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DOI: 10.1055/s-0037-1610717
Single-Step Dual Functionalization: One-Pot Bromination-Cross-Dehydrogenative Esterification of Hydroxy Benzaldehydes with CCl3Br – A Comparison with Selectfluor
Publication History
Received: 07 April 2019
Accepted after revision: 27 May 2019
Publication Date:
19 June 2019 (online)
Dedicated to Professor Ganesh P. Pandey on the occasion of his 63rd birthday.
Abstract
Bromination of phenolic compounds without directly using molecular bromine possesses much importance. In this article an IrIII/CCl3Br-assisted single-step double functionalization of hydroxy benzaldehydes is reported. It involves simultaneous esterification of the aldehyde group and bromination of the aryl ring of phenolic aldehydes in one-pot. The reaction proceeds under mild conditions in the presence of 445 nm blue LED light to obtain highly functionalized bromo hydroxy benzoates in moderate to good yields. In comparison, Selectfluor as an oxidant gives only non-bromo phenolic esters.
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Key words
photoredox catalysis - iridium - cross-dehydrogenative coupling - aldehydes - bromo hydroxy benzoates - SelectfluorBecause of the mild and environmentally benign conditions photoredox reactions generally require, they have wide applicability in synthesis to photochemically ‘activate’ a variety of organic molecules which seldom absorb visible light.[1] The growing interest in transition-metal-based photoredox catalysis is evident by the exponential growth of reports in bibliography recently.[1]
Esters are undoubtedly one of the most important functional groups in organic synthesis[2] and cross-dehydrogenative coupling is a very convenient strategy to furnish different esters from aldehydes.[3] Moreover, bromination of phenols without direct use of molecular bromine is an interesting subject of investigation to avoid direct handling of this hazardous halogen.[4] A critical study on this has been performed by Xia et al. who used CBr4 with RuII photoredox catalysis.[4c] Although photocatalytic cross-dehydrogenative esterification is well known for both phenolic and nonphenolic aldehydes,[5] photocatalytic esterifications of phenolic aldehydes, in particular accompanied with aromatic ring bromination in one pot have not been reported till date.
After the successful synthesis of esters from aldehydes by an IrIII[df(CF3)ppy]2(dtbbpy)PF6-mediated photocatalytic redox pathway by the author,[5a] herein, a simple and one-step strategy for dual bromination-dehydrogenative esterification of hydroxy benzaldehydes is reported to give 3-bromo-4-hydroxy benzoates in good to excellent yields (Scheme [1]) in the presence of CCl3Br (2) as the additive. Bromotrichloromethane (2) acts as a dual agent here, both as an oxidizing and brominating agent towards this smooth transformation.
In this context, it is worth mentioning that 2-bromophenol derivatives are important marine-derived biologically active agents.[6]
When 4-hydroxy benzaldehyde (1) was treated with 1.0 equivalent of CCl3Br (2) in the presence of n-butyl-TBDMS ether[5a] (3a) and an iridium photocatalyst in dry acetonitrile as the solvent at room temperature under an inert atmosphere, the ester 3-bromo-4-hydroxy butyl benzoate (4a) was formed in 68% yield after eight hours of irradiation under an array of 445 nm blue light emitting diodes (LED) (Scheme [2]). The presence of the free intact phenolic –OH group in 4a was detected by a D2O exchange experiment (see Supporting Information).
a 1 mmol of 1 was taken, c = 0.4 M.
b Isolated yield.
c IrIII means Ir[df(CF3)ppy]2(dtbbpy)PF6.
d Butyl butanoate was formed as the major product (detected in GC–MS of the crude mixture) by the photoredox reaction between 3a and butyraldehyde formed by the photo-oxidation of the solvent.
e RuII means Ru(bpy)3Cl2.
f Reaction was carried out throughout in the presence of air.
g Reaction was carried out under reflux.
h n-Butyl alcohol was used instead of silyl ether 3a.
i Reaction was carried out in the dark.
