Stereospecific ligand reactions

This chapter, in previous volumes, concentrated on mechanistic studies of the stoichiometric reactions of coordinated a- and tt-hydrocarbons with nucleophiles and electrophiles. In order to provide a more comprehensive overview of the reactivity of coordinated ligands in general, related ligand reactions in classical coordination complexes are now also included. The stereospecificity of such processes and their potential for asymmetric synthesis has continued to attract increasing attention, and it is therefore appropriate to collect them all together in one chapter. There are three subsequent sections. The first is concerned with cobalt(III) complexes, and the second with complexes of other metals. The last section deals with the ligand reactivity of organometallic compounds. 

Heck reactions can be carried out in the absence of phosphine ligands.141 These conditions usually involve Pd(OAc)2 as a catalyst, along with a base and a phase transfer salt such as tetra-n-butylammonium bromide. These conditions were originally applied to stereospecific coupling of vinyl iodides with ethyl acrylate and methyl vinyl ketone. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

Occasionally, however, stereospecific results are encountered in the literature which clearly implicate ligands about the transition metal in steric control. For example, when a typical catalyst system based on WCle was modified by the addition of triphenylphosphine, Dall Asta found (77) that the reaction of c/s-2-pentene led very selectively to the formation of tr n.s-olefinic products. On the other hand, Katz demonstrated (75) that when (CO)5W=C(Ph)2 was used, e/.v-2-pentene afforded butenes and hexenes having about 95% cis structure, and notably that this specificity persisted even for reactions carried to near-equilibrium. 

The reaction is carried out with aryl triflates and other details such as solvent and base used are also important. Intramolecular additions of aryl halides or triflates to alkenes in a side-chain leading to cyclic compounds have been reported by Overman [24], Rather complicated ring structures can be made stereospecifically. While initially BINAP seemed the best ligand for this conversion, the number of useful ligands is increasing [25],... 

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