An expedient osmium(VI)/K₃Fe(CN)₆-mediated selective oxidation of benzylic, allylic and propargylic alcohols

Abstract

A chemoselective osmium(VI) catalyzed oxidation of benzylic, allylic and propargylic alcohols using K₃Fe(CN)₆ as a secondary oxidant is described. This protocol is operationally simple and exhibits excellent chemoselectivity favouring the oxidation of benzylic alcohols over the aliphatic alcohols. A larger scale reaction was also found to be compatible.

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Introduction

Oxidation of alcohols is an important method for functional group transformations in synthetic organic chemistry¹ as well as in industry.² Many methods have been developed to accomplish this elementary reaction. Some of the notable methods involve the use of chromium reagents,³ manganese(IV)oxide,⁴ (COCl)₂/DMSO (Swern oxidation),⁵ hypervalent iodine reagents,⁶ TEMPO,⁷ etc. An alternative and even more practical approach using metal complexes as catalyst in combination with terminal oxidants has been reported. Examples include Cu,⁸ Pd,⁹ Ru,¹⁰ Au,¹¹ W,¹² Mn,¹³ Fe,¹⁴ Co,¹⁵ V,¹⁶ etc. An attractive alternative is the use of O₂ as primary oxidant in transition metal catalyzed alcohol oxidations.¹⁷ In addition, Iwabuchi and co-workers reported an elegant method, in which the oxidation of various primary and secondary alcohols into corresponding carbonyl compounds is mediated by a novel alkoxyamine-type organocatalyst, 3-methyl-4-oxa-5-azahomoadamantane with NaOCl as the primary oxidant.¹⁸ However, the organocatalyst shows high reactivity and is not suitable for chemoselective oxidation of allylic, benzylic, and propargylic alcohols. Recently, Stahl and co-workers showed that Cu(I)/ABNO or TEMPO catalytic system exhibits high chemoselectivity for unhindered primary alcohols.¹⁹ A few reports are documented to address the chemoselective oxidation of allylic and benzylic alcohols. Notable examples include DDQ/NaNO₂,²⁰ DDQ/Mn(OAc)₃,²¹ NBS/thiourea,²² vanadium complexes,²³ Pd-catalyst/α-bromo sulphoxide,²⁴ Pt black/H₂O₂,²⁵ Fe-catalyst/Na₂CO₃ ²⁶ and N-hydroxyindole/CuCl.²⁷

While many methods are known in literature for alcohol oxidation, the selectivity issue is still a concern and there is scope for developing newer methods, particularly for chemoselective oxidation of allylic, benzylic, and propargylic alcohols. Although osmium(IV) is frequently used for the dihydroxylation of olefins,²⁸ Beller and co-workers for the first time reported Os(IV)/DABCO catalyzed oxidation of alcohols using O₂ as a primary oxidant.²⁹ However, this protocol is not suitable for chemoselective oxidation of allylic and benzylic alcohols. Later Brown and co-workers achieved a practical Os/Cu co-catalyzed air oxidation of allylic and benzylic alcohols.³⁰ Recently Shah and co-workers developed osmium/chloramine-T catalyzed oxidation of allylic and benzylic alcohols.³¹ Working on similar lines we explored the K₂OsO₄·2H₂O/K₃[Fe(CN)₆]-mediated chemoselective oxidation of various allylic, benzylic and propargylic alcohols to carbonyl compounds in good to excellent yields. Primary and secondary unactivated alcohols are unreactive towards this oxidation system (Scheme 1).

Scheme 1 Oxidation of benzylic, allylic and propargylic alcohols.

Results and discussion

To begin this study, 4-methoxy benzyl alcohol (1) was chosen as a model substrate to optimize the reaction conditions (Table 1). Initially, 1 was added to a well stirred solution of K₂OsO₄·2H₂O (0.4 mol%), K₃[Fe(CN)₆] (3.0 equiv.), K₂CO₃, (3.0 equiv.), pyridine (2.0 mol%), and MeSO₂NH₂ (2.0 equiv.) in a mixture of tBuOH : H₂O (1 : 1). The reaction was performed in a closed vessel and stirred for 72 h at room temperature. The desired product 4-methoxy benzaldehyde (2a) was isolated in 67% yield (Table 1, entry 1). When we increased the catalyst loading to 0.6 mol% and 0.8 mol% (Table 1, entries 2 and 3), after 48 h, under similar conditions the yield of 2a was improved to 73% and 78% respectively. With the increase in the amount of secondary oxidant, K₃[Fe(CN)₆] and K₂CO₃ to 6.0 equiv. each, 87% of 2a was isolated (Table 1, entry 4). However to our delight, increasing the Os(VI) catalyst loading to 1.0 mol%, after 13 h, 2a was obtained quantitatively (entry 5). To improve the oxidation efficiency, we kept the Os(VI) loading to 1 mol%, and decreased the amount of secondary oxidant and K₂CO₃ to 3.0 equiv. each. This resulted in 2a in 93% yield (entry 6). Further decrease in the amount of oxidant and K₂CO₃ (2.0 equiv. each) required longer reaction time and gave lower yield (Table 1, entry 7). The requirement of additives like pyridine and MeSO₂NH₂ was further investigated. With no MeSO₂NH₂ added, the reaction was completed in 18 hours giving 2a in 96% yield (entry 8). Similarly, with no pyridine added, 2a was obtained also in 96% yield (entry 9). Gratifyingly, in the absence of both pyridine and MeSO₂NH₂, the reaction yielded 2a quantitatively (entry 10). With no primary oxidant K₂OsO₄·2H₂O there was no reaction (entry 11). Similarly, with no K₃[Fe(CN)₆] or no K₂CO₃, the reaction did not work (entries 12 and 13). This strongly indicated the need of secondary oxidants.

