Molecules 2010, 15, 7732-7741; doi:10.3390/molecules15117732
molecules
ISSN 1420-3049
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Communica ion
Applica ion o Phosphine-Phosphi e Ligands in he I idium
Ca alyzed Enan ioselec i e Hyd ogena ion o 2-Me hylquinoline
Miguel Rubio and An onio Pizzano *
Ins i u o de In es igaciones Químicas, Consejo Supe io de In es igaciones Cien í icas and
Uni e sidad de Se illa, A da. Amé ico Vespucio 49, 41092 Se illa, Spain
* Au ho o whom co espondence should be add essed; E-Mail: piz[email p o ec ed];
Tel.: +34-954-489-556; Fax: +34-954-460-565.
Recei ed: 6 Oc obe 2010; in e ised o m: 26 Oc obe 2010 / Accep ed: 28 Oc obe 2010 /
Published: 29 Oc obe 2010
Abs ac : The hyd ogena ion o 2-me hylquinoline wi h I ca alys s based on chi al
phosphine-phosphi es has been in es iga ed. I has been obse ed ha he eac ion is e y
sensi i e o he na u e o he ligand. Op imiza ion o he ca alys , allowed by he highly
modula s uc u e o hese phosphine-phosphi es, has imp o ed he enan ioselec i i y o
he eac ion up o 73% ee. The in luence o addi i es in his eac ion has also been
in es iga ed. Con a y o he bene icial in luence obse ed in ela ed ca aly ic sys ems,
iodine has a dele e ious e ec in he p esen case. O he wise, a yl phospho ic acids
p oduce a posi i e impac on ca alys ac i i y wi hou a dec ease on enan ioselec i i y.
Keywo ds: asymme ic hyd ogena ion; chi al ligands; phosphines; phosphi es; i idium;
quinolines
1. In oduc ion
The use o wo coo dina ing unc ions o di e en na u e in a chi al ligand cons i u es as a e y
powe ul app oach in he ield o asymme ic hyd ogena ion [1]. Thus, excellen ca alys pe o mance
has been achie ed in a ple ho a o eac ions by he use o ligands which app op ia ely combine di e se
C, N, S and P dono g oups [2-5].
The ca aly ic asymme ic hyd ogena ion o quinolines o p oduce op ically ac i e e ahyd o-
quinolines is a e y in e es ing eac ion due o he impo ance o he esul ing p oduc s. Fo ins ance,
chi al e ahyd oquinolines a e ubiqui ous p oduc s in Na u e (Figu e 1) which exhibi , in addi ion, a
OPEN ACCESS
Molecules 2010, 15
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wide ange o biological p ope ies o in e es o he pha maceu ical indus y [6-12]. A a ie y o
ca aly ic sys ems, mos ly based on I complexes and chi al chela ing ligands wi h ei he equi alen
(C
2
symme ic) o non equi alen (C
1
symmme ic) coo dina ing unc ions, ha e p o ided sa is ac o y
esul s o his ans o ma ion. In his ega d, Zhou e al. ha e desc ibed a ca aly ic sys em o
[I Cl(cod)]
2
, MeO-Biphep and I
2
ha hyd ogena es a a ie y o subs i u ed quinolines wi h high
enan ioselec i i ies [13]. Likewise, Chan e al. ha e success ully applied o he diphosphines such as
P-phos and Di luo phos in his eac ion [14-15]. In e es ingly, his ans o ma ion can also be
e ec i ely ca alyzed by species based on less dono diphosphoni es, as shown by Ree z e al. [16].
Mo eo e , C
1
symme ic phosphine-oxazoline, phosphine-sul oximine o phosphine-phospho amidi e
ligands ha e also led o e icien ca alys s [17-19]. In addi ion, complexes based on a combina ion o a
monoden a e chi al phospho amidi e and an achi al phosphine, which p o ide good le els o ac i i y
and enan ioselec i i y, ha e been desc ibed by Fe inga e al. [20]. Despi e he excellen esul s
ob ained in hese p eceden s, he knowledge abou his eac ion is s ill limi ed. Then, s udies aimed o
unde s and he in luence o impo an ea u es like ligand basici y, bi e angle o he in luence o
addi i es a e highly in e es ing [13-21].
