Zirconacyclopentadiene complex

Figure 7.8. Crystal structure of the novel zirconacyclopentadiene complex, 8,8-bis(cyclopentadienyl)-7,9-bis-(trimethylstannyl)-8-zirconabicyclo[4.3.0]nona-l (9),6(7)-die ne 109. Adapted by the authors.

The reactions between the zirconacyclopentadienes Cp2 CR=CMe-CMe=CR (R = Me, SiMes) and MYX2 (M = Sb, Y = Ph M = Bi, Y = Ph, Br) afford various pnictoles and bismoles YM CR=CMe-CMe=CR 0 xhe reaction of Cp2ZrCl2 with bis(SiMc3) a,(0-diynes in which hetero groups are located between the alkyne units results in metallacyclic zirconacyclopentadiene complexes. ... 

Fig. 17. Molecular structure of a zirconacyclopentadiene complex (hydrogen atoms omitted, methyl...

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

When an alkynylsilyl group is present at the a-position of zirconacycles such as zircona-cyclopropene, zirconacyclopentene, or zirconacyclopentadiene, an unusual rearrangement proceeds to give zirconacyclosilacyclobutene derivatives 132, as shown in Eq. 2.78 [57]. The zirconacyclosilacyclobutene fused complex 132 and the zirconacydohexadiene silacy-clobutene fused complex 137 have been characterized by X-ray analysis. 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

An easier route to the same unsymmetric zirconacyclopentadiene 20 is the use of Cp2ZrEt2 as reagent (easily prepared from Cp2ZrCl2 with two equivalents of EtMgBr). This reagent (Scheme 9), equivalent to a zirconocene-ethene complex 24, reacts similarly to the Negishi-type reagent, which was prepared in the... 

JCD3087>. Another zirconacyclopentadiene 180 (R = R = H, R = R = SiMe3) follows from the same starting zirconium(iv) complex and trimethylsilylacetylene in the presence of sodium amalgam. 

Phospholyl anions are also isoelectronic to Cp ligands, and these species readily complex Zr. A reduction reaction of (C4Me4P)2ZrCl2 by magnesium in THF gives transient diphosphazirconocene species which can be trapped by CO, bis trimethylsilyl acetylene, or 2-butyne to yield the corresponding dicarbonyl (67), zirconacyclopropene (68), and zirconacyclopentadiene (69). ... 

Transmetalation to Cu, Ni, or Zn is sometimes necessary to release the organic compounds. For instance, treatment of zirconacyclopentadiene with CuCl generates an unstable dicopper complex, which can couple in situ with allyl, benzyl, aryl, thienyl, alkynyl, and alkenyl halides (Scheme 32). ... 

Rearrangement processes of alkyltitanocene dichlorides that occur under electron impact have been investigated using deuterium labelling. A novel type of zirconium-mediated coupling reaction of alkynes with vinyl bromide to afford 2,3-disubstituted dienes has been reported (see Scheme 105), and an inter-intramolecular reaction sequence has been proposed for the observed formation of vinylcyclohexadienes and/or methylenecycloheptadienes from the copper-catalysed reaction of zirconacyclo-pentadienes with allylic dichlorides. The essential step in these processes appears to be transmetallation of the zirconium-carbon bond of the zirconacyclopentadiene to produce a more reactive copper-carbon bond. New phosphorus heterocycles, e.g. (417), have been constructed by the thermal rearrangement of a [l,4-bis(trimethylsilyl)->/ -cyclooctatetraene]- ,3,5-triphospha-7-hafhanorbomadiene complex (416). 

Reactivity studies reveal that the bis(indenyl)zirconium sandwich complexes serve as isolable, modular precursors to the rich chemistry of divalent zirconocene. Addition of 1 atm of carbon monoxide generates the bis(indenyl)-zirconocene dicarbonyl complex, 307, while treatment with N,N-dimethylaminopyridine results in C-H activation to form the zirconocene pyridyl hydride complex 308. Crystallographic characterization of both complexes as well as multinuclear NMR spectroscopy establishes that the more familiar 77s, 77s hapticity for the indenyl ligands has been restored. Alkyne coupling reactions are also observed as addition of 2-butyne to 302 allows observation of the alkyne complex, 208, and ultimately the zirconacyclopentadiene 309 (Scheme 50).104... 

Transition metal mediated or catalyzed benzene formation reactions have been reported using various metals. However, the use of three different alkynes is difficult [38], In many cases, a mixture of several benzene derivatives is obtained. In 1974, Wakatsuki and Yamazaki used three different alkynes with Co complexes [27b], but isomers were formed and a tedious chromatographic separation was necessary. The first selective coupling of three different alkynes in high yields was reported in 1995 using a combination of unsymmetrical zirconacyclopentadienes and CuCl, as shown in Eq. 2.52 [7k]. 

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