Scientific paper
Chemistry of Organo Halogenic Molecules. Part 229. The Role of Iodine in Acetyl Group Transfer to Oxygen-containing Molecules under Solvent-free Reaction Conditions
Marjan Jereb,1* Dejan Vražič2 and Marko Zupan1
1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, A{ker~eva 5, 1000 Ljubljana, Slovenia
2 Present address: Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
* Corresponding author: E-mail: marjan.jereb@fkkt.uni-lj.si
Received: 09-12-2008 Dedicated to Professor Blanko Stanovnik on the occasion of his 70'' birthday
Abstract
Iodine was shown to be an efficient catalyst for the conversion of phenyl-substituted aldehydes to the corresponding 1,1-diacetate derivatives under solvent-free reaction conditions (SFRC), which are superior to the classical solution conditions. It was demonstrated that the order of the addition of reactants was of fundamental importance; the ability of substituents on the phenyl ring modified reactivity irrespectively to electronic properties, the pentafluorophenyl group significantly reduced reactivity of the aldehyde. Alcohols yielded acetates; acetic anhydride was found to be the most efficient reagent; isopropenyl acetate and vinyl acetate were less reactive; however the pentafluorophenyl group enhanced reactivity with the latter two reagents. Beside the esterification of benzyl alcohol and its pentafluorophenyl analogue, the formation of acetals was also observed.
Keywords: Iodine, catalyst, solvent-free, acetylation.
1. Introduction
Rapidly changing climatic and environmental circumstances have a significantly growing impact on the life on Earth. Consequently, the field of chemistry has been turning to 'green chemistry'; endeavoring to reduce the waste, to minimize the costs, to simplify and optimize reaction protocols.1 One of the important contributions in this respect is functionalization without the use of sol-vent.2 Considerable attention should be paid to exothermic and reactions with extensive gas evolution, since there is no medium to relieve the heat or pressure shock; furthermore, scale-up might be a challenging task.3 Another critical aspect could be associated with the heterogeneity of the reaction mixture and insufficient stirring; particularly when only solid reactants are involved. Acetylation has been extensively investigated;4 its products are important intermediates in the synthesis, the acetyl group frequently serves as a protecting group of hydroxy, amino
and thiol functionality in biologically-important molecules; at the same time, acetylated products have found broad application in industry. The transformation of aldehydes to 1,1-diacetate analogues has been widely studied: without catalyst,5 H2NSO3H,6 Fe2(SO4)3 ■ xH2O,7 KHS04,8 heteropolyacids,2 P2O5/Al2O3,10 HBF4-Si02,11 silica sulfuric acid,
12
2 ^^ ii^i 4
^^^ /c;r> 14
solid
HClO4-SiO2,13 FeCl3/SiO silica sulfuric acid,15 [Yb(OPf)3],16 LiOT^,17 Zr(SO4)2
4H2O/SiO2,18 zeolites,19 Bi(OTf)3 ■ xH2O,20 RuCl3,21 InBr3,22 tetrabutylammonium tribromide,23 and others. However, many procedures employed heavy-metallic, hazardous, strongly-acidic and moisture-sensitive catalysts. Iodine has several advantages over the existing catalysts: it is mild and remarkably versatile catalyst in organic che-mistry,24 and one of its major advantages is neutrality. It has high affinity to the molecular oxygen and oxygen-functional groups; it is able to discriminate between H2O2, MeOH and H20, and hydroxy, hydroperoxy and methoxy groups.25 The iodine-catalyzed transformation of aldehydes to 1,1-diacetate derivatives has already been publis-
hed, however in a CHCl3 solution and with a huge excess of Ac2O.26 The acetylation of alcohols catalyzed by I2 was accomplished using Ac2O,27 isopropenyl acetate (IPA)28 and vinyl acetate (VA).29 Here, we report on iodine-catalyzed acetyl group transfer to aldehydes and alcohols, comparing the reactivity of acetic anhydride, isopropenyl acetate and vinyl acetate under SFRC.
