Cobalt(II)-catalyzed oxidative esterification of aldehydes: a cooperative effect between cobalt and iodide ions

Abstract

The efficient cobalt(II) catalyzed oxidative alkoxylation of aldehydes, a method directly leading to the corresponding esters, is presented. Mechanism studies provide extensive insights into the cobalt mediated decomposition of TBHP in the presence of the iodide ion. The in situ generated hypoiodites (IO⁻/IO₂⁻) mainly from conversion of I₃⁻ to IO₃⁻ account for a cooperative effect with the oxygen centered radical species to make the important hydrogen abstraction of hemiacetal intermediates more selective, thereby offering high efficiency.

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Introduction

Developing cobalt catalysts for the selective functionalization of C–H bonds is a conceptually and economically attractive strategy that overcomes the need for precious metals and enables new transformations.¹ Cross-coupling reactions by low-valent cobalt have emerged as a versatile approach.^2–6 However, the requisite reductive conditions limit the functional group tolerance.^4–6 As an alternative strategy, the high-valent-cobalt-catalyzed coupling reaction is attracting increasing attention and was developed very recently.^7–9 For instance, Co(II)-catalyzed C–H bond activation in the presence of an oxidant has been applied in the construction of various C–O, C–N and C–C bonds.^8,9

Despite this significant progress, only two examples of Co(I)-catalyzed functionalization of the formyl C–H bond in aldehydes have been reported (eqn (1) and (2)). These include Brookhart's oxidative C–H activation³ and Dong's oxidative cyclization,⁶ respectively. Interestingly, the unusual β-hydride elimination¹ occurred on the cyclic [Co(III)–O–CH] intermediate in the latter case, probably due to the ring tension thereof. Independent of this, the oxidative strategy by using an external oxidant, albeit representing an efficient mode of reactivity towards aldehydes,¹⁰ appears reluctant in the desired cross-coupling with other organic substrates. Therein, the dehydrogenative cross-coupling of aldehydes and alkanols (eqn (3)) is more challenging because of the fact that alkanols are easily transformed to the corresponding aldehydes (further to acids) or ketones through radical mechanisms.¹¹ Even the intermediate hemiacetal–metal complexes were formed before the undesired decomposition of each substrate, the subsequent β-hydrogen elimination leading to esters might be a limitation in the case of cobalt¹ compared with palladium or nickel catalysis.¹² Therefore, cobalt-catalyzed direct oxidative esterification of aldehydes with alcohols warrants further study.

Previous studies demonstrated that organic radical species, generated from the reactions of some transition-metal complexes or iodides with tert-butyl hydroperoxide (TBHP), exhibit high reactivity as oxidizing agents in the oxidative coupling of aldehydes/alcohols.¹³ Chang and coworkers once reported Co(II)/T-HYDRO catalyzed dehydrogenative C–N cross-coupling in the presence of AcOH (eqn (4), T-HYDRO is the trademark name for 70 wt% tert-butyl hydroperoxide solution in water).^9b Nucleophilic attack of amines was found to be more facile by using an acid additive. Noteworthily, subtle differences in the ligands of cobalt complexes were known to result in extreme differences in the catalytic cycles of peroxide decomposition,¹⁴ thereby tuning the efficiency of hydrogen abstraction. Based on these results, we speculated that certain Co(II) species would catalyze the oxidative esterification of aldehydes when they are used in combination with TBHP and proper acids. Herein, we disclose the first CoI₂-catalyzed direct alkoxylation of aldehydes (eqn (3)). Interestingly, investigations attribute the high efficiency to a cooperative effect between the iodide chelating ligand and the cobalt ion in the catalytic cycle.

Results and discussion

Our study started with developing a Co(II)/TBHP system in the oxidative coupling of benzaldehyde (1a) and MeOH (2a). As expected, the choice of acid additives and their amount employed turned out to be crucial for the selectivity (Table 1, entries 1–9). The Brønsted acid AcOH tends to form an undesired carboxylic acid (entry 1), the origin of which will be discussed later in the paper. In contrast, the Lewis acid AlCl₃ was the most effective in the desired product ester formation among the various acid co-catalysts screened (entries 2–5), although the mass balance still consisted of a part of carboxylic acid (4aa). Moreover, cobalt species other than CoI₂ exhibited reduced catalytic activity (entries 9–13). And the ester yields were not improved by either decreasing or increasing the equivalent of MeOH (entries 8 and 9) and TBHP (entries 14 and 15). Under the optimized conditions, unsubstituted benzaldehyde (1a) smoothly reacted with MeOH (2a) to provide methyl carboxylate 3aa in a high yield of 94% (entry 13).

