Indian Journal of Chemi stry
Vol. 43B, May 2004, pp. 936-946
Mild and selective oxidation of alcohols and deoximation of oximes over
supported quinolinium fluorochromate
G Abraham Rajkumar, V Si va muruga n, Banumathi Arabindoo & V Murugesan*
Department of Chem istry, Anna Uni ve rsity, Chennai 600025 , Ind ia
E- lIlail: v_lIl urllgll @!zollllail .colll . Fax: +09 / -44-22200660
Recei ved 6 Septelllber 2002; accepted (revised) 29 lilly 2003
Quinolinium nuorochro mate sup ported over sili ca gel selecti vely brin gs abo ut the ox idati on of a wide va riety of alcohols and ox idative deox imati on of certain aldox imes and ketox imes to the corresponding carbonyl compounds in good yield.
The reagent selecti vely ox idises pri mary hydroxy l gro up in the presence of seconda ry hydroxy l gro up. The selectivit y is also
exhi bited in the oxidati on of ax ial hydroxy l gro up of the cis-4-t-butyl cyclohexanol in preference to eq ui toria l hydroxy l
gro up. Further the reage nt is stab le even after a peri od of twelve wee ks of it s storage.
IPC : Int.CI. 7 C 07 B 31/00, C 07 C 31100
Oxidation is one of the most important reacti ons in
functional group transformati ons in sy nthetic organi c
chemistry. The oxidati ve transformati on of alcohols into
aldehydes and ketones is of paramount importance in
organic chemistry both for laboratory scale ex periments
and manufacturing processes. Though several reagents
and methods are available, conti nuous attention is
drawn to newer and selective meth ods to acco mplish
this transformation. I Hexavalent chromium reagents are
hi ghl y valuable oxidising agents fo r the ox idati on of
organic compounds? In the past, reagents such as Jones,
San-ett and Collins prepared from chromi c ac id and
chromium trioxide were wi dely used for the ox idation
of alcohols to the cOlTesponding carbonyl co mpounds. 3-5
However, drastic reaction conditions paved way for a
new class of mild chromium (VI) reagents viz.,
heterocyclic halochromates.
Corey and Suggs introdu ced pyri dinium chl orochro mate (PCC) fo r the ox idati on of alcohols 6
Pyridinium flu orochro mate (PFC/, quinolinium
chl oroc hromate (QCC)8, bipyridin ium chl oroc hromate
(BPCC)9 and quin olinium fluoroc hromate (QFC) IO".b
are other notabl e halochrom ates. Though the heterocycli c halochromates were foun d to be better in their
reacti vity and selecti vity co mpared to the co mmon
Cr(VI) oxidants, th eir utility is restri cted due to th e
di ffi culty in product isolati on, low reacti vity, long
reaction time and hi gh acidity.
The co ncept of supportin g Cr(VI) based reagents
onto inert inorgani c and polymeric matrices has circum vented so me of the problems assoc iated with
these reagents. II The survey of I iterature has revealed
th at there are a number of Cr(VI) reagen ts supported
on inert solid supports. The supported Cr(V I) based
reagents ca n be broadl y catego ri sed as fo llows:
(1 ) Chromium triox ide supported on inert ino rgani c
supports. 12a .i
(2) Heterocyclic haloc hrom ates supported on inert
..
l3a·d
1I10rganl c supports.
(3) Polymeric analogues of heterocyclic haloc hromates. 14a -d
Though these reage nts have advantages they ca nnot
be easil y prepared .1 2a.b.C. 14a.d Furth er they un dergo
rapid deacti vatio n on storage l2b .e and large excess of
th e reage nt I2b.C.13n-d is required bes ides lo nger reacti on
time.1 2g. 14b.d Thu s, there still ex ists scope for an effecti ve, selecti ve and a stab le supported Cr(VI) reagent
to accompli sh th e ox idatio n of alco hols and related
ox idative transfo rm ati ons.
In co ntinu at ion of our investi gati ons on the utility
of quin olinium fluoroc hromate supported on inert in.
d
· report the preparatI.on
organIC
suppo rt 13c.,
we herell1
and sy nth eti c utili ty of quinolinium Ouorochro mate
supported on sili ca ge l (QFC-silica gel) as a selec ti ve,
stable and versatile ox ida nt in th e oxidati on of alcohols and ox idati ve deox imati on of certai n al dox imes
and ketox imes to the co rresponding carbonyl compounds.
