Aryne-metal complexes

Two areas of aryne chemistry, aryne-metal complexes and aryne radical ions, have been omitted from this review. 

Although a large number of aryne-metal complexes have been prepared and characterized, and they have an interesting chemistry of their own, that chemistry is in general quite different from the types of reactions usually associated with arynes. One might have hoped that aryne-metal complexes could act as shelf-stable storehouses for arynes, that could then be released on demand to do their usual chemistry (as in the manner, for example, of cyclobutadiene complexes), but to date this has not proved to be the case. The synthesis and chemistry of aryne-metal complexes has been reviewed, and the interested reader is also provided with a few references to work that has appeared since those reviews. ... 

The first mention of the formation of aryne-metal complexes is due to Wittig, who formulated these species as possible intermediates in the formation... 

The intermediacy of aryne-metal complexes in many organometallic reactions has been postulated on the basis of the structures of the reaction products. This mechanistic hypothesis received strong support in 1979 with Schrock and co-workers isolation of complex 8 and its structural characterization by X-ray crystallography [ 10]. Since then, many aryne complexes of early transition metals have been prepared, including complexes of Ti, Zr, Nb, Mo, W, Re, V, and even the actinoid elements U and Th. Figure 3 shows some representative examples of aryne complexes of early transition metals that have been characterized by X-ray diffraction studies. 

Despite all this work, until the end of the 1990s the only aryne-metal complexes for which synthetic applications had been developed were a few zirconium complexes. The principal drawback of this chemistry was the use of stoichiometric amounts of metal complexes and, as a consequence, its poor atom economy (in both ligands and metals) and high cost. 

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... 

Jones, William M., and Klosin, Jerzy, Transition-Metal Complexes of Arynes,... 

Many stable metal complexes of arynes are known but in most of their reactions of synthetic interest, the yields are poor. For example, thermolysis of titanocene (Cp2TiPh2) at 80-100 C gives rise to a tita-nium-benzyne complex which reacts with molecular nitrogen to afford aniline with low efficiency.29 However, procedures are available for in situ generation of zirconium complexes (14) and for their coupling reactions to synthesize functionalized aromatic compounds in preparatively useful yields (Scheme I).30 Whether such complexes should be regarded as ir-bonded benzynes or o-bonded o-phenylenes, remains a debatable point.31... 

Most early transition-metal complexes of arynes have been prepared by thermally induced /3-hydrogen elimination from appropriately substituted aryl sigma complexes [Eq. (I)]2,8... 

The reader is referred to an earlier review2 for a discussion of the IR and NMR spectroscopy of metal complexes of arynes. 

To date, no mononuclear transition metal complexes of three- or four-membered cycloalkynes have been reported. A number of polynuclear complexes of cyclobutyne have been prepared.90 However, since this article is limited to complexes in which the aryne, alkyne, or cumulene is coordinated to a single metal atom, a review of this topic is not included. 

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