Table 1 Optimization of the cobalt catalyst system for the oxidative esterification of benzaldehyde (1a) with MeOH (2a)^a

Entry	Cat.	2a (equiv.)	Additive	Conv. (%)	Yield^b (%)
					3aa	4aa
a General conditions: 1a (1 mmol), cat. (5 mol%), additive (5 mol%), TBHP (2 equiv.), 100 °C, 24 h. b Yields determined by GC with biphenyl as the internal standard. c AlCl3 (10 mol%). d TBHP: 1.5 equiv. e TBHP: 4.0 equiv.
1	Co(OAc)2·4H2O	8.0	HOAc	100	15	84
2	Co(OAc)2·4H2O	8.0	InBr3	95	57	35
3	Co(OAc)2·4H2O	8.0	BF3·Et2O	78	41	31
4	Co(OAc)2·4H2O	8.0	In(OTf)3	62	39	20
5	Co(OAc)2·4H2O	8.0	AlCl3	98	59	38
6	Co(OAc)2·4H2O	8.0	—	97	37	58
7^c	Co(OAc)2·4H2O	8.0	AlCl3	75	29	40
8	Co(OAc)2·4H2O	4.0	AlCl3	100	42	53
9	Co(OAc)2·4H2O	16.0	AlCl3	87	55	30
10	CoCl2	8.0	AlCl3	88	64	23
11	Co(acac)2	8.0	AlCl3	81	60	15
12	Co(OAc)2	8.0	AlCl3	95	62	31
13	CoI2	8.0	AlCl3	98	94	3
14^d	CoI2	8.0	AlCl3	89	83	1
15^e	CoI2	8.0	AlCl3	100	52	44

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To explore the substrate scope, we examined a range of aldehydes (1) in the coupling with MeOH (2a) under the optimized reaction conditions (Table 2, entries 1–16). It was observed that electronic variation of the substituents at the para- and meta-positions of benzaldehyde did not significantly affect the reaction efficiency. The corresponding methyl carboxylate products of benzaldehyde substituted with a methoxy, nitro, and methyl group were obtained in satisfactory yields (entries 2–8). Benzaldehyde bearing electron-donating groups such as methoxy at the ortho-position could also be employed as facile substrates that provided the corresponding ester in a high yield (entry 10). However, the one with electron-withdrawing groups at the same position such as chloride performed poorly in conversion (entry 11). Not only methyl aromatic carboxylates but also methyl aliphatic carboxylates were readily obtained in acceptable yields (entries 12 and 13). In addition, various heteroaromatic aldehydes were tolerated in such a catalytic system and afforded their methyl carboxylates in moderate yields (entries 14–16). We next examined the scope of alcohol reactant in the cobalt-catalyzed oxidative esterification of aldehydes (Table 2, entries 17–20). In the case of benzaldehyde (1a), the desired alkoxylate products were obtained in moderate yields when branched alcohols were employed instead of linear ones.

Table 2 Scope of CoI₂-catalyzed oxidative esterification of aldehydes with alcohols^a

Entry	Substrate 1 (R^1)	Substrate 2 (R^2)	Conv. (%)	Product 3 (yield^b (%))
a General conditions: 1 (1 mmol), 2 (8 mmol), cat (5 mol%), AlCl3 (5 mol%), TBHP (2.0 mmol), 100 °C, 24 h. b Isolated yields. c Yields determined by GC-MS.
1	1a (Ph)	2a (Me)	97	3aa (94)
2	1b (4-NO2C6H4)	2a (Me)	100	3ba (80)
3	1c (4-OMeC6H4)	2a (Me)	100	3ca (94)
4	1d (4-MeC6H4)	2a (Me)	100	3da (89)
5	1e (4-ClC6H4)	2a (Me)	96	3ea (88)
6	1f (3-NO2C6H4)	2a (Me)	95	3fa (92)
7^c	1g (3-OMeC6H4)	2a (Me)	100	3ga (92)
8	1h (3-MeC6H4)	2a (Me)	100	3ha (93)
9	1i (3-ClC6H4)	2a (Me)	100	3ia (94)
10	1j (2-OMeC6H4)	2a (Me)	89	3ja (85)
11	1k (2-ClC6H4)	2a (Me)	54	3ka (49)
12	1l (n-C6H13)	2a (Me)	97	3la (65)^c
13	1m (Ph(CH2)2)	2a (Me)	95	3ma (72)
14	1n (4-pyridine)	2a (Me)	93	3na (75)
15	1o (2-furyl)	2a (Me)	91	3oa (66)
16	1p (2-thiophene)	2a (Me)	97	3pa (69)
17	1a (Ph)	2b (n-Et)	98	3ab (96)
18	1a (Ph)	2c (n-Bu)	94	3ac (94)
19	1a (Ph)	2d (i-Pr)	90	3ad (73)
20	1a (Ph)	2e (t-Bu)	100	3ae (58)

