Other Leaving Groups

Interest is increasing in aquations in which the leaving group is uncharged. This has been mentioned earlier in this chapter in connection with cobalt(m) complexes and dimethyl sulphoxide as a leaving group. The aquation of 

This is a common form of rate law for aquations of aquo-chromium(ra) complexes. Activation enthalpies are greater than 30kcalmol- for both aquation routes for this 3-picoline complex.  

The kinetics of aquation of the [Cr(OH2)5(Ns)] + cation over the range 10 M [HC104] IM have previously been examined. Now the kinetic pattern over the range 1M [HC104] IIM has been established, both to try to find the p of the co-ordinated azido-ligand in this complex and to see if a correlation of reactivity with Hammett values exists. The rate law in these relatively strong perchloric acid media is 

Multidentate Leaving Groups.— Three previously published papers have reported kinetic parameters for the aquation of the cw-[Cr(mal)a(OH2)2] anion. They agreed tolerably well on the activation enthalpy for this reaction, but reported very different rate constants. Now a fourth paper on the kinetics of this aquation, which follows the rate law 

Composite activation parameters are reported, and a similar aquation mechanism to that for cw-[Cr(mal)2(OH2)2] is suggested.  

One extreme in a displacement reaction would be where the leaving group has already departed, and a preformed stable carbocation reacts with the nucleophile. In this event, the cross-coupling reaction might be either direct polar combination of the carbocation with the Grignard reagent or electron transfer to the carbocation followed h radical recombination. 

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It should be mentioned here that if no other leaving group is present, sulfonyl can act as its own leaving group in hydroxide- or alkoxide-catalyzed elimination from sulfones. Carbanion formation is not involved in this but the promotion of the ionization of a C—H bond by the sulfonyl group is seen at the /1-carbon rather than the a-carbon, e.g. equation 21. 

RH ->RHt), or the detachment of other leaving groups or even more complex molecular motion... 

The leaving group is most often a proton. This is a simple acid-base reaction, and a base is required to remove the proton. However, other leaving groups are known (see Chapter 12) ... 

Alkyl halides, treated with thioethers, give sulfonium salts. Other leaving groups have also been used for this purpose. ... 

Other leaving groups are sometimes used. Sulfates, sulfonates, and epoxides give the expected products. Acetals can behave as substrates, one OR group being replaced by ZCHZ in a reaction similar to 10-101. Ortho esters behave similarly, but the product loses R OH to give an enol ether. ... 

The reaction has also been applied to compounds with other leaving groups. Diazo ketones, diazo esters, diazo nitriles, and diazo aldehydes react with trialkylboranes in a similar manner, for example. 

The same products are obtained (though in different proportions) when Na or K is omitted but the solution is irradiated with near-UV light.In either case other leaving groups can be used instead of halogens (e.g., NR3, SAr) and the mechanism is the SrnI mechanism. Iron(II) salts have also been used to initiate this reaction. The reaction can also take place without an added initiator Enolate ions of ketones react with Phi in the dark. " In this case, it has been suggested that initiation... 

For examples with other leaving groups, see Sicher, J. Jan, G. Schlosser, M. Angew. Chem. Int. Ed. Engl., 1971, 10, 926 Zavada, J, Pankova, M. Collect. Czech. Chem. Commun., 1980, 45, 2171 and cited references. 

Many other leaving groups are known, however, e.g. SR2, S02R, 0S02Ar, etc. 1,2-Eliminations are, of course, the major route to alkenes. 

By far the most common leaving group is X = H where, as above, it is a proton that is removed, (1)—> (2), though other leaving groups are also known, e.g. C02 from the decarboxylation (p. 285) of RC02e... 

Fluorine and chlorine are the only important choices for the labile substituents on a heterocyclic reactive system but numerous other leaving groups have been patented, mostly as substituent on an s-triazine ring. These include sulpho, cyano, thiocyanato, azido and trichloromethyl [26], as well as more elaborate groupings (Figure 7.2). Such derivatives are... 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

Substitution of scheme a forms a well-documented class of reactions (see Costentin et al. 1999,2000, Costentin and Saveant 2000, Corsico and Rossi 2000, 2002, 2004, Adcock et al. 2001, Vanelle and Crozet 2002, Medebielle et al. 2002, Galli and Rappoport 2003, Rossi et al. 2003, Vanelle et al. 2004, and Bnden et al. 2005 and references therein). In contrast to conventional nucleophilic substitution, the nncleophile, Nu-, reacts not with the substrate, RX, to give a product but with the radical R. The latter emerges as a result of R-X bond cleavage. Snbstitnent X is very often a halogen atom, bnt other leaving groups can also be used (see section 7.8.1). In the majority of aromatic Sr I reactions, the anion-radical RX- (R=Ar) is the observable intermediate. It is depicted in scheme a. With aliphatic snbstrates, snbstitntion takes place rather than 8 2 or S l substitutions, and the concerted mechanism depicted in scheme b is feasible. 

Of the halogens, the fluoro-substituent is a favourite in photosubstitution (c/. Section 2). It shows a curious selectivity in that as a rule it is not easily substituted by cyanide ion, a nucleophile that functions so well with other leaving groups. 

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