Table 1 Optimization of alcohol oxidation varying the amounts of Os catalyst, Fe complex, K₂CO₃ and additives^a

Entry	K2OsO4·2H2O (X mol%)	K3[Fe(CN)6] (Y equiv.)	K2CO3 (Z equiv.)	Pyridine (mol%)	MeSO2NH2 (equiv.)	t (h)	Yield^b (%)
a Reaction conditions: substrate (0.5 mmol), K2OsO4·2H2O (X mol%), K3[Fe(CN)6] (Y equiv.), K2CO3 (Z equiv.), pyridine, MeSO2NH2, in tBuOH (1.5 mL) and H2O (1.5 mL) at room temperature.b Isolated yields. NR = no reaction.
1	0.4	3.0	3.0	2.0	2.0	72	67
2	0.6	3.0	3.0	2.0	2.0	48	73
3	0.8	3.0	3.0	2.0	2.0	48	78
4	0.8	6.0	6.0	2.0	2.0	48	87
5	1.0	6.0	6.0	2.0	2.0	13	Quant.
6	1.0	3.0	3.0	2.0	2.0	18	93
7	1.0	2.0	2.0	2.0	2.0	72	62
8	1.0	3.0	3.0	2.0	—	18	96
9	1.0	3.0	3.0	—	2.0	15	96
10	1.0	3.0	3.0	—	—	15	Quant.
11	—	3.0	3.0	—	—	15	NR
12	1.0	—	3.0	—	—	15	2
13	1.0	3.0	—	—	—	15	NR

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Encouraged by these results, we carried out the screening of solvent and temperature conditions (Table 2) using the optimum requirement from Table 1, entry 10. Of the solvent mixtures (with water, Table 2, entries 1–7) tested we found CH₃CN : H₂O (1 : 1) was the best combination which delivered aldehyde 2a quantitatively, with substantially reduced reaction time (entry 6, 1.5 h). Further, from the temperature study (Table 2, entries 8–12) the reaction in CH₃CN : H₂O (1 : 1) at 60 °C reduced the reaction time to just 15 min, producing aldehyde 2a quantitatively. The oxidation of 1a on gram scale (1 g, 7.23 mmol) gave 2a without much change in reaction time and yield (98%, entry 13). Thus, with the optimized condition which include the use of K₂OsO₄·2H₂O (1.0 mol%), K₃[Fe(CN)₆] (3.0 equiv.), K₂CO₃ (3.0 equiv.) in CH₃CN : H₂O (1 : 1) at 60 °C, we explored the scope and limitations of this oxidation protocol (Table 3).

Table 2 Optimization of alcohol oxidation varying solvent and temperature^a

Entry	Solvent (1 : 1)	T °C	t	% yield^b
a Reaction conditions: substrate (0.5 mmol), K2OsO4·2H2O (0.005 mmol), K3[Fe(CN)6] (1.5 mmol), K2CO3 (1.5 mmol) in solvent (1.5 mL) and H2O (1.5 mL).b Isolated yields.c Reaction on 1 g, 7.23 mmol of 1.
1	tBuOH : H2O	rt	15 h	Quant.
2	DMF : H2O	rt	5 h	78
3	DMSO : H2O	rt	4 h	88
4	THF : H2O	rt	27 h	67
5	Toluene : H2O	rt	15 h	73
6	CH3CN : H2O	rt	1.5 h	Quant.
7	(CH3)2CO : H2O	rt	6 h	92
8	tBuOH : H2O	45	13 h	97
9	tBuOH : H2O	60	12 h	95
10	CH3CN : H2O	45	30 min	Quant.
11	CH3CN : H2O	60	15 min	Quant.
12	CH3CN : H2O	80	12 min	Quant.
13	CH3CN : H2O	60	20 min	98^c

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Table 3 Oxidation of various aryl alcohols by K₂[OsO₄·2H₂O]/K₃[Fe(CN)₆] system^a

Entry	Product		t	Yield^b (%)
a Reaction conditions: substrate (0.5 mmol), K2OsO4·2H2O (0.005 mmol), K3[Fe(CN)6] (1.5 mmol), K2CO3 (1.5 mmol) in CH3CN (1.5 mL) and H2O (1.5 mL) at 60 °C.b Isolated yields.
1	4-MeO-benzaldehyde	2a	15 min	Quant.
2	4-Me-benzaldehyde	2b	20 min	98
3	2,5-Dimethoxybenzaldehyde	2c	45 min	Quant.
4	3,4-Dimethoxybenzaldehyde	2d	30 min	Quant.
5	4-NO2-benzaldehyde	2e	10 min	Quant.
6	3-NO2-benzaldehyde	2f	15 min	98
7	4-Cl-benzaldehyde	2g	45 min	87
8	2-Cl-benzaldehyde	2h	45 min	81
9	α-Napthaldehyde	2i	20 min	96
10	Piperonal	2j	25 min	Quant.
11	2-OH-benzaldehyde	2k	45 min	65
12	4-OH-3-methoxy benzaldehyde	2l	45 min	88
13	Thiophene-2-carbaldehyde	2m	2 h	78
14	1H-Indole-3-carbaldehyde	2n	2 h	78
15	Furfural	2o	2 h	82
16	Pyridine-3-carbaldehyde	2p	6 h	36
17	Pyridine-2-carbaldehyde	2q	10 h	38
18	Quinoline-4-carbaldehyde	2r	18 h	40
19	3-Methyl-1-phenyl-1H-pyrazole-4-carbaldehyde	2s	18 h	42
20	1H-Imidazole-4-carbaldehyde	2t	18 h	51
21		2u	4 h	73
22	Acetophenone	2v	15 min	Quant.
23	Benzophenone	2w	15 min	Quant.
24		2x	30 min	Quant.
25		2y	8 h	91