Figu e 1. Some na u ally occu ing chi al e ahyd oquinolines.
N
Me
N
Me O
ON
Me OMe
OMe
Angus u eine Galipinine Cuspa eine
Among chela ing ligands wi h unequal coo dina ing g oups, we ha e ocused on phosphine-
phosphi es (P-OP) and hei applica ion in asymme ic ca aly ic hyd ogena ion eac ions. In e es ingly,
he dissimila elec onic p ope ies o hei P unc ionali ies p o ides an e icien di e en ia ion
be ween coo dina ion posi ions, which educes he numbe o eac ion in e media es and allows a
be e s e eocon ol [22-23]. Mo eo e , he highly modula s uc u e o he P-OP de i a i es de eloped
in ou labo a o y (Figu e 2), enables a de ailed ca alys sc eening co e ing he in luence o phosphine,
phosphi e and backbone agmen s. This app oach has success ully been applied in he Rh ca alyzed
enan ioselec i e hyd ogena ion o se e al ypes o ole ins [24-26]. In addi ion, we ha e demons a ed
he use ulness o P-OP ligands in he I ca alyzed hyd ogena ion o N-a yl imines [27]. As an ex ension
o he scope o chi al phosphine-phosphi es in he hyd ogena ion o C=N bonds, we desc ibe he ein
p elimina y esul s abou he applica ion o hese ligands in he I ca alyzed asymme ic hyd ogena ion
o 2-me hylquinoline.
Figu e 2. Gene al s uc u e o P-OP ligands.
O
P
PO
O
R'
R'
R
R
O
P
PO
O
R'
R'
R
R
R = alkyl, a yl
R' = H, Bu
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2. Resul s and Discussion
In a i s s age, we ha e ca ied ou a se o eac ions o sea ch o app op ia e condi ions o he
ca aly ic hyd ogena ion o 2-me hylquinoline (Eq 1). Thus, unde 40 ba o hyd ogen p essu e and a
oom empe a u e, he ca alys gene a ed om [I (Cl)(COD)]
2
and ligand (S)-1a (Figu e 3), p oduced
only a mode a e con e sion and a low enan ioselec i i y (en y 1, Table 1). In e es ingly, he p esence
o a mo e dona ing phosphine g oup in (S)-1b led o an impo an inc ease in ca alys ac i i y, wi h a
sligh imp o emen on enan ioselec i i y (en y 2). Mo eo e , a ca ionic ca alys p ecu so based on
(S)-1b gene a ed a poo e ca alys (en y 3). In e es ingly, he neu al ca alys s p oduced an inc eased
con e sion a a lowe p essu e bu again wi h low selec i i ies (en ies 4, 5). F om his p elimina y
sc eening, condi ions o he la e eac ions we e chosen o analyze he in luence o ligand s uc u e on
enan ioselec i i y (Table 2).
Figu e 3. P-OP ligands used in he p esen s udy.
N
H
2
[I (Cl)(COD)]
2
+ P-OP N
H
(1)
O
P
PO
O
Bu
Bu
R
R
O
P
PO
O
R
R
R = Ph [(S)-1a], P
i
[(S)-1b]R = P
i
[(R)-2b)], Bu
[(R)-2c],
2-MeC
6
H
4
[(R)-2d],
1-naph hyl [(R)-2e]
O
P
PO
O
Bu
Bu
Ph
Ph
(S)-4a
O
P
PO
O
Ph
Ph
(R)-5a
O
P
PO
O
R
R
R = Ph [(S)-6a], P
i
[(S)-6b]
O
P
PO
O
Bu
Bu
Ph
Ph
(S)-3a
A compa ison o he ca alys pe o mance along he se ies o ligands allows one o ex ac some
in e es ing obse a ions. Mos ema kably, he use o less hinde ed phosphi e g oups has a posi i e
e ec on he eac ion. Thus, ca alys p epa ed wi h ligand (R)-2b p oduced an impo an inc ease on
enan ioselec i i y o e (S)-1b, om 16 o 62% ee, while main aining a good con e sion (en ies 2, 3).
