Microscopic reversibility substitution

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

The assumption of a definite location for the negative charge on the j8-carbon requires that 59a and 60a both be on the lowest energy path for substitution and isomerization. That this location should be formed anti derives from orbital symmetry, i.e. microscopic reversibility suggests that anti entry of one halide and syw-departure of the other are improbable. We deduce, therefore, that the substitutions in (142) require at least three elementary steps, e.g. cis-1 - 59a -> 60b -v cis-2 isomerizations with exchange require at least three steps, e.g. cis-l 59a —> 59b - trans-2 isomeriza-... 

It might appear that the simplest possible type of system to which ALPH might apply is a system with one lone pair only, and which does not involve a proton transfer. Reactions of a-substituted amines, or their microscopic reverse, nucleophilic additions to iminium ions, thus suggest themselves as suitable testbeds for ALPH, but the information on such systems is sparse. Stevens (1984) has studied the addition of carbon nucleophiles to tetra-hydropyridinium salts from the standpoint of utility in organic synthesis, and found a preference for axial attack, in accord with ALPH (Scheme 4). 

Stereochemistry of CO Dissociation. If a complex has nonequivalent CO groups, the stereochemistry of the dissociative process can be defined. This has prompted many studies of exchange reactions with Mn(CO)5Br. Substitution reactions always lead to m-Mn(CO)4LBr, but substitution products do not necessarily provide information on the site of CO dissociation. In an exchange reaction, the products must reflect the site of dissociation by the principle of microscopic reversibility. One would predict that the site of CO dissociation would be cis to the bromide in Mn(CO)sBr since each cis-CO (equatorial) is trans to CO and should not be as strongly jt bonded as the axial CO, which is trans to Br. However, both the axial and equatorial positions were emiched when reacted withC Oor CO ... 

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