Results and Discussion
Quinolinium tluoroc hromate suppo rted on sil ica
gel (QFC-sili ca gel) was prepared ill situ adopting th e
procedu re simil ar to the preparati on of si lver carbonate on celite. 15 The average loading of the reage nt was
RAJKUMAR et al.: QFC-SILICA GEL AS A SELECTIVE, STATIC AND VERSATILE OXIDANT
found to be 1.4-1.5 millimoles of QFC per gram of
silica gel. The effect of the solvent was evaluated by
carrying out the oxidation of benzyl alcohol (chosen
as model substrate) in a series of solvents of varying
polarity such as dichloromethane, chloroform, benzene, tetrahydrofuran and hexane. The maximum
conversion of benzyl alcohol to benzaldehyde occurred in a shorter reaction period in hexane and dichloromethane. Hence hexane was chosen as the solvent for most of the oxidation studies excepting for a
few cases in which dichloromethane was used.
The substrate/oxidant mole ratio was optimised by
carrying out a series of oxidation studies with a set of
chosen alcohol s. The oxidation of 10 millimoles of
each of I-heptanol, 2-octanol, benzyl alcohol and
cyclohexanol with varying amounts of QFC-silica gel
at room temperature and under reflux conditions was
carried out. The optimum substrate/oxidant mole ratio
for these alcohols have been found to be 1: 1.25, 1: l.5 ,
1:l.25 and l:l.S, respectively.
The oxidation of a series of aliphatic primary alcohols with QFC-silica gel reveals that the lower homologues up to l-octanol (la-Ie) are readily converted
to the corresponding aldehydes at room temperature
while the higher homologues such as I-nonanol (If)
and I-decanol (lg) require reflux condition
(Scheme I) . Further, it is interesting to note that the
Table 1-
937
R1
>-
OH
R2
QFC I SG .
Scheme I
oxidation of primary alcohols with an alkyl group at
C-2 position such as 2-ethyl I-hexanol (lh) and
neopentyl alcohol (li) proceeds smoothly at a faster
rate than I-hexanol (lb) and I-pentanol (Ie). The results of the oxidation of alcohols are significant since
aliphatic alcohols either undergo overoxidation or
give a low yield of products. ' 6 It has been observed
that the oxidation of secondary alcohols (3a-e) with
QFC-silica gel proceeds smoothly only under reflux
conditions resulting in high conversion of alcohols to
the corresponding ketones . The results are given in
Table I.
QFC-silica gel oxidises alicyclic alcohols in hexan e
(Table II) under reflux conditions to the corresponding ketones in high yield. The observed order of reactivity is cyclopentanol > cycloheptanol > cyclooctanol
> cyclohexanol. The results are in accordance with the
17
qualitative interpretation suggested by Brown et al.
on the basis of I strain in these rings. The oxidation of
(-) menthol with QFC-silica gel gave exclusivel y
(-) menthone in 72% yield.
Oxid ation * of aliphati c primary and seco nda ry alcohol s
Substrate
RI
R2
Substrate/Oxida nt
mole ratio
Product"
Reaction
.
