Phosphine exchange

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Equations analogous to Eq. (21) will be written showing the metastable isomer as lying on the pathway for phosphine exchange (Scheme 4) and substitution (Scheme 5) in MeReO(mtp)PR3 compounds. 

The third and last example in this chapter illustrates a case where kinetic data were used to derive relative enthalpies for a series of similar reactions. Consider the ligand (phosphine) exchange reaction 15.15 (see figure 14.5 forthe structure of the complex),... 

Using 31P NMR as analytical technique, Bryndza et al. investigated the kinetics of this reaction in benzene for a variety of ligands X (see figure 15.5) [326,327]. Their experiments demonstrated that the phosphine exchange proceeds by a dissociative pathway, that is,... 

H. E. Bryndza, P. J. Domaille, R. A. Paciello, J. E. Bercaw. Kinetics and Mechanism of Phosphine Exchange for Rutheniumill) Complexes in the Series (r 5-CsMes)(PMeo,)2RuX. Ancillary Ligand Effects on Dative Ligand Dissociation. Organometallics 1989, 8, 379-385. 

However, the reaction of RuCl2(PPh3)4 with only HC = CCPh20H directly led to the indenylidene complex VIII, which gave, after phosphine exchange with PCy3, the indenylidene complex IX [42]. 

Most successful of the diphosphine derivatives (e.g. 27, 28) are the cationic, edge-bridged dimers of type 28 developed by the Hofmann group, and described in a recent review.Routes to the neutral diphosphine complexes commonly utilize 22b as precursor (Scheme 4) 134-136 commercial availability, and consequent synthetic convenience, counterbalancing its slower rate of phosphine exchange, relative to 22a or 2 Vinylalkylidene... 

It was shown (17) that phosphine exchange occurred in [AuMe(PR3)l complexes, but species with a coordination number greater than two were not observed, even with bidentate phosphorus ligands. Although such species were not observed, an associative mechanism was thought to be operative (16, 18). With trimethylphosphite also, only the two-coordinate complex [AuMe P(OMe)3 l was detected in solution (19). 

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