Oxidation of benzylic alcohols to carbonyls using N-heterocyclic stabilized λ³-iodanes

Introduction

The oxidation of alcohols to aldehydes and ketones is an essential transformation in organic chemistry [1,2]. Generating aldehydes is particularly challenging as they are easily overoxidized to carboxylic acids. Over the past decades a variety of methods have been developed, utilizing toxic heavy metals such as pyridinium dichromate (PDC) [3-5] or manganese dioxide (Figure 1) [6,7]. Molecular oxygen [8] and peroxides [9,10] can also be used as inexpensive terminal oxidants in combination with transition-metal catalysts. Metal-free methods employ chlorodimethylsulfonium compounds as the reactive species and have gained great popularity under the name Swern oxidation or the Corey–Kim oxidation [11]. Hypervalent iodine compounds have also been studied and are well established in several oxidative transformations including the synthesis of complex molecules and drugs [12,13]. The most prominent examples are the pentavalent derivatives 2-iodoxybenzoic acid (IBX) and Dess–Martin periodinane (DMP) [14,15]. Although mild and selective oxidants, these highly oxidized λ⁵-iodanes have drawbacks, in particular low solubility and moisture sensitivity [11]. Hypervalent iodine compounds in a lower oxidation state (λ³-iodanes), such as iodosobenzene (PhIO)_n or phenyliodine(III) diacetate (PIDA) have been reported in alcohol oxidations but they often result in overoxidation to the corresponding carboxylic acids [16]. Additives such as bromide salts or Al₂O₃ can eliminate this problem and allow selective oxidation to some extent [17-20].

During the past years, N-heterocycle-stabilized iodanes (NHIs) were demonstrated as suitable tools for various applications among them group transfer reactions [21] and as building blocks [22-24]. The synthetic potential of NHIs has been previously studied in model transformations such as thioanisole oxygenation, oxidative lactonization, or diacetoxylation of alkenes [25-28]. In this work, we want to apply NHIs in a mild oxidation of primary and secondary benzylic alcohols to aldehydes and ketones as an alternative to λ⁵-iodanes.

Results and Discussion

Initially, we investigated a variety of pyrazole-, triazole-, and oxazole-substituted hydroxy-NHIs previously developed by our group [25]. However, none of them proved to be effective in a model oxidation reaction of n-octanol (2). Since previous investigations have repeatedly shown that the number of heteroatoms in the N-heterocycle correlates with the NHIs activity, a series of tetrazole- and tetrazine-substituted NHIs 1a–e was synthesized (Figure 2) [29,30]. A crystal structure was additionally obtained for tetrazine 1c. Bond lengths and angles were similar to those of known five-membered NHIs [25], including a strong intramolecular interaction between the nitrogen of the tetrazine and the hypervalent iodine atom (I1–N1: 2.44(4) Å; the sum of VdW radii: 3.61 Å [31]).

Beginning with the electron-deficient and thereby highly reactive NHIs 1a and 1c, we explored the potential for a ligand-exchange process on the iodane via ¹H NMR spectroscopy by combining equimolar quantities of NHI and n-octanol (2). When the tetrazole-substituted hydroxy(aryl)iodane 1a was added, no significant shifts in the NMR spectral signals were detected, probably due to the poor solubility of the iodane. Conversely, with the addition of the red tetrazine salt 1c, a significant downfield shift was observed for the alpha-carbon protons from 3.51 ppm to 4.55 ppm, as illustrated in Figure 3a.

This indicates a ligand exchange of the hydroxy group resulting in a loss of electron density and the formation of the alkoxy-NHI 2'. The chemical shift is consistent with previously measured alkoxyiodanes [32].

The experiments were repeated using activated p-tolylmethanol (3a), again showing no reaction with iodane 1a. Utilizing the tetrazine 1c, p-methylbenzaldehyde (4a) was observed as a new species at 9.94 ppm (Figure 3b). The reaction reached 31% conversion after 72 h, however, p-methylbenzoic acid (4a’) was formed in 35% as well, showing an undesired overreaction. In this experiment no formation of an alkoxyiodane was observed, indicating that the formation of this ligand-exchanged intermediate is slower than the dehydrogenation. As a consequence, we attempted to accelerate the ligand exchange through the addition of a Lewis acid and the performance of the NHIs was compared with common iodine(III) reagents by ¹H NMR spectroscopy (Figure 4). After 60 h the measurements revealed a higher yield of aldehyde 4a using 1a (68%) compared to 1c (30%) under the influence of AlCl₃. As a comparison, the use of PIDA (5b) and IBA (5c) with the additive resulted in a significantly lower oxidation of the alcohol. Only small amounts of benzoic acid 4a’ were observed in all reactions with additional AlCl₃, suggesting that the additive inhibits the previously observed overoxidation.

Surprisingly AlCl₃ activated the cyclic tetrazole iodane 1a but had almost no influence on the reactivity of the tetrazine salt 1c. Based on these results, the reaction conditions were further optimized using NHI 1a with the benzyl alcohols 3a (electron-rich) and 3b (electron-poor) as the model substrates. First, the reaction temperature was increased, finding 60 °C to be the optimal value in EtOAc (Table 1, entry 1). At this temperature, the reaction time was significantly reduced to 2.5 h. A variety of other additives were tested next, revealing TsOH or NaOTs inhibiting the reaction (Table 1, entries 2 and 3). The addition of tetrabutylammonium halides showed the chloride salt being superior, giving comparable or even better yields than AlCl₃ (Table 1, entries 4–7). Investigation of other chloride sources resulted in a reduced yield in the case of ammonium chloride and an improved yield of 82% of 4a when concentrated aqueous HCl was added (Table 1, entries 8 and 9). Other solvents did not further increase the yield (see the full table in Supporting Information File 1).