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To begin investigating the mechanism of this Co-catalyzed oxidative coupling of alcohols with aldehydes, the single electron transfer (SET) process was considered. The formation of ester was completely suppressed when two equivalents of TEMPO were introduced into the reaction under standard conditions, but no TEMPO adduct of acyl radicals was obtained (eqn (5)). Importantly, cross-coupling did not occur between the aldehyde and the C=C bond in the presence of 16 equiv. of methyl acrylate (eqn (6)), thus disfavoring an acyl radical pathway.¹⁵ Moreover, it was also demonstrated that the carboxylic acid (4aa) was formed as a side product rather than an intermediate in the oxidative esterification since only traces of the corresponding methyl carboxylate (3aa, conversion: 10%, yield: 8%) were obtained under standard conditions by reacting 4aa with MeOH (8 equiv.). Combined with the fact that the reaction is accelerated by a strong-Lewis acid, these results indicate that the reaction may proceed via a hemiacetal^13a pathway.

Notably, some alkali¹⁶ and organic iodides¹⁷ readily react with TBHP. As an alternative to the conventional understanding, pioneering work reported by Ishihara disclosed that the in situ generated hypoiodites (IO⁻/IO₂⁻) are the catalytically active species for the oxidative carbon–oxygen cross-coupling.^17a Encouraged by this discovery, many achievements on the new bond formation have been obtained recently by employing the iodide/TBHP oxidation system.^17b Interestingly, transition-metal catalysis in the presence of iodide either as a chelating ligand or additive often shows distinguished reactivity from the others.¹⁸ Although it was ever interpreted as a result of the iodine-assisted effect,^18h,i the reaction details were unclear. Indeed, in our study, both n-Bu₄NI and KI facilitated this transformation well, albeit providing different yields with respect to specific substituents (Table S1†). Thus, the uncertainty of whether Co(II) was involved in the catalytic cycle arose.

To address this issue, we performed various control experiments and spectroscopic analysis. A cyclic voltammogram (CV) obtained from an acetonitrile (CH₃CN) solution of CoI₂ (1 mM) revealed the presence of two reversible signals at E_1/2 −0.07 V and E_1/2 0.32 V (vs. Ag⁺/Ag) corresponding to 3I⁻/I₃⁻ and I₃⁻/3/2I₂, respectively, and a broad irreversible Co²⁺/Co³⁺ wave at a relatively higher positive E_ox ∼ 1.80 V (vs. Ag⁺/Ag) (Fig. 1). These iodide oxidative features are nearly the same as those observed with a solution containing non-transition-metal iodides (NaI and n-Bu₄NI, Fig. S2†), however, the oxidative potential of the Co²⁺/Co³⁺ couple is much higher compared with that detected for the chlorine-ligated analogue (CoCl₂). These CV data indicated that the iodide chelating ligand is relatively more sensitive to oxidants other than the Co(II) ion, on the other hand, Co(III)I₂ is capable of oxidizing I⁻ and I₃⁻ ions if formed.

Fig. 1 Cyclic voltammograms of 1 mM CoI₂ and CoCl₂. Conditions: 0.1 M LiClO₄ in CH₃CN, under Ar, 50 mV s⁻¹ scan rate.

The lack of reactivity between halogen-ligated Co(II) and TBHP was subsequently demonstrated by UV-visible spectroscopic study, indicating oxidation did not occur in the reaction with CoCl₂ at 100 °C for 30 min (Fig. 2A). But it readily proceeded in the case of CoI₂ even at lower temperature. Representative spectra were obtained during oxidation of CoI₂ in CH₃CN at 60 °C (Fig. 2B). The absorption band of the I⁻ ion at 247 nm decreased with time. Instead, the transient iodide species referenced at 289 nm and 362 nm¹⁹ emerged and reached their potential concentration limit within 5 min. Importantly, the absorption band at a range from 600 nm to 800 nm typically assigned to tetrahedral complexes of halogenated Co(II)²⁰ deceased sharply, coinciding with a broad signal appeared at around 650 nm. Moreover, the obtained spectrum was also different from Co(II)I₂ plus I₂ (Fig. S5†). The charge variation on the cobalt ion was, therefore, most likely involved.

