Mechanistic changeover

A similar mechanistic changeover is observed, though considerably sooner, in traversing the series ... 

If a NH3 molecule was included in the exchange process, we obtained an interchange type of mechanism. Therefore, we conclude that the larger molar volume of ammonia compared to water is responsible for the mechanistic changeover from an associative (A) to an associative interchange (Ia) type of mechanism. 

An interesting, pH-dependent mechanistic changeover was reported in the H2A-02-[Fem(TPPS)] (TPPS = 5,10,15,20-tetrakis(p-sulfonatophe-nyl)porphyrinate) system in aqueous solution (23). This water-soluble metalloporphyrin exists as a monomer under slightly acidic conditions and reacts with ascorbic acid on the time-scale of several hours to produce [Fen(TPPS)] in the absence of dioxygen. The formation of the [Fem(TPPS +)] radical was also reported at pH 5, but it is not clear how... 

Although only four examples of trigonal bipyramidal copper(II) complexes have been studied in the literature so far, these investigations have shown that substitution generally occurs via an 7a mechanism with a six-coordinate transition state. In the case of the Me6tren complex, the steric hindrance prevents the formation of a six-coordinate transition state. The consequence is a dramatic reduction in la-mechanistic changeover toward an 7d mechanism. It... 

Besides having smaller oxidation potential values than substituted benzyl alcohols (E° > 1.4 V/NHE), the DMAs have larger energy values (90-92 kcalmoD ) for the NC—H bond with respect to C—H bond energies around 75-85 kcalmoD of the benzyl alcohols (Scheme 12). Both factors disfavour the operation of the radical HAT route for PINO with the DMAs, and cause a mechanistic changeover to the ET route, as opposed to the reactions with the benzylic substrates listed in Table 4. 

The LFMM reaction barriers correlate very well with experiment and even predict a mechanistic changeover from associative interchange for Mn(II) to dissociative interchange for Fe(II) and Co(II) in agreement with the change in signs of the volumes of activation (Fig. 33). 

In general pyridines with pK.A > 1 nitrate as conjugate acids at the a- or (3-position depending on the orientating effect of the attached substituents, while derivatives with pKa < — 2.5 nitrate as free bases. Those pyridines with intermediate pKa values often show a mechanistic changeover, with change in pH (H0). 

If sufficient rate data are unavailable for acidities below H0 -8.5, extrapolated rate coefficients may be obtained from the high-acidity region of the rate profiles by a different procedure. This latter may also be used for the extrapolation of data for compounds which show a mechanistic changeover [75JCS(P2)1600]. 

Arylalkene bromination is a typical electrophilic addition to form an a-carbocation, but markedly non-linear structure-reactivity relationships were observed for brominations of styrene [32], trans-stilbene [37] and a-methylstilbene [38] (Ruasse and Dubois, 1972, 1974 Ruasse et al., 1978 Ruasse and Argile, 1983). Some of these curvatures could not be interpreted directly by the Y-T equation but some were related to a mechanistic changeover based on a multipathway scheme (Ruasse, 1990). Three pathways leading to the and carbocations and to the bromonium ion in the bromination of trans-stilbene (Ruasse and Dubois, 1972, 1974) are shown in Scheme 10. 

Ru-O distances which are 212 pm in both complexes. For [Cp M(H20)3] (M = Rh, Ir) there is an even more dramatic increase of the water exchange rate constant (14 orders of magnitude) when compared with the corresponding hexaaqua-ion [M(H20)6p. This increase in reactivity is also related to a mechanistic changeover from an mechanism towards a more dissociative activation mode. In both cases the rhodium compound is significantly more reactive than the iridium one. 

Thus, these studies of 1-arylethyl systems provide excellent insight into the behavior of reactive systems where mechanistic changeover occurs. 

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