Zinc compounds alkynylation

A synthesis of dienediynes from 2-(hydroxymethylene)cyclopentanone involves the di-enol ditriflate [bis(trifluoromethanesulphonate)] 120, which reacts with an alkyne in the presence of a palladium catalyst to give 121, accompanied by only small amounts of the regioisomer 122. The major product on treatment with the same or different alkyne yields the dienediyne 123 2-(Alkyn-l-yl)-l-methylpyrrolidines 125 (R = Me, pentyl, MesSi or Ph) have been obtained from the thiolactam 124 by sequential alkylation, treatment with a lithium acetylide and reduction with LAH. The phenylthio group in a-(phenylthio)-lactams is displaced by an alkynyl group by the action of an alkyn-l-ylzinc chloride or of di(alkyn-l-yl)zinc compounds. Thus the j -lactam 126 gave 127 and the azaprostacyclin II 128 gave 129 1... 

In an early example of this reaction aryl and benzyl zinc halides were coupled with aryl halides (equation 40) (102). Alkynyl zinc compounds cross-couple with aryl halides (iodides or bromides) in the presence of a Pd catalyst (equation 41) (103) or alkynyl iodides can be coupled with aryl and alkenyl zinc compounds (equation 42) (100). 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

The hydroboration of allylic silanes proceeds with high diastereoselectivity as demonstrated by Fleming and Lawrence.87 It is difficult to use the newly formed carbon-boron bond for making new carbon-carbon bonds due to its moderate reactivity. However, the B/Zn exchange converts the unreactive carbon-boron bond to a reactive carbon-zinc bond, as in compound 24. A further transmetallation with the THF soluble salt CuCN-2LiCl provides copper reagents, which can be allylated, alkynylated, or acylated (Scheme 6). 

Alkynylation of zinc-copper compounds has been used for the synthesis of polyfunctional acetylenic ethers [84] and for the preparation of building blocks for pharmaceutically active compounds [85]. Whereas cross-coupling between non-activated iodoalkenes and zinc-copper reagents only proceeds at elevated temperatures and in polar solvents such as NMP or DMPU (60 12 h) [86], alkenyl... 

Vinyllead and alkynyllead tricarboxylates can only be prepared by metal-metal exchange, such as mercury-lead,ll6 ll7 tin-lead,ll6,ll7 boron-lead i or zinc-lead.l In the case of tin-lead exchange, cleavage of the methyl-tin bond is much slower than cleavage of the vinyl-tin or alkynyl-tin bond. Therefore (trimethyl)vinyltin derivatives are preferred to the corresponding (tributyl)vinyltin analogues for the synthesis of the unstable vinyllead and alkynyllead compounds. 

The often difficult conjugate addition of alkynyl organometallic reagents to enones is greatly facilitated by TMSOTf. In particular, alkynyl zinc reagents (normally unreactive with a, 8-unsaturated carbonyl compounds) add in good yield (eq 22). The proportion of 1,4-addition depends on the substitution pattern of the substrate. 

T -Complexes contain a metal-carbon single bond (Figure 1.1). The organic group may be alkyl, vinyl, alkynyl, aryl or acyl. With the exception of the acyl complexes, there are analogous compounds of more familiar metals, such as magnesium and zinc. It is also possible to have complexes with metal-carbon double and triple bonds these are known as carbenes and carbines. Cumulenes are also known, such as in vinylidene complexes. 

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