Fu he examina ion o di e en phosphine g oups did no allow us o imp o e he alues achie ed by
(R)-2b (en ies 4-6). As an al e na i e, we conside ed ligands based on e hane b idged examples, as
his backbone has a posi i e e ec in he hyd ogena ion o N-a yl imines [27]. Con a y o ou
expec a ions, he enan ioselec i i ies wi h hese ligands we e a he low (en ies 7-9). In an a emp o
Molecules 2010, 15
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inc ease he p ac ical u ili y o he p esen sys em we ha e also analyzed he pe o mance o BINOL
based ligands (S)-6a [28] and (S)-6b. These compounds a e s uc u ally simila o 2, bu conside ably
easie o syn hesize, as he equi ed chlo ophosphi e can be p epa ed in one s ep om comme cially
a ailable BINOL [29]. New (S)-6b was eadily p epa ed by condensa ion be ween 2-hyd oxyphenyl-
diisop opyl phosphine and BINOL chlo ophosphi e (see Expe imen al).
Table 1. Ca aly ic asymme ic hyd ogena ion o 2-me hylquinoline wi h P-OP ligands.
1
En y Ca . P ecu so P/a m %Con % ee Con
1 ½ [I (Cl)(COD)]
2
+ (S)-1a 40 48 7 S
2 ½ [I (Cl)(COD)]
2
+ (S)-1b 40 84 27 S
3 [I (COD)(1b)]BF
4
40 9 6 n.d.
4 ½ [I (Cl)(COD)]
2
+ (S)-1a 20 72 0 -
5 ½ [I (Cl)(COD)]
2
+ (S)-1b 20 96 16 S
1
Reac ions we e ca ied ou a oom empe a u e in oluene a a S/C = 100 and 0.6 M subs a e
concen a ion. Ca alys p ecu so was gene a ed om [I (Cl)(COD)]
2
and P-OP ligand a a
I :P-OP =1:1.1 a io unless o he wise s a ed. Con e sion was de e mined by
1
H-NMR and
enan iome ic excess (ee) by chi al HPLC. Con igu a ion was de e mined by compa ison o op ical
o a ion o he li e a u e alue.
Table 2. Ca aly ic asymme ic hyd ogena ion o 2-me hylquinoline wi h P-OP ligands.
1
En y Ligand P/a m %Con % ee Con
1 (S)-1a 20 72 0 -
2 (S)-1b 20 96 16 S
3 (R)-2b 20 88 62 S
4
2
(R)-2c 20 7 56 S
5
2
(R)-2d 20 30 46 S
6 (R)-2e 20 34 20 S
7 (S)-3a 20 98 9 S
8 (S)-4a 20 100 0 -
9
2
(R)-5a 20 6 10 S
10 (S)-6a 20 28 45 R
11 (S)-6b 20 82 65 R
12 (S)-6b 40 63 73 R
13 (S)-6b 10 40 63 R
1
Reac ions we e ca ied ou a oom empe a u e in oluene a a S/C = 100 and 0.6 M subs a e
concen a ion unless o he wise s a ed. Ca alys p ecu so was gene a ed om [I (Cl)(COD)]
2
and
P-OP ligand a a I :P-OP =1:1.1 a io. Con e sion was de e mined by
1
H-NMR and enan iome ic
excess (ee) by chi al HPLC. Con igu a ion was de e mined by compa ison o op ical o a ion o he
li e a u e alue.
