2.4 Results and Discussion
2.4.3 Oxidation of n-octane
The catalytic activity of complexes 1 and 2 were explored in the oxidation of n-octane. All catalytic runs were carried out (in duplicate or triplicate in some cases) at 80 °C in acetonitrile with TBHP (t-BuOOH) as the oxidant under argon atmosphere. Preliminary work showed low conversions with no significant change in product selectivity at lower temperatures. At lower reaction times poor selectivity to the alcohols and low conversion was observed. TBHP as an oxidant has been used in a number of oxidation reactions and has the advantage over other oxidants in that it has higher solubility in organic solvents, which contain dissolved hydrophobic hydrocarbons.62 Optimization of the substrate to oxidant ratio was carried out, by investigating n-octane to TBHP ratios of 1:2.5; 1:5; 1:7.5 and 1:10, where the ratio of 1:5 gave the highest conversion with good selectivity. Control experiments were carried out in the absence of the catalyst and oxidant respectively. In the former, a 2% conversion was observed with the highest selectivity to 2-octanone (Fig. 2.3). However, the latter reaction showed a 0% conversion. Blank studies with oxidant and catalysts showed an observable color change from green to blue for catalysts 1 and no change in color for catalysts 2.Testing was also carried out with cobalt chloride (CoCl2.6H2O) under the same catalytic conditions. A 2% conversion was observed, with selectivity to the over oxidized products, namely the ketones and octanoic acid. This is the same conversion as for the blank reaction with TBHP, only but with a greater selectivity to the over oxidized products. It is known that with the simple salts of iron, low activities have been exhibited in oxidation reactions.33
Comparing the activity of catalysts 1 and 2 (Fig. 2.4), the latter show a higher conversion with the highest being for 2a with 14% conversion. This can be related to the bite angle (P-
Co-P), where catalysts 2 have a much larger bite angle of 116.32° (2c) in comparison to catalysts 1, ~71.19°. The more sterically hindered catalysts limit the activity in comparison to the more flexible catalysts 2. The bite angle is known to have an impact on the activity and selectivity of catalytic reactions.63 It has been reported that the effect of bite angle on C-X bond activation originates from an electronic factor, where the donor-acceptor orbital interactions (metal d orbitals to the substrate σ*c-x) stabilize the transition state.[64] As the metal-ligand d-hybrid orbital is driven to smaller bite angles, the transition state becomes more stabilized.64 Interestingly, a decrease in the activity for both catalysts 1 and 2 is observed, as one moves from the cyclohexyl (a) to the iso-propyl (c) substituent. Noteworthy, for these ligands used in ethylene tetramerisation, a similar trend is observed in terms of their activity with chromium as the active metal.20 This can be attributed to the basicity of the substituent on the nitrogen atom.20
Figure 2.3. Selectivity of the blank reaction (no catalyst) to products of oxidation at a ratio of 1:5 of substrate to oxidant at 80 °C in acetonitrile.
Figure 2.4. Conversion of n-octane by catalysts 1 and 2 at a ratio of 1:5 of substrate to oxidant at 80 °C in acetonitrile.
- 10 20 30
1- octanol 3-octanol 4-octanol 2-octanone 3-octanone 4-octanone octanoic acid
Selectivity/ mol%
Products of oxidation
0 2 4 6 8 10 12 14 16
a b c
% Conversion
Catalyst
1 2
Both sets of catalysts (1 and 2) are highly selective to the ketones, with 2-octanone being the dominant product (Fig. 2.5 and 2.6). This indicates that the catalyst causes oxidation of the internal carbons more readily than the terminal carbon. Such cases, with 2-octanone being the dominant product, have been observed in the biological hydroxylation of alkanes catalyzed by methane monooxygenase.13 The C(2) position is three times as active as the C(1), and is most reactive in linear alkane chains as reflected by the regioselectivity in reactions for n-heptane, as well as n-octane.5 Over-oxidation is highly prevalent at the C(1) position of the n-octane chain for both catalysts, with higher selectivity to octanoic acid and very low selectivity to 1- octanol and no selectivity to octanal. Ketone formation is also observed with ruthenium phosphine compounds, in which cases only 2 and 4-octanone are observed with a 4%
conversion.47,48 Chen and White have reported yields to 2 and 3 octanone with no selectivity to primary products using a bulky iron electrophillic catalyst.3
Figure 2.5. Selectivity of catalyst 1 to the products of oxidation at a ratio of 1:5 of substrate to oxidant at 80 °C in acetonitrile.
Following the method of Shul’pin and coworkers, addition of triphenylphosphine to a filtered (through a plug of silica to remove the catalyst) aliquot of the reaction mixture 10 mins prior to GC analysis was performed, since an increase in the alcohol peak and a decrease in the ketone peak can result.33 This was the true concentration of the alcohols and ketones, since the alkyl hydroperoxides that are present are completely reduced to the corresponding alcohols. For catalysts 1, a change in the selectivity to the alcohol and ketone before and after addition of the PPh3 was observed, however, for catalysts 2 no observable change was noted.
