Ring permutation

Fig. 16. Some of the isomerization pathways of benzene and xylene through ring permutation.

Barltrop, J. A., Colinday, A., Ring Permutations Novel Approach to Aromatic Phototransposition Reactions, J. Chem. Soc., Chem. Commun. 1975, 177 179. 

This process moves the departing Y (MeaN) into an apical position where its leaving (presumably favored by microscopic reversibility) gives product of inverted phosphorus configuration as is observed experimentally. An Mif (ring) permutation of the type 27 -> 28 would not alter substitution stereochemistry, but neither would it place the MeaN apical. 

For a discussion of permutation pattern analysis in aromatic phototransposition chemistry and its application to the analysis of the phototransposition reactions of five-membered heteroaromatics, see (a) Barltrop, J.A. and Day, A.C., Ring permutations a novel approach to aromatic phototransposition reactions, /. Chem. Soc., Chem. Commun., 177, 1975 and (b) Barltrop, J., Day, A.C., Moxon, P.D, and Ward, R.R., Permutation patterns and the phototranspositions of 2-cyanopyrroles, /. Chem. Soc., Chem. Commun., 786,1975. 

Valence Isomerism. Restricted Rotation and Permutational Isomer i sm. - The variable temperature spectra of bicyclic tetraphosphines (29) has been analysed in terms of valence isomerism involving the corresponding two coordinate open-chain isomers.62 A report on ab initio MO calculations of the inversion barriers for a series of methyl phosphines includes a discussion of the electronic consequences of steric effects.63 Inversion barriers of 1,2-diphosphinobenzenes are in the range 100 - 110 kJ mol". 66 The ring inversion barrier for a dibenzophosphorin has also been measured.63... 

Triheterapentalenes have numerous structural variations of the generic structure 6. They can possess either S, Se, or Te as its central hypervalent atom Y. The number of permutations increases dramatically since each of the atoms X and Z can be replaced by S, Se, O, or NR. Also, one or more of the remaining ring carbons can also undergo aza replacements. 

Following publication of CHEC-II(1996) there have been a large number of reports on different fused 5 6 5 heterocyclic rings with wide structural variations <1996CHEC-II(8)967>. As such it is pertinent to organize them in a logical fashion, due to the plethora of possible permutations that these structures may include. 

Treatment of these complexes with excess phenylacetylene leads ultimately to the symmetrical triphenyl benzene as the main product, but the intermediate adduct IV, Os3(CO)9L2, exists in three isomeric forms which are considered to be related to the osmacyclopentadiene complex Os3(CO)9[C2(C6H5)2]2, obtained from reaction of diphen-ylacetylene with the Os3(CO)i2. The structure of this complex has been established by X-ray crystallography (132) to be that shown in Fig. 21, with R = Rl = Rz = R3 = Ph. The three isomeric derivatives in the case of the phenylacetylene adduct arise from the various permutations of the phenyl and hydrogen groups around the osmacyclopentadiene ring. 

Figure 14 gives graphical illustrations of some of the permutations in classes Ci-C and Figure 15 shows the 16 isomers of a spirophosphorane with two identically substituted (R = R1) five-membered rings which, in isomer 12, are linked at 1,5 and 2,4. The reasons for this choice are that it shows ... 

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