2. Results and Discussion
The pentafluorophenyl ring often exhibits uncommon and intriguing behavior; it is frequently employed as molecular marker in crystal engineering, biological recognition and supramolecular assemblies.30 The pentafluorop-henyl group can also significantly modify the reactivity of substrates; little information is available on its effect on the reactivity on transformations under SFRC. We have examined the role of the structure of the acetylation reagent on I2-catalyzed transformation of benzaldehyde 1a and pen-tafluorobenzaldehyde 2 under SFRC (Scheme 1).
Ar
C6H5
C6F5
Y	Reaction conditions^		Conv.' (%)
	T (°C)	t (min)	
COMe	25	25	95
CMeCH2	85	480	91c
chch2	85	960	0
COMe	25	1440	25
CMeCH2	85	480	0
chch2	85	960	0
2 mmol of VA and 0.03 mmol I2. b) Determined by 'H NMR. c) A mixture of 3 and 5 in a ratio of 23:77.
Sheme 1
It was established that 1a and 2 could be converted to their 1,1-diacetate derivatives 3a and 4 using Ac2O, where 1a was remarkably more reactive than 2. Transformation of 1a with isopropenyl acetate gave an unexpected result; beside 3a, 5 was obtained as well, whereas 2 did
not react under these conditions. In an independent experiment, a 17% conversion of 3a to 1a in the presence of 3 mol % I2 (7 h at 85 °C, SFRC) was noted; after 27 h, conversion rose to 39%. Transformation of 3a with IPA in the presence of 3 mol % of I2 (7 h at 85 °C, SFRC) furnished 5 (31%) and 1a (6%). Vinyl acetate was not sufficiently reactive to convert 1a and 2 to 3a and 4.
In order to understand the effect of reaction conditions on the functionalization of organic molecules under SFRC, it is reasonable to compare reactivity in various solvents. We further examined the role of solvent on the transformation of 1a to 3a in the presence of 3 mol % of I2; the results are given in Table 1.
Table 1: The effect of iodine and solvent on transformation of benzaldehyde 1a to 1,1-diacetoxy-1-phenylmethane 3a with acetic
anhydride.a		
Solvent	I2	Conversionb
	(mol %)	(%)
CH2Cl2	0	0
	3	42
CHClj	0	0
	3	5
CH3CN	0	0
	3	75
h2o	0	0
	3	0
SFRC	0	0
	3	95
'' 1 mmol of 1a, 1.1 mmol of Ac2O, 0.03 mmol I2 , 2 mL of solvent; r.t. = 25 min; T = 25 °C. b) Determined by "H NNMR.
It is evident that reactions under SFRC gave superior results; conversions in solution were lower, while water was not suitable at all, and the presence of iodine was found to be indispensable for the functionalization. Iodine is capable of coordinating organic molecules in a different fashion; one of the decisive moments could be sequence of the addition of reactants. Therefore, we have examined the role of reaction protocol on the I2-catalyzed transformation of 1 and 2 (Scheme 2).
In general, the best results were obtained following the protocol a where I2 was added to Ac2O; the mixture was heated to dissolution, cooled to room temperature, and aldehyde was added last. Transformation of 1a was found to be independent of the reaction protocol; substituted aldehydes 1b and 1c exhibited higher differences in reactivity. Substituents containing oxygen atom(s) on the aromatic ring are capable of additional complexation of iodine; the transformation may not be straightforward and reactivity opposite from expected. The protocol ß (aldehyde and I2 heated to dissolution and cooled to room temperature) gave similar results to what a did; except for 1c which is solid, in contrast to other tested substrates.
b
Aldehyde
Reaction protocol^ Conv.c (%)
a ß Y
95 98 97
a ß
Y
54
49 32
a ß
Y
94 0 0
a ßb
Y^
67 66 54
Reaction conditions: 1 mmol of ArCHO, 1.1 mmol of Ac2O, 0.05 mmol I2; r.t. = 25 min; T = 25 °C. R.t. = 24 h. c) Determined by 1H NMR.
