Oxidation of the Ruthenium(0) Sandwich Complex 91

First, the oxidation with Cu(OAc)2 was analyzed. Interestingly, no oxidation was observable when adding solely Cu(OAc)2 to the sandwich complex. Also the addition of sodium naphthoate (87b) and DMAP as ligand to regenerate the five-membered ruthenacycle 89bb did not suffice. The addition of

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KPF6, which was used as additive in the catalytic reaction developed by Ackermann and coworkers,^[17]

cleanly formed the product and also acetic acid as additive led to product formation in quantitative yield.

Scheme 59. Oxidation of sandwich complex 91ba with copper acetate as the oxidant.

As the complex 91be and 91ce derived from alkyl substituted alkynes proved to be instable under aerobic conditions, the question arose if oxygen can be used as an environmentally friendly oxidant for the aromatic system as well. Also in this case the simple addition of oxygen did not generate the product or induce any observable reaction, while the addition of acetic acid and oxygen yielded the product 3ba and also [Ru(OAc)2(p-cymene)]. If the experiment was conducted under air and not under an oxygen enriched atmosphere, a somewhat longer reaction time along with a higher temperature were required. These results show that on the one hand the benzoate 87 is needed to achieve the cycloruthenation of the benzoic acid 1 but on the other hand acetic acid is required for the oxidation of the ruthenium(0) complex 91. Thus, the reaction requires conditions that allow for the formation of the benzoate 87 in the presence of acetic acid.

Scheme 60. Oxidation of 91ba applying oxygen as the oxidant.

39 The excess of acetic acid is crucial for the oxidation step. If only ten equivalents are used a conversion of only 17% is observed under otherwise identical conditions. Interestingly, the reactivity increases again when the molarity of the reaction is increased. Thus, moving from a concentration of 0.017 M to 0.068 M the conversion went up to 62%, again under otherwise identical conditions (Scheme 61). This showed that the concentration of acetic acid in solution and not only the overall amount is important for the efficiency of the reaction.

Scheme 61. Influence of the molarity on the oxidation of 91ba.

A reasonable explanation for the crucial role of the acid would be an oxidative addition of acetic acid to ruthenium, but the addition of acetic acid without oxygen did not lead to any reaction (Scheme 62). Hence, an oxidative addition, like it is known for aryl^[30] and alkyl halides^[104] and also aryl ethers,^[105] seems to be unlikely. The oxidation simultaneously requires oxygen and acetic acid. The reason for this either lies in a simultaneous process or a reversible first step, in which the addition of the second required chemical shifts the equilibrium to the product side via a nonreversible reaction.

Scheme 62. Role of acetic acid in the oxidation.

Interestingly, when controlling the oxidation process by ¹H NMR studies a paramagnetic side product was observed. To further check this observation EPR studies were performed. It turned out that indeed a paramagnetic ruthenium species could be observed. The radical displays orthorhombic features and the g-value suggested that the radical character is most likely not based on the ligand system (giso = 2.41, gx≠ gy ≠ gz). Aging of the reaction mixture led to a decrease of the observed radical, while a new organic radical evolves (g = 2.006; ge= . ; ∆ = . . Ruthe iu I spe ies are very scarce in the literature^[106] and it is therefore most likely that this side product is a result of

40

an over oxidation process to ruthenium(III). However, upon subjecting ruthenium(II) acetate complex 96 to the oxidation conditions no paramagnetic species was formed (Scheme 63).

Scheme 63. Attempted oxidation of [Ru(OAc)2(p-cymene)].

Cyclic voltammetry studies of the sandwich complex were conducted to gain more insight into the oxidation process (Figure 12). The CV studies suggested an irreversible oxidation process in which only one oxidation step was visible. The special role of the acetic acid in the oxidation process can be seen in a decreased oxidation potential in case of the addition of acetic acid.

0.0 0.5 1.0

Figure 12. Cyclic voltammogram of ruthenium(0) sandwich complex 91ba.

Interestingly, when adding a large excess of acetic acid a second oxidation process becomes observable, thus strengthening the theory of an over oxidation leading to a paramagnetic ruthenium(III) species (Figure 13).

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0.5 1.0

0.0 0.5 1.0 1.5

Current density / Am-2

Potential vs Fc/Fc⁺ / V

91ba + 200 equiv HOAc

Figure 13. Cyclic voltammogram of ruthenium(0) sandwich complex 91ba with 200 equivalents acetic acid.

The two following oxidation pathways are reasonable (Scheme 64).

Scheme 64. Possible pathways for the oxidation of the ruthenium(0) sandwich complex 91.

The oxidation itself is probably proceeding via a fast single electron transfer process producing a peroxo ruthenium species, which further reacts to the acetate complex (Scheme 64a). Calculations from Fu, Lin and coworkers^[107] regarding the formation of Ru(II) peroxo species suggested an initial formation of an end-on η¹-O2 complex. The dangling oxygen atom then attacks the metal to form the side-on η²-O2 peroxo species. However, the calculations have not been conducted for η⁶-arene

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complexes. The in situ formed hydrogen peroxide is then consumed during oxidation of another ruthenium(0) complex. An alternative would be represented by an oxidation involving two ruthenium centers, forming a dioxygen-bridged complex (Scheme 64b). This may either be directly attacked by acetic acid or split into two oxo-ruthenium species, which subsequently react with acetic acid.

To probe the feasibility of hydrogen peroxide as oxidant, C. Kornhaaß^[108] tested it in the catalytic reaction under an inert atmosphere and the product indeed formed in good yield with only one equivalent of hydrogen peroxide (Scheme 65).

Scheme 65. Ruthenium(II)-catalyzed alkyne annulation with hydrogen peroxide as oxidant.

Nevertheless, a test for hydrogen peroxide with peroxide test stripes after the reaction was negative.

The detection limit of these stripes is 1 mg peroxide per liter. The result may either indicate that no hydrogen peroxide is formed during the reaction or that the in situ produced hydrogen peroxide is a better oxidant than oxygen itself and is therefore consumed directly after its formation.