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Various benzyl alcohols with aryl substitution involving electron donating and withdrawing groups like methoxy, methyl, nitro, chloro and hydroxyl were compatible with the reaction conditions, delivering the desired aryl aldehydes 2a–l in good to quantitative yields within the short reaction times of 10–45 min (Table 3, entries 1–12). Benzyl alcohols with strong electron withdrawing nitro groups exhibited higher reactivity compared to benzyl alcohols having the electron donating groups (Table 3, e.g. entry 5 vs. 3). In some cases a mere filtration of the reaction mixture and concentration of the filtrate afforded virtually pure products. α-Hetero aryl methyl alcohols delivered the corresponding aldehydes 2m–u (entries 13–21) in good yields with the exception of N-based heterocycles like 2p–t obtained in moderate 36–51% yields (entries 16–20). It appears that N-based heterocycles are poor substrates with the exception of 1H-indole-3ylmethanol giving 2n in good yields (78%, entry 14, Table 3). The dibenzofuran based aldehyde 2u was obtained in good yield (73%, entry 21) after 4 h reaction. Secondary benzylic alcohols were also oxidized under the present oxidation protocol delivering aryl ketones 2v–y in good to quantitative yields (entries 22–25).

The method was further explored towards oxidation of benzylic and/or allylic and propargylic alcohols (Table 4). With the optimized conditions, the primary allyl alcohols delivered the unsaturated aldehydes 4a and b in good yields (Table 4, entries 1 and 2). The secondary allyl alcohols provided the unsaturated ketones 4c–f in moderate to good yields (entries 3–6). Similarly, the primary propargyl alcohols afforded the alkynals 4g and h in good yields (entries 7 and 8). The benzylic and secondary propargyl alcohols gave the corresponding ketones 4i–l in good to excellent yields (entries 9–12). The selective oxidation of benzylic over unactivated primary aliphatic alcohols resulted in only benzylic alcohol oxidation delivering the β-hydroxyalkyl aryl ketones 4m–p in good yields (entries 13–16). The oxidation of unactivated primary or secondary alcohols failed to deliver the corresponding carbonyl compounds 4q–t (entries 17–20) indicating that the present protocol is mild and selective towards oxidation of benzylic, allylic and propargylic alcohols.

Table 4 Oxidation of allylic, benzylic and propargylic alcohols^a

Entry	Product		T (h)	Yield^b (%)
a Reaction conditions: substrate (0.5 mmol), K2OsO4·2H2O (0.005 mmol), K3[Fe(CN)6] (1.5 mmol), K2CO3(1.5 mmol) in CH3CN (1.5 mL) and H2O (1.5 mL) at 60 °C.b Isolated yields. NF = not formed.
1		4a	3	87
2		4b	14	88
3		4c	18	77
4		4d	18	58
5		4e	18	47
6		4f	18	68
7		4g	3	85
8		4h	18	65
9		4i	10	88
10		4j	10	68
11		4k	10	76
12		4l	10	64
13		4m	10	84
14		4n	12	85
15		4o	12	69
16		4p	12	77
17	Dodecanal	4q	48	NF
18	Phenylacetaldehyde	4r	48	NF
19		4s	48	NF
20		4t	48	NF

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Conclusions

In conclusion, we have developed an efficient method for the selective oxidation of benzylic, allylic and propargylic alcohols with catalytic Os(VI) and using K₃[Fe(CN)₆] as secondary oxidant. The method is mild, operationally simple and can be carried out in a closed flask. High yields of aryl and unsaturated aldehydes and good chemoselectivity are other advantages of this method.

Experimental section

General information

Solvents and reagents were purified by standard methods. Thin-layer chromatography was performed on EM 250 Kieselgel 60 F254 silica gel plates. The spots were visualized by staining with KMnO₄ or under UV lamp. ¹H and ¹³C NMR were recorded with a Bruker, AVANCE III 500 or 400 spectrometer and the chemical shifts are based on TMS peak at δ = 0.00 pm for proton NMR and CDCl₃ peak at δ = 77.00 ppm (t) in carbon NMR. IR spectra were obtained on Perkin Elmer Spectrum One FT–IR spectrometer and samples were prepared by evaporation from CHCl₃ on CsBr plates. High-resolution mass spectra (HRMS) were obtained using positive electrospray ionization by TOF method.