Scheme 2: The role of reaction protocol on the I2-catalyzed transformation
The protocol Y, where I2 was added last, was found to be the least favorable; it worked well only in the case of 1a. Iodine was separately dissolved in Ac2O, in 1a, and in 1b and IR spectra of the mixtures were recorded; however, no perceivable differences were noted when compared with spectra of the pure reactants.
Additionally, we studied the role of the amount of I2 on the transformation of aldehydes with Ac2O, Table 2.
The reactivity pattern was not uniform; substituents exhibited a strong, but atypical influence. No general threshold of I2 amount was observed; as low as 1 mol % of I2 was a sufficient amount for almost complete transformation of 1a to 3a within 25 min at room temperature. 1b exhibited a controversial reactivity; increasing conversion
Table 2: The effect of aldehyde structure and quantity of iodine on transformations to geminal diacetatesa
Aldehyde
I2 (mol %)
Conv.b (%)
95 95 97 0
10 5 3 1 0
37 54 71
69 0
94 54 0 0
100 100 77 0
10 5
3 0
94c 67c 25c 0c
Reac. cond.: 1 mmol of ArCHO, 1.1 mmol of Ac2O and I2, r.t. = 25 min; T = 25 °C. b) Determined by 'H NMR.c) R.t. = 24 h.
with decreasing amount of I2. The methoxy group obviously plays a unique role in complexation with I2. 4-Nitro-benzaldehyde 1c required 5 mol % of I2 to achieve high conversion; in the case of 1d, only 3 mol % was needed, but no appreciable difference in reactivity against 1a was noted. Pentafluoro analogue 2 was the least reactive substrate, requiring 10 mol % of I2 and 24 h at room temperature to reach high conversion.
I2-catalyzed acetylation of alcohols using Ac2O, iso-propenyl acetate and vinyl acetate has been already pub-lished;27-29 here we present a comparison of their reactivity on selected alcohols under SFRC, Table 3.
It is not clear which reactant is activated by iodine; alcohol or acetyl group donating reagent or both. For this reason, we studied the esterification of alcohols whose activation could involve carbocations upon activation with iodine; consequently, rearranged products would be formed. £xo-norborneol (6a) and enrfo-norborneol (6b) were suitable targets in this respect, but no rearranged products were formed. The exo-isomer 6a gave the exo-acetate 7a and the endo-alcohol 6b furnished the endo-acetate 7b. 2-Adamantanol 6c could also yield rearranged products, but only 2-adamantyl acetate 7c was obtained. Benzyl alcohol 6d and its pentafluoro congener 6e yielded the corresponding acetate derivatives 7d and 7e, both exhibiting surprisingly similar reactivity with Ac2O. Moreover, the fluorinated analogue displayed higher reactivity in reaction with IPA and VA. Interestingly, ethanal formed from VA
Table 3: The effect of reagent on iodine induced acetyl transfer to alcohols
Alcohol
Reaction conditions^
Ac2O/25 °C/5 min IPA/85 °C/8 h VA/85 °C/16 h
Ac2O/25 °C/5 min IPA/85 °C/8 h VA/85 °C/16 h
Ac2O/25 °C/5 min IPA/85 °C/8 h VA/85 °C/16 h
Ac2O/25 °C/5 min IPaA/25 °C/10 min VA/85 °C/1 h
Ac2O/25 °C/5 min IPA/25 °C/10 min VA/85 °C/30 min
Conv. (%)
100 96 79
100 87 67
100 82 75
99 27 53b
100 85 85c
a) 1 mmol of 6, 0.03 mmol and 1.1 mmol of Ac^O, IPA or VA
stirred at given conditions. b) A mixture of 7d and 8d in a ratio of
1:1. c) A mixture of 7e and 8e in a ratio of 47:53.
to column chromatography. Flash-column chromato-graphy was carried out using Fluka 60 silica gel (63-200 pm, 70-230 mesh ASTM) and monitored by thin-layer chromatography on Merck 60 F254 TLC plates, utilising mixtures of light petrol ether (b.p. 40-60 °C) and t-butyl methyl ether. NMR spectra were recorded on a Bruker Avance 300 DPX instrument. The following procedures are the same regardless of the aggregate state of aldehyde or alcohol.
