Alkynylmagnesium reagents

The Ni-catalyzed cross-coupling reaction of alkynyl Grignard reagents 60 with ( )- or (Z)-dichloroethene 61a,b has been applied to a simple procedure for the preparation of the protected form 62a,b of a highly unstable synthon, as hexa-3-ene-l, 5-diyne [Eq. (23). Separation of the diastereomers 62a and 62b is facile since the former is an oil and the latter is a solid [35]. 

The Pd-catalyzed cross-coupling reactions of vinyl iodides 63 with alkynylmagnesium halides 64 have been found to produce enynes 65 with retention of configuration ( 97%) [Eq. (24)] [36]. Using this procedure, the thienylacetylene 66, which is an intermediate for the synthesis of a natural antifungal and nematicidal agent can be prepared [Eq. (25)] [37]. 

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Disubstituted 1,2,3-triazoles are formed in 1,3-dipolar cycloaddition of alkynylmagnesium reagents to azides. This reverse regioselectivity is also achieved in ruthenium-catalyzed cycloadditions. Examples of such reactions can be found in Section 5.01.9. 

In addition to their use as precursors for alkynylboron, tin or zinc compounds, alkynylmagnesium reagents show a moderate reactivity toward the coupling with haloarenes and haloalkenes. They are often commercially available, or easy to prepare. Their main drawback is their low chemoselectivity and high nucleophilicity, which implies incompatibilities with functional groups such as nitro and carbonyl. 

An example of a non-palladium-based system was reported by Madec et al. They made use of Ni(PPh3)2Gl2 for the coupling of vinylcarbamates and alkynylmagnesium reagents in benzene at higher temperatures (70 °G), and obtained good yields of product. ... 

Typical catalysts for the couplings of alkynylmagnesium reagents rely on Ni or Pd complexes. Oxidative addition in the standard catalytic cycle is usually a rapid process for comparatively simple Ni/hgand catalyst systems, such as NiClj/PPhj (Scheme 9.7). On the other hand, Pd mostly requires rather expensive bulky ligands to suppress undesired side reactions. 

Figure 9.3 Bisphosphine-ligated Fe(ll) complex 128 catalyzing the cross-coupling of alkyl halides with alkynylmagnesium reagents [348].

As shown in Table 1, a wide variety of metal countercations have been shown to participate in this reaction. Under the same reaction conditions, alkynylmetals containing Zn exhibit the highest reactivity (Entfy 6), which is followed by those of Mg (Entries 4 and 5), Al (Entries 12-14), and Sn (Entries 16 and 17). Although the product yield observed with 1-heptynytmagnesium bromide was modest (49%), alkynylmagnesium reagents have since been shown to be very satisfactory in many cases. ... 

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