b
time
(hr)
Yi e ld"
(%)
la
CH )(CH 2)2
H
I: l.25
2a
2
63
Ib
CH 3(C H2) 3
H
I: 1.25
2b
4
81
Ie
CH )(C H2) 4
H
I: 1.25
2c
5
79
Id
CH 3(CH 2)s
H
I: 1.25
2d
6
83
Ie
CH )(CH 2) 6
H
I: 1.25
2e
5
80
If
CH )(CH 2h
H
I: 1.25
2f
4
7 1'
Ig
CH )(CH 2)g
H
I: l.25
2g
4
86'
Ih
CH )(CH 2k CH-C 2 Hs
H
I: 1.25
2h
4
81
Ii
(CH J))C
H
I: 1.25
2i
2
79
3a
(}H 3
CH )CH 2
I: 1.5
4a
2
64'
3b
CH )
CH )(CH 2) )
I: 1.5
4b
4
64'
3c
CH )
CH )(CH 2)4
I: 1.5
4c
5
84'
3d
CH )
CH 3(CH 2)5
I: 1.5
4d
6
88'
3e
CH 3
CH 3(CH 2h
1:1.5
4e
3
83'
" oxidation at room te mperature and under reflux conditions; Solve nt: Hexan e
·confirmed by GC, IR a nd IH NMR ; bdete rmined by GC analy sis
"isolated yield
INDIAN J. CHEM. , SEC B, MAY 2004
938
Table II-Oxidation * of alicyclic alcohols with QFC - silica gel
Substrate
Substrate/
Oxidant
mole ratio
Product"
6
1: 1.5
6
OH
6
4
S5
711130
(1 39/760)
7
71
93/100
(155/760)
4
77
95/100
(164/760)
5
SO
lOS/SO
(179/760)
6.5
74
40 (42)
(m.p.)
2
72
102/20
(2 10/760)
b.p./m.p.
(lit b.p./m.p.)
°C/mm Hg
CH
0'
0
1: 1.5
OH
6
0
1: 1.5
0c~
0
OH
6
OOH
1: 1.5
1: 1.5
6
0°
C~
CH3
6 0H
~
Yield "
(%)
0
OH
H3C
Reaction
time b
(hr)
CH3
1: 1.5
60
~
H3C
C~
*oxidation under reflux conditions; Solvent : Hexane
"confirmed by IR, IH NMR and GC analysis ; bdetermined by GC analysis
Cisolated yield
Oxidation of benzyl alcohol with QFC-silica gel in
hexane proceeded smoothly at room temperature
(Scheme II). However, the presence of a substituent
on the aromatic ring has a significant effect in the reaction time and product yield. Thus, substrates with
an electron releasing group in 2 or 4 position (Se, f, h)
were readily oxidised in a shorter reaction period than
the substrates with an electron withdrawing group in 2
or 4 position (Sb, dI, g). On the other hand, the differR'>-OH
QFC/SG.
ROO
Scheme n
RAJKUMAR et al.: QFC-SILICA GEL AS A SELECTIVE, STATIC AND VERSATILE OXIDANT
939
Table III-Oxidation* of aromatic primary and secondary alcohols
Substrate
R'
R"
Substrate/Oxidant
mole ratio
Product"
Reaction
time b
Yield c
(%)
(hr)
Sa
C6H5
H
1: 1.25
6a
2(2)
93(96)
Sb
2-N02-C 6H4
H
I: 1.25
6b
4(4)
49(70)
56(75)
Se
3-NO r C6H4
H
I: 1.25
6e
4(4)
Sd
4-NOrC 6H4
H
1: 1.25
6d
4.5(4.5)
53(71 )
Se
H
I: 1.25
6e
3(3)
68(88)
Sf
2-CH r C 6H4
4-CH 3-C6 H4
H
I: 1.25
6f
3(3)
74(90)
Sg
4-CI-C 6H4
H
I: 1.25
6g
6(5)
48(74)
Sh
4-CH30-C6H4
H
1: 1.25
6h
4(3)
76(95)
Si
C6H5CH 2
H
1: 1.25
6i
6(4)
82(90)
7a
C 6H5
CH)
1:1 .5
8a
7(4)
50(78)
7b
C 6H5
C 6H5
1:1.5
8b
4.5(3)
59(89)
7e
C6H5 (CH)OH
C6H5
1:1 .5
8e
3(3)
88(89)
*oxidation at room temperature and under reflux conditions; Solvent: Hexane
"confirmed by GC, IR and 'H NMR; bdetermined by GC and TLC analysis
Cisolated yield; values in parentheses ( ) refer reaction time and yield obtained under reflux condition
ence in reactivity between 2-phenyl ethanol (S i) and
benzyl alcohol suggests that QFC-silica gel has more
affinity towards the oxidation of a primary side chain
hydroxyl group attached to an aromatic ring with a
less number of spacer methylene groups. The results
are summarized in Table III. Among the aromatic
secondary alcohols (Table III) the oxidation of benzhydrol (7b) proceeded more readily at a faster rate
than the oxidation of (±) I-phenyl ethano l (7a) .
