V o l. 13 N o. 4 ( 1997)
Chem ica l R esea rch in Ch inese U n iversit ies
Sa lt- induced E lectron Tran sfer Photoox ida t ion
of Cholesterol in Nonpolar Solven ts
3
YAN G Chen 2gen
(D ep a rtm en t of O rg an ic C hem istry , A g ricu ltu ra l C olleg e,
Y ang z hou U n iv ersity , Y ang z hou , 225009)
W AN Yan and J IAN G Zh i2qin
(A na ly sis and R esea rch C en ter, T ong j i U n iv ersity , S hang ha i , 200092)
(R eceived Sep t. 24, 1996)
Ke yw o rds Cho lestero l, Pho tooxygena t ion , Sa lt 2induced, E lect ron t ran sfer
In troduction
[ 1—3 ]
. How ev2
Sa lt effect s a re often u sed to con t ro l o r p robe o rgan ic react ion s
er, on ly recen t ly som e sa lt s have been em p loyed in the invest iga t ion of p ho to 2
[ 4—6 ]
. T hough m uch a t ten t ion ha s been a t t racted to elect ron
p hy sica l p rocesses
[ 7—9 ]
, w h ich p roceed s v ia rad ica l ion s of o rgan ic sub 2
t ran sfer p ho tooxygena t ion
st ra te, p roduced by elect ron t ran sfer to excited sta te of a sen sit izer, the reac2
t ion s have a lm o st been lim ited in po la r so lven t s. In nonpo la r so lven t s, energy 2
w a st ing retu rn ing elect ron t ran sfer often occu rs in com p et it ion w ith chem ica l re2
[ 10—12 ]
. H erein w e w ill repo rt the p ho tochem ica l resu lt s of a study of tet ra 2
act ion s
n 2bu ty lamm on ium tet rafluo robo ra te ( R 4NB F 4 ) 2induced elect ron t ran sfer p ho 2
toox ida t ion of cho lestero l ( 1 ) , w h ich is of b io log ica l im po rtance, in
d im ethoxyethane (DM E ).
Exper im en ta l
H NM R sp ect ra w ere reco rded on a B ruker AM 2400 sp ect rom eter and in 2
fra red sp ect ra on a N ico let 5DXF T 2IR sp ect rop ho tom eter. M a ss sp ect ra w ere
44SGS g
reco rded on a M A T g
M Sg
SS sp ect rom eter.
1　M a te ria ls
Cho lestero l ( 1 ) ( F luka ) w a s recry sta llized from m ethano l. 9, 102D icyanoan 2
th racene, DCA ( Kodak ) w a s recry sta llized from ho t to luene, m. p. 324—326 ℃.
B ip heny l (B P ) ( H ip k in w illam s ). D im ethoxyethane (DM E ) w a s d ist illed from
P 2O 5. T et ra 2n 2bu ty lamm on ium tet rafluo robo ra te (R 4NB F 4 ) w a s p rep a red a s p re2
[ 13 ]
viou sly described , the sa lt w a s recry sta llized from w a ter, w a ter 2a lcoho l m ix 2
1
3 To w hom co rrespondence shou ld be addressed.
tu res and from ethy l aceta te, m. p. 161. 8℃.
2　P ho tooxyg e na tion of 1 in a D CA g
BPg
R 4NB F 4 g
DM E S ys tem
DCA 2sen sit ized react ion s w ere irrad ia ted a t > 390 nm w ith a 500 W h igh
p ressu re m ercu ry lam p. A ll so lu t ion s fo r p ho tooxygena t ion w ere p laced in p y rex
- 4
test tubes. A m ix tu re of 97 m g of 1 ( 0125 mm o l) , DCA ( 410×10 m o lg
L ) , BP
- 2
( 510×10- 2 m o lg
)
(
)
L and sa lt R 4NB F 4 315×10 m o lg
L w a s irrad ia ted a t 20 ℃,
(
)
in oxygen 2sa tu ra ted d im ethoxyethane DM E, 20 mL fo r 4 h. T he react ion m ix 2
tu re w a s reduced by 170 m g of t rip heny l p ho sp h ine after the irrad ia t ion. T he
m ix tu re of the p roduct s w a s sep a ra ted on a co lum n con ta in ing ba sic silica gel
(M erck ) w ith to luene g
ethy l aceta te ( 17∶8) a s eluen t. Ch rom a tog rap hy affo rded
raw m a teria l 1 and th ree p roduct s a s fo llow s: 319 m g of 1, m. p. 146—148 ℃;
219 m g of cho lest 262en 23Β, 5Α2d io l ( 2 ) , m. p. 147—148 ℃; M S, m g