2
0.2 M subs a e concen a ion.
Mos ema kably, (S)-6b led o simila esul s as hose ob ained wi h (R)-2b. On he o he hand,
(S)-6a p oduced a mode a e enan ioselec i i y, bu lowe han he
i
P de i a i e. As shown by ligands 1,
a mo e elec on dona ing ligand leads o a mo e ac i e ca alys . Finally, examina ion o ca alys based
on (S)-6b a di e en p essu es exhibi ed an inc ease in enan ioselec i i y up o a 73% ee a 40 a m,
bu a lowe alue a 10 a m (en ies 12, 13). In e es ingly, p oduc con igu a ion depends on he na u e
Molecules 2010, 15
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o he phosphi e agmen . Thus, he S p oduc is a ou ed by ligands wi h a e -bu yl-subs i u ed S
phosphi e g oup. On he con a y, ligands wi h an unsubs i u ed S phosphi e agmen (e.g., (S)-6a) and
(S)-6b), gi e p edominan ly he R amine.
As men ioned, he p esen hyd ogena ion is e y sensi i e o he p esence o addi i es. In pa icula ,
excellen esul s ha e been epo ed in he li e a u e by he use o iodine as coca alys [13-17]. Based
on hese p eceden s we p epa ed a se o eac ions using di e en ligands in he p esence o I
2
(Table 3).
As obse ed be o e, he coca alys p oduces an impo an inc ease in eac i i y, leading o eac ion
comple ion in all cases. No ewo hily, he p esence o iodine p oduces a e e sal o p oduc
con igu a ion in eac ions pe o med wi h ligands 1. Howe e , he enan ioselec i i ies we e
decep i ely low in all cases. P esumably, coo dina ion o π-acidic phosphi e agmen in he I (III)
species gene a ed by iodine addi ion [30], should no be a ou ed, which may e ode he chi al
induc ion exe ed by he P-OP ligand.
Table 3. Hyd ogena ion o 2-me hylquinoline in p esence o iodine.
1
En y Ligand P/a m %Con % ee Con
1 (S)-1a 40 100 30 R
2 (S)-1a 20 100 16 R
3 (S)-1b 20 100 7 R
4 (S)-3a 20 100 0 -
5 (S)-4a 20 100 11 R
6 (S)-6b 40 100 0 -
7 (S)-6b 20 100 5 R
1
Reac ions we e ca ied ou a oom empe a u e in oluene a a S/C = 100 and 0.6 M subs a e
concen a ion. Ca alys p ecu so was gene a ed om [I (Cl)(COD)]
2
, P-OP ligand and I
2
a
a I :P-OP:I
2
=1:1.1:10 a io. Con e sion was de e mined by
1
H NMR and enan iome ic excess (ee) by
chi al HPLC. Con igu a ion was de e mined by compa ison o op ical o a ion o he li e a u e alue.
In o de o imp o e he pe o mance o ca alys based on (S)-6b, we ha e nex s udied he in luence
o o he addi i es men ioned in he li e a u e (Table 4). Se e al sal s we e es ed i s , al hough hey
did no p o ide a bene icial e ec o e he e e ence sys em (en ies 1-5). On he con a y, an
in e es ing e ec is p o ided by phospho ic acids [31-33]. Thus, diphenylphospho ic acid p oduced a
mo e ac i e ca alys , al hough less enan ioselec i e (en y 6). F om his esul and he epo ed
asymme ic educ ion o quinolines ca alyzed by binaph hol based phospho ic acids [34], we ha e
es ed he wo enan iome s o 1,1’-binaph hyl-2,2’-diylphospho ic acid (BINOL-PO
2
H, en ies 7, 8).
In e es ingly, hese acids also ha e a bene icial e ec on ca alys eac i i y and main ain he
enan ioselec i i y. Howe e , hey did no show any in luence o he con igu a ion o he acid on
enan ioselec i i y. P obably, mo e elabo a ed binaph hyl agmen s, including a oma ic g oups in
3,3’ posi ions, a e needed o p o ide a syne gis ic e ec o imp o e he ca aly ic sys em [35].

Molecules 2010, 15
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Table 4. In luence o di e se addi i es in he hyd ogena ion o 2-me hylquinoline wi h
ligand (S)-6b.