Further the oxidation of be nzoin (7c) to the corresponding diketone benzil (8c) under ambient temperature and shorter reaction time is illustrative of the
mildness of QFC - silica gel.
The oxidation of cinnamyl alcohol with QFC-silica
gel in hexane furnished exclusively cinnamaldehyde
without the formation of either bond cleavage product
such as benzaldehyde or side products such as epoxide. cis and trans-Hexen-l-ol afforded the respective
cis and trans aldehydes in reasonable yield and thus
no cis-trans isomerization was observed . Further the
oxidation of citronellol gave citronellal in high yield
(88 %) without the formation of pulegone, which may
arise due to cationic cyclisation . The results are presented in Table IV.
Oxidation of steroidal homoallylic alcohols is a
crucial step in the synthesis of most of the steroidal
harmones. Oxidation of cholesterol with PCC in
benzene under reflux condition gave cholest-4-en-3one with cholest-5-en-3-one as the intermedi ate. 18
However, the oxidation of cholesterol with QFC-sili ca
gel gave exclusively cholest-5-en-3-one in 70% y ield
without any side product due to the isomerisation of
the double bone\. On the other hand, for the oxidation
of cholesterol with unsupported QFC, a ten-fo ld
excess of the reagent was required and the yield o f
cholest-5-en-3-one was low « 50%). The oxidati on
of 3-~-cholestanol with QFC-silica gel gave
cholestanone in 60% yield . These reaction s clearl y
indicate the potential utility of QFC-silica gel in
biosynthesis. Further the utility of QFC-silica gel as a
mild oxidising reagent is revealed in the oxidation o f
1,2,5,6-0-dicyclohexylidene a-D-glucofuranose to
the corresponding ketone in 60% yield. The stability
of acid labile cyclic aceta l groups towards QFC-sili ca
gel oxidation conditions is noteworthy. QFC-silica gel
is quite effective for the oxidation of heterocycli c
alcohols also. Thus, furfurol was oxidised to furfural
in high yield . On the other hand the oxidation o f 3pyridyl methanol afforded 3-pyridyl carboxaldeh yde
in 65 % yield along with 20% quinoline (due to ligand
exchange). This is a significant result, since the
oxidation of the same substrate with unsupported
QFC failed to give the aldehyde and led to an
intractable black tarry deposit. The results are
presented in Table V .
INDIAN 1. CHEM., SEC B, MAY 2004
940
Table IV -
Substrate/Oxidant
mole ratio
Substrate
©-CH
H3C
= CH-CHO
Lf%
~
H3C
H3C
Oxidation* of unsaturated alcohols with QFC - silica gel
OH
~~OH
(H 3ChC=CH--CH 2OH
1: 1.5
Product a
© - CH
= CH-CH2OH
Reaction
time b
(hr)
Yield c
2
84
141/30
(248n60)
3
88
105/20
(207/760)
i
67 2
37/8
(37-38/8)
5'
62 2
39/10
50/30
3'
462
50/30
( J33-35n60)
b.p.im .p.
(lit. b.p.im.p.)
°C/mm Hg
(%)
CH3
d
1:1.5
CH
,;:?
3
H3C
\:1 .5
1:1.5
~CHO
H3C
~
CHO
H3C~
1: 15
/
(H 3ChC=CH--CHO
*Oxidation under reflux conditions; Solvent: Hexane; 'oxidation at room temperature; Solvent: Dichloromethane
aconfirmed by GC, IR and 'H NMR; bdetermined by GC analysis; cisolated yield; 2determined by GC analysis
The oxidising ability and the selectivity of other ions
such as perrnanganate have been studied extensively.'9.2o Metal supported heterocyclic perrnanganate,
bis(pyridine)silver perrnanganate though selectively
oxidises the alcohols, amines and oximes, it is relatively
unstable and rather expensive. The transition metals
such as copper and zinc incorporated heterocyclic
permanganates such as tetrakis(pyridine) copper (II)
permanganate and tetrakis(pyridine) zinc (II) permanganate have been reported?' The oxidants are highly
hygroscopic and zinc pemlanganate is also inflammable
which make them unsuitable for mild and clean
oxidation process. The same authors introduced
bis(2,2' -bipyridyl) copper(II) permanganate22. 23 and
found it is far superior than that of Mn02 with respect to
oxidizing ability and easy handling, but it cleaves the
benzylic double bonds.