To optimize the yield of the reaction, a number of other conditions were tested and the isolated yields of 4a are reported in Table [1] (2.5 mL of solvent per 1 mmol of 1). It was observed that upon doubling the concentration of CCl3Br the reaction gave 4a with an increment in yield (Table [1], entry 1 vs 2). The negative effect of n-butyl alcohol as the solvent was noteworthy due to the formation of butyraldehyde by photooxidation and subsequently butyl butanoate (detected in GC–MS of the crude mixture) thus consuming 3a (entry 3).[5b] [f] Changing the photocatalyst to RuII(bpy)3Cl2 or increasing the solvent polarity did not give better results either (entries 4 and 5, respectively). Carrying out the reaction throughout in the presence of air while keeping the other conditions identical gave the best yield of 4a with a shorter reaction time (entry 6).[4c] When heated to reflux, the product yield was good but not as promising as that at room temperature, although the reaction rate was faster (Table [1], entry 7). Using an acetonitrile/water 1:1 mixture as the solvent did not yield any product (entry 8). Use of n-butyl alcohol instead of its silyl ether gave the product in a much lower amount even after a prolonged period of reaction (entry 9). This may be due to the more facile formation of the alkoxy radical (G, Scheme [3]) because of the higher stability of the silyl cation that was generated upon interacting with IrIV (TBDMS+, F, Scheme [3]). The reason for that is probably the lower electronegativity (i.e. more metallic character) of silicon compared to that of hydrogen and also the lower O–Si bond energy relative to that of O–H. Lastly, no product was obtained either in the absence of photocatalyst, light, or CCl3Br (entries 10, 11, and 12, respectively).[5a] Interestingly, no unbrominated product was ever obtained irrespective of the reaction conditions screened.
After obtaining the optimized conditions for the reaction, a number of saturated silyl alcohols or silyl ethers 3 were screened and the results are shown in Scheme [3].[7] Simple aliphatic alcohol silyl ethers, including cyclic ones and those containing long alkyl chains underwent smooth conversions to give their corresponding bromo esters 4. The reaction speed decreased upon increasing the alkyl chain length in the alcohol. The best result (with 89% yield) was obtained with ethyl silyl ether (3c). The halogenated substrates 3d and 3f also participated in the reaction with equal efficiency. In this line of transformations, it is noteworthy to mention that esters with a trifluoro group have always been important as acylating agents.[8] Propargyl alcohol silyl ether (3h) gave the corresponding 3-bromo ester (4h) in 61% yield. The TBDMS ether of benzyl alcohol gave only benzaldehyde as the sole product, rather than the desired ester. Tertiary ether tert-BuOTBDMS afforded only a trace amount of ester product. When the reaction was performed with unsaturated alcohol silyl ethers, the corresponding silyl ethers of allyl alcohols did not participate in the reaction at all.
On the basis of the results obtained and by combining the mechanisms described by Xia[4c] with the author’s previous work,[5a] a general mechanistic pathway can be proposed as shown in Scheme [4]. The final brominated product is obtained after involving two IrIII-photocatalytic cycles. The first cycle gives the ester through cross-dehydrogenative coupling (CDC, Scheme [4]). The next is responsible for the bromination of the products. Upon irradiation, first, IrIII forms the excited [IrIII]* (B). It donates an electron to 2 to become IrIV. The aerial oxygen speeds up the process of formation of IrIV from IrIII, thus accelerating the formation of Br2 from Br– in turn by IrIV. Compound 2 forms a CCl3 radical (E) along with a bromide anion (D). IrIV strips a single electron from silyl ether 3 to become IrIII with concomitant formation of radical G and TBDMS+ (F). G attacks aldehyde 1 to give radical intermediate H,[5a] which is oxidized by E to afford ester 4'. In the second catalytic cycle, IrIII oxidizes the bromide anion to molecular bromine in situ, which reacts with 4' giving the final ortho-bromo hydroxy benzoate ester product 4 along with hydrobromic acid.[4c] , [5] , [9] A Kharasch-type bromination by radical/homolytic cleavage of CCl3Br can also be proposed.[10] Additionally, the other possibility of IrIII photocatalytic bromination occurring before the CDC ester formation (interchange of first and second catalytic cycles) cannot be completely ruled out as well (no formation of any unbrominated product in Scheme [3]).