A typical general procedure for I2-catalyzed transformation of aldehydes with acetic anhydride, isopro-penyl acetate, or vinyl acetate.
Iodine (0.03 mmol, 7.6 mg) was dissolved in acetic anhydride (1.1 mmol, 112 mg) or in isopropenyl acetate (2 mmol, 200 mg) or in vinyl acetate (2 mmol, 172 mg), benzaldehyde (1a, 1 mmol, 106 mg) was added, and the reaction mixture stirred until the TLC showed complete conversion. The crude reaction mixture was diluted with t-butyl methyl ether, washed with aqueous Na2S2O3, Na2CO3 (only in the case of Ac2O) and water and dried over anhydrous Na2SO4. The solution was filtered and solvent removed under reduced pressure. The crude products were purified by column chromatography and pure products were obtained.
during acetylation underwent acetalation to 8d and 8e with both benzyl alcohols 6d and 6e, the ratio acetate/ace-tal being approximately 1/1. Observation of acetal and ke-tal formation was reported during acetylation of sacchari-des using VA and IPA.28a
3. Conclusion
We have established that the reactivity of acetyl transfer agents (acetic anhydride, isopropenyl acetate and vinyl acetate) towards aldehydes and alcohols differs considerably; the best conversions were obtained under solvent-free reaction conditions. It was found that the sequence of the addition of reactants importantly influences the reaction outcome in the case of aldehydes; however no general reactivity pattern was observed, pentafluoroben-zaldehyde was significantly less reactive than benzaldehyde. Alcohols as stereochemical probes, underwent acety-lation without rearrangements, pentafluorobenzyl alcohol exhibited surprisingly high reactivity in comparison with benzyl alcohol.
4. Experimental
Reactions were performed under an air atmosphere in conical reactors using a small stirring bar. Chemicals were obtained from commercial sources and were used as received. Crude reaction mixtures were directly subjected
1,1-Diacetoxy-1-phenylmethane (3a).26b Column chromatography (157 mg, 75%), mp 43.9-44.3 °C (lit. 45-46 °C). IR (neat) v 1751, 1377, 1246, 1210, 1013, 991, 948, 762, 701 cm-1. 1H NMR (300 MHz, CDCl3) 5 2.13 (s, 6H), 7.40-7.53 (m, 5H), 7.68 (s, 1H).
1,1-Diacetoxy-1-(4-methoxyphenyl)methane (3b).26b
Column chromatography (138 mg, 58%), mp 65.0-67.2 °C (lit. 67 °C). IR (neat) v 1749, 1619, 1522, 1378, 1244, 1206, 1169, 1062, 1018, 935, 832, cm-1. 1H NMR (300 MHz, CDCl3) 5 2.11 (s, 6H), 3.82 (s, 3H), 6.92 (d, J = 9 Hz, 2H), 7.45 (d, J = 9 Hz, 2H), 7.62 (s, 1H).
1,1-Diacetoxy-1-(4-nitrophenyl)methane (3c).26b Column chromatography (226 mg, 89%), mp 126.0-126.4 °C (lit. 125 °C). IR (neat) v 1762, 1611, 1528, 1376, 1351, 1230, 1204, 1063, 976, 944, 857, 831, 698 cm-1. 1H NMR (300 MHz, CDCl3) 5 2.16 (s, 6H), 7.70 (d, J = 8.7 Hz, 2H), 7.73 (s, 1H), 8.27 (d, J = 8.7 Hz, 2H).
