Associative mode of activation

When an associative mode of activation is indicated it is instructive to examine the ways in which reactivity, as measured by the second-order rate constant k2, depends upon the nature of the nucleophile and if a large number of substrates show the same pattern of preference it is useful to consider scales of nucleophilicity. A systematic study of the substitution reactions of fra/is-[Pt(py)2Cl2] (it appears that pyridine and piperidine were used interchangeably) in methanol at 30 °C (displacement of chloride) led to the establishment of an n scale, nm d= log,o(k2/kI) for the standard reaction,447 and later the more dimensionally correct Up,0 scale448-449 ( p,° = log,0(li2/ki)[MeOH]) so that npt° = npt+1-41 (unfortunately this distinction has not been strictly adhered to in the literature).449 A collection of n 0 values will be found in Table 14. 

Ruthenium(III) complexes are seen in Tables 8.2 and 8.3 to be kinetically comparable to the corresponding chromium(III) species, despite a larger Dq for Rura, because the LFAE factor is lower for low-spin d5 than for d3. For Ru(H20)63+, AVjx indicates a much more associative mode of activation than for Ru(H20)62+, which seems typical of octahedral transition metal complexes... 

This rate law is consistent with an associative mode of activation. Base attack on the metal center leads to the expansion of the coordination number of the metal from 5 to 6. The supposed stronger Tc=0 bond generates larger kinetic trans effects and makes associative attack by CH3O less favorable than the somewhat weaker Re=0 bond. The six-coordinate complexes mentioned above may react by a dissociative activation mode [67]. 

The first possibility, (11), clearly is concerned with a dissociative mode of activation (d). The second, (12), mxghihsassociative led by water attack. But, this interpretation of (12) involves commitment to the proposition that no nucleophile (possibly excepting OH ) has been discovered which is better than water, i.e. the associative attack must always involve water in the first instance. The proposition... 

The results of our recent study of water exchange on iron(III) (8), summarized above, strongly indicate a change in the mode of activation from associative to dissociative on re-... 

NMR has been used to study ligand exchange in non-coordinating solvents for a series of Be(C104)2-4L (L = trimethyl phosphate, DMSO, DMA, DMF, NMA, 1,1,3,3-tetramethylurea, dimethyl methylphosphonate and dimethyl phenylphosphonate) complexes. 3H NMR studies show that the mode of activation for exchange on beryllium varies from dissociative to associative depending upon L and the nature of the non-coordinating solvent.127 128... 

All systems studied in this way until now have a spherical distribution of the non-bonding electrons, so no account need be taken of the symmetry relationships between the ground state and the possible transition states. Consequently, the size of the reaction centre, the coordination number and the effective size and packability of the ligands determine whether the mode of activation is associative or dissociative. There is, as yet, no suitable evidence to decide whether the nucleophilicity of the entering group plays a significant part in deciding the mode of activation. 

Although the evidence for the direct attack on the substrate by the nucleophile is overwhelming it was necessary to examine the ki pathway in depth to decide whether it represented a parallel dissociative process or associative solvolysis. Trapping,417 AF+ measurements446 and the general observation that kj and k2 respond in a similar fashion to electronic and steric variations in the other ligands in the substrate suggest that both pathways possess the same mode of activation. The possibility of dissociative activation in cfl planar complexes is discussed separately below (Section 7.1.7.6) and further consideration of this question is best reserved until then. 

It has always been of some interest to examine the extent to which associative activation dominates the mechanism of substitution of four-coordinate planar cP metal complexes. The coordination unsaturation of these formally 16-electron valence shell complexes predicts that a substitution pathway with increased coordination number (18-electron valence shell) will be favoured over one with a reduced coordination number (14 electrons). This was well understood by workers in the field438 long before Tolman94 published his rules. The first attempt to force a dissociative mechanism was made by Basolo and Baddley513>514 who reasoned that since the steric requirements of associative substitution (rates reduced by steric hindrance from the cis position) were opposite to those of a dissociative mechanism (rates increased or unchanged by increased steric hindrance), sufficient congestion in the substrate should reduce the rate of the associative process to the point where dissociative activation took over. If this did not produce a change in mechanism it could at least indicate a lower limit to the difference of the two modes of activation. 

An alternative generation of a ruthenium catalyst has also emerged which is not based on the benzylidene structural motif. Easily accessible catalyst 8 is typical of a class of cationic catalyst from the groups of Furstner and Dixneuf [17]. This species can promote highly efficient RCM reactions and has the flexibility associated with both thermal and photochemical modes of activation [18]. 

Keeping this limitation in mind, it may be desirable to associate pressure with another mode of activation. This coupling will be called biactivation throughout this chapter. The aim of the chapter is to give the present status of the biactivation process considered as the combination of pressure (physical activation) with (a) chemical activation and (b) physicochemical activation. 

Here we will look at some findings that highhght potential modes of activity as well as problems associated with looking at organotin activity. Further, essentially no mechanistic studies have been carried out on tin polymers, so results from studies of small molecule-containing organotin agents will be examined. 

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