In order to evaluate additional capabilities of QFCsilica gel, its utility in oxidative deoximation was
studied (Scheme III). Oxidative deoximation of aromatic aldoximes and ketoximes with QFC-silica gel
proceeded rapidly under reflux conditions in dichloromethane and the carbonyl compounds were obtained in a quantitative yield. However, aliphatic aldoximes such as heptanaloxime failed to undergo the
transformation. Aldoximes and ketoximes with acid
R
R
)=N-OH
R)=O
R'
Scheme III
sensItive groups such as 4-methoxy benzaldoxime
(9b), cinnamaldoxime (9c), salicylaldoxime (ge) and
4-hydroxy acetophenone oxime (90 were al so
smoothly converted to the corresponding carbonyl
compounds in high yield (> 90% ). The results
(Table VI) of ox.idative deoximation are comparable
with that obtained with unsupported QFC and are better than the results obtained with pce, pee, H 2 0 2 and
trimethyl ammonium chlorochromate 24 -26 Oxidation
of oximes to the corresponding carbonyl compounds
has been reported by many workers 27 as oximes can
be prepared from non-carbonyl compounds. Because
of the limited solubility of the KMn04 in organic solvents it has to be activated by impreganation onto
supports such as zeolite. But not all the zeolites convert oximes to carbonyl compounds. Sodium periodate and tetrabutylammonium periodate require manganese (III) tetra phenyl porphyrin catalyst for the
regeneration of carbonyl compounds from oximes at
room temperature. 28
RAJ KUMAR et al. : QFC-SILICA GEL AS A SELECTIVE, STATIC AND VERSATILE OXIDANT
941
Table V - Oxidation * of heterocyclic, steroidal alcohols and carbohydrates with QFC-silica gel
Product'
Substrate/
Oxidant
mole ratio
Substrate
ODWH
Yieldc
(%)
3
65
OQfO
I : 1.5
GOWH
Reaction
time b
(hr)
96/15
(95-97/15)
Gc~
1:1.5
0
b.p./m.p.
(lit. b.p./m.p.)
°C/mm Hg
3
1
79
74/30
(1621760)
0
1:6
5
70
126 (125-127)
(m.p.)
1:4
8
60
129 (128-130)
(m.p.)
1:5
6
60
93
(m.p.)
*oxidation under reflux condition and oxidation at room temperature; Solvent: Hexane
aconfirmed by GC, IR and IH NMR; bdetermined by GC analysis; cisolat.ed yield
Table VI -
Oxidative* deoximation of aldoximes and ketoximes
Substrate
R
R
Substrate/Oxidant mole ratio
Producta
Reaction time b
(hr)
Yield c
(%)
9a
9b
9c
9d
ge
9r
9g
9b
4-N02-C 6 H4
4-CH)O-C 6 H4
H
H
1:2
1:2
3
3
89
95
C6 HsCH=CH
H
1:2
lOa
lOb
10c
10d
lOe
lOr
109
lOb
3
93
C6 Hs
CH 3
1:2
2-HO-C 6 H4
H
1:2
4-HO-C 6H4
H
1:2
R=R ' = -(CH 2)s
1:2
CH 3(CH 2)s
H
1:2
*under reflux conditions; Solvent: Dichloromethane
aconfirmed by GC, IR and IH NMR; bdetermined by TLC analysis
cisolated yield
4
94
5
73
5
84
5
93
5
INDIAN 1. CHEM., SEC B, MAY 2004
942
Table VII -
Oxidation * of mixture of alcohols with QFC - silica gel
Substrate/
Oxidant
mole ratio
Substrate
Products
1- Octanol
Reaction
time"
(hr)
Yield"
(%)
1- Octanal
+
1:2
+
2 - Octanol
6
56
14
5
29
66
5
69(49)
19(29)
2 - Octanone
1- Heptanol
1 - Heptanal
+
+
1:2
Benzyl alcohol
Benzyl alcohol
Benzaldehyde
Benzaldehyde
+
+
1:2
Cyclohexanol
Cyclohexanone
*oxidation at room temperature; Solvent: Hexane
'determined by GC analysis; values in parentheses( ) refer to yield obtained under renux condition
The selectivity of QFC-si lica gel in the oxidation of
alcohols was ascertained by carrying out the competitive oxidation on a mixture of alcohols. The results
(Table VII) reveal that an aliphatic primary hydroxyl
group is preferentially oxidised in the presence of an
aliphatic secondary hydroxyl group. Further, an aromatic side chain primary hydroxyl group is oxidised
more preferentially than an aliphatic primary hydroxyl gro up and an alicycli c hydroxy l group.