General procedure for oxidation of alcohols

To a well stirred solution of K₂OsO₄·2H₂O (1.8 mg, 0.005 mmol, 1.0 mol%), K₃[Fe(CN)₆] (554 mg, 1.5 mmol, 3.0 equiv.), K₂CO₃, (207 mg, 1.5 mmol, 3.0 equiv.) in CH₃CN (1.5 mL) and H₂O (1.5 mL) was added the substrate alcohol (0.5 mmol) at room temperature. The reaction mixture was warmed to 60 °C and stirred for specified time (see Tables 3 and 4). It was then quenched with aq. saturated solution of. Na₂SO₃ (1.0 mL) and the solvent partially evaporated. The reaction mixture was then filtered through a small pad of silica gel and washed with EtOAc (3 × 10 mL). The filtrate was concentrated and the residue in some cases contained virtually pure compound and no further purification was necessary. In other cases the residue was purified by silica gel column chromatography using petroleum ether/EtOAc as an eluent to afford the carbonyl compounds.

4-Methoxybenzaldehyde (2a). Isolated yield of 2a, (68 mg, quant.). Colorless oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.89 (s, 1H), 7.84 (d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H), 3.90 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 190.6, 164.4, 131.8, 129.7, 114.1, 55.4.

4-Methylbenzaldehyde (2b). Isolated yield of 2b, (59 mg, 98%). Colorless oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.96 (s, 1H), 7.77 (d, J = 8.1 Hz, 2H), 7.33 (d, J = 7.8 Hz, 2H), 2.43 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 191.9, 145.4, 134.1, 129.7, 129.6, 21.7.

2,5-Dimethoxybenzaldehyde (2c). Isolated yield of 2c, (83 mg, quant.). Yellow crystalline solid, mp 44–46 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.43 (s, 1H), 7.32 (d, J = 3.3 Hz, 1H), 7.13 (dd, J = 9.0, 3.3 Hz, 1H), 6.93 (d, J = 9.0 Hz, 1H), 3.89 (s, 3H), 3.79 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 189.6, 156.7, 153.6, 124.9, 123.5, 113.3, 110.4, 56.1, 55.8.

3,4-Dimethoxybenzaldehyde (2d). Isolated yield of 2d, (83.0 mg, quant.). White solid, mp 41–42 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.84 (s, 1H), 7.45 (dd, J = 8.2, 2.0 Hz, 1H), 7.40 (d, J = 1.8 Hz, 1H), 6.97 (d, J = 8.2 Hz, 1H), 3.96 (s, 3H), 3.93 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 190.9, 154.4, 149.6, 130.1, 126.8, 110.3, 108.9, 56.1, 56.0.

4-Nitrobenzaldehyde (2e). Isolated yield of 2e, (75.5 mg, quant.). Yellow solid, mp 102–104 °C; ¹H NMR (500 MHz, CDCl₃/TMS) δ 10.16 (s, 1H), 8.40 (d, J = 8.6 Hz, 2H), 8.07 (d, J = 8.6 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 190.3, 151.1, 140.0, 130.5, 124.3.

3-Nitrobenzaldehyde (2f). Isolated yield of 2f, (74 mg, 98%). Yellow solid, mp 54–56 °C; ¹H NMR (500 MHz, CDCl₃/TMS) δ 10.12 (s, 1H), 8.73–8.71 (m, 1H), 8.49 (d, J = 8.2 Hz, 1H), 8.24 (d, J = 7.6 Hz, 1H), 7.77 (t, J = 7.90 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 189.7, 148.7, 137.3, 134.6, 130.3, 128.5, 124.3.

4-Chlorobenzaldehyde (2g). Isolated yield of 2g, (61.1 mg, 87%). White solid, mp 45–47 °C; ¹H NMR (500 MHz, CDCl₃/TMS) δ 9.98 (s, 1H), 7.83 (d, J = 8.6 Hz, 2H), 7.52 (d, J = 8.3 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 190.8, 140.9, 134.6, 130.8, 129.4.

2-Chlorobenzaldehyde (2h). Isolated yield of 2h, (56.9 mg, 81%). Colorless oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.50 (s, 1H), 7.93 (d, J = 7.7 Hz, 1H), 7.54–7.51 (m, 1H), 7.47–7.45 (m, 1H), 7.40 (t, J = 7.5 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 189.9, 137.9, 135.1, 132.3, 130.5, 129.3, 127.2.

1-Naphthaldehyde (2i). Isolated yield of 2i, (75 mg, 96%). Colorless oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.41 (s, 1H), 9.26 (d, J = 8.6 Hz, 1H), 8.10 (d, J = 8.2 Hz, 1H), 7.99 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.2 Hz, 1H), 7.72–7.70 (m, 1H), 7.68–7.58 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 193.6, 138.9, 136.7, 135.3, 133.7, 131.4, 130.5, 129.1, 128.5, 126.9, 124.9.

Piperonal (2j). Isolated yield of 2j, (75 mg, quant.). White solid, mp 35–37 °C; ¹H NMR (500 MHz, CDCl₃/TMS) δ 9.81 (s, 1H), 7.41 (d, J = 7.9 Hz, 1H), 7.33 (d, J = 1.5 Hz, 1H), 6.92 (d, J = 7.9 Hz, 1H), 6.07 (s, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 190.2, 153.0, 148.6, 131.8, 128.6, 108.3, 106.8, 102.0.

2-Hydroxybenzaldehyde (2k). Isolated yield of 2k, (39.7 mg, 65%). Colorless oil; ¹H NMR (500 MHz, CDCl₃/TMS) δ 11.02 (s, 1H), 9.89 (s, 1H), 7.57–7.50 (m, 2H), 7.04–6.98 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 196.6, 161.6, 137.0, 133.7, 120.6, 119.8, 117.6.

