proceeds smoothly in diethyl ether, furnishing exclusively vinylallenes 48 with variable substituent patterns (Scheme 2.17) [42],... 

The ring opening of pyridazine 1-oxide (770) by aryl Grignard reagents has been found to yield exclusively terminal ( )-enynes (773) (79JCS(P1)2136, 63CPB83). With more... 

The coupling reaction of tellurides with Grignard reagents promoted by Ni(ll) was described many years ago by LJemura.143 Recently, bis-vinylic tellurides were reacted with alkynes in the presence of catalytic amounts of Ni(dppe)Cl2 and Cul leading to enynes with retention of the (Z)-stereochemistry (Scheme 128).306... 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

Grignard reagents add to enynes with electron-donor substituents in the allylic position. The highly hindered olefin in Eq. (10) gave simple addition of r-butylmagnesium chloride [21]. 

An entirely different method for effecting, overall, an jp - p -coupling of aryl Grignard reagents is by reaction of the latter with pyridazine A-oxide (Scheme 20). In the simple cases studied, yields of enynes were 52-77%. The possibility of applying this approach to more highly substituted substrates does not appear to have been examined. 

The synthesis and characterization of a family of mono-Gp dichloro complexes with disubstituted aryloxo ligands has been reported, and their molecular structures provide some means of quantifying the number of electrons donated to the metal center by an aryloxide ligand. These complexes can be reduced by Grignard reagents or LiBu11 in the presence of enynes. The formation of metallacyclic derivatives (Scheme 348) was observed for the Cp but not for the Cp complexes, as deduced by NMR spectroscopy. The complexes have been investigated as catalysts... 

Negishi E, Cederbaum FE, Takahashi T (1986) Reaction of zirconocene dichloride with alkyllithiums or alkyl grignard reagents as a convenient method for generating a zirconocene equivalent and its use in zirconium-promoted cyclization of alkenes, alkynes, dienes, enynes, and diynes. Tetrahedron Lett 27 2829-2832... 

In the presence of the complex MnCl2.2LiCl (3%), soluble in THF, conjugated chlorodienes and chloroenynes stereoselectively react with alkyl Grignard reagents in THF-DMPU to afford, respectively, enynes and dienes in good to excellent yields [41]. The chemoselectivity of the reaction allows preparation of various mul-tifunctionalized polyunsaturated compounds (Scheme 13.57). The selectivity observed in the presence of aliphatic chlorides and aromatic bromides must be especially underlined. 

---