z 384
- 1
π
(M —H 2O , 30% ) , 366 (M —2H 2O , 100% ) , 351 ( 3662M e, 20% ) ; IR , Μ
m ax g
cm :
3 636, 3 423, 3 305 ( b, O —H ) , 2 981, 2 867 ( s, C —H ) , 1 636 (w , C
C),
1
1 381; H NM R , ∆, 0171 ( 3H , s ) , 0183 ( 6H , s ) , 0191 ( 3H , s ) , 1126 ( 3H , s ) ,
0180—2140 ( 28H , m ) , 4110 ( 1H , m ) , 5160 ( 2H , d ) [ 14 ]. 47 m g of cho lest 252en 23Β,
+
7Β2d io l ( 3) , m. p. 174—176℃; M S, m g
z : 402 (M ) , 384 (M —H 2O , 100% ) , 366
- 1
πm ax g
(M —2H 2O , 40% ) , 351 ( 3662M e, 10% ) ; IR , Μ
cm : 3 377, 3 363 ( b, O —H ) ,
1
2 934, 2 867 ( s, C —H ) , 1 662 (w , C
C ). H NM R , ∆: 0168 ( 3H , s) , 0189 ( 6H ,
d ) , 0193 ( 3H , d ) , 1105 ( 3H , s) , 1110—2100 ( 26H , m ) , 2130 ( 2H , m ) , 3160 ( 1H ,
[ 14 ]
m ) , 3185 ( 1H , m ) , 5162 ( 1H , d ) ; 2917 m g of cho lest 252en 272one 23Β2o l ( 4 ) ,
+
m. p. 152—153℃; M S, m g
z 400 (M , 100% ) , 382 (M —H 2O ) , 367 ( 3822M e ) ,
- 1
πm ax g
342 ( 3672M e ) ; IR , Μ
cm : 3 401, 3 391 ( b, O —H ) , 2 945, 2 936 ( s, C —H ) ,
1
1 672 ( s, C
O ) , 1 630 (w , C
C ). H NM R , ∆: 0168 ( 3H , d ) , 0182 ( 6H , d ) ,
1119 ( 6H , b ) , 1152—2149 ( 26H , b ) , 3103 ( 1H , m ) , 3166 ( 1H , m ) , 5168 ( 1H ,
[ 15 ]
s) .
3　P ho tooxyg e na tion of 1 in O the r S ys tem s
T he cond it ion s of these p ho tochem ica l react ion s w ere the sam e a s tho se of
react ion in the DCA g
BPg
R 4NB F 4 g
DM E sy stem excep t tha t these sy stem s w ere
irrad ia ted fo r 18 h.
T he p ho tooxygena t ion of 1 in a DCA g
DM E sy stem gave 1215 m g of 1,
718 m g of 2, 56 m g of 3 and 1317 m g of 4. T he p ho tooxygena t ion of 1 in a DCA g
BPg
DM E sy stem gave 1017 m g of 1, 616 m g of 2, 5717 m g of 3 and 1618 m g of
4. T he p ho tooxygena t ion of 1 in a DCA g
R 4NB F 4 g
DM E sy stem gave 1113 m g of
1, 617 m g of 2, 55 m g of 3, 1512 m g of 4.
Results and D iscuss ion
T he m ix tu re of 9, 102d icyanoan th racene (DCA ) , b ip heny l (B P ) , cho lestero l
( 1) and sa lt R 4NB F 4 w a s irrad ia ted in oxygen 2sa tu ra ted d im ethoxyethane (DM E )
fo r 4 h. T he react ion m ix tu re w a s reduced by t rip heny l p ho sp h ine after the irra 2
393
d ia t ion, 1 w a s quan t ita t ively con sum ed and ox ida t ion p roduct s cho lest 262en 23Β,
5Α2d io l ( 2) , cho lest 252en 23Β, 7Β2d io l ( 3 ) , cho lest 252en 272one 23Β2o l ( 4 ) w ere ob 2
ta ined in excellen t y ield s a s show n in T ab le 1 together w ith sim ila r resu lt s ob 2
ta ined in o ther react ion sy stem s. W ithou t sen sit izer DCA , the react ion d id no t
take p lace.
T ab le 1　Y ield s of DCA 2sen sit ized p ho toox ida t ion p roduct s 2, 3, 4
and effect s of added sa lt R 4NB F 4 o r co sen sit izer B P in DM E
R un
A dded R 4NB F 4 o r B P
R eaction tim e g
h
1
2
3
4
N one
BP
R 4NB F 4
R 4NB F 4 , B P
18
18
18
4
Y ield ( % )
2
10. 1
8. 1
8. 7
3. 6
3
72. 1
71. 1
71. 5
58. 8
Conversion ( % )
4
17. 8
20. 8
19. 8
37. 3
87
89
88
96
A rem a rkab le sa lt effect w a s ob served w hen the irrad ia t ion w a s ca rried ou t
in the nonpo la r so lven t DM E. T he DCA 2B P co sen sit ized p ho toox ida t ion of
cho lestero l ( 1) w a s d ram a t ica lly accelera ted by sa lt R 4NB F 4. T he d ist ribu t ion of
the p roduct s w a s a lso changed by sa lt R 4NB F 4 , esp ecia lly the y ield of p roduct 4.