However, when electron-deficient p-chlorobenzyl alcohol (3b) was used the highest yield of 4b (69%) was achieved with TBACl as the chloride source in MeCN (Table 1, entry 10). These optimizations lead to the conclusion that AlCl₃, as proposed in the initial experiments is not a Lewis acid activator but just a chloride source. Further optimization studies improved the yield to 78% of 4b using a concentration of 0.20 M of the alcohol and 1.4 equiv of 1a (see Supporting Information File 1). Finally, all NHIs were tested under the optimized conditions, revealing the tetrazole-substituted iodane 1a to be the best oxidant for this reaction (Table 2).

The two suitable methods (A: HCl in EtOAc; B: TBACl in MeCN) were then applied to a variety of activated alcohols. The best option is shown in Figure 5. Model substrate 4a could be isolated in a high yield of 84% with reisolation of the 5-(2-iodophenyl)-1H-tetrazole (6) in 90% yield. Other para-halogenated benzaldehydes 4b–f were isolated in good yields of up to 88%. ortho-Substitution led to a lower yield of the iodinated product 4g (43%) compared to the para-iodinated analogues 4d (75%). The ortho-phenyl-substituted aldehyde 4h was isolated in 85% yield, while the ortho-methoxy substrate did not convert to 4i. The ortho-, meta- and para-permutation of a CF₃ group showed lower reactivity for the ortho-substituted 4j (53%), while the meta- and para-derivatives 4k and 4l gave higher yields of 84% and 71%, respectively. The steric inhibition of a doubly substituted phenyl ring was observed in a diminished formation of 2,6-dichlorobenzaldehyde (4m) in 39% yield. Naphthalen-2-ylmethanol gave aldehyde 4n in 44% yield. Pyridines 4o and 4p were also compatible and gave good yields of 87% and 64%, respectively. Unfortunately, the synthesis of vanillin (4q) was unsuccessful due to undesirable oxidation reactions of the electron-rich arene. The cyclopropane derivative 4r was generated from the cyclopropylmethanol in 53% yield. The acetylene derivative 4s could not be isolated due to undesired oxidations of the triple bond. The behavior of secondary benzylic alcohols was tested next, giving 4-methylacetophenone (4t) in an excellent yield of 97% and 1-indanone (4u) in 46%. It is worth noting that for some derivatives oxidized by method A, an acylation of the alcohol was detected as a side reaction via mass spectrometry. Vinyl alcohols were also studied, giving carvone (4v) in 74% yield without oxidation of the double bonds. Finally, other heterocyclic benzylic alcohols were investigated, which led to undesired chlorinations in the case of benzimidazoles 3w and 3x and decomposition for thiophenylmethanol 3y.

Regarding the reaction mechanism, two plausible pathways can be discussed based on literature examples (Scheme 1, path a [17] and path b [33]). In either path, initial ligand exchange to the hydroxy(chloro)iodane I-OH is proposed. For getting an indication of a chloride-activated iodane of this type, a mixture of NHI 1a and HCl in EtOAc was stirred for 1 h at 60 °C and an ESI(−) mass spectrum was recorded afterward, showing an ion I-OMe with m/z 337.0 [1a – OH + MeO + Cl]⁻ (Scheme 1c). It is known that methanol, which is used as a solvent in the mass spectrometer, can be exchanged with the hydroxy group of the NHI [21]. No such ion was measured in the mixture before heating. This ion therefore indicates an I–Cl bond in the activated iodane. Starting from I-OH, in a potential path a) formation of hypochlorous acid is suggested, which consequently oxidizes the alcohol through the alkyl hypochlorite IIa. The second mechanism (path b) requires a direct ligand exchange of I-OH with the alcohol and subsequent β-elimination of the alkoxy(hydroxy)iodane IIb to form the desired aldehyde 4.

Conclusion

In conclusion, this study has successfully introduced N-heterocycle-stabilized iodanes (NHIs) as effective λ³-iodane oxidants for the selective synthesis of ketones and aldehydes, avoiding overoxidation to carboxylic acids. The developed protocols proved particularly effective for benzylic alcohols, yielding good to excellent results. The beneficial role of chloride salt additives was investigated, potentially leading to the formation of a hydroxy(chloro)iodane intermediate. This intermediate either liberates hypochlorous acid as the terminal oxidant or undergoes a direct ligand exchange with the alcohol, followed by oxidative elimination to form the aldehyde. Thus, these reagents offer a viable alternative to traditional aryl-λ⁵-iodane-based oxidants, although further studies are necessary to fully understand their reaction mechanisms.

Experimental

General procedure for oxidation of benzylic alcohols

1a (700 µmol, 201 mg, 1.40 equiv), benzylic alcohol (3, 500 µmol, 1.00 equiv) and method A: aqueous HCl (37%, 500 µmol, 41.6 µL, 1.00 equiv) in EtOAc (2.5 mL) or method B: TBACl (500 µmol, 137 mg, 1.00 equiv) in MeCN (2.5 mL), respectively, were stirred at 60 °C for 2.5 h, quenched with Me₂S (2.00 equiv) and the reaction mixture was purified via flash column chromatography on silica.