Pocket’ porphyrins

Steric encumbrance on both faces of the porphyrin ring must prevent the bimolecular oxidation pathway but often still allows ligand binding. This approach has been proposed by several groups there are the two-pocket porphyrins of Amundsen and Vaska [67], Cense and Le Quan [68] and Suslick and Fox [69] the double-sided porphyrins of Tsuchida and coworkers [70, 71] the basket-handle porphyrins of Momenteau et al. [72-75], Rose and coworkers [76], Walker and coworkers [77], Mansuy et al. [78] and Reddy and Chandrashekar [79] and the jellyfish porphyrins of Uemori and coworkers [80, 81],... 

For the pocket porphyrin (Figure 4.23), structural data are available on the... 

For the iron(II) pocket porphyrins, the coordination state of the iron depended on the size of the pocket. Visible absorption and MCD spectral data showed that Fe (Poc-Piv) derived from 171a remained five coordinate even in the presence of excess base. Although the medium and large size pockets showed increasing six-coordination, concentration ranges could be determined within which five-coordinate iron(II) was the dominant species. The O2 and CO binding of the pocket and picket-fence porphyrins were comparedWhile the O2 affinities for both systems were similar, the pocket porphyrins showed a reduced CO affinity. Since electronic and solvent effects were similar in the two model systems, the reduction in CO affinity was attributed to the steric hindrance of the cap which distorted the Fe-C-0 unit from linearity. 

We have already seen that sterically encumbered porphyrins may still be susceptible to p-0X0 bridge formation if any four-coordinate species in solution binds oxygen on the open face. Steric encumbrance on both faces of the porphyrin, as in the bis-pocket porphyrin systems of Amundsen and Vaska and Suslick may prevent this bimolecular oxidation pathway. 

Fig. 2 Dioxygen-binding myoglobin analogues as synthesized by Collman and coworkers.[ (A) Picket-fence porphyrin. (B) Pocket porphyrin. (C) Picnic-basket porphyrin.

Collman, J.P. Brauman. J.I. Collins, T.J. Iverson, B. Sessler. J.L. The pocket porphyrin A hemo-protein model with lowered CO affinity. J. Am. Chem. Soc. 1981, 103... 

Suslick and his coworkers examined the shape selective oxidation of alkanes and alkenes catalyzed by the iron and manganese complexes of the bis-pocket porphyrin (Mn(TTPPP) and Fe(TTPPP) where TTPPP is tetrakis(2,4,6-triphenylphenyl)porphyrin) with a variety of oxidants such as mCPBA, PhIO, F5-PhIO [290]. The selectivities for terminal hydroxylation of n-alkanes are found to be very similar. While the details of the reactive intermediates are not clear, a peracid-Fe TTPPP adduct was obtained as a stable complex at -78°C to measure IR spectrum (Vc=o- 1744 cm l) [139]. However, an expected structure of an oxidant-metalloporphyrin complex could be too crowded to react with olefins. Uponwarming the solution temperature up to room temperature, the... 

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