1
En y Addi i e P/a m %Con % ee Con
1 none 20 82 65 R
2 pipe idine·HCl 20 19 66 R
3 Bu
4
NI 20 63 62 R
4 KCl 20 80 36 R
5 NaBF
4
20 69 49 R
6 (PhO)
2
PO
2
H 20 92 49 R
7 (R)-BINOL-PO
2
H 20 89 70 R
8 (S)-BINOL-PO
2
H 20 92 67 R
1
Reac ions we e ca ied ou a oom empe a u e in oluene a a S/C = 100 and 0.6 M subs a e
concen a ion. Ca alys p ecu so was gene a ed om [I (Cl)(COD)]
2
, ligand 6b and addi i e a a
I :6b:addi i e =1:1.1:10 a io. Con e sion was de e mined by
1
H-NMR and enan iome ic excess (ee)
by chi al HPLC. Con igu a ion was de e mined by compa ison o op ical o a ion o he li e a u e alue.
3. Expe imen al
3.1. Gene al
All eac ions and manipula ions we e pe o med unde ni ogen o a gon, ei he in a B aun
Labmas e 100 glo ebox o using s anda d Schlenk- ype echniques. All sol en s we e dis illed unde
ni ogen using he ollowing dessican s: Sodium-benzophenone-ke yl o benzene, die hyle he (E
2
O)
and e ahyd o u an (THF); sodium o pe oleum e he and oluene; CaH
2
o dichlo ome hane
(CH
2
Cl
2
) and NaOMe o me hanol (MeOH). NMR spec a we e ob ained on B uke DPX-300, DRX-400
o DRX-500 spec ome e s.
31
P{
1
H} NMR shi s we e e e enced o ex e nal 85% H
3
PO
4
, while
13
C{
1
H} and
1
H shi s we e e e enced o he esidual signals o deu e a ed sol en s. All da a a e
epo ed in ppm down ield om Me
4
Si. HPLC analyses we e pe o med by using a Wa e s 2690
Sys em. HRMS da a we e ob ained using a Jeol JMS-SX 102A mass spec ome e . Op ical o a ions
we e measu ed on a Pe kin-Elme Model 341 pola ime e .
(S)-2-(Diisop opylphosphino)phenyl-1,1’-binaph hyl-2,2’-diyl phosphi e [(S)-6b]. A solu ion o
(S)-2,2’-bisnaph oxyphospho us chlo ide (0.35 g, 1.0 mmol) in oluene (10 mL) was added d opwise o
(2-hyd oxyphenyl)diisop opyl phosphine (0.21 g, 1.0 mmol) and NE
3
(0.15 mL, 1.1 mmol) dissol ed
in oluene (10 mL). The esul ing suspension was s i ed o 24 h, he mix u e il e ed and ola iles
emo ed. The solid ob ained was dissol ed in oluene and passed h ough a sho pad o neu al
alumina. Solu ion was e apo a ed yielding a whi e solid (0.25 g, 50%). [α]
D20
= +222 (c 0.5, THF).
1
H-
NMR (CDCl
3
, 500 MHz): δ 0.93 (m, 6H, 2 CH
3
,
i
P ), 1.08 (dd, J
HP
= 14 Hz, J
HH
= 7 Hz, 3H, CH
3
,
i
P ),
1.12 (dd, J
HP
= 14 Hz, J
HH
= 7 Hz, 3H, CH
3
,
i
P ), 2.18 (m, 2H, 2 CH,
i
P ), 7.12-7.22 (m, 2H, 2 H
a om), 7.25-7.35 (m, 3H, 3 H a om), 7.37-7.50 (m, 5H, 5 H a om), 7.57 (d, J
HH
= 8.3 Hz, 1H, H a om),
7.60 (d, J
HH
= 8 Hz, 1H, H a om), 7.87-7.95 (m, 3H, 3 H a om), 8.00 (d, J
HH
= 8.6 Hz, 1H, H a om).
