Salophen

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Yoon I, Narita M, Shimizu T, Asakawa M (2004) Threading-followed-by-shrinking protocol for the synthesis of a [2]rotaxane incorporating a Pd(II)-salophen moiety. J Am Chem Soc 126 16740-16741... 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

MPa O2). The role of the encapsulated [Co(salophen)] complexes is to catalyze the aerobic oxidation of hydroquinone to p-benzoquinone, which in turn oxidizes Pd(0). For the oxidation of 1,3-cyclohexadiene to l,4-diacetoxy-2-cyclohexene, the most active catalyst system involved the encapsulated complex [Co(tetra-tert-butyl-salophen)], which afforded product yields of 85-95% after 3 h at room temperature with greater than 90% trans-selectivity. This complex displayed significantly higher activity than the encapsulated [Co(salophen)] complex (72% yield in 3h) and the analogous homogeneous complex (86% yield in 5h). The increased activity of the t-butyl substituted catalyst was attributed to distortion of the bulky complex by the... 

Monooxoruthenium(V) complexes [Ru(0)(salophen)X]" [salophen = V,V -bis(salicylidene)-o-phenylenediaminato X = Cl, Im(Imidazole), 2-Me-Im(2-methyl-imdazole)f and [Ru(0)(edta)f have also been claimed, but further characterization of these species is desirable. 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

As a last point, it should be mentioned that cobalt(II) salen (26) and cobalt(II) salophen (27) are known to interact with molecular oxygen in solution. Dioxygen forms a peroxo bridge between two cobalt centers, resulting in an often-undesired dicobalt(III) species that can attenuate the efficiency of electrogenerated cobalt(I) salen or cobalt(I) salophen for other reactions of interest, for example, the catalytic reduction of alkyl halides discussed in the following text. 

On the other hand, the two-to-one adduct arising from the spontaneous reaction between dioxygen and a pair of either cobalt(II) salen or cobalt(II) salophen species provides an avenue for the catalytic reduction of dioxygen to the superoxide ion, 02 [120]. 

IR (KBr) 3040 w, 2974 w, 1647 s, 1550 s, 1477 s, 1340 s, 756 s. The synthesis for AlLCl L = SalenCl, Acen, Salophen, ° Salen Bu, Salpen Bu, Salben Bu, Salo-phen Bu, Salomphen Bu is similar and leads to organic soluble compounds. The preparations for L = Salen, SalenCl, Acen, and Salophen are facilitated by the insolubility of the compounds, which precipitate essentially quantitatively from toluene. 

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