4-Hydroxy-3-methoxybenzaldehyde (2l). Isolated yield of 2l, (66.9 mg, 88%). White solid, mp 81–82 °C; ¹H NMR (500 MHz, CDCl₃/TMS) δ 9.83 (s, 1H), 7.44–7.42 (m, 2H), 7.04 (d, J = 8.5 Hz, 1H), 6.21 (s, 1H), 3.97 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 191.0, 151.8, 147.2, 129.7, 127.5, 114.4, 108.8, 56.0.

Thiophene-2-carbaldehyde (2m). Isolated yield of 2m, (43.7 mg, 78%). Pale yellow oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.95 (s, 1H), 7.80–7.77 (m, 2H), 7.24–7.21 (m, 1H); ¹³C NMR (100 MHz, CDCl₃) δ = 183.0, 144.0, 136.3, 135.1, 128.3.

1H-Indole-3-carbaldehyde (2n). Isolated yield of 2n, (56.6 mg, 78%). Brown solid, mp 193–195 °C; ¹H NMR (400 MHz, DMSO-d₆) δ 9.89 (s, 1H), 8.23 (s, 1H), 8.07 (d, J = 7.8 Hz, 1H), 7.51 (d, J = 7.6 Hz, 1H), 7.27–7.19 (m, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 186.0, 139.2, 137.5, 124.4, 124.1, 122.8, 121.3, 118.6, 112.9.

Furfural (2o). Isolated yield of 2o, (39.4 mg, 82%). Yellow oil; ¹H NMR (500 MHz, CDCl₃/TMS) δ 9.69 (s, 1H), 7.72–7.71 (m, 1H), 7.27 (d, J = 0.4 Hz, 1H), 6.63 (dd, J = 3.6, 1.7 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 177.7, 152.8, 148.0, 121.0, 112.5.

Pyridine-3-carbaldehyde (2p). Isolated yield of 2p, (19.3 mg, 36%). Yellow oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.11 (s, 1H), 9.07 (d, J = 1.2 Hz, 1H), 8.84 (dd, J = 4.8, 1.4 Hz, 1H), 8.16 (td, J = 6.0, 2.0 Hz, 1H), 7.48 (dd, J = 7.8, 4.9 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 190.7, 154.7, 152.0, 135.8, 131.4, 124.1.

Pyridine-2-carbaldehyde (2q). Isolated yield of 2q, (20.4 mg, 38%). Yellow oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.10 (s, 1H), 8.80 (d, J = 4.4 Hz, 1H), 7.97 (d, J = 7.7 Hz, 1H), 7.88 (td, J = 7.6, 0.7 Hz, 1H), 7.55–7.52 (m, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 193.1, 152.5, 149.9, 136.9, 127.7, 121.5.

Quinoline-4-carbaldehyde (2r). Isolated yield of 2r, (31.4 mg, 40%). White solid, mp 45–47 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.53 (s, 1H), 9.20 (d, 4.2 Hz, 1H), 9.02 (dd, J = 8.5, 1.0 Hz, 1H), 8.23 (d, J = 8.4 Hz, 1H), 7.85–7.80 (m, 2H), 7.74 (td, J = 7.7, 1.3 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 192.9, 150.4, 149.2, 136.8, 130.2, 130.0, 129.4, 125.8, 124.4, 123.9.

3-Methyl-1-phenyl-1H-pyrazole-4-carbaldehyde (2s). Isolated yield of 2s, (39.1 mg, 42%). White solid, mp 57–59 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.99 (s, 1H), 8.34 (s, 1H), 7.68 (dd, J = 8.6, 1.1 Hz, 2H), 7.49 (t, J = 7.9 Hz, 2H), 7.38–7.34 (m, 1H), 2.59 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 184.3, 151.9, 139.0, 131.7, 129.6, 127.6, 122.9, 119.5, 13.0.

1H-Imidazole-4-carbaldehyde (2t). Isolated yield of 2t, (24.5 mg, 51%). Pale yellow solid, mp 174–176 °C; ¹H NMR (400 MHz, CD₃OD) δ 9.77 (s, 1H), 7.93 (s, 1H), 7.90 (s, 1H); ¹³C NMR (100 MHz, CD₃OD) δ 183.4, 138.8, 134.9, 129.5.

Dibenzo[b,d]furan-4-carbaldehyde (2u). Isolated yield of 2u, (71.6 mg, 73%). White solid, mp 94–96 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 10.6 (s, 1H), 8.21 (dd, J = 7.6, 1.2 Hz, 1H), 8.01–7.95 (m, 2H), 7.70 (d, J = 8.1 Hz, 1H), 7.54 (td, J = 7.8, 1.1 Hz, 1H), 7.49 (t, J = 7.5 Hz, 1H), 7.42 (t, J = 7.5 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 188.4, 156.6, 155.9, 128.1, 127.5, 126.6, 126.0, 123.5, 122.9, 122.8, 121.2, 120.8, 112.1.

Acetophenone (2v). Isolated yield of 2v, (60 mg, quant.). Colorless oil; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.97–7.92 (m, 2H), 7.57–7.51 (m, 1H), 7.48–7.41 (m, 2H), 2.59 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 198.1, 137.0, 133.0, 128.5, 128.2, 26.5.

