Zirconium complexes alkynes

Alkenyl zirconium complexes derived from alkynes form C—C bonds when added to aHyUc palladium complexes. The stereochemistry differs from that found in reactions of corresponding carbanions with aHyl—Pd in a way that suggests the Cp2ZrRCl alkylates first at Pd, rather than by direct attack on the aUyl group (259). 

In this method, one alkyne is treated with Schwartz s reagent (see 15-17) to produce a vinylic zirconium intermediate. Addition of MeLi or MeMgBr, followed by the second alkyne, gives another intermediate, which, when treated with aqueous acid, gives the diene in moderate-to-good yields. The stereoisomer shown is the one formed in usually close to 100% purity. If the second intermediate is treated with I2 instead of aqueous acid, the 1,4-diiodo-1,3-diene is obtained instead, in comparable yield and isomeric purity. This reaction can also be done intramolecularly Diynes 56 can be cyclized to ( , E) exocychc dienes 57 by treatment with a zirconium complex. 

Stable zirconate complexes of type 31 have also been described by the [3 + 2] cycloaddition of a-phosphino zirconaindene with alkyne derivatives, and the method has been extended to the synthesis of zirconium complexes from various heterocumulenes X=C=Y (X,Y=0,S,CyN) (Scheme 15) [69,70]. 

As Scheme 16 shows, vinyl(methyl)zinc compounds 17 were obtained from the hydrozirconation of terminal alkynes with Cp2ZrHGl, followed by treatment with dimethylzinc. The initially formed vinyl zirconium complex undergoes rapid transmetallation with ZnMe2 to yield the product.44... 

The bis(trimethylphosphino)zirconium complex 92 undergoes unusual reactions with alkynes.114 With diynes, metallocyclic products 93 are formed, whereas acetylene affords 94 whose nonplanar boratanaphthalene ligand is... 

A zirconium complex of tropyne has been prepared by hydride abstraction from a mixture of 231a-c (Scheme 28).87 This compound is extremely thermally unstable, decomposing to a complex mixture of products above —50°C. Identification of both 230 and 232 rests exclusively on low-tempera-ture, multinuclear NMR spectra. Attempts to couple 232 with reagents such as alkenes, alkynes, nitriles, or ketones gave only complex mixtures of products. 

Synthesis, reactions, and physical properties of stable mononuclear platinum and zirconium complexes of cyclohexyne reported prior to the late 1980s have been comprehensively covered in earlier reviews.2-8 More recently, reaction of the zirconocene complex of cyclohexyne with trimethyla-luminum and trimethylgallium has been reported to give 247 and 248, respectively [Eq. (36)].57-58 These products are novel because four atoms (carbons Cl and C3, the transition metal, and the main group metal) are covalently bonded to an sp2 hybridized carbon (C2) in a planar tetracoordi-nate fashion. Synthesis of this type of complex, which Erker describes as anti-van t Hoff/LeBel, does not require the strained cycloalkyne ring zirconocene complexes of acyclic alkynes react similarly.57-58... 

Notably, phospholes with controlled regioselectivity can also be obtained from zirconium complexes using silylated alkynes. For example, sequential treatment of Schwartz reagent 167 with 2-butyne, MeLi, and silylated alkynes... 

Here is an example from the McMurry flexibilene synthesis quoted in chapter 1. An alkyne 127 with a protected aldehyde group reacts with Cp2ZrHCl to give a vinyl zirconium complex 128 which is coupled to a palladium-allyl complex in the next step. The double bond so produced is present with the same E configuration in the final product. It is marked 129 with an arrow in the diagram.28... 

A variety of ansahvxAged complexes such as (143) resembling those of ansa-zirconocenes have also been reported. Moreover, reduction of the zirconium complexes with n-BuLi in the presence of PMea provides an entry point to Zr(II) complexes (144) their reactivity toward alkynes has been studied in detail. ... 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

A variety of chiral, non-racemic zirconium complexes were explored in attempts to develop an enantioselective variant of this reaction (Scheme 3) [7-9]. Eor example, when allyUc amines lla,b were treated with EtMgCl and 10% of C2-symmetric BINOL-zirconium bis(tetrahydroindenyl)ethane (12, Brintzinger s catalyst [10], BINOL is l,l -binaphthalene 2,2 -dioate), chiral ethylated products 13a,b were obtained in 34-39% yield with enantiomeric excesses (ee) of ca. 26% [8]. Use of a (neomenthylindene)ZrCpCl2 catalyst 14, designed to improve the steric differentiation of the diastereomeric transition states, improved the chemical yields of amines at lower catalyst loadings (2-4%) and increased the ees of the reactions by a factor of three in the case of 11a [8,9]. Similar reactivity is observed in zirconocene dichloride-catalyzed cyclization of 1,6- and 1,7-enynes with 12.5% Cp2ZrCl2 using EtsAl as the stoichiometric reductant. For these substrates, the alkyne coordinates... 

The first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

Our own investigations into the chemistry of the heavier group 4 analogs have primarily focused on the hydrazinediido-zirconium complex [Zr(N2 NpP (NNPh2)(py)l (Figure 13.1) [17]. The latter has displayed similar stoichiometric and catalytic reactivity toward unsaturated substrates as found for titanium. However, its reactivity toward alkynes appears to be characterized by the absence of a hydrohydrazination step in favor of N-N bond cleavage before the formation of a hydrazone and a reaction pathway similar to that depicted in Scheme 13.8 [24]. 

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