Cyclam iron complex

The uses of the aza-erown macrocycles are multiplying as shown above. New research is reported almost daily. The titles of a few recent articles are instructive (1) Macrocyclic Polyamines [IbjN, and [21]N4 Synthesis and a Study of Their ATP Complexation by P Nuclear Magnetic Resonance Spectroscopy (Prakash et al., 1991) (2) Removal of Dyes from Solution with the Macrocyclic Ligands (Buschmann et al., 1991a, 1991b) (3) Iron-Cyclam Complexes as Catalysts for the Epoxidation of Olefins by 30% Aqueous Hydrogen Peroxide in Acetonitrile and Methanol (Nam et al.,... 

In a typical reaction, a stock solution containing styrene (0.45 mmol), substituted styrene (0.45 mmol), chlorobenzene (0.45 mmol, internal standard), and iron cyclam complex (0.03 mmol) was prepared. The solution was divided into three parts of 5 mL each. Oxidants (0.12 mmol) were added to the reaction solution, and the reaction mixture was stirred for 15 min. The amounts of olefins before and after reactions were determined by CjC/MS. The relative reactivities were determined using eq 3,... 

Figure 2. Time course for the formation of dj-stilbene oxide, rran -stilbene oxide, and benzaldehyde in the oxidation of dj-stilbene by molecular oxygen in the presence of an aldehyde and an iron cyclam complex.

Table 4. Comparison of stereoselectivity, regioselectivity, and competitive reactivities in iron cyclam complex-catalyzed epoxidations by various oxidants. 

Possible mechanisms for oxygen transfer from H2O2, PhIO, and MCPBA to olefins in the presence of an iron cyclam complex are depicted in Figure 5. If the formation of a high-valent iron 0x0 complex is faster (Figure 5, pathway A and B) than the direct reactions of olefins with intermediates 2, 3, and 4 (Figure 5, pathway C and D), the... 

Figure 5. Possible mechanisms for olefin epoxidations by H2O2, MCPB A, and PhIO catalyzed by the iron cyclam complex.

Since it has often been suggested that the observation of substantial levels of 1 0 incorporation from labeled H2 0 into the oxygenated organic products is evidence for the intermediacy of the high-valent iron 0x0 complex 1,44-46 measured the extent of incorporation into the products of iron cyclam complex-catalyzed epoxidations of cyclohexene with H2O2, PhIO, and MCPBA when H2 0 was added to the reaction mixture.47 We observed that labeled oxygen was fully incorporated into cyclohexene oxide in the iodosylbenzene reaction (eq 4) but not at all in the H2O2 and MCPBA reactions (eq 5). 

Tetraza-macrocycles of the right ring size are expected to give very high inplane ligand field strengths. Fe(II) complexes based on such ligands are therefore expected to be either low-spin or spin-crossover. Busch and his coworkers [17] have synthesized six coordinate iron(II) complexes [Fe (tet-a)X2], where tet-a is one of the isomers of hexamethyl cyclam, a 14-membered tetraza-macrocycle (8). 

Of other related systems, molybdenum(V) porphyrin exhibits very high stereoselectivity with tert-BuOOH as the oxygen source (97% m-2-hexene oxide and 99% trans-2-hexene oxide from cis- and trans-2-hexene, respectively).329 Nonporphyrin complexes of iron were found to be stereoselective in the epoxidation of stil-bene isomers. Iron cyclam, a nonporphyrin iron complex, gives the corresponding cis and trans epoxides in epoxidation with H202.330 Fe(acac)3, in contrast, yields the trans epoxide from both stilbene isomers.331... 

Because of the success with iron cyclam, our search for other non-heme iron complexes involved a survey of cyclam like ligands including HMCD (5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazocyclodec-4,l 1-diene), TIM (2,4,9,10-tetramethyl-l,4,8,ll-tetraazocyclodec-l,3,8,ll-tetraene), TDO (1,4,8,ll-tetraazocyclodecane-5,7-dione), etc. Under the same reaction conditions as for iron cyclam, the iron complexes of these other ligands also proved to be efficient catalysts for olefin epoxidation with PhIO and MCPBA, but not for H2O2. These results further support the notion that the three oxidants behave differently in their reactions with these non-heme iron complexes. 

Scheme 17 Iron cyclam iodosylarene complex (left) and chiral iodosylarene (right)...

A number of water-soluble calcium salts provide convenient vehicles for the administration of therapeutic anions. Probably the most widely encountered is the acetylsalicylate of soluble aspirin (patented 1935) the urea complex of calcium acetylsalicylate (water-solubility 231 gdm-3 at 310K, pH 4.8 (717)) is also widely used. Other examples include calcium bromide and bromolactobionate (sedatives), calcium 2-hydroxy-3-mercapto-l-propanesulfonateaurate(III) (chrysanol, antiarthritic), and calcium Af-carbamoylaspartate (tranquillizer). Calcium iron(II) citrate has been used to remedy iron deficiency - it has the advantage of being stable to air oxidation of the iron(II). The cyclamate anion is certainly not therapeutic, but is relevant here. 

---