Bicyclo octyl bridge

The charge photoseparation in porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge, ensuring a distance between the centres of P and Q of about 16 A, has been studied [57]. The rate constant of intramolecular electron transfer from P to Q was found to depend on the dielectric properties of the medium and reached 3.3 x 107s 1 for a solution of P-L-Q in propionitrile. 

Fig. 10. Structure of porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge.

To determine the parameters of electron tunneling from P to Q, the kinetics of P fluorescence decay was studied for P-Lr-Q compounds where the rigid bridge was made up of one or two bicyclo[2.2.2]octyl fragments [58,... 

At T = 77 K in MTHF, the kinetics of fluorescence decay of P-L-Q with a bridge containing one bicyclo[2.2.2]octyl is of a non-exponential character. This effect can be explained by the coexistence in the frozen solution of several rotational conformations of the P-L-Q molecule (rotation of the porphyrin fragment around the a bond in its meso position is meant here). The characteristic time of the fluorescence decay for the predominant portion of the P-L-Q particles at 77 K, r 1.1 x 10 1°s, virtually coincides with the value of r = l/k(e1 at 298 K, i.e. the rate of tunneling from P to Q is independent of temperature. The exponential character of the fluorescence decay curve at 298 K indicates that, at this temperature, the rate of rotation exceeds k(e1. ... 

Since evo-2-norbornyl arenesulfonates apparently do solvolyze with anchi-meric assistance, the less strained 2-bicyclo[2.2.2]octyl derivatives (92) are of interest. In this system no comparison of the. rates of exo and endo derivatives can be made, since a substituent exo to one bridge is endo to t fief other. Stereochemical studies are, however, possible. If the carbonium ion (93) were formed from chiral 92 by participation of the —C6 bond, it would be chiral. Scheme 5 shows that attack at C2 of 93 should give retained bicyclo[2.2.2.]octyl solvolysis products (92a), and attack at Cx should give chiral exo-2-bicyclo[3.2.1]octyl... 

Solvolyses of 2-bicyclo [2.2.2] octyl tosylate (814) and exo-2-bicydo [3.2.1 ] octyl tosylate (815) give mixtures of (277) and (278) having the same composition (Table 18)555. These results are compatible with the view that (814) and (815) give rise to the same bridged ion (818). The products derived from optically active tosyla-tes, however, are partially racemic555 In all cases the [2.2.2] isomer (277) is less... 

Deuterium label distributions in the deamination of exo- and e c/o-tricyclo[3.3.0.0 ] octyl-6-amine (7.122 and 7.123, respectively) were determined in a joint investigation of Bentley and Kirmse (Bentley et al., 1988). The results indicate that the bicyclo-hexyl-type bridged ion structure 7.124 and the open structure 7.125 are approximately equivalent with respect to energy level, but that the norbornyl-type ion 7.126 is not involved in the deamination, a result that is in accord with MM2 force field calculations (Scheme 7-37). 

When bicyclo[2.2.2]octyl brosylate was solvolyzed in acetic acid containing sodium acetate, the products were a mixture of bicyclo[2.2.2]octyl acetate and bicyclo[3.2.1]octyl acetate, each of which was optically active. The stereochemistry of formation of bicyclo[2.2.2]octyl acetate was concluded to be 82 15% retention of configuration, in accord with that expected for a bridged-ion intermediate. ... 

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