Screening a substituted 4-hydroxy benzaldehyde, vanilin (5) underwent smooth reactions with silyl ethers 3a, 3e, and 3i to produce the respective 3-bromo esters 6a–c in good yields (Scheme [5]) with no bromination at the ortho or para position to the methoxy functionality.
When salicaldehyde (7) was taken as a substrate with a hydroxyl group in 2-position, 5-bromo ester 8 was obtained in low yield (36%) as the only product (no 3,5-dibromo) under the reaction conditions with 3a (Scheme [6]). In contrast, isovanilin (3-hydroxy, 9) yielded the unbrominated ester 10 as the major product, when a reaction was performed with 3e, along with the minor product 2-bromo isovanilin 11. In 10, bromination did not take place, probably because of steric crowding by the large ester group blocking both the 2- and 6-positions of the ring that are ortho/para to the hydroxyl group with no influence from the methoxy group (no bromination at 5-position, ortho to methoxy). Monobromo aldehyde 11 was a minor product in this case (34%) with 79% combined yield. Compound 11 was the result of bromination of 9 at 6-position because it is the least crowded position (compared to the 2-position).
Quite similar to CCl3Br in nature, the additive Selectfluor 12 has been proven to be a very effective oxidant in photoredox catalysis,[11] but its use in dehydrogenative esterification of aldehydes has not been reported, yet. As to compare its reaction efficiency towards 4-hydroxy benzaldehyde a number of silyl ethers were tested to afford very good yields of phenolic esters 4a'–i' (Scheme [7]). The products in this case were non-bromo phenolic esters only. The chloroethoxy and trifluoro ethyl ethers (3d and 3m, respectively) also gave the corresponding benzoates (4a',e') easily and in good yields. All acyclic and cyclic silyl ethers (entries 3a,e,g,j,n,o) underwent the reaction to smoothly afford the corresponding 4-hydroxy benzoates. Salicaldehyde gave the corresponding butyl ester in 40% yield. Decreasing the amount of 12 to less than 2.0 equivalents resulted in low yields of esters 4'.
The mechanism of this Selectfluor-mediated photocatalytic oxidative transformation is quite similar to that with CCl3Br and is discussed in Scheme [8].[11b] [c] The mechanism denotes the final abstraction of the hemiacetal radical H by the diazabicyclo radical cation E', formed by the reduction of 12 by the photoexcited IrIII species (B) to give the ester 4'.
When treated with non-hydroxy benzaldehydes 13a–c, Selectfluor afforded the corresponding esters 14a–c in moderate yields (Scheme [9]). A steady decrease in product yield along with an increase in the reaction speed was observed with increasing electron-pulling properties of the 4-substituents.
To summarize, this report depicts the reaction outcomes of an IrIII/CCl3Br catalyst/reagent-mediated photocatalyzed cross-dehydrogenative esterification-ring bromination of 2-hydroxy, 3-hydroxy, and 4-hydroxy benzaldehydes. The transformations occurred through dehydrogenative coupling at the aldehyde functionality along with 3-bromination at the aromatic nucleus in good to excellent product yields. Salicaldehyde, however, gave solely the 5-bromo adduct. The 3-hydroxy benzaldehyde isovanilin produced unbrominated benzoate ester along with 2-bromo isovanilin. When Selectfluor was used in the reactions as an additive instead of CCl3Br under an inert atmosphere, non-bromo ester derivatives were obtained. Compounds 4d,f,h,j,l are reported here for the very first instance. The bromination/esterification of other electron-rich heteroatom-based aldehydes with a higher degree of substitution with functional groups is being carried out in our laboratory along with theoretical studies (DFT energy calculation of the intermediates and kinetics of the photophysical processes) to get more insights in the mechanism.
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Acknowledgment
The author would like to thank CBMR, Lucknow for the financial support.
