Benzyne-nickel complexes

A more sophisticated procedure has recently allowed the preparation of palladium complex 34, based on an intramolecular Suzuki coupling. The key intermediate 39 is generated by oxidative addition of palladium(O) to 38, and treatment of 39 with base promotes a Suzuki-type intramolecular reaction leading to 34 (Scheme 7) [26]. The same procedure is applicable to the synthesis of benzyne-nickel complexes with a variety of ligands (PPh3, PCy3, PEt3) [26]. 

A mononuclear nickel complex, which was prepared in very low yields by trapping free benzyne, was also observed spectroscopically.2... 

OXIMES AND SEMICARBAZONES Ceric ammonium nitrate. CLEMMENSEN REDUCTION Zinc dust. COMPLEX OF BENZYNE. Nickel carbonyl. n-COMPLEXES Diiron nonacarbonyl. CONDENSING AGENT Calcium carbide. CONJUGATE ADDITION Dimethylcop-perlithium. Nickel carbonyl. Tetrakisjiodo-(tri-ff-butylphosphine)copper(I)]. CONVERSION OF PHENOLS TO ARYL BROMIDES Triphenylphosphine dibromide. 

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... 

Nickel complexes of benzynes and other strained alkynes ... 

With the exception of a nickel -benzyne complex (13) [Eq. (5)],32,33 which is probably formed by trapping the free intermediate with the coordinatively unsaturated metal,34 all complexes of nickel and platinum have been prepared by reduction of preformed (2-haloaryl)halometal complexes, which in turn have been synthesized by oxidative addition of M(0) to o-diha-loarenes.2 35-37 Typical of this procedure is preparation of the bis(triethyl-phosphine)nickel-benzyne complexes 19 shown in Eq. (6). 

New nickel-benzyne complexes (143-147) have been prepared by reaction of o-dihaloarenes with Ni(COD)2 in the presence of a trialkylphosphine followed by reduction of the oxidative addition product with either Li or 1% Na/Hg in ether [e.g., Eq. (23)]. The oxidative addition reaction depends on the nature of substituents on the arene and fails to occur when strong electron-donating groups are present. Based on NMR and mass spectrometry (MS) data, the new complexes were formulated as monomeric. It had been... 

A. A true benzyne complex of nickel has been reported (D. XXVII) but as yet no crystal structure determination has been carried out, 26). 

The quaternization of (iS)-( - )-benzylmethylphenylphosphine with aryl bromides, by the complex salt method using nickel bromide, proceeds with predominant retention of configuration at phosphorus. Optically active phosphonium salts are also obtained by quaternization of optically active triarylphosphines with benzynes. ... 

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... 

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. 

The symmetrically-substituted 1,3-diyne 228a reacts with nickel(0)-benzyne complexes in the presence of an excess of triethylphosphine with regioselective formation of the 2,3-dialkynylnaphthalene 229 (Equation 12.56) [123]. The high... 

In view of the analogy between nickel-benzyne and nickel-carboryne complexes, we envisage that they may share chemical properties in reaction with unsaturated substrates. In fact, in situ generated ( j -C2BjoHio)Ni(PPh3)2 (1)... 

Attempts to prepare benzyne complexes by the reactions of metal complexes with established benzyne precursors have been uniformly unsuccessful. In some cases, complexes derived from only partial decomposition of the precursors have been obtained (Cook and Jauhal, 1968). An earlier claim of the preparation of a nickel-benzyne complex by the reaction of Ni(CO)4 and diiodobenzene (Gowling et al, 1968) has been shown more recently to be erroneous (Bailey et al., 1971). The reactions between CgFsMgBr and several metal carbonyls have been studied briefly (Roe and Massey, 1970). In the reaction with Co2(CO)g, a complex of stoichiometry (alkyne)Co4(CO)io was obtained which underwent thermolysis to give perfluorofluorenone. 

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