1,1-Diacetoxy-1-(4-trifluoromethylphenyl)methane
(3d).31 Column chromatography (215 mg, 78%), mp 27.0-28.0 °C (lit. 31-33 °C). IR (neat) v 1763, 1326, 1238, 1201, 1126, 1068, 1010 cm-1. 1H NMR (300 MHz, CDCl3) 5 2.14 (s, 6H), 7.62-7.69 (m, 4H), 7.71 (s, 1H).
1,1-Diacetoxy-1-(2,3,4,5,6-pentafluorophenyl)methane
(4).32 Column chromatography (236 mg, 79%), mp 63.8-64.4 °C (lit. 64-65 °C). IR (neat) v 1769, 1508, 1376, 1233, 1196, 1155, 1014, 947 cm-1. 1H NMR (300
MHz, CDCl3) 5 2.14 (s, 6H), 7.90 (s, 1H). Anal. Calcd. for C11H7F504 (298.16): C, 44.31; H, 2.37; Found: C, 44.49;
H,	2.43.
4-Acetoxy-4-phenyl-2-butanone (5).33 Column chromatography, oily product (107 mg, 52%). IR (neat) v 1739, 1720, 1371, 1240, 1163, 1045, 757, 701 cm-1. 1H NMR (300 MHz, CDCl3) 5 2.04 (s, 3H), 2.15 (s, 3H), 2.83 (dd, J = 16.6 Hz, J = 5.(3 Hz, 1H), 3.11 (dd, J = 16.6 Hz, J = 8.6 Hz, 1H), 6.19 (dd, J = 8.6 Hz, J = 5.0 Hz, 1H), 7.28-7.37 (m, 5H). 13C NMR (75.5 MHz, CDCl3) 5 204.5, 169.7, 139.6, 128.6, 128.2, 126.4, 71.6, 49.8, 30.3, 21.0.
A typical general procedure for I2-catalyzed transformation of alcohols with acetic anhydride, isopropenyl acetate, or vinyl acetate.
Benzyl alcohol (6d, 1mmol, 108 mg) was dissolved in acetic anhydride (1.1 mmol, 112 mg) or in isopropenyl acetate (1.1 mmol, 110 mg) or in vinyl acetate (1.1 mmol, 95 mg) and iodine (0.03 mmol, 7.6 mg) was added and the reaction mixture stirred until the TLC showed complete conversion. Isolation and purification procedure were the same as described above.
-2-Norbornyl acetate (7a).34 Column chromatography, oily product (106 mg, 69%). IR (neat) v 1736, 1449, 1369, 1246, 1072, 1018, 988 cm-1. 1H NMR (300 MHz, CDCl3) 5 1.03-1.21 (m, 3H), 1.33-1.60 (m, 4H),
I.64-1.78	(m, 1H), 1.98 (s, 3H), 2.15-2.32 (m, 2H), 4.51-4.60 (m, 1H). MS m/z (%): 139 (M+-Me, 1), 111 (31), 94 (47), 79 (34), 71 (19), 66 (100).
-2-Norbornyl acetate (7b).34 Column chromatography, oily product (111 mg, 72%). IR (neat) v 1733, 1449, 1361, 1246 cm-1. 1H NMR (300 MHz, CDCl3) 5 0.92-1.02 (m, 1H), 1.21-1.45 (m, 4H), 1.49-1.66 (m, 1H), 1.67-1.81 (m, 1H), 1.91-2.06 (m, 4H), 2.17-2.26 (m, 1H), 2.43-2.50 (m, 1H), 4.84-4.95 (m, 1H). MS m/z (%): 154 (M+, <1), 111 (36), 94 (57), 79 (55), 71 (21), 66 (100).