Supporting Information
- Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0037-1610717.
- Supporting Information
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- 7 Synthetic Route to 3-Bromo-4-hydroxy Butyl Benzoate (4a); Typical Procedure A 10 mL double-necked round-bottomed flask equipped with a magnetic stirring bar was charged with 4-hydroxybenzaldehyde (1) (244 mg, 2.0 mmol), photocatalyst IrIII[df(CF3)ppy]2(dtbbpy)PF6 (45 mg, 0.04 mmol), CCl3Br [2, 793 mg, 4.0 mmol (to synthesize 4a'–l', 4.0 mmol of Selectfluor 12 was used instead, with 2.0 mmol of 1 or non-hydroxy benzaldehydes 13a–c and the inert atmosphere was maintained)], n-BuOTBDMS (3a) (1.13 g, 6.0 mmol) and acetonitrile (5.0 mL) at room temperature. The mixture was irradiated under 445 nm blue LED array for 7 h in air. The reaction was monitored by TLC. After completion of the reaction the solvent was evaporated under reduced pressure. The residue was distributed in water (10 mL) and extracted with ethyl acetate (20×3 mL). The combined organic layers were dried with anhydrous MgSO4 and concentrated under reduced pressure. The crude product was purified by column chromatography by using 100–200 mesh silica gel with use of 5–10% ethyl acetate in hexanes as the eluent to afford pure 3-bromo-4-hydroxy butyl benzoate (4a) as a white crystalline solid in 85% yield (464 mg). A similar methodology was executed with the other substrates. Spectral Data of New Compounds 2,2,2-Trifluoroethyl 3-bromo-4-hydroxybenzoate (4d): Yield: 465 mg (78%); Rf 0.39 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 53–55 °C; 1H NMR (400 MHz, CDCl3): δ = 4.68 (q, J H–F = 8.0 Hz, 2 H), 6.14 (br s, 1 H), 7.08 (d, J = 8.0 Hz, 1 H), 7.96 (dd, J = 8.0, 4.0 Hz, 1 H), 8.21 (d, J = 2.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 61.0 (q, J C–F = 36.3 Hz), 110.5, 116.2, 122.3, 123.2 (q, J C–F = 275.7 Hz), 131.7, 134.5, 157.2, 163.6; HRMS (ESI): m/z [M + H]+ calcd for C9H7BrF3O3: 298.9531; found: 298.9526. 3-Chloropropyl 3-bromo-4-hydroxybenzoate (4f): Yield: 446 mg (76%); Rf 0.35 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 92–94 °C; 1H NMR (400 MHz, CDCl3): δ = 2.22 (quin, J = 8.0 Hz, 2 H), 3.68 (t, J = 8.0 Hz, 2 H), 4.45 (t, J = 8.0 Hz, 2 H), 6.15 (br s, 1 H), 7.05 (d, J = 8.0 Hz, 1 H), 7.91 (dd, J = 8.0, 4.0 Hz, 1 H), 8.17 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 31.8, 41.4, 62.1, 110.3, 116.0, 124.0, 131.2, 134.1, 156.5, 165.1; HRMS (ESI): m/z [M + Na]+ calcd for C10H10BrClNaO3: 314.9400; found: 314.9398. Prop-2-yn-1-yl 3-bromo-4-hydroxybenzoate (4h): Yield: 311 mg (61%); Rf 0.43 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 144–146 °C; 1H NMR (400 MHz, CDCl3): δ = 2.52 (s, 1 H), 4.90 (d, J = 2.0 Hz, 2 H), 6.01 (br s, 1 H), 7.06 (d, J = 8.0 Hz, 1 H), 7.95 (dd, J = 8.0, 2.0 Hz, 1 H), 8.22 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 52.7, 75.3, 77.7, 