Alkynylmetal reagent

Alkynylmetal reagents, in i/>-hydridized carbon polymerization, 11, 673 Alkynyl(methoxy)borates, preparation, 9, 212 Alkynyl oligosilanes, preparation, 3, 433 Alkynylsilanes... 

One characteristic feature of the MPV alkynylation is the chemoselective transfer of functionalized alkynyl groups to aldehyde carbonyls. Reaction of chloral with functionalized A1 reagent 28 in CH2CI2 proceeds nicely at room temperature to furnish alcohol 29 in good yield, leaving the keto functionality intact (Sch. 13) [34]. Such transformation is not easily realized by use of ordinary alkynylation procedures because of the difficulty of generating functionalized alkynylmetal reagents. 

Cross-coupling reactions of alkynylmetal reagents with sp carbon atoms.. 203... 

Cross-coupling Reactions of Alkynylmetal Reagents with sp Carbon Atoms... 

Allylic organozinc reagents display a unique reactivity towards alkenyl-, allenyl- and alkynylmetals. Allenic organozinc reagents which are in metallotropic equilibrium with their propargylic counterparts (equation 150) also share some similarities with allylic organozinc reagents in their behavior towards alkenes and alkynes. 

Alkynylation of aldehydes.3 The reaction of alkynylmetals with chiral aldehydes shows no or slight diastereoselectivity, but in the presence of TiCl4, alkyn-yltributyltin reagents react with the steroid aldehyde 1 with about 9 1 diastereoselectivity. TiCl4-catalyzed reaction of 1 with allyltrimethylsilane or allyltributyltin also shows similar and comparable diastereoselectivity. 

Negishi and co-workers also reported in 1978 Pd-catalyzed alkynylation reactions with alkynylmetals containing Mg, B, A1 and Sn. But B and Sn versions have sometimes been known as the Suzuki and Stille alkynylation reactions, respectively. Negishi and co-workers during 1977-1978 reported the Pd-catalyzed alkynylation reactions of alkynylzinc reagents with 1-halo-l-alkynes and aryl halides. 

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 (Entry 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-heptynylmagnesium bromide was modest (49%), alkynytmagnesium reagents have since been shown to be very satisfactory in many cases. ... 

Pd-catalyzed alkyne cross-coupling has become one of the most satisfactory methods for the synthesis of alkynes. As discussed in Sect. HI.2.8, there are two related but discrete protocols, that is, Sonogashira coupling, which does not involve formation of alkynylmetals as preformed and discrete reagents (Sect. HI.2.8.1), and the general... 

In cases where alkynylmetals are generated either in situ or in a separate step as discrete reagents. Mg, Zn, B, and Sn have been the four widely used metals. Comparative studies have shown that Zn is generally the most favorable among them (Sect. III.2.8.2). Here again, however, Sn has been the most frequently used metal. Similar comments on Sn as those made in Sect. D are also applicable to these cases. Particularly noteworthy are examples of the intramolecnlar cyclization of alkynyltins, as in the synthesis of neocarzinostatin. cyclization via double alkynylation with haloalkynes, as in the synthesis of calicheamicinone and dynemicin and... 

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