The studies on the stereochemical preference of
QFC-silica gel in the oxidation of cis (11) and trans4-t-butyl cyclohexanol (12) indicate that the axial hydroxyl group of the cis isomer (GC; R t .6.42) is oxidised faster in high yield than the equatorial hydroxyl
group of trans isomer (GC; R t.7.00) (Scheme IV).
The stability and effectiveness of QFC-silica gel
was evaluated by carrying out the oxidation of benzyl
alcohol during different periods of storage of the
reagent. The results were compared with those
obtained with QFC, PCC and PFC-alumina. The
results (Table VIII) reveal that QFC-silica gel is
(CI-h),c~
11
quite stable and effective even after a period of twelve
weeks of its preparation (as indicated by the
consistent yield of benzaldehyde), while QFC, PCC
and PFC-alumina te nd to lose their effectiveness on
storage.
The mechanism of oxidation of alcohols is proposed based on the fo ll owing observations (i) The
faster oxidation of primary alcohols with an alkyl
substituent at C-2 carbon such as 2-ethyl-l-hexanol
and neopentyl alcohol than the un substituted counterparts suggests that there is steric acceleration of rate
(ii) The faster ox idation of alicyclic alcohols with I
strain also supports steric acceleration of rate. This
implies that the reaction proceeds throu gh a tran sition
state in which the strain is relieved . Thi s fact is further
supported in the oxidation of 4-r-buty l cyc lohexanol
as discussed previously (ii i) enhanceme nt of the rate
in the oxidation of substituted benzyl alcohols with an
electron releasing substituent suggests that the oxidation takes place through the formation of chromate
ester intermediate (Scheme V) .
OFC-Silica gel
Ref, 2h, 92%
(Cl-hbC~
OH
_O_F_C-_S_ili_ca_ge_I_~/
13
Ref, 4.5h, 63%
12
Scheme IV- Oxidation of cis (11) and trans-4-t-butyl cyclohexanol (12)
o
RAJKUMAR et at.: QFC-SILICA GEL AS A SELECTIVE. STATIC AND VERSATILE OXIDANT
Table VIII -
Oxidation * of benzyl alcohol with QFC. QFC - silica gel. PCC and
PFC-alumina during storage
Substratel
Oxidant
mole ratio
Substrate
1:1.5
QFC - Silica gel
I: 1.5
Storage period
weeks
Reaction time'
(hr)
Yield'
2
90
90
87
90
91
2
4
6
12
1
2
4
89
85
3
6
71
12
1
62
90
81
2
1:1.5
4
69
2
61
54
6
12
1
2
PFC-alumina
1: 1.5
(%)
84
72
4
3
64
6
52
12
44
•oxidation at room temperature; Solvent: Hexane
Isolvent: Dichloromethane
'determined by GC analysis
R'
+
I
I
H
R-C-OH
...
iiiiihiiiiiiiiiiiiiiiiii
liiiihiiiiiiiiiiiiiiiiii
SG
SG
,
/
1
C=O
R'
i1iiihiiiiiiiiiiiiiiiili
SG
iiiiihiiiiiiiiiliiiiiiiiiiiiiiiiiiiiiiiiiiliiiiii
SG
Scheme V- Mechanism of oxidation of secondary alcohol
#
943
944
INDIAN 1. CHEM., SEC 8, MAY 2004
Further ESR spectral analysis of the reduced product of the QFC-silica gel showed a single band with a
g value of 1.982 and this value shows the presence of
2
d Cr (IV) species. This indicates that a two electron
transfer is involved in the oxidation. From these observations the mechanism of oxidation of aicohols is
proposed to occur through the following steps. (1)
QFC-silica gel attacks the hydroxyl group to give the
chromate ester (2) chromate ester then undergoes cyclic hydride transfer to give the carbonyl compound
and the reduced Cr(IV) species as shown in the
mechanism.
