Allyl ligands classification

In contrast to soft nucleophiles which attack the allyl face opposite the palladium complex, hard nucleophiles (e.g., organozinc reagents) first coordinate to the metal center and then are transferred intramolecularly to the allyl ligand (see, e.g.. Table 1 in [13]). Therefore, the reaction of allyl-palladium complexes with hard nucleophiles usually involves retention of configuration. However, the classification as soft and hard nucleophiles is not always unambiguous. With acetate as the nucleophile, e.g., the stereoselectivity depends on the reaction conditions and both overall inversion as well as retention have been observed [18]. 

In addition to direct determination of the stereochemistry of the allylation process, alternative means of classification of nucleophiles as to those that proceed by ligand or metal addition have been advanced. In one case the classification is made based on the regiochemistry of addition to a specific unsymmetri-cal allyl complex,383 and in a second by the ability of a particular allyl complex to react only by ligand addition,386 and therefore to be inert to nucleophiles that add via attack at the metal. 

The mass spectra of a considerable number of 77--bonded complexes of the group VIA metals have been reported, but in many cases mass spectrometry has only been used to determine the molecular weight, so that a detailed examination of the fragmentation processes involved has not been attempted, and only the molecular ion and perhaps a few other major peaks are reported. Within this section it is more convenient to discuss the compounds in terms of the attached ligands rather than in terms of the central metal atom. The classifications are (A) cyclopentadienyl compounds (B) arene compounds and (C) olefin, acetylene, and allyl compounds. 

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