110.3, 116.0, 123.4, 131.5, 134.3, 156.7, 164.4; HRMS (ESI): m/z [M + H]+ calcd for C10H8BrO3: 254.9657; found: 254.9661. Cyclohexyl 3-bromo-4-hydroxybenzoate (4j): Yield: 395 mg (66%); Rf 0.39 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 119–121 °C; 1H NMR (400 MHz, CDCl3): δ = 1.31–1.48 (m, 3 H), 1.53–1.60 (m, 3 H), 1.77–1.79 (m, 2 H), 1.92–1.94 (m, 2 H), 4.99 (quin, J = 4.0 Hz, 1 H), 6.04 (bs, 1H), 7.04 (d, J = 8.0 Hz, 1 H), 7.93 (d, J = 12.0 Hz, 1 H), 8.18 (s, 1 H); 13C NMR (100 MHz, CDCl3): δ = 23.9, 25.6, 31.8, 73.5, 110.1, 115.8, 125.0, 131.1, 134.0, 156.2, 164.7; HRMS (ESI): m/z [M + Na]+ calcd for C13H15BrNaO3: 321.0102; found: 321.0101. Undecyl 3-bromo-4-hydroxybenzoate (4l): Yield: 379 mg (51%); Rf 0.58 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 59–61 °C; 1H NMR (400 MHz, CDCl3): δ = 0.87 (t, J = 8.0 Hz, 3 H), 1.26–1.41 (m, 15 H), 1.70–1.78 (m, 3 H), 4.28 (t, J = 8.0 Hz, 2 H), 6.22 (br s, 1 H), 7.04 (d, J = 8.0 Hz, 1 H), 7.91 (dd, J = 8.0, 2.0 Hz, 1 H), 8.18 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 14.3, 22.8, 26.1, 28.8, 29.4, 29.5, 29.6, 29.7 (2), 32.0, 65.5, 110.2, 115.9, 124.5, 131.1, 134.1, 156.4, 165.4; HRMS (ESI): m/z [M + Na]+ calcd for C18H27BrNaO3: 393.1041; found: 393.1044.
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For reviews on visible light photoredox catalysis, see:
For the importance of esters, see:
For the syntheses of aldehyde to ester, see:
For the photocatalytic syntheses of aldehyde to ester, see:
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- 7 Synthetic Route to 3-Bromo-4-hydroxy Butyl Benzoate (4a); Typical Procedure A 10 mL double-necked round-bottomed flask equipped with a magnetic stirring bar was charged with 4-hydroxybenzaldehyde (1) (244 mg, 2.0 mmol), photocatalyst IrIII[df(CF3)ppy]2(dtbbpy)PF6 (45 mg, 0.04 mmol), CCl3Br [2, 793 mg, 4.0 mmol (to synthesize 4a'–l', 4.0 mmol of Selectfluor 12 was used instead, with 2.0 mmol of 1 or non-hydroxy benzaldehydes 13a–c and the inert atmosphere was maintained)], n-BuOTBDMS (3a) (1.13 g, 6.0 mmol) and acetonitrile (5.0 mL) at room temperature. The mixture was irradiated under 445 nm blue LED array for 7 h in air. The reaction was monitored by TLC. After completion of the reaction the solvent was evaporated under reduced pressure. The residue was distributed in water (10 mL) and extracted with ethyl acetate (20×3 mL). The combined organic layers were dried with anhydrous MgSO4 and concentrated under reduced pressure. The crude product was purified by column chromatography by using 100–200 mesh silica gel with use of 5–10% ethyl acetate in hexanes as the eluent to afford pure 3-bromo-4-hydroxy butyl benzoate (4a) as a white crystalline solid in 85% yield (464 mg). A similar methodology was executed with the other substrates. Spectral Data of New Compounds 2,2,2-Trifluoroethyl 3-bromo-4-hydroxybenzoate (4d): Yield: 465 mg (78%); Rf 0.39 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 53–55 °C; 1H NMR (400 MHz, CDCl3): δ = 4.68 (q, J H–F = 8.0 Hz, 2 H), 6.14 (br s, 1 H), 7.08 (d, J = 8.0 Hz, 1 H), 7.96 (dd, J = 8.0, 4.0 Hz, 1 H), 8.21 (d, J = 2.