Association nucleophilic path

So as to determine the optimum path of approach of a nucleophile to the nitroso group, model reactions of the associative nucleophilic substitution in the molecules of nitroxyl and nitrosyl flouride were studied by the MINDO/3 method [163] ... 

The k2 term suggests a simple bimolecular process in which nucleophilic attack by Y leads to a SN2 reaction. Associative paths will involve a 5-coordinate (sp or tbp) intermediate, and the relative rarity of isolable 5-coordinate plati-num(II) species - compared with 4-coordinate - is not inconsistent with their involvement as reactive intermediates (Figure 3.81). 

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. 

As shown in equation 90, the ionic intermediate can follow several reaction routes. The product distribution is therefore controlled not only by the nature of the intermediate, whether bridged or weakly bridged, but also by association with its nucleophilic partner and by the rate ratios derived from the different reaction paths. All these factors depend on the alkene structure, the electrophile and the reaction conditions (solvent, added salts, temperature). 

The replacement of X by pyridine in the complex PtX(dien)+ to give Pt(dien)py2+ (equation 549) has been studied under controlled conditions with a range of leaving groups X. These data are shown in Table is.1 87-1988 From these data the leaving group order is NOj > H20 > Cl- > Br > I- > N3 > SCN- > NO2 > CN-. Reactions such as these must be carried out under thermal conditions for accurate comparison since photoaquation can occur, albeit with a rather low quantum yield.1989 The volumes of activation of these reactions (equation 550) are all negative. An associative mechanism is proposed for the nucleophilic dependent path, but for the nucleophile independent pathway both associative and dissociative mechanisms need to be considered.1990... 

There is no doubt that the solvent plays an important role in process (1). It seems to be accepted that activation energy for charge separation should be reduced with increasing polarity Various nucleophiles, e.g. SCN , I, MeO, amines, etc., do, in fact, react faster in polar than in non-polar solvents Eliminations, the microscopic reverse of additions, take the syn path more often in non-polar or associating solvents . All of this is supportive of the idea that polar solvents favour anti attack, other things being equal. 

Figure 46 Representative reaction path potentials for Cl + CH3CI -> CICH3 + Cl and Cl + CH3Br —f CICH3 + Br nucleophilic substitution. Reproduced, with permission of the American Association for the Advancement of Science, from Ref. 362.

The earliest claims centered on complexes of the types [PdX(Et4-dien)F+ and [PtX(dien)]. The anomalous reactivity of certain nucleophiles, including hydroxide, and the observation of a two-term rate law for anation reactions (X = H2O) led to the postulate that when associative ligand replacement was very slow, then a dissociative reaction path could be seen to compete. Because of this, these sterically hindered molecules were often termed pseudooctahedral. This early evidence has been reviewed (6, 12), and it is now known that these apparent anomalies can be interpreted instead in terms of the usual associative pathways. For example, the original detection of a two-term rate law for the anation of [Pt(OH2)(dien)] is probably a consequence of examining the reaction as it approached equilibrium (142). 

The loose TS on the SnVct path is associated with a large energy of deformation (A energy required to deform the reactant, vinyl chloride, to its geometry in the TS without interaction with the nucleophile). This should lead to a high activation barrier. The difference in the A def values for the two routes (Table 3, last column) must be closely related to the relative ease of the a and tt attack. The... 

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