Indian Journal of ChemistryVol. 36B, March 1997, pp. 272-274

Selective oxidation of primary alcoholswith quinolinium chlorochromate

Jasvinder Singh", G L Kad, Shikha Vig, Munisha Sharma &BRChhabra'

Department of Chemistry, Panjab University,Chandigarh 160 014, India

'Department of Chemisry, Panjab Agricultural University,Ludhiana, India

Received 28 May 1996; revised and accepted 21 October 1996

Selective oxidation of primary alcohol in the pres-ence of a secondary alcohol with quinolinium chlor-ochromate (QCC) has been achieved with a mild,stable and very efficient reagent. Its mechanistic andcomparative study with isoquinolinium chlotochrom-ate (IQCC) has also been reported.

Though many reagents are available for the oxi-dation of alcohols, there still exists a need fordeveloping new methods which show great selec-tivity, high yields and can be used under mildconditions. Chemoselective oxidation of a primaryor a secondary hydroxyl group in the presence ofboth is a useful transformation in synthesis. A fewmethods are available for such selective oxidationof primary hydroxyl groups but there are severalreports regarding the chemoselectivity of variousregents for the oxidation of secondary alcohols tothe corresponding carbonyl compounds. Pyridinein combination with chlorine', tritylfuloroborate-,silver carbonate", sodium bromate' in the presenceof catalytic cerium ammonium nitrate or sodiumhypochlorite' in aq. acetic acid are suitable forthe oxidation of secondary alcohols in the pres-ence of primary alcohols. Potassium ruthenate'' inthe presence of potassium peroxodisulfate select-ively oxidizes allylic and benzylic alcohols but notthe saturated alcohols. Recently we have foundthat Se02 TBHP Si02 (ref. 7) is a very good rea-gent for the oxidation of a primary allylic alcoholto the corresponding a, fJ-unsaturated aldehydewhile secondary allylic, benzylic, saturated pri-mary or secondary alcohols remain unaffected.

We have already reported" that quinoliniumchlorochromate, which resembles very much thewell known pyridinium chlorochromate (PC:C)9,possesses excellent selectivity to oxidise primaryalcohols in the presence of secondary alcoholsunder normal reaction conditions. With this rea-

Note

RtR-HH interne tion

R HtH-H~H intercctien

Primaryalcohol

Secondaryalcohol

Chart I

gent primary alcohols undergo oxidation to thecorresponding aldehydes in a reaction time of 1-2hr, while it takes around 12 hr for the oxidationof secondary alcohols in almost 80-90% yields.This happens irrespective of the fact whetherboth hydroxyl groups are present in the samemolecule or are present in different molecules. Asa part of our ongoing studies we want to reportadditional details and some new findings relatingto mechanistic and comparative study of oxida-tion of different alcohols with quinolinium chlor-ochromate (QCC) and isoquinolinium chloroch-romate (IQCC). Quinolinium chlorochromate is ayellowish brown solid not appreciably hygrosco-pic. It is stable (their oxidation ability is the sameafter a few months of storage at ambient tempera-ture) and possesses excellent selectivity to oxidiseprimary alcohols in the presence of secondary al-cohols. This difference in the rate of oxidation isenvisaged due to steric factors (see mechanism).

MechanismThe detailed mechanism of oxidation is com-

plex and is perhaps not very clear. It has, how-ever, been reported'? that after the chromate esterformation the abstraction of hydrogen is intra orintermolecular which leads to the correspondingcarbonyl compounds. If the abstraction is intram-olecular then there should be no difference in therate of oxidation of primary and secondary alco-hol. On the other hand, if the proton abstractionis intermolecular it can then be assumed that

NOTES 273

Table I -Oxidation of primary alcohols with quinolinium chlorochromate

Sl. Alcohol Product Time" % Conversion of alcoholsNo.

QCC· PCC IQCC1.

