Notes Bull. Korean Chem. Soc. 2009, Vol. 30, No. 12 3107 DOI10.5012/bkcs.2009.30.12.3107
Oxidation of Benzylic Alcoh
이s with Iodine and Lithium Carbonate in Ionic Liquid
Seung Bae LeeandJong Chan Lee*
Departmentof Chemistry, Chung-Ang University,Seoul 156-756,Korea. * E-mail: jclee@cau. ac.kr Received August 6, 2009, Accepted October 19, 2009
KeyWords: Benzylic alcohol, Iodine, Ionicliquid, Oxidation
The oxidation of benzylic alcohols to the corresponding car bonylcompounds isa very importanttransformation in organic chemistry and a variety of methods havebeenreported.Nume rous reagents have been utilized for the oxidation of benzylic alcohols. Forexamples transition metals complexes such as chromium(VI) complexes,1ruthenium reagents,2 and manganese dioxide3 are widely utilized. In addition, hypervalent iodine(V) compounds such as Dess-Martin periodinate4 and o-iodoxy- benzoicacid,5 iodosobenzene,6 and (diacetoxyiodo)benzene7 received muchattentiondue to theiroutstandingreactivities.
However, above mentionedreagentssuffered from several draw
backsin terms of harsh reaction conditions, high cost, instability, high toxicity, and operational difficulty.Moleculariodine can be considered one of the mostideal oxidant in organic reactions dueto itsmanydesirable properties like low cost, safe to handle,
and ready availability. However, the utilization of iodine for theoxidation of alcoholstothecorrespondingcarbonylcom pounds has been veryscarce.Furthermore,due to the intrinsic low reactivity of molecular iodine comparedtothe otherhighly reactive hypervalent iodine reagents,oxidation of alcohols with molecular iodine generally required the concomitant use of activatingagents. For examples, previously reported iodine in
ducedoxidativemethods had been invariablycarried out inthe presenceof activating substancessuchas 2,2,6,6,-tetramethyl-1- piperdinyloxyl (TEMPO),8 potassiumiodide,9 or(diacetoxyio- do)benzene.10 To thebest of ourknowledge,successfulutiliza tionof molecular iodine in the absence of any additional promo ters in the oxidationof benzylic alcohols tothecorresponding carbonylcompoundshas been unprecedented.
Herein, we wish toreport our findings on the facile oxidation
Table 1. Oxidation of Alc사iols to Aldehydes and Ketones with h/LizCOa
Entry Substrate
1 PhCH2OH
2 P-CH3C6H4CH2OH 3 P-FC6H4CH2OH 4 p-ClC6H4CH2OH 5 p-BrC6H4CH2OH 6 p-CH3OC6H4CH2OH 7 Piperonyl alcohol
8 PhCH(CH3)OH
9 PhCH(CH2CH3)OH
10 p-ClC6H4CH(CH3)OH
11 PhCH(CN)OH
12 (Ph)2CHOH
13 p-CH3OC6H4CH(CH3)OH
Product Time (h)/Temp (oC) Yield。(%)
O
PhCH 6/60 85
O P-CH3C6H4CH
o
P-FC6H4CH
4/60 81
4/60 81
O
p-ClCgHqCH 4/60 82
O p-BiCt^CH
O
4/60 81
p-CH3OC&H4CHII 24/60 83
Piperonyl aldehyde 24/60 71
o
PhCCH3 12/r.t 88
O
PhCCH2CH3 12/r.t 83
O
p-ClC6H4&H3 12/r.t 87
PhCCN
o
12/r.t 81(Ph)2C=O 12/r.t 84
o
p-CH3OC&H4CCH3
12/r.t 81
“Isolated yields.
3108 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 Notes
of benzylicalcohols using\2fLi2CO3 in ionicliquidasa useful oxidizing system for theoxidation of benzylicalcohols.Thus, reactionofbenzyl alcohol and iodine (1.5 equiv)at 60 oC for 6 h in the presenceof lithium carbonate (1.0 equiv)in 1-butyl- 3-methylimidazoliumtetrafluoroborate, [bmim]BF4, exclusi
vely yielded thebenzaldehyde in85%. Replacement ofLizCQ by other bases such as K2CO3, EtsN, and 2,6-lutidine gave always inferioryields ofdesired products. At the present reaction conditions, various primary benzylic alcohols underwent smooth
ArCHR'OH ——
MS
_ArCHR'Br[bmim]BF4
oxidation toprovide high yields of the correspondingaldehydes as summarized in Table 1. Over-oxidizedproductswerenot detected atthe presentreaction conditions.Next,ourattention wasdirectedtowardsoxidationof secondary alcohols.Asexem plifiedin Table 1, secondarybenzylicalcohols(entries 8-13) reacted efficiently giving high yields of ketones. Interestingy, in cases of oxidation of secondary benzylicalcohols, strong tem
perature dependence of selectivity ofreactions werefoundwith the highest atroom temperature and somewhatlowered yields wereobtained atincreasedtemperatures.Atthe temperatureof 60 oC complicated product mixtures wereobtained. Electron
donating and withdrawingsubstituent in aromatic rings had relativelyminorinfluenceontheoutcome of theoxidation of substitutedbenzylic alcohols.In allthe attemptedexamples, our conditions were highlyselective and efficient, and no over oxidation products weredetected. Thisappearsto be thefirst example of theoxidationofbenzylicalcohols as performed by only molecular iodine withoutany additional activation reagent.
Inconclusion, we havedemonstrated a simple and effective transition metal-free protocol for theoxidation of benzylical coholsto thecorresponding aldehydes and ketones. Thesimple procedure togetherwith use of readily available, safe to use, and
inexpensive molecular iodine should make present protocol an importantalternativetoother classicalmethods.
ExperimentalSection
The 1-butyl-3 -methylimidazolium tetrafluoroborate,[bmim]
BF4,wasobtainedfrom Aldrich.).Thin layer chromatography wasperformedon Merck silica gel 60 F-254 plates (0.25 mm).
Merck silicagel 60 (230 - 400 mesh)was usedfor flashcolumn chromatography.
Generalprocedure. Benzylic alcohol(1.0mmol) wasadded to a well-stirred mixture ofiodine (0.381g, 1.5 mmol)andlithi um carbonate (0.073 g, 1.0 mmol) in solution of [bmim]BF4
(1 mL). The mixture was stirred at 60oC over6h.Oncompletion of the reaction,themixture wasdissolvedindichloromethane (40 mL) and washed with 5% sodiumthiosulfate (20 mL). The organiclayer wasseparated and driedoverMgSO4 and evapo ratedunderreduced pressure. The residue was purified by flash column chromatography (EtOAc : Hexane =1: 3) on silica gel to obtain purecarbonylcompounds.
Acknowledgments. This research was supported by the Chung-Ang University ResearchGrants in 2009.
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