Negishi’s reagent

The formation of cyclic boriozirconocenes 63 has been observed by treating l-alkynyl-6-alkenyl diboronates 64 with Negishi s reagent (Scheme 7.20) [133]. Hydrolysis of 63 with HC1 in Et20 gave the 1-alkenyl-l-alkyl diboronate 65 in 80% yield. 

Thus, an alternative reaction mechanism must be considered. It is known that dibutylzirconocene, prepared from Cp2ZrCl2 and 2 equiv. of BuLi, gives zirconocene coordinated by a butene ligand (Negishi s reagent, Eq. 15) [23]. 

Scheme 9.57) [118]. Cyclic l-alkenyl-4-alkyl diboronates were available from the monoborylated 1,6-hexa- or heptadiyne and Negishi s reagent, Cp2ZrCl2/n-BuIi [119]. 

In the case of (n-Bu)2Zn as the organometallic reagent (R = L = n-Bu m = 1), the product (n-Bu)CH2SiMe2Ph (5a) was formed in 61% yield. A 1,2-migration mechanism was proposed on the basis of kinetic data and NMR experiments. Following Negishi s pioneering work, many other examples of related reactions were reported, the most important of which are described below. 

The hydrozirconation of alkynes with Cp2ZrHCl (Schwartz s reagent) provides ready access to vinylzirconium reagents, which have been shown to be suitable partners in palladium-catalyzed cross-couplings ( zirconium-Negishi reaction ) with aryl and vinyl electrophiles [1]. Recently, Fu disclosed the first... 

Tetrahedron Lett 30 3927 1989], and has been useful for the synthesis of a wide range of early transition-metal complexes and organometallic compounds [Negishi TskalaasYa Aldrichimica Acta 18 31 1985, see also selected volumes of Fieser Fieser s Reagents for Organic Synthesis],... 

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