2-Adamantyl acetate (7c).35 Column chromatography, oily product (146 mg, 75%). IR (neat) v 1735, 1449, 1368, 1243, 1025, 985 cm-1. 1H NMR (300 MHz, CDCl3) 5 1.50-1.61 (m, 2H), 1.70-1.90 (m, 8H), 1.94-2.06 (m, 4H), 2.07 (s, 3H), 4.86-4.94 (m, 1H). MS m/z (%): 194 (M+, <1), 151 (<1), 134 (100), 105 (15), 92 (98), 79 (32).
Benzyl acetate (7d).36 Column chromatography, oily product (119 mg, 79%). IR (neat) v 1742, 1497, 1454, 1379, 1229, 1027, 746, 698 cm1. 1H NMR (300 MHz, CDCl3) 5 2.09 (s, 3H), 5.09 (s, 2H), 7.26-7.37 (m, 5H). MS m3 /z (%): 150 (M+, 35), 108 (100), 91 (59), 77 (15).
2,3,4,5,6-Pentafluorobenzyl acetate (7e).37 Column chromatography, oily product (197 mg, 82%). IR (neat) v
1753, 1657, 1509, 1225, 1134, 1058, 1033, 939 cm-1. 1H NMR (300 MHz, CDCl3) 5 2.07 (s, 3H), 5.16 (s, 2H). MS m/z (%): 240 (M+, 33), 197 (17), 181 (100). HRMS Calcd. for: C^HjFjO, 240.0210; Found 240.0213.
Acetaldehyde bis(pentafluorobenzyl) acetal (8e). Compound 8e was formed in the reaction of pentafluorobenzyl alcohol (6e, 198 mg, 1 mmol) with vinyl acetate (95 mg, 1.1 mmol) in the presence of 3 mol % Ij in 30 minutes at 85 °C, following the procedure described above. Separation on column chromatography (SiO2, petrol ether/t-butyl methyl ether) yielded pure oily product 8e (91 mg, 43%). IR (neat) v 1655, 1509, 1129, 1056, 939 cm-1. 1H NMR (300 MHz, CDCl3) 5 1.43 (d, J = 5.4 Hz, 3H), 4.62 (td, J = 11 Hz, J = 1.7 Hz, 2xC^H, 2H), 4.73 (td, J = 11 Hz, J = 1.7 Hz, 2xCHH, 2H), 4.96 (q, J = 5.4 Hz, CH, 1H). 13C NMR (75.5 MHz, CDCl3) 5 145.6 (m), 141.4 (m), 137.5 (m), 111.1 (m), 99.9, 53.9, 19.0. HRMS Calcd. for: C15H5F1o02 407.0142 (M+-Me); Found 407.0130. Anal.
Calcd. ^or C1gH8F10O2 (422.22): C, 45.51; H, 1.91; Found: C, 45.57; H,126 .080.10 2
5. Acknowledgement
We are grateful to Dr. B. Kralj and Dr. D. Zigon at the Mass Spectroscopy Centre at the 'Jožef Stefan' Institute in Ljubljana, to Prof. A. Petrič for using nmr instrument and to T. Stipanovič and Prof. B. Stanovnik for elemental combustion analysis.
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Povzetek
Pokazali smo, da je jod učinkovit katalizator za pretvorbo fenil-substituranih aldehidov v ustrezne 1,1-diacetate pod reakcijskimi pogoji brez topil (RPBT), ki so ustreznejši od klasičnih pogojev v raztopini. Ugotovili smo, da vrstni red dodajanja reaktantov igra ključno vlogo; substituenti, ne glede na elektronske lastnosti, na aromatskem obroču vplivajo na reaktivnost; pentafluorofenilna skupina močno zmanjša reaktivnost aldehida. Alkohole smo pretvorili v acetate; ace-tanhidrid je bil najučinkovitejši reagent, izopropenil acetat in vinil acetat sta bila manj reaktivna, vendar pentafluorofe-nilna skupina poveča reaktivnost s slednjima reagentoma. Poleg esterifikacije benzil in pentafluorobenzil alkohola smo opazili tudi nastanek acetalov.