31
P{
1
H} NMR (CDCl
3
, 202.4 MHz): δ -2.3 (b , P-C), 143.1 (d, P-O, J
PP
= 30 Hz);
13
C{
1
H} NMR
(CDCl
3
, 125.8 MHz): δ 19.5 (d, J
CP
= 10 Hz, Me,
i
P ), 19.6 (d, J
CP
= 10 Hz, Me,
i
P ), 20.0 (Me, iP ),
20.2 (Me, iP ), 23.2 (d, J
CP
= 13 Hz, CH,
i
P ), 23.3 (d, J
CP
= 13 Hz, CH,
i
P ), 120.1 (d, J
CP
= 11 Hz, CH
Molecules 2010, 15
7738
a om), 121.9 (2 CH a om), 122.9 (C
q
a om), 123.8 (CH a om), 124.4 (d, J
CP
= 5 Hz, C
q
a om), 124.9
(CH a om), 125.1 (CH a om), 126.1 (CH a om), 126.3 (CH a om), 127.0 (CH a om), 127.1 (CH
a om), 127.6 (d, J
CP
= 23 Hz, C
q
a om), 128.2 (CH a om), 128.3 (CH a om), 129.7 (CH a om), 130.1
(CH a om), 130.3 (CH a om), 131.2 (C
q
a om), 131.6 (C
q
a om), 132.6 (C
q
a om), 132.9 (C
q
a om),
135.0 (d, J
CP
= 7 Hz, CH a om), 147.2 (C
q
a om), 148.0 (d, J
CP
= 4 Hz, C
q
a om), 155.7 (dd, J
CP
= 14, 6 Hz,
C
q
a om); HRMS (FAB): m/z 525, 1766, [M+H]
+
(exac mass calcula ed o C
32
H
31
O
3
P
2
: 525.1748).
3.2. Gene al Hyd ogena ion P ocedu e
In a glo ebox, o a 2 mL glass ial was added 2-me hylquinoline (0.3 mmol), he app op ia e
phosphine-phosphi e ligand (3.15 µmol), [I Cl(COD)]
2
(1.5 µmol) and he addi i e (30 µmol) in
oluene (0.5 mL). Vials we e placed in a model HEL CAT18 p essu e eac o ha holds up o eigh een
eac ions. The eac o was pu ged h ee imes wi h H
2
and inally p essu ized. A e 24 h, he eac o
was slowly dep essu ized, solu ions we e e apo a ed and con e sions we e de e mined by
1
H-NMR.
The esul ing mix u es we e dissol ed in a 95:5 n-hexane/isop opanol mix u e and il e ed h ough a
sho pad o silica o emo e he ca alys . Enan iome ic excesses o 2-me hyl-1,2,3,4-
e ahyd oquinoline we e analyzed by chi al HPLC (Chi acel OJ-H, low 0.5 mL/min, n-
hexane:isop opanol 95:5).
4. Conclusions
We ha e epo ed a p elimina y s udy abou he I ca alyzed hyd ogena ion o 2-me hylquinoline wi h
phosphine-phosphi e ligands 1-6. The sc eening indica es an impo an in luence o he ligand s uc u e
and has led o a con enien ca aly ic addi i e- ee sys em which achie es a good con e sion and an
enan ioselec i i y o up o 73% ee. Complemen a y s udies on he in luence o addi i es indica es a
dele o ious e ec o iodine. On he con a y, phospho ic acids ha e a posi i e in luence on ca alys
eac i i y, wi hou a ec ing enan ioselec i i y in he case o binaph hyl phospho ic acids. S udies o deep
in o hese obse a ions o he imp o emen o his ca aly ic sys em a e cu en ly unde in es iga ion.
Acknowledgemen s
We g a e ully acknowledge MICINN (CTQ2009-11867 and CONSOLIDER-INGENIO, CSD2007-
00006, FEDER suppo ) and Jun a de Andalucía (2008/ FQM-3830). M. R. also hanks CSIC o a
JAE pos doc o al con ac .
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Sample A ailabili y: Samples o he compounds a e a ailable om he au ho s.