~20" ~O lh 40 92 90

2. OH 0 lh No Reaction 100 95

6 01 to" tOHO lh 40 85 85

4. H3C....... H3C,.,CHCH20H >CHCHO lh 25 100 100

H3C H3C

C2H5 C2H5I I

5. CHJ (CH2)3 C H CH20H CH3(CH2)3 CH CHO Ih 20 100 95

6 H3C",:, HJC_CHCH2CH20H ::;:CHCH2CHO lh 30 100 95

H3C H3C

7. P P 2h 80

CH20H CHO

~~

2h '95 90

fr~ frO 2h 98 958.

qOH qo 2h 90 909.

lQ

~H20H ~~

It! 40 90 85

All the compounds were isolated and identified by IR, NMR, elemental analysis and were estimated as their2,4-dinitrophenylhydrazones*The reaction was found to be complete in each case after 12 hr.

274 INDIAN J. CHEM. SEC B, MARCH 1997

there will be steric interaction between the peri-hydrogen of quinoline and the group present onC (may be H or R) other than the H which has tobe abstracted as depicted in Chart 1.

If this theory is correct then the rate of oxida-tion of primary and secondary alcohols should besame if we use pyridine (no reference is availablewhich mention the difference in the rate of oxida-tion of primary and secondary alcohols with PCC)or isoquinoline in which the nitrogen lone-pairorbital is free of any steric interaction withthe peri-hydrogen in comparison to quinolinewhere this type of interaction exists.

Keeping this in mind, isoquinolinium chloroch-romate (IQCC) was prepared, which is a brown-ish black solid, m.p. 73-75°C. It is interesting tomention that all the oxidations with this reagentunderwent smoothly and no difference in the rateof oxidation of primary and secondary alcoholswas observed, thereby supporting the above theo-ry.

In order to explore this reaction as well as theproposed mechanism, a comparison of the rate ofoxidation of primary and secondary alcohols withall the three reagents viz., PCC, QCC and IQCCwas undertaken. As is evident from Table I, therate of oxidation as well as the yields are compar-able when PCC and IQCC were used as the oxi-dising agents under similar reaction conditions,whereas no reaction took place with QCC in caseof secondary alcohols (entry 2, 8, 9). Primary al-cohols having substitutionts) at C-2 and C-3 wereoxidised (QCC) from zero to 70% under the reac-tion conditions (entry 1,3,4,5,6,7,10). From thesestudies it may be concluded that the rate of oxi-

. dation of alcohol is very much sensitive to substi-tutions on the subsequent. carbons which may beattributed to the increased strain in the transitionstate (I) and the suggested order of rate of oxida-tion is

Experimental SectionPreparation of quinolinium chlorochromate

(QCC).Quinoline (13 mL, 109 mmole) wasadded dropwise to a cooled (O°C)stirred mixtureof chromium trioxide (10 g, 100 mmole) andcone, HCI (11 mL), in water (10 mL). The yellow-ish brown solid obtained was collected in a sin-

tered glass funnel and dried in vacuo affording22.4 g (77%) of reagent, m.p. 65-66°C (Found: C,40.28; H, 3.09; N, 5.19. Calc. C, 40.67; H, 3.01;N,5.27%).

Preparation of isoquinolinium chlorochromate(IQCC)Jsoquinoline (26 mL, 218 mmole) wasgradually added to a cooled stirred solution ofchromium trioxide (20 g, 200 mmole) in water(20 mL) and cone. HCI (22 mL). The brownishblack solid was collected on a sintered glass fun-nel and dried in vacuo affording 42.4 g (73%) ofIQCC, m.p. 73-75°C (Found: C, 40.36; H, 2.90;N, 5.20. Calc. C, 40.67; H, 3.01;N, 5.27%).

General procedure of oxidation. In a dry 50mL round bottom flask, QCC (1.27 g, 4.7 mmole)was suspended in anhyd. dichloromethane (20mL). The solution of alcohol (4 mmole) in anhyd.dichloromethane was added all at once to a mag-netically stirred suspension. The mixture wasstirred for the time as indicated in Table I. Theprogress of the reaction was monitored by 11£.After the period as mentioned in Table I, it wasdiluted with n-hexane (15 mL), allowed to standat room temp for 10 min and then passed througha short column of silica gel]The [solvent was eva-porated 'leaving the product (purity checked byIH NMR) product mixtures which was separatedby column chromatography using silica gel as ad-sorbent.

Following the similar procedure oxidation withisoquinolinium chlorochromate was done.

AcknowledgementFinancial assistance from UGC, New Delhi is

highlyacknowledged.

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