Ring closure activation parameters

Figure 27 shows plots of all the available EM s for closures of small- and common-sized saturated carbocycles and heterocycles by intramolecular nucleophilic displacement. Clearly, a-values as small as 0.1 would be required in order to calculate extrathermodynamically from (67) EM-values comparable to those actually observed for ring-sizes 3 and 4, and an even smaller value would be necessary for ring-size 5. This would lead to the conclusion that the effect of ring strain on cyclisation rates is insignificant. The same conclusion was recently drawn by Benedetti and Stirling (1983), based on rates and activation parameters for the cyclisation of bis-sulphonyl-stabilised carbanions to 3-, 4-, and 5-membered bis-sulphonylcyloalkanes. 

A // = 40kJmol-1 AS = —204JK 1mol 1). The activation parameters indicate associative activation for both steps (212). Associative chelate ring closure here is consistent with the mechanistic pattern established in organometallic systems (213), where this process is associative for [M(CO)5(diimine)] where M = Mo or W (though dissociative for M = first-row Cr (214)) and the diimine does not carry bulky substituents. 

Generation of [(i7 -S S)W(CO)4(CB)] by flash photolysis in chlorobenzene (CB) enabled rates of ring closure to be obtained and the activation parameters are shown in Table 10.3. For x = 1 (in dichloroethane) and 2, ring closure is probably associative but for x = 3-5 the values of correspond to those expected for dissociative loss of CB and the values of A5 are probably related to entropy changes imposed by freezing of internal rotations when the transition state is formed. 

Table 10.3. Activation Parameters for Ring Closure in [(j] -DTA)W(CO)4(CB)] in Chlorobenzene [DTA = Me3CS(CH2)xSCMe3]...

It is interesting to note that it is apparently an enthalapy of activation effect that results in the relative slow rate of formation of the azetidine from N-y-bromopropylaniline (see Table 2) since the measured AS values for three- and four-membered ring formation are identical. In each case a high yield of cyclic product is obtained, and hence the activation parameters are for the ring closure reaction. 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

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