0%
5%
10%
15%
20%
25%
30%
selectivity / mol %
Products of oxidation 1a 1b 1c
Figure 2.6. Selectivity of catalyst 2 the products of oxidation at a ratio of 1:5 of substrate to oxidant at 80 °C in acetonitrile.
The regioselectivity parameters (Table 2.4) further indicate that the C(2) position is the most activated carbon of the n-octane chain, with the C(1) being the least activated. The catalysts with the pentyl and iso-propyl (b and c) substituents are most selective to the alcohols, with catalysts 2 being more selective to the alcohols than 1.
Table 2.4 Selectivity parameters in the oxidation of n-octane by catalysts 1 and 2.
Catalyst Alcohola C(1):C(2):C(3):C(4)
Ketoneb C(2):C(3):C(4)
Totalc
C(1):C(2):C(3):C(4)
1a 0:1.5:1.7:1 1.2:1:1.2 1:5.5:5:4.8
1b 0:1.6:1.8:1 1.2:1:1.1 1:7.4:6.5:6.1
1c 0:2:1.6:1 1.1:1:1.0 1:7.6:6.5:6.1
2a 1:0:3.3:3.3 1.4:1:1 1:5.2:4.1:4.2
2b 1:4.2:4.2:2.9 1.1:1:1 1:4:3.5:3.4
2c 1:4.2:2.9:2.9 1.2:1:1 1:4.1:3.4:3.5
a Parameters C(1):C(2):C(3):C(4) are the relative reactivities of hydrogen atoms at carbon 1, 2, 3 and 4 of the n-octane chain.
b The calculated reactivities from the % selectivity are normalized, i.e. calculated taking into account the number of hydrogen atoms at each carbon.
c Includes the % selectivity of octanoic acid, alcohols and ketones and the values are normalized.
When using TBHP as an oxidant, the ketone product forms from the oxidation of the alcohol (over oxidation).35 Since the selectivity to the ketones is much greater, it is likely that the oxidation reaction proceeds via the formation of hydroxyl radicals where the metal complex activates the oxidant, t-BuOOH, forming a reactive species, the hydroxyl radical, which
0%
5%
10%
15%
20%
25%
30%
35%
40%
Selectivity/ mol%
Products of oxidation
2a 2b 2c
attacks n-octane. Taking into consideration the metal and the substrate being activated we propose a mechanism which is consistent to that present in literature for the oxidation of alkanes using TBHP as an oxidant.5,66-70 We assume that the reaction takes place in the coordination sphere of the metal complex which would explain the influence of the ligand system on the reaction. The t-BuO is formed by the reduction of t-BuOOH by the Co(II)L which generates a hydroxo-Co(III) species (eqn 1).66,67
The hydroxo species reacts further with t-BuOOH (eqn 2) which then decomposes to form the t-BuOO radical (eqn 3) which gives a further t-BuO radical after dismutation (eqn 4).66,67
The oxygen centered radicals generated in eqn 1 and 4 attack the n-octane with the formation of octyl radicals (eqn 5).5,66,67
The octyl peroxy radical is formed (eqn 6) upon reaction of R with oxygen which reacts with n-octane to form ROOH (an organo-hydroperoxide, eqn 7) and thereafter undergoes homolytic decomposition via the cobalt(II)L complex to form the RO (an organooxyl radical, eqn 8).5,66,67
By H-abstraction from the octane, the RO radical forms the octanol (ROH, eqn 9) and the ROO decomposes to octanol and octanone (eqn 10) or is regenerated via eqn 7. Increase in the ketone selectivity occurs when the ROO attacks the t-BuOO through mixed molecular Russel termination (eqn 11).71 Deeper oxidation of octanol forming octanones is generated by the Co(II)L complex (eqn 12).5,66,67
(1) Co(II)L + t-BuOOH Co(III)(L)OH + t-BuO
Co(III)(L)OH + t-BuOOH Co(III)(L)(OOBu-t) + H2O (2) Co(II)(L) + t-BuOO (3) Co(III)(L)(OOBu-t)
t-BuOO t-BuO + 1/2 O2 (4)
t-BuOH + R (5)
t-BuO + RH
ROO (6)
R + O2
(7)
ROO + RH ROOH+
(8) R
Co(III)(L)OH + RO Co(II)L + ROOH
RO + RH (9)
2ROO ROH+ R-H=O + O2 (10)
(11) ROH + R
2ROO + t-BuOO R-H=O + t-BuOH + O2 ROH + t-BuOOH Co(II)L R-H=O (12)