Thus quinolinium f1uorochromate supported on silica gel on account of its versatility has the following
advantages over the existing supported chromium(VI)
reagents: (a) simple preparation (b) good selectivity
(c) prolonged stability (d) substrate/oxidant mole ratio
is minimum (e) work-up of the reaction mixture is
very simple (f) reaction times are reasonable (g) useful in the oxidation of acid sensitive substrates (h)
does not require other complementary techniques
such as microwave ilTadiation or the presence of catalysts such as Lewis acids?9 No leaching of chromium
species into the reaction mixture. This is very important with respect to the env ironmental concern. Thus
QFC-silica gel is one of the best and least hazardous
Cr(VI) reagents and a valuable addition to the existing
reagents.
Materials and Methods
Alcohols used in the present work were of extrapure quality (E. Merck, Fluka and Aldrich) and
were distilled under reduced pressure/recrystallised
from suitable solvent prior to use. cis and trans
t-Butyl cyclohexanols were prepared by reported
methods. Oximes were prepared by adopting the standard procedures. 3D Reagents such as pyridiniulTl
chlorochromate (PCC)6, quinolinium fluorochromate l 3c and pyridinium f1uorochromate supported on
alumina l3c were prepared adopting the reported procedures. The products of oxidation were identified by
comparison with authentic samples (IR, GC, IH
NMR , b.p. and m.p.). IH NMR spectra were recorded
on Bruker DPX-200 (200 MHz) high-resolution FTNMR spectrometer using CDCI 3 as solvent and
tetramethylsilane (TMS) as internal standard . IR spectra were recorded in Perkin-Elmer infrared spectrophotometer (model: Hitachi 270-50). GC analyses
were performed on Hewlett Packard 5890A gas
chromatograph using flame ionisation detector. The
columns employed in the study were carbo wax 20M
and OV-l7 packed columns. TLC analyses were performed on precoated silica gel (silica gel F 254 ) plates.
TLC plates were developed in an iodine chamber
and/or by spraying a solution of 2,4-dinitrophenyl
hydrazine. The surface area of silica ge used was determined using Micromeritics pulse chemisorb. Melting points of the products were determi ned on a Raga
hot stage apparatus.
Experimental Section!
Preparation of quinolinium tluorochromate
supported on silica gel. To an ice cold mixture of
Cr(VI) oxide (l5g, 0. 15 mole) and 40% hydrofluoric
acid (11.3 mL, 0.23 mo le), silica gel (45g, 60-120
mesh; surface area: 253 m2/g) activated at 100°C prior
to use was added with stirring usin g a mec hanical stirrer. Quinoline (17 .7 mL, 0.15 mole) was then added
dropwise and the resulting yellow orange solid was
filtered, washed with water and cold acetone and then
dried in vacuum for 2 hr.
General procedillre for the oxidation of alcohols/oximes. To a stirred slurry of QFC-s ilica gel in
hexane (approximately 2 mL per gram of the suppOlted
reagent), a solution of the alcohol in hex ane was added
and the reaction mixture was stirred vigorously at room
temperature/reflux condition. The course of the reaction was followed by GC / TLC analysis after every
hour. After completion of the reaction , the reaction
mixture was diluted with dry diethyl ether (30-40 mL)
and filtered through a short column of silica gel (2 cm).
The solid residues were thoroughly washed with dry
diethyl ether (4 x 20 mL) . The combined filtrate on
evaporation in a rotary evaporator gave the crude product, which was purified by distillation under reduced
pressure (or) recrystallisation in case of solid products.
For the oxidative deoximation of oxirnes the above
procedure was followed in dichloromethane as solvent
under reflux condition. The course of the reaction was
followed by TLC analysis (silica gel, pet. ether: ethyl
acetate 9: I).