Fig. 2 UV-visible spectra acquired from monitoring the reaction time courses of the oxidation of CoI₂ and CoCl₂ by TBHP ((A) 0.05 mM CoCl₂, CoCl₂ : TBHP = 1 : 26, reaction at 100 °C; (B) 0.05 mM CoI₂, CoI₂ : TBHP = 1 : 26, reaction at 60 °C; insert: CoI₂: 4.0 mM, CoI₂ : TBHP = 1 : 7, reaction at 25 °C.).

Co2p XPS (X-ray photoelectron spectroscopy) spectra were then acquired for the reaction residue of CoI₂ and TBHP (20 equiv.) to elucidate the electronic state variation of the cobalt ion during the catalytic cycle. Upon reaction at 60 °C for 6 h, the sample showed a Co2p spectrum of the main peak at 781.0 eV, integrating about 49% of the overall signal area, and minor satellites at 783.5, 787.0 and 790.5 eV, with ΔE_SO (spin-orbital splitting) contributions of 15.4 eV (Fig. 3A). The XPS Co2p spectral features of CoI₂ (Fig. 3B), on the other hand, showed a higher ΔE_SO (16.0 eV), with high intensity satellites, as expected for 3d ions with unquenched orbital momentum.²¹ This difference in ΔE_SO of the Co2p spectrum suggested the presence of ls-Co(III) (ls = low spin) in the oxidized sample.²²

Fig. 3 The Co2p (left) and the I3d (right) XPS spectrum of the catalyst residue and raw CoI₂ along with deconvolution of the photopeaks and the corresponding best fitting lines (reaction conditions for residue a: 0.1 M CoI₂, 2.0 M TBHP, CH₃CN as solvent, 6 h at 60 °C; reaction conditions for residue a: 2.6 × 10⁻³ M; CoI₂ : TBHP = 1 : 20, CH₃CN as solvent, 1 h at 60 °C).

In addition, the corresponding I3d XPS spectral study of the residue (Fig. 3C and D) showed two separated bands at both I 3d_5/2 and 3d_3/2 levels. The narrow and asymmetric band at the relatively lower level was deconvoluted into two peaks of intensity ratio ca. 1.4 : 1 with different binding energies, containing values assigned to I₂ at 620.0 eV (ref. 23) and I⁻ at 619.2 eV. Note that the I₃⁻ ion is a resonating system consisting of the I⁻ ion and I₂ molecule and the 3d core-level spectra for some I₃⁻ show a band composed of 2 : 1 double-component peaks. This detected iodine species was, therefore, attributed to a mixture of I₃⁻ and unconverted I⁻ ions. Independent of this, the higher energy iodate, IO₃⁻, was directly formed (Fig. 3C) as illustrated by the I 3d_5/2 band typically located at 624.3 eV.²³ In comparison, this band for IO₃⁻ was not detected in the residue of n-Bu₄NI with TBHP under identical conditions (Fig. S6†). These observations can be interpreted as a result of an oxidative conversion from I⁻ to IO₃⁻via I₃⁻, leading to a mixture with different ratios depending on the conditions. It's understandable that no iodate emerged in the residue (Fig. 3D) after the reaction of the substrates of reduced concentration (CoI₂: 2.6 × 10⁻³ M; CoI₂ : TBHP = 1 : 20) for a shorter time (1 h). Noteworthily, the rapid generation and subsequent disproportionation of the transient hypoiodites (IO⁻/IO₂⁻) account for the transformation either from I⁻ to I₃⁻ or the subsequent from I₃⁻ to IO₃⁻.^17a,19 Thus, albeit undetected, hypoiodites were poised to be in the catalytic cycle.

Kinetic evidence that CoI₂ is quite different from KI and n-Bu₄NI as the catalyst in the decomposition of TBHP was obtained by comparing their specific rate on iodide oxidation. Indicated by the literature and our cross experiments (Fig. 3, S3 and S4†), the oxidative conversion from I⁻ to I₃⁻ was supposed to occur in each case at the initial stage, especially with employing dilute substrates. These oxidations were monitored at 362 nm by the UV-visible spectrum which was assigned to the in situ generated transient hypoiodites (IO⁻ and IO₂⁻) and relatively stable I₃⁻.¹⁹