Benzophenone (2w). Isolated yield of 2w, (91.1 mg, quant.). White solid, mp 47–49 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.81 (d, J = 7.0 Hz, 4H), 7.61–7.57 (m, 2H), 7.49 (t, J = 7.8 Hz, 4H); ¹³C NMR (100 MHz, CDCl₃) δ 196.7, 137.5, 132.4, 130.0, 128.2.

9H-Fluoren-9-one (2x). Isolated yield of 2x, (90.1 mg, quant.). Yellow solid, mp 80–82 °C; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.66 (d, J = 7.4 Hz, 2H), 7.53–7.46 (m, 4H), 7.31–7.27 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 193.7, 144.3, 134.5, 134.0, 128.9, 124.1, 120.2.

1-(Benzo[d][1,3]dioxol-5-yl)butan-1-one (2y). Isolated yield of 2y, (87.5 mg, 91%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3079, 2964, 2928, 2906, 2873, 2784, 1674, 1605, 1504, 1488, 1443, 1359, 1303, 1248, 1141, 1114, 1089, 1039, 998, 935, 911, 807, 648, 620, 577; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.56 (dd, J = 8.2, 1.6 Hz, 1H), 7.44 (d, J = 1.7 Hz, 1H), 6.84 (d, J = 8.2 Hz, 1H), 6.03 (s, 2H), 2.86 (t, J = 7.3 Hz, 2H), 1.77–1.71 (m, 2H), 0.99 (t, J = 7.4 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 198.1, 151.3, 147.9, 131.7, 123.9, 107.54, 107.50, 101.6, 40.0, 17.7, 13.6; HRMS (ESI-TOF) calcd for [C₁₁H₁₂O₃ + Na]⁺ 215.0679, found 215.0677.

Cinnamaldehyde (4a). Isolated yield of 4a, (57.5 mg, 87%). Colorless oil; ¹H NMR (500 MHz, CDCl₃/TMS) δ 9.71 (d, J = 7.7 Hz, 1H), 7.57 (dd, J = 7.0, 2.4 Hz, 2H), 7.49 (d, J = 16.0 Hz, 1H), 7.45–7.43 (m, 3H), 6.57 (dd, J = 16.0, 7.7 Hz, 1H); ¹³C NMR (125 MHz, CDCl₃) δ 193.7, 152.8, 134.0, 131.3, 129.1, 128.6, 128.5.

(E)-Oct-2-enal (4b). Isolated yield of 4b, (55.5 mg, 88%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 2956, 2930, 2862, 1688, 1635, 1465, 1382, 1146, 1097, 976, 909, 650; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.50 (d, J = 7.9 Hz, 1H), 6.89–6.81 (m, 1H), 6.15–6.08 (m, 1H), 2.36–2.30 (m, 2H), 1.55–1.47 (m, 2H), 1.36–1.25 (m, 4H), 0.90 (t, J = 6.9, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 194.2, 159.1, 132.9, 32.7, 31.3, 27.5, 22.4, 13.9; HRMS (ESI-TOF) calcd for [C₈H₁₄O + K]⁺ 165.0676, found 165.0681.

1-Phenylprop-2-en-1-one (4c). Isolated yield of 4c, (50.9 mg, 77%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3065, 3020, 1733, 1672, 1609, 1580, 1448, 1404, 1286, 1233, 1180, 1072, 994, 698; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.95 (d, J = 7.1 Hz, 2H), 7.58–7.56 (m, 1H), 7.45 (t, J = 7.1 Hz, 2H), 7.16 (dd, J = 17.1, 10.6 Hz, 1H), 6.44 (dd, J = 17.1, 1.6 Hz, 1H), 5.94 (dd, J = 10.6, 1.7 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃) δ 191.0, 137.2, 132.9, 132.3, 130.1, 128.6, 128.5; HRMS (ESI-TOF) calcd for [C₉H₈O + Na]⁺ 155.0467, found 155.0473.

Dec-1-en-3-one (4d). Isolated yield of 4d, (44.7 mg, 58%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 2980, 2929, 2857, 1703, 1684, 1616, 1466, 1402, 1478, 1263, 1184, 1081, 985, 962, 911; ¹H NMR (400 MHz, CDCl₃/TMS) δ 6.35 (dd, J = 17.7, 10.5 Hz, 1H), 6.21 (dd, J = 17.6, 1.2 Hz, 1H), 5.81 (dd, J = 10.5 Hz, 1H), 2.57 (t, J = 7.5 Hz, 2H), 1.61–1.57 (m, 2H), 1.32–1.26 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 200.8, 136.4, 127.6, 39.5, 31.5, 29.0, 28.9, 23.8, 22.4, 13.9; HRMS (ESI-TOF) calcd for [C₁₀H₁₈O + Na]⁺ 177.1250, found 177.1245.

2-Methyl-5-(prop-1-en-2-yl)cyclohex-2-enone (4e). Isolated yield of 4e, (35.3 mg, 47%). Colorless oil; ¹H NMR (500 MHz, CDCl₃/TMS) δ 6.75–6.74 (m, 1H), 4.76 (d, J = 19.8 Hz, 2H), 2.71–2.63 (m, 1H), 2.59–2.54 (m, 1H), 2.46–2.07 (m, 3H), 1.77 (d, J = 1.2 Hz, 3H), 1.73 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 199.8, 146.7, 144.7, 135.4, 110.4, 43.1, 42.4, 31.2, 20.5, 15.7.