Schem e
In po la r so lven t aceton it rile, ou r p reviou s stud ies
of DCA 2B P co sen si2
t ized p ho toox ida t ion of cho lestero l ( 1 ) suggested tha t p roduct s 3 and 4 w ere
fo rm ed v ia elect ron t ran sfer m echan ism ( Schem e 2). B P quenched sing let excited
3
+ ・
[ 18 ]
DCA to genera te the rad ica l ca t ion B P
efficien t ly ( <= 018) , w h ich ox id ized
+ ・
cho lestero l ( 1 ) to the rad ica l ca t ion 1 , w h ich, sub sequen t ly, dep ro ton ized to
・’
・’
the rad ica l 1 . T he react ion of the rad ica l 1 w ith oxygen gave the p roduct s 3
1
and 4. In the p resence of DCA a lone, the react ion of 1 w ith sing let oxygen O 2
[ 15—17 ]
394
affo rded cha racterist ic p roduct s 2 and 3 in aceton it rile and benzene.
DCA 3
DCA
DCA 3
(DCA —B P ) 3
+ BP
B P + g + DCA BP+ g + 1
B P + 1+ g
+g
g’
1
1 + H+
1g’+ O 2
……3 + 4
Schem e 2
g
In nonpo la r so lven t DM E, how ever, the p roduct s of DCA 2B P co sen sit ized
p ho toox ida t ion of 1 w ere the sam e a s tho se fo rm ed by react ion w ith sing let oxy 2
1
gen genera ted by a typ ica l O 2 sen sit izer Ro se B enga l (RB ). T he sa lt R 4NB F 4 led
to the accelera t ion of DCA 2B P co sen sit ized p ho toox ida t ion of 1 and the change of
p roduct s d ist ribu t ion. W ithou t co sen sit izer B P , sa lt d id no t induce elect ron
t ran sfer p ho to react ion of 1 bo th in aceton it rile (M gC lO 4 ) and in d im ethoxyethane
(R 4NB F 4 ). P reviou s resu lt s of the stud ies [ 16 ] of DCA fluo rescence quench ing il2
3
lu st ra ted clea rly tha t excip lex (DCA —B P ) w a s fo rm ed in DM E and rad ia t ion re2
laxa t ion w a s a dom inan t energy 2w a st ing p rocess. In the p resence of sa lt
R 4NB F 4 , cho lestero l ( 1 ) reacted a t app reciab le ra tes in DM E a s w ell a s in
3
M eCN. T hese ob serva t ion s ind ica te tha t the react ion of excip lex (DCA —B P )
w ith agg rega ted o r quad rupo la r sa lt (R 4NB F 4 ) g ives rad ica l ion s, p robab ly by an
ion 2p a ir exchange p rocess. T h is sa lt 2induced elect ron t ran sfer m echan ism
( Schem e 3) can accoun t fo r the p ho tochem ica l resu lt s.
(DCA —B P ) 3 + R 4NB F 4
R 4N + DCA - g + B P + gB F _4
B P + gB F -4 + 1
1+ gB F -4 + B P
Schem e 3
O ne p a rt icu la rly in t rigu ing find ing w a s tha t 1, 42d im ethoxy 2benzene (DM B )
u sed a s a quencher com p letely supp ressed sa lt induced DCA 2B P co sen sit ized p ho 2
toox ida t ion of 1 bu t no t the sing let oxygen react ion (DCA sen sit ized a lone ).
DM B w a s w idely u sed a s a select ive inh ib ito r of the p ho tooxygena t ion, w h ich
[ 19, 20 ]
. T he in it ia lly
p roceed s v ia rad ica l ca t ion p roduced by elect ron t ran sfer
+ ・
+ ・
fo rm ed rad ica l ca t ion B P
or 1
is reduced by DM B ( Schem e 4 ) , w h ich ha s a
+ ・
low er ox ida t ion po ten t ia l. T hen DM B
react s w ith DCA , g iving m a in ly reverse
elect ron t ran sfer. T h is find ing confirm s tha t w ith sa lt R 4NB F 4 w h ich can induce
elect ron t ran sfer to g ive rad ica l ion s in nonpo la r so lven t s, the rad ica l ion m echa 2
n ism p redom ina tes, affo rd ing the p roduct s 3, 4 of the p ho tochem ica l react ion.
B P + gB F -4 + DM B
DM B + gB F -4 + B P
DM B + gB F -4 + DCA - gR 4N +
DM B + DCA + R 4NB F 4
Schem e 4
T he p relim ina ry w o rk on the DCA 2B P co sen sit ized p ho toox ida t ion of cho les2
tero l ( 1 ) show ed tha t sa lt is u sefu l bo th fo r con t ro lling the p ho tochem ica l reac2
t ion s and fo r p erfo rm ing the p u ta t ive elect ron t ran sfer react ion s.
395
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396