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 61.0 (q, J C–F = 36.3 Hz), 110.5, 116.2, 122.3, 123.2 (q, J C–F = 275.7 Hz), 131.7, 134.5, 157.2, 163.6; HRMS (ESI): m/z [M + H]+ calcd for C9H7BrF3O3: 298.9531; found: 298.9526. 3-Chloropropyl 3-bromo-4-hydroxybenzoate (4f): Yield: 446 mg (76%); Rf 0.35 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 92–94 °C; 1H NMR (400 MHz, CDCl3): δ = 2.22 (quin, J = 8.0 Hz, 2 H), 3.68 (t, J = 8.0 Hz, 2 H), 4.45 (t, J = 8.0 Hz, 2 H), 6.15 (br s, 1 H), 7.05 (d, J = 8.0 Hz, 1 H), 7.91 (dd, J = 8.0, 4.0 Hz, 1 H), 8.17 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 31.8, 41.4, 62.1, 110.3, 116.0, 124.0, 131.2, 134.1, 156.5, 165.1; HRMS (ESI): m/z [M + Na]+ calcd for C10H10BrClNaO3: 314.9400; found: 314.9398. Prop-2-yn-1-yl 3-bromo-4-hydroxybenzoate (4h): Yield: 311 mg (61%); Rf 0.43 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 144–146 °C; 1H NMR (400 MHz, CDCl3): δ = 2.52 (s, 1 H), 4.90 (d, J = 2.0 Hz, 2 H), 6.01 (br s, 1 H), 7.06 (d, J = 8.0 Hz, 1 H), 7.95 (dd, J = 8.0, 2.0 Hz, 1 H), 8.22 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 52.7, 75.3, 77.7, 110.3, 116.0, 123.4, 131.5, 134.3, 156.7, 164.4; HRMS (ESI): m/z [M + H]+ calcd for C10H8BrO3: 254.9657; found: 254.9661. Cyclohexyl 3-bromo-4-hydroxybenzoate (4j): Yield: 395 mg (66%); Rf 0.39 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 119–121 °C; 1H NMR (400 MHz, CDCl3): δ = 1.31–1.48 (m, 3 H), 1.53–1.60 (m, 3 H), 1.77–1.79 (m, 2 H), 1.92–1.94 (m, 2 H), 4.99 (quin, J = 4.0 Hz, 1 H), 6.04 (bs, 1H), 7.04 (d, J = 8.0 Hz, 1 H), 7.93 (d, J = 12.0 Hz, 1 H), 8.18 (s, 1 H); 13C NMR (100 MHz, CDCl3): δ = 23.9, 25.6, 31.8, 73.5, 110.1, 115.8, 125.0, 131.1, 134.0, 156.2, 164.7; HRMS (ESI): m/z [M + Na]+ calcd for C13H15BrNaO3: 321.0102; found: 321.0101. Undecyl 3-bromo-4-hydroxybenzoate (4l): Yield: 379 mg (51%); Rf 0.58 (EtOAc/petroleum ether, 1:4); white crystalline solid; mp 59–61 °C; 1H NMR (400 MHz, CDCl3): δ = 0.87 (t, J = 8.0 Hz, 3 H), 1.26–1.41 (m, 15 H), 1.70–1.78 (m, 3 H), 4.28 (t, J = 8.0 Hz, 2 H), 6.22 (br s, 1 H), 7.04 (d, J = 8.0 Hz, 1 H), 7.91 (dd, J = 8.0, 2.0 Hz, 1 H), 8.18 (d, J = 4.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3): δ = 14.3, 22.8, 26.1, 28.8, 29.4, 29.5, 29.6, 29.7 (2), 32.0, 65.5, 110.2, 115.9, 124.5, 131.1, 134.1, 156.4, 165.4; HRMS (ESI): m/z [M + Na]+ calcd for C18H27BrNaO3: 393.1041; found: 393.1044.
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For reviews on visible light photoredox catalysis, see:
For the importance of esters, see:
For the syntheses of aldehyde to ester, see:
For the photocatalytic syntheses of aldehyde to ester, see:
