Alkenyl magnesium halide

The role of the halide is crucial for the stereoselectivity of the reaction the syn- or am/-product arises from a common alkenyl magnesium halide intermediate [Eq. (78)]. 

The first such catalytic reaction was reported in 2001 by Miller. Aryl cyanides can be coupled with aryl-, alkyl-, and alkenyl-magnesium halides pretreated with... 

In the majority of cases, organolithium compounds and Grignard reagents contain polarized but covalent carbon—metal bonds. Lithioalkanes, -alkenes, and -aromatics, on the one hand, and alkyl, alkenyl, and aryl magnesium halides, on the other hand, are therefore formulated with a hyphen between the metal and the neighboring C atom. Only lithiated alkynes and alkynyl Grignard reagents are considered to be ionic—that is, species with carbon, metal bonds similar to those in LiCN or Mg(CN)2. 

Triorganoaluminums with unsaturated C—C bonds adjacent to AI can be prepared from the respective organolithium, -sodium or -magnesium halide reagents. However, the solvate-free compounds (R = alkenyl, alkynyl) are unstable and, therefore, should be isolated as ether adducts ... 

Chen and Wang described a nickel-catalyzed carbonylative Negishi coupling reactions [82]. In the presence of a catalytic amount of nickel chloride and 4,4 -dimethoxyl-2,2 -bypyridyl under carbon monoxide atmosphere, various enones were produced from enol triflates and diorganozinc reagents (Scheme 4.44). They demonstrate that the rate of CO insertion is increased by the addition of lithium or magnesium halides and the use of polar solvents. Alkenyl iodides can also be used instead of enol triflates. 

C.ii.b. Stille Couplings. A polymer-bound tin hydride has been used to hydrostannylate alkynes under palladium catalysis hydrostaimylation to give polymer-bound alkenyl-stannanes.t Alternatively, the latter could be prepared from a polymer-bound tin chloride and an alkenyllithium or -magnesium halide reagent. These alkenylstannanes were employed in intermolecular as well intramolecular Stille reactions. The inter-molecular reactions provided the coupling products in good yields. In addition, the... 

In 1979 Wenkert reported the cross-coupling reaction of alkenyl methyl ethers with phenyl and methyl magnesium halides via the activation of C(sp )-OMe bonds by using NiCl2(PPh3)2 as a catalyst precursor [Eq. (2)] [12, 13]. This alkenyl ether cross-coupling was further extended to silyl enol ethers [14] and cyclic ethers, such as dUiydrofurans [15-23], which offer a unique method for the stereoselective construction of alkenes (see Sect. 4). 

I n this chapter, we wUl focus on the formation ofJunctionalized alkyl-, alkenyl-, aryl-and heteroaryl-magnesium halides and describe the scope and limitations of their ap-plications in carbon-carbon and carbon-heteroatom bond forming reactions [13]. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Alkenyl, alkynyl, and aryl halides, like alkyl halides, can be converted to the corresponding magnesium and lithium compounds. However, the reaction conditions, such as choice of solvent, can be critical. Bromoethene, for instance, can be converted to ethenylmagnesium bromide in good yield if the solvent is oxacyclopentane [tetrahydrofuran, (CH2)40] ... 

---