Electronically equivalent groups

The concept of electronically equivalent groups can also be extended to 5-electron main group elements [Group 15 (VA)] and 15-electron organometallic species. For example, phosphorus and Ir(CO)3 both form tetrahedral tetramers, as shown in Figure 15-1. The 15-electron Co(CO)3, which is isoelearonic with lr(CO)3, can replace one or more phosphorus atoms in the P4 tetrahedron, as also shown in this figure. 

Pyrazole Equivalent Electron-withdrawing group prefers 3-position... 

Not much information has been added in recent years to the earlier studies of tautomeric equilibria of benzimidazoles based on basicity measurements [76AHC(S1), p. 292]. For 5(6)- and 4(7)-substituted benzimidazoles and 2-methyl-5(6)-substituted benzimidazoles values are very close to 1, which indicates near equivalence in the stability of N1(H) and N3(H) tautomers. The tautomeric equilibria of 2-substituted (H, NH2, OMe, CN) 5-nitrobenzimidazoles and 4-nitrobenzimidazoles were analyzed with the use of semiempirical MINDO/3 and INDO methods. It was predicted that electron-releasing groups in position 2 shifted the equilibria to the 6-NO2 and 4-NO2 tautomers, respectively. 

Imidazole solution, and solid state Equivalent the IH tautomer Electron-withdrawing group prefers 4-... 

Benzothiepin 1-oxides 17, stabilized by electron-withdrawing groups, are available from the 1 -benzothiepins in good yields when only one equivalent of 3-chloroperoxybenzoic acid is used. 

Most electrophilic substitutions in benzimidazole (31 R = H) occur primarily in the 5-position. In multiple bromination the order followed, 5 > 7 > 6,4 > 2, parallels molecular orbital calculations. In benzimidazole itself the 4(7)- and 5(6)-positions are tautomerically equivalent. Fusion of a benzene ring deactivates C-2 to electrophilic attack to such an extent that it is around 5000 times less reactive than the 2-position of imidazole. Strong electron donors at C-5 direct halogenation to the 4-position, whereas electron-withdrawing groups favor C-4 or C-6 substitution (84MI21). 

If existence of a persulphide or other potentially electron accepting sulphur group is confirmed, this might explain why redox titration experiments have shown the number of electron equivalents which the xanthine oxidase molecule can accept to be greater than is required for reduction of the three non-protein components (58, 91). Certainly, this interpretation seems more probable than the original suggestion (58, 91) that the molybdenum can be reduced to lower oxidation states than Mo(IV) by some substrates. 

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