Ind ian Journal or Chcmistry Vol. 43B, Fcbruary 2004, pp. 430-43 1

Sulfuric ac id adsorbed on sil ica gel and chromium(VI) oxide: Rapid and selective

ox idation of alcohols in solvent-free condition

Majid M Herav i"" !', Nasrin Farhangi". Yahya Sh. Beheshti ha, "Mitra Ghassemzadehh &

Koroush Tabar- I--! ydarh "Dcpartmcnt of Chcmistry. School of Scicnccs, Azzahra Univcr-

sit y.Vanak. Tchran. Iran (E-mail : mmhcrav i @azzahra.ac.ir) hChcmistry and Chcmical Enginccring Rcscarch

Centcr of Iran. P.O. Box 1 43~5 - 186 . Tchran. Iran /?ece il 'ed 5 .1111.1' 2002 ; IIccepled ( rel'ised) 22 .lilly 2003

A rapid and sc lcc ti vc Il1cthod 1'0 1' thc ox idati on of alcoho ls to carbonyl compounds that occurs on si l ica gc l-sulruri c acid sup-portcd CrOl in solvcntlcss systcm is describcd. Thc rcactions can he expcd itcd by microwavc irradiation.

IPC: Int.CI.7 C 0713 33/00

The ox idati on of hyd roxy group to a carbony l func-tionality is an important react ion ill organic synthesis anc! severa l methods are ava ilable under a variety of reacti on conditions to accompl ish this conversion I . Chromium based reagents have been ex tensively used in organic symhesis ic .

The use of supported reagents has gai ned popular-ity because of the improved selecti vity, reacti vity and assoc iated ease of manipulati on2. In continuati on of our ongoing program to develop environmentally fr iendly conditions for chemical reacti on under sol-ventl ess systems3 we explored the ox idation of neat alcohols with an eq uimolar amount of e rO} supported onto silica gel-su l furic acid that results in the forma-tion of carbony l compounds upon grinding them in a watch glass .

In view of the current emphasis on solid state syn-thes is and on green chemistr/ there is a merit in de-velop ing a selecti ve and inexpensive oxidati on of al-cohols in solvent- free conditions. Organic solvents are not only expensive but they are hazardous to envi-ronment. Literature survey showed a report on the ox idati on of alcohols in so lventless system using chromium tri ox ide using microwave irradiationS.

We have recentl y used sul furic acid adsorbed on sili ca gel as an ef f icient catal yst for solvent- free tetra-hydropyranylation of alcohols and phenols6. Armed with thi s experience \-\le supported chromium triox ide

T able I-Ox idation or alcohols to carbony l compoullli s by CrO, supportcd onto su l furic ac id adsorbcd on si l ica gc l

A lcohol

Benzyl 2-N itrobcnzy l 2-Mcthy lbcnzy l Bcnzohydrol Cyc lohcxano l II-A mylalcohol isu-Amylal cohol

Rcaction timc Product (scc) 60 Benzaldchyde 60 2-Ni trobenzaldchydc 120 2-Mcthylbcnza ldchydc 120 Bcnzophenonc 120 Cyclohexant)nc 180 II-Amylaldchydc 180 iso-Amylaldehydc

Yicld" (%)

99(92) 100(95) 97(88) 99(93) 99(93) 97(88) 97(85)

" Yi clds dctcrmincd by GLC; numbcr in pa r . .II1thesis refcrs to y icld of isolatcd products.

onto sul furic acid adsorbed on sil ica gel to oxidize alcohols in solventless system.

T he reacti on is conducted by mi x ing su l furi c ac id adsorbed on sili ca ge l w i th chromium tri ox ide and neat alcohols. T he reac ti ons are completed upon simple mi x ing. By th ese mi xing in most cases an exothermi c reacti on ensues and completed al-most immediately as conf i rmed by TLC (pet. ether : ethy l ace tate, 8:2). The reacti ons are re lative ly clean w ith no tar fo rmati on typi ca l fo r many ch ro-mium tri ox ide ox idati on. N o overox idati on to car-boxyl ic ac id was observed. Oxid at ion of cinnamy l alcohol gave moderate y ield of cinnamaldehyde (70%) and some benza ldehyde was also detected indicating carbon-carbon dou Ie bond is prone to cleavage with thi s method. To estab li sh the genera l-ity of thi s method a vari ety of ( Icohols were ox i-dized to the co rresponding carbony l compounds (Table I).

It is noteworthy to mention that in the absence of sul furic acid and use of neat sil ica ge l the reactions do not proceed. However, they proceed when the mi xtu re was exposed to microwave irradiat ion. Oxidation of alcohols with su lfuric acid adsorbed on silica ge l mi xed w ith chromium triox ide was expedited by mi-crowave irradiati on. Placing the above mixture in a microwave oven however is not recommended due to its fl ammability . In conclusion, mixture of sul furic acid adsorbed onto silica gel and chromium triox ide efficiently and rapidl y convert alcohols to carbony l compounds in excellent y ields. The method employs relati vely simple conditions since the consumption of solvent is minimum.

NOTES 431

Experimental Section Products were characteri zed by physical and spec-

troscopic comparison with those of authentic samples. Yields are based on GLC and isolated products6.

Preparation of sulfuric acid adsorbed on silica gel. To a solution of acetone (20 mL) conc. sulfuric acid ( I mL), silica gel ( 10 g, Aldrich Brockmann 1- 150 mesh) was added. This suspension was stirred at room temperature for I hr. This catalyst can be kept in air at room temperature for at least 3 months without losing its acti vity .

Preparation of chromium trioxide supported onto silica gel-sulfudc acid. The above cata lyst (2.4 g) was mi xed thorough ly with chromium trioxide (0.8 g, 8 mmol) using spatula and mortar. This re-agent should be kept in dissicator under nitrogen.

Oxidation of alcohols: General procedure In a watch glass the above suppolted reagent (0.6 g)

and an appropriate alcohol (1 mmole) was mi xed thor-Ollgh ly using a spatu la. An exothermic reaction ensues with darkening the color of the reagent and is completed almost immediately as confirmed by TLC using pet. ether: ethyl acetate (8:2). The product was extracted with CH2Cl2 ( 10 mL). The solution was passed through a sil-ica gel pad. The solvent was evaporated to afford the corresponding carbonyl compound (Table I). Caution

Although we did not have any accident with this reacti on, it is hi ghl y recommended to do the reaction in an efficient hood and special care.

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