Cage pair intermediates

A description of a bond thermolysis in solution requires at least two elementary steps. This is due to the fact that the immediate product of the bond cleavage step is a pair of radicals which are initially restricted to remain as neighbors by the surrounding solvent molecules. We follow Rabinowitch and Wood and term this reactive species the cage pair intermediate . Scheme 1 depicts a simple chemical model for this kinetic sequence. Subsequent to the... 

The center of our focus is the effective energy surface leading to and from cage pair intermediates. Figure 1 shows a schematic comparison of the Gibbs free energy and enthalpy profiles for a Scheme 1 reaction in the case where the barrier to recombination of the cage pair is entirely entropic (AS < 0). ... 

KOENIGETAL. Cage Pair Intermediates and Activation Parameters... 

Photochemical Cage Pairs--Laser methods have progressed to the point where direct observations of cage pair intermediates are feasible. With pulse widths in the picosecond (psec) regime, the time course of the formation and decay of cage pair intermediates can be recorded as first demonstrated by Endicott and Netzel for aqueous solutions of coenzyme recent report d on the 354.7... 

The direct observation of the reactive intermediates by the use of time-resolved picosecond spectroscopy and fast kinetics (Figure 4) enables the course of CT osmylation to be charted in some de. The analysis proceeds from the mechanistic context involving the evolution and metamorphosis of the CT ion pair, as summarized in Scheme 5 (the brackets denote solvent-caged pairs) for the critical initial stq> (equation 24) to form the 1 1 adduct to a benzene donw. 

It has been proposed that the decarbonylation of aldehydes by the Wilkinson catalyst [RhCl(PPh3)3] involves a radical pair disproportionation or recombination reaction. A radical pair intermediate in solution is equivalent to a cage reaction (Scheme 6). Table 15 shows the results obtained from the decarbonylation of a series of chiral cyclopropyl aldehydes ... 

There is evidence for a radical pair intermediate in this reaction (observation of a CIDNP) and the formation of carbonylation and hydrogenation products can be rationalized in terms of a reaction within the solvent cage (hydroformylation) or outside the cage (hydrogenation). 

Photolysis of the 1-naphthyhnethyl ester of phenylacetic acid (327) in methanol, for example, affords the 1-naphthylmethyl cation carboxylate anion pair in addition to 1-naphthylmethyl radical acyloxy radical pair intermediates, which, after decarboxylation, form an adduct with methanol (328, formed along with 329) or an in-cage radical coupling product 330 (Scheme 6.146).1030 The competition between the radical and ionic pathways was found to be very dependent upon the substituents on the naphthalene ring. 

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