Metal Complexes of Diazonium and Diazo Compounds

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

Metal — dinitrogen complexes are discussed briefly in this book, because they are structurally related to diazo and diazonium coupounds. In these complexes a diazo group is bonded to a metal and not to a C-atom, as in organic compounds. Addition products of aliphatic and aromatic diazo compounds will be reviewed in Chapter 10. 

This chapter demonstrates that this statement is applicable to diazo compounds. Therefore, the reader should not be surprised to find here two sections on diazo and diazonium compounds as ligands in metal complexes and, in the volume on aromatic diazo chemistry (Chapt. 11), a discussion of host-guest complexation chemistry. 

It is generally accepted that a typical carbonyl yhde reaction proceeds as shown in Fig. 2. Interaction of diazo compound 1 with the metal forms diazonium complex 2, which then extrudes nitrogen forming carbenoid intermediate 3. Reaction of 3 with the carbonyl group present in the substrate forms intramolecular carbonyl yhde 4 (or an intermolecular carbonyl yhde) in which the metal catalyst may or may not remain associated with the ylide [13]. Finally, the [3+2]-cycloaddition and regeneration of the active cat-... 

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