Cyclohex-2-en-1-one (4f). Isolated yield of 4f, (32.7 mg, 68%). Colorless oil; ¹H NMR (500 MHz, CDCl₃/TMS) δ 6.98 (td, J = 10.1, 4.2 Hz, 1H), 6.00 (td, J = 10.1, 2.0 Hz, 1H), 2.43–2.39 (m, 2H), 2.36–2.32 (m, 2H), 2.04–1.98 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 199.8, 150.7, 129.9, 38.1, 25.6, 22.7.

3-Phenylpropiolaldehyde (4g). Isolated yield of 4g, (55.3 mg, 85%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 2926, 2855, 2738, 2240, 2189, 1660, 1596, 1489, 1444, 1388, 1283, 1260, 1178, 1160, 1070, 1027, 1002, 978, 922, 688, 617; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.41 (s, 1H), 7.59 (dd, J = 8.3, 1.1 Hz, 2H), 7.51–7.46 (m, 1H), 7.39–7.37 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 176.8, 133.2, 131.2, 128.7, 119.3, 95.1, 88.4; HRMS (ESI-TOF) calcd for [C₉H₆O + H]⁺ 131.0491, found 131.0494.

Oct-2-ynal (4h). Isolated yield of 4h, (40.3 mg, 65%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3022, 2959, 2934, 2862, 2740, 2284, 2201, 1669, 1466, 1424, 1387, 1341, 1327, 1139, 1106, 1064, 1028, 976, 929, 908, 882, 812, 668; ¹H NMR (400 MHz, CDCl₃/TMS) δ 9.18 (s, 1H), 2.41 (t, J = 7.1 Hz, 2H), 1.62–1.57 (m, 2H), 1.42–1.33 (m, 4H), 0.91 (t, J = 7.1 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 177.3, 99.4, 81.6, 30.9, 27.2, 22.0, 19.0, 13.8.

1-(Benzo[d][1,3]dioxol-5-yl)-3-phenylprop-2-yn-1-one (4i). Isolated yield of 4i, (110 mg, 88%). Pale yellow solid, mp 92–93 °C; IR (CHCl₃, cm⁻¹): ν 3018, 2904, 2785, 2208, 1732, 1630, 1600, 1504, 1485, 1447, 1373, 1359, 1251, 1140, 1107, 1097, 1043, 1014, 997, 936, 908, 885, 822, 688, 668; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.90 (dd, J = 8.2, 1.6 Hz, 1H), 7.68–7.63 (m, 3H), 7.48–7.40 (m, 3H), 6.92 (d, J = 8.2 Hz, 1H), 6.09 (s, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 176.1, 152.8, 148.2, 132.9, 132.0, 130.6, 128.6, 127.2, 120.1, 108.2, 108.0, 102.1, 92.3, 86.7; HRMS (ESI-TOF) calcd for [C₁₆H₁₀O₃ + Na]⁺ 273.0522, found 273.0523.

1-(Benzo[d][1,3]dioxol-5-yl)oct-2-yn-1-one (4j). Isolated yield of 4j, (83.7 mg, 68%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3076, 2961, 2932, 2791, 2210, 1742, 1637, 1602, 1503, 1487, 1444, 1359, 1319, 1275, 1260, 1226, 1157, 1116, 1077, 1038, 934, 913, 888, 853, 806, 677, 648; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.79 (dd, J = 8.2, 1.7 Hz, 1H), 7.55 (d, J = 1.6 Hz, 1H), 6.87 (d, J = 8.1 Hz, 1H), 6.06 (s, 2H), 2.47 (t, J = 7.2 Hz, 2H), 1.67 (q, J = 7.3 Hz, 2H), 1.48–1.32 (m, 4H), 0.92 (t, J = 7.2 Hz, 3H) ppm; ¹³C NMR (100 MHz, CDCl₃) δ 176.2, 152.5, 147.9, 131.9, 126.9, 108.1, 107.7, 101.9, 95.9, 79.3, 30.9, 27.3, 21.9, 18.9, 13.7; HRMS (ESI-TOF) calcd for [C₁₅H₁₆O₃ + Na]⁺ 267.0992, found 267.0991.

1-(Benzo[d][1,3]dioxol-5-yl)-4-(benzyloxy)but-2-yn-1-one (4k). Isolated yield of 4k, (111.8 mg, 76%). Pale yellow solid, mp 69–70 °C; IR (CHCl₃, cm⁻¹): ν 3066, 3032, 2903, 2787, 2250, 2229, 1638, 1601, 1504, 1486, 1445, 1360, 1262, 1227, 1155, 1105, 1091, 1039, 932, 910, 857, 822, 806, 648, 605; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.82 (dd, J = 8.1, 1.7 Hz, 1H), 7.55 (d, J = 1.6 Hz, 1H), 7.40–7.31 (m, 5H), 6.88 (d, J = 8.2 Hz, 1H), 6.08 (s, 2H), 4.69 (s, 2H), 4.44 (s, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 175.6, 153.1, 148.3, 136.7, 131.6, 128.6, 128.1, 127.5, 126.8, 108.2, 108.0, 102.1, 89.3, 84.1, 72.2, 57.2; HRMS (ESI-TOF) calcd for [C₁₈H₁₄O₄ + Na]⁺ 317.0784, found 317.0785.

