Substitution mechanisms and pressure effect

General Discussion—Pressure Effects and Substitution Mechanisms... 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

The pressure dependence for the substitution of pyridine and substituted pyridine is reported in Figure 23, from which it follows that the larger effects observed for the 4-acetyl- and 4-cyanopyridine complexes are consistent with the volume difference expected between the LF and MLCT excited states. The positive volumes of activation support the operation of a dissociative D substitution mechanism as a result of LF excitation [101], The photosubstitution reactions of complexes of the type M(CO)4phen (M = Cr, Mo, W) have received attention from various groups, mainly because of possible roles played by both LF and MLCT excited states in such processes (Fig. 24). For the overall reaction given in Eq. (30),... 

A volume expansion is expected for homolytic bond dissociations as already pointed out in the Introduction. This expectation has been confirmed for several homolytic bond cleavages showing positive activation volumes near AV - +10 cm mol [123]. The analysis of the pressure effect on the cleavage of azo compounds is however, complicated by the possibility of one- and two-bond scission processes [124]. The benzylic and benzhydrylic 1,4-shifts in the substituted pyr-idiminiumoxides (Scheme 2.29, entry (1)) [125] illustrate the utility of high pressure for the distinction between a pericyclic and dissociative mechanism comparable to the rearrangement of l,3,4,6-tetraphenyl-l,5-hexadiene which has already... 

Croup VI Carbonyl complexes In the case of substitution of neutral ligands by neutral ligands, pressure effects can be better correlated with the intrinsic volume changes associated with the mechanism. One such study dealt with the photosubstitution reactions of the hexacarbonyls M(CO)6 (M = Cr, Mo, W) to give M(CO)sL (Eq. 6.22) and M(CO)4L2, where L is a ligand such as pyridine [61]. For each M, Oco decreased with increasing pressure. Under the risky assumption that kn is independent of P, the pressure dependence of laser flash photolysis techniques have shown that CO loss to form the 5-coordinate intermediate M(CO)s occurs in less than 1 ps. For this reason, one cannot treat the ligand substitution pathway from the reactive ES in terms of mechanisms elucidated for bound excited states. Instead the positive... 

The situation is considerably different for the iron(III) analog (TPPS)Fe "(H20)2 which is hexacoordinate in aqueous solution. In this case the reaction with NO is considerably slower (kon = 4.5 x 10 s ) and kojr (500 s ) is sufficiently large to be measurable by the flash photolysis method as well [92]. Temperature and hydrostatic pressure effects were probed and AH, AS and AV values of 69 + 3 kj mol , 95 10 J mol K and +9+1 cm mol were determined for the "orf reaction and 76 + 6 kJ mol , 60 + 11 ) mol and 18 + 2 cm mol respectively, for the off reaction [93j. The activation parameters for the on reaction compare very favorably with those measured for exchange between coordinated and solvent water for aqueous solutions of (TPPS)Fe "(H20)2 [94] and indicate that kinetics for the reaction of NO with this complex are dominated by the lability of the coordinated water. Furthermore, the large and positive values of AVj and ASj point to a dissociative substitution mechanism as described in Eq. (6.43). 

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