Oxidation of 1-heptanol (1 d) to 1-heptanal (2d)
as a typical exampl,e. QFC-silica gel (8.6 g, 12.5
mmole) was made into a slurry with hexane (35 mL)
in a two necked RB flask of 100 ml capacity fitted
with a reflux condenser and a mechanical stirrer. The
solution of I-heptano l (I.l6g, 10 mmo le) in hexane
(3 mL) was added to the slurry and the mixture was
stirred vigorously at room temperature. The course of
the reaction was followed by GC analysis (Carbowax
20M column, Injecti on temperature: 180°C; Column
temperature: 100°C) after every hour. After comple-
RA JKUM A R et al.: QFC-S ILI CA GEL AS A SEL ECTIV E, STATIC AN D VERSATILE OX IDANT
ti o n of th e reacti o n (minimum peri od in which max imum conversio n was achi eved) the reacti o n mi xture
was stirred fo r a further peri od of 30 minutes, diluted
with dry di ethy l eth er (40 mL) and filte red thro ug h a
sho rt co lumn of sili ca ge l (2 e m). Th e so lid res idue
was tho rou ghl y washed with d ry di ethyl ethe r
(4 x 20 mL). Th e co mbin ed filtrates o n evapo rati o n
gave th e crude produ ct, whi c h was di still ed throu g h a
short vigereux co lumn to give I-he ptanal (b.p.
62°C13 0 mm Hg) in 83% y ie ld . IR : 2940(w),
2850( m), 2710(m), 17 LO(s) a nd 1470 (m) e m-I.
Oxidation of cholesterol to 5-cholesten-3-one. The
ox idati on was carri ed o ut with cho lesterol (J g, 2.6
mmole) and Q FC-sili ca gel (1 0 .13 g, 15 mmole) in 35
mL hexane. The reacti o n mi xture was reflu xed and th e
course of the reacti o n was fo ll o wed by TLC (s ili ca gel;
10% diethyl ether in hexane). After co mpl eti o n of th e
reacti on the prod uct was iso lated by colu mn chrom atography (s ilica ge l, 5% dieth yl ether in hexane) . 5cholesten-3-o ne (m.p. 126 °C) was iso lated in 70%
yie ld (0.68 g). IR: 2960(s), 1725(s) and 1620(m) e m-I;
IH NMR (CDC I}): 8 0.70 (s, 3 H), 0.87 (s,3H), 0. 93
(s,3 H), 1.02 (s,3H), 1.02- 1.60 (m, 18H), 1.50-2.20 (m,
7H ), 2.25 (s, I H), 2.35 (s, I H) and 5.37 (m, IH ); Mass:
M/z 384 (M, 56%), 369 (M-C H}, 29%), 27 1 (M-s ide
chain ), 229 (55 %) and 124 ( 100%).
Oxidation of citronellol. The reacti o n was pe rfo rmed with c itro ne ll o l ( 1. 56 g, 10 mmo le) and QFCsili ca gel (1 0.1 4 g, 15 mmo le) in hexane (20 mL ). Th e
reactio n mixture was refl uxed a nd th e course of the
reactio n was fo ll owed by GC ana lys is (Carbowax
20M co lumn ; Inj ecti o n te mperature: 300°C, Co lumn
te mperature: 180°C) . T he produ c t was iso lated by di stil latio n under redu ced press ure. C it ro ne ll a l (b.p.
105 °C/20 mm Hg) was obtained in 88% (1. 37 g) y ie ld.
IR: 2930 (s), 2700 (s), 1700 (s), 1640 (w), 1450 (m)
and 1375 (m) em-I; IH NM R (C DCl 3): 8 0.98 (d, 3 H),
1.08 -1 .47 (m, 3 H), 1.6 1 (s, 3 H), 1.70 (s, 3H), 2.03(m,
2H), 2.36 (dd, 2 H), 5. 10 (t, IH ) and 9.78 (s, IH).
Acknowledgement
This research was suppo rted by Unive rs ity Gra nts
Commiss io n (UGC), New De lhi . G A Rajku mar
thanks CS IR , New Delhi fo r th e award of Seni o r
Researc h Fe ll ows hip . The auth o rs are g rateful to
Prof. K K Balasubrama ni an, li T , Chen nai, fo r th e
valub le suggestio ns.
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