1-(Benzyloxy)dec-2-yn-4-one (4l). Isolated yield of 4l, (82.7 mg, 64%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3032, 2950, 2929, 2214, 1676, 1496, 1455, 1399, 1351, 1256, 1228, 1154, 1092, 1028, 907, 698; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.39–7.31 (m, 5H), 4.62 (s, 2H), 4.33 (s, 2H), 2.57 (t, J = 7.4 Hz, 2H), 1.69–1.57 (m, 2H), 1.34–1.27 (m, 6H), 0.88 (t, J = 7.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 187.4, 136.7, 128.4, 128.2, 128.0, 87.3, 85.3, 72.0, 56.8, 45.3, 31.4, 28.5, 23.8, 22.3, 13.9; HRMS (ESI-TOF) calcd for [C₁₇H₂₂O₂ + Na]⁺ 281.1512, found 281.1509.

1-(Benzo[d][1,3]dioxol-5-yl)-3-hydroxypropan-1-one (4m). Isolated yield of 4m, (81.6 mg, 84%). White solid, mp 85–87 °C; IR (CHCl₃, cm⁻¹): ν 3447, 3019, 2971, 2896, 1670, 1604, 1506, 1490, 1444, 1355, 1257, 1109, 1097, 1041, 936, 878, 669; ¹H NMR (400 MHz, CDCl₃/TMS) δ 7.57 (dd, J = 8.2, 1.7 Hz, 1H), 7.43 (d, J = 1.7 Hz, 1H), 6.86 (d, J = 8.2 Hz, 1H), 6.06 (s, 2H), 4.02–3.98 (m, 2H), 3.15 (t, J = 5.3 Hz, 2H), 2.71 (t, J = 6.4 Hz, OH); ¹³C NMR (100 MHz, CDCl₃) δ 198.4, 152.0, 148.2, 131.4, 124.5, 107.9, 107.6, 101.9, 58.1, 40.0; HRMS (ESI-TOF) calcd for [C₁₀H₁₀O₄ + Na]⁺ 217.0471, found 217.0472.

1-(2,5-dimethoxyphenyl)-3-hydroxypropan-1-one (4n). Isolated yield of 4n, (89.3 mg, 85%). Colorless oil; IR (CHCl₃, cm⁻¹): ν 3436, 3000, 2946, 2906, 2837, 1669, 1610, 1582, 1496, 1465, 1443, 1414, 1358, 1280, 1223, 1182, 1165, 1049, 1017, 912, 815, 648; ¹H NMR (500 MHz, CDCl₃/TMS) δ 7.32 (d, J = 3.2 Hz, 1H), 7.05 (dd, J = 8.9, 3.2 Hz, 1H), 6.92 (d, J = 8.9 Hz, 1H), 3.97 (t, J = 5.3 Hz, 2H), 3.87 (s, 3H), 3.79 (s, 3H), 3.26 (t, J = 5.3 Hz, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 201.4, 153.3, 153.1, 127.2, 120.4, 113.5, 112.9, 58.1, 55.7, 55.5, 45.8; HRMS (ESI-TOF) calcd for [C₁₁H₁₄O₄ + Na]⁺ 233.0784, found 233.0791.

3-Hydroxy-1-(4-nitrophenyl)propan-1-one (4o). Isolated yield of 4o, (67.3 mg, 69%). Yellow solid, mp 88–90 °C; IR (CHCl₃, cm⁻¹): ν 3415, 2923, 2851, 1679, 1642, 1599, 1514, 1404, 1341, 1212, 1102, 1048, 912, 598; ¹H NMR (500 MHz, CDCl₃/TMS) δ 8.35 (d, J = 9.0 Hz, 2H), 8.15 (d, J = 8.9 Hz, 2H), 4.10 (q, J = 5.7 Hz, 2H), 3.30 (t, J = 5.3 Hz, 2H), 2.42 (t, J = 6.5 Hz, 1H, OH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 198.6, 150.5, 140.9, 129.1, 123.9, 57.7, 41.1; HRMS (ESI-TOF) calcd for [C₉H₉NO₄ + Na]⁺ 218.0424, found 218.0428.

1-(4-Chlorophenyl)-3-hydroxypropan-1-one (4p). Isolated yield of 4p, (71.1 mg, 77%). Yellow oil; IR (CHCl₃, cm⁻¹): ν 3437, 2945, 2891, 1681, 1591, 1569, 1488, 1465, 1401, 1361, 1250, 1209, 1175, 1093, 1063, 1014, 998, 976, 909, 876, 818, 649; ¹H NMR (500 MHz, CDCl₃/TMS) δ 7.91 (d, J = 8.6 Hz, 2H), 7.45 (d, J = 8.6 Hz, 2H), 4.03 (t, J = 5.3 Hz, 2H), 3.20 (t, J = 5.3 Hz, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 198.8, 139.7, 134.8, 129.3, 128.8, 57.6, 40.4; HRMS (ESI-TOF) calcd for [C₉H₉ClO₂ + Na]⁺ 207.0183, found 207.0190.

Acknowledgements

We thank the Department of Science and Technology, New Delhi, India (Grant no. SB/S1/OC-42/2013) and Department of Chemistry, IIT-Bombay for financial support. V. B. thank the Council of Scientific and Industrial Research (CSIR) New Delhi, India for research fellowships.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR spectra for all the compounds. See DOI: 10.1039/c4ra07500e

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