Aryne-nickel complexes

Aryne-nickel complexes, which were carefully studied by Bennett [6, 7], show a different reactivity, since following the insertion of a first unsaturated species, the metallacycles so formed usually undergo a second insertion and subsequent reductive elimination (Scheme 9). Thus, complex 44 undergoes the insertion of two molecules of 3-hexyne to afford 43 in good yield, and double insertion of the asymmetric alkyne t-butylacetylene into complex 44 yields naphthalene 45 with a high regioselectivity attributed to steric factors. Interestingly, the reaction of 44 with the more electron-deficient alkyne hexafluoro-... 

It is believed that such reactions proceed through aryne intermediates. These aryne intermediates have been confirmed by reactions of an isotopically labeled chlorobenzene with potassium amide in liquid ammonia1011. Additionally, aryne intermediates have been observed in flash-photolysis experiments and in mass spectrometry12 and trapped as a stable nickel complex (Figure 1), which was characterized by 111 NMR spectroscopy13. 

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]. 

In 1998, Guitian, Perez and co-workers reported that the cyclotrimerization of benzyne (1) to triphenylene (56) is efficiently catalyzed by palladium(O) complexes (Scheme 11) [39]. This was the first time an aryne had been the substrate in a metal-catalyzed process. The success of the reaction was based on a judicious choice of the reaction conditions, particularly with regard to the catalytic system and the method of generating the aryne. Some significant results of this seminal study are shown in Table 1. After some preliminary experiments in which nickel complexes were used as catalysts (entry 1), it was found that nucleophilic palla-dium(O) complexes efficiently promote the desired [2+2+2] cycloaddition of the strongly electrophilic substrate 1 (entries 2-6) [40]. At the same time, a systematic study of benzyne generation conditions showed that Kobayashi s method... 

The triple bond in ortHo-benzyne can be stabilized by complexation with transition metals. Aryne-metal complexes were originally proposed as intermediates in the decomposition of various aryl derivatives of early transition metals, and the first fuUy characterized mononuclear ortho-benzyne complex, TaMe2(q -C5Me5) (q -CjH4), was prepared. Although this method does not appear general for all transition metals, various complexes of zirconium, rhenium, and niobium have been characterized. More recently, complexes of nickel and platinum have also been... 

A. A. Bennett, M. R. Kopp, E. Wenger, A. C. Willis, Generation of Nickel(0)-Aryne and Nickel(II)-Biphenyldiyl Complexes via in situ Dehydrohalo-genation of Arenas. Molecular Structures of [Ni(2,2 -C6H4C6H4)dcpe)] and C2-Hexabenzotriphenylene, J. Organo-met. Chem. 2003, 667, 8-15. 

Although details of the metal-mediated cycloaddition chemistry of arynes are well known [110], the use of arynes was for a long period of time restricted to stoichiometric reactions. An excellent example of this is the [2-I-2-I-2] cycloaddition reaction developed by Bennett and Wenger [111], in which one of the participating triple bonds is a nickel-benzyne complex that reacts with 2 equiv. of an alkyne to generate substituted naphthalenes (Equation 12.50). The development of new methods of generating arynes under mild conditions has led to the exploitation of arynes in metal-catalyzed cycloaddition reactions. 

It has been shown that carbon monoxide can insert into a nickel-aryne bond in a stoichiometric manner [134]. The catalytic carbonylation reaction of arynes induced by transition metals was first reported in 2001 (Equation 12.68) [135], and cobalt carbonyl complexes have since been found to be the catalysts of choice in this transformation. For example, anthraquinone was obtained in 82% yield in the presence of 2mol% of Co4(CO)i2. 

Nickel(0)-aryne complexes readily undergo insertion reactions with unsaturated substrates such as CO, CO2, olefins, and alkynes. " The latter often undergo double insertions. For instance, the complex Ni(r7 -C6H4)(PEt3)2 undergoes double insertion of alkynes to form substituted naphthalenes (Scheme Z) while double insertion of thioalkynes gives various 2,3-thioether naphthalenes, as shown in Scheme 13. A recent report has also demonstrated the application of this insertion reaction to various diynes such as dialkynyl naphthalenes. The factors that determine the regiochemistry of these insertions have been reviewed. 

In 2006, it was found that [NiBr2(PEt3)j can act as catalyst for the transformation of a nickel aryne compound into the dinickel(I) biarylyl 44 in the presence of sodium amalgam (Scheme 10.17) [20]. A detailed study revealed that [NiBr2(PEt3)j was first reduced into [Ni(0)(PEt3)2], which subsequently formed a dinickel aryne complex and acted as the true catalyst. After oxidative... 

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