It was found that the cobalt mediated iodide oxidation proceeds much faster than the other two non-transitional metal cases from the initial slope of the plots (Fig. 4A). Additionally, the saturated capacity of transient iodides at equilibrium in the case of either KI or n-Bu₄NI was not significantly affected (Fig. 4B). In contrast, the oxidized species of iodide was found to be quickly predominant in the solution of CoI₂ at a lower equivalent of TBHP and reached the potential limit with around only two equivalents to [I⁻], a 20-fold lower present in the catalytic reaction. Specifically, with increased concentration of TBHP to eight equivalents, the absorption at 362 nm decreased sharply (Fig. 4B). It's attributable to the second oxidative conversion from I₃⁻ to IO₃⁻. Thus, the iodide chelating ligand in CoI₂ was prone to be quickly converted to an inert I₃⁻ ion and the hypoiodites thereof formed were not kinetically competent to serve as the active site unless the subsequent generation occurred during the deep oxidation to IO₃⁻.

Fig. 4 (A) Plot of the initial slope acquired from monitoring the reaction time course of the oxidation of iodides by TBHP at 25 °C. (B) Plot of the potential amount limit of IO⁻/I₃⁻ produced by the reaction of iodides ([I⁻]: 0.4 mM) with various concentrations of TBHP at 60 °C.

Collectively, the reaction of CoI₂ with TBHP was poised to start with oxidative transformation of the iodide chelating ligand to the active intermediate, while the charge variation on cobalt remained at this stage (Scheme 1). It seems likely that the active Co(II)OH species were thereof formed and initiated another pathway to decompose TBHP. In this emerged hydroxyl system, the cobalt is cycling between oxidation states +II and +III. And as soon as the Co(III) species were formed, they reacted as one-electron oxidant to accelerate the transformation speed of I⁻ to I₃⁻ and thus making the second oxidative conversion to IO₃⁻ possible.

Scheme 1 Proposed mechanism for CoI₂ catalyzed oxidative esterification of aldehydes.

The subsequent control experiment by using Co(OH)₂ instead of CoI₂ as the catalyst proceeded in a reduced efficiency leading to 3aa in a yield of 70% (Table 3, entry 2). We, therefore, could not completely rule out a catalytic role of the iodide. And the in situ generated hypoiodites account for the catalytically active species making the important hydrogen abstraction from hemiacetals more selective (Scheme 1), however, they are probably mainly from the second oxidative conversion from I₃⁻ to IO₃⁻ under Co(II) mediation. In consistence with this assumption, the reaction by using a combination of Co(OH)₂ with either n-Bu₄NI or n-Bu₄NI₃ as the catalyst provided a comparable yield as CoI₂ (entries 5 and 6). But the performance was not improved upon direct addition of either molecular iodine or iodate additive (entries 3 and 4). In these cases without efficient iodide additives, carboxylic acid 4aa was formed as the main side product (entries 2, 3 and 4).

Table 3 Mechanistic study^a

Entry	Cat.	Additive (mol%)	Yield^b (%)
a General conditions: 1a (1 mmol). b Yields determined by GC with biphenyl as the internal standard. c Yield of 4aa: 2: 29%; 3: 31%; 4: 30%.
1	CoI2	—	94
2^c	Co(OH)2	—	70
3^c	Co(OH)2	I2 (5)	68
4^c	Co(OH)2	AgIO3 (10)	66
5	Co(OH)2	n-Bu4NI3 (10)	90
6	Co(OH)2	n-Bu4NI (10)	89

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Conclusions

The present study reports a practical CoI₂ catalyzed system for the oxidative C–O cross-coupling of various aldehydes and alcohols. It is the first homogeneous inexpensive cobalt catalyst system for oxidative esterification of aldehydes. The methods are compatible with substrates bearing a variety of functional groups, including electron-poor or rich aromatic aldehydes, as well as aliphatic and heterocyclic aldehydes. And, the methods exhibit highly favorable practical characteristics: a solvent free system, a rather low alcohol/aldehyde ratio and the catalyst components are inexpensive and commercially available reagents. More importantly, this study has provided extensive insights into the cobalt mediated decomposition of TBHP in the presence of the iodide ion. The in situ generated hypoiodites mainly from I₃⁻ to IO₃⁻ account for a cooperative effect with oxygen centered radical species to offer the highly efficient conversion.

Acknowledgements

The authors are grateful to the National Key Basic Research Program of China (or 973 Program, No. 2015CB251401), the National Natural Science Foundation of China (No. 21476240), the Special Funds of the National Natural Science Foundation of China (No. 21127011) and the CAS 100-Talent Program (2014), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07070600) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5qo00293a

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