Polymer bound Pd(O) phosphine catalyst for homogeneous catalysis

Indi an Jou rnal of Chem istry
Vo l. 43 8 , April 2004, pp. 852-856
Polymer bound Pd(O) phosphine catalyst for homogeneous catalysis
Nirmal Koshli *, Shubhang i Naik & Bharal Parab
Galaxy Research Centre, Galaxy Su rfaeta nt s Ltd. , CA9I2 , lTC Ind'i Area, Pawnc, Navi Mumbai 400 7 3, India
Received 2 A I/g us/ 2002; ocr-eli/ell (re vised ) 13 May 2003
Facilc sy nthesis of wa ter-solu ble polymer bound Pd(O) phosphine cata lys t is desc ri bed. Th is recycla ble polymeri c
catalyst is solu ble in aqueous or aqueo us/o rga ni c media and has hi gh activity in nucleophilic all yl ic substituti on, Hcck
react ion and hydroge nation.
IPC: IntCI.? C 01 G 55/00
Monomeri c as well as polymer bound phosphines
have been employed successfully in homogeneo us
catalys is. Poly (N-i so propyl acrylamide) (PNIPAM)
bound phosphines have been used to make efficient
and recoverablc catalysts by Bergbreiter et al.) PN 1PAM 2•3 has lower critical soluti on temperature
(LeST) of 31 D
e and it phase-separates from its aqueous solution when heated above its lower critical solution temperature. These PNIPAM bound catal ysts are
soluble in polar solvents like THF, aceto nitrile,
DMSO and water. And hence, PNIPAM bo und catalyst also show simil ar behav iour which is ex ploited in
recovering and recycling of th e po lymer bo und catalyst. However, due to relatively low LeST, th e utility
of PNIPAM bound catalyst in aq ueous med ium is severely restri cted to ambi ent temperature. Neverth eless, in other polar so lvents (T HF, acetonitril e,
DMSO) or solvent water mi xed systems these polymeric catalysts have bee n shown to be extremely effi cient for Hec k reaction , allylic substituti ons, Suzuki
couplin g, etc. and even for asymmetric hydrogenation
using chiral phosphine Rh odium complex. 4 In general ,
PNIPAM bound catalysts are recovered by either
heating aqueous reaction mixture above LeST, addition of the same to brine or by solvent precipitation.
To anchor aryl phosphines on to hydrophili c
poly(N-i sopropyl acrylamide), Bergbreiter used
Whitesides's5.6 strategy of using amino terminated
phosphines and Winnik ' s7 copo lymer co ntaining Nisopropyl acrylamide and N-acryloxy succinimide.
The active ester units of this copolymer are th en reacted with th e nucl eophile to anchor the li gand on
thc polymer. This strategy of functi ona li sation of
PNJPAM suffers from a major problem of purificati on because the di splaced N-hydroxy succinimide
(N HS ) remains partly suspe nd ed or eve n solubili sed
in reac ti on mi xture. Purifi cati on by di sso lving functi onali sed po lymer in water an d reprec ipitat ing by
hea tin g the aqu eo us so luti on abov e LeST is th e on ly
way of gettin g rid of di sp laced NHS. The polymer
that prec ipitates from its aqu eo us sol ution has to be
co mpl ete ly dried before its actual use. Drying of thi s
wct pol ymer is another cumberso me step. More importantly , wi th air-se nsiti ve li ga nds such as pol ymer
bound ph osp hi nes, nu mber of puri fication steps inev itabl y lead to ox idati on of phosph ine to the correspondi ng ox ide.
Results and Discussion
To overco me these difficulti es we des igned PNIPAM bound phosphines by using copolymers of Niso propyl acrylamide and 2-v in yl 4,4'-dimeth yl 2oxazo li ne 5-one com monl y referred to as vin yl
aza lactone (VAL). In thi s co mmuni ation , we report
poly(N- isopropyl acrylamide) bou d phosphine Pd(O)
catal yst th at ca n be used in aqueous or in oth er po lar
so lvents ran ging from THF, acetonitrile, N-meth yl
pyrro lidinone, etc. for effecting a variety of sy nthetic
tran sFormati ons.
Vinyl aza lac tone is co mmerciall y avai lable an d is
hi ghl y reactive towards all sorts of nucleop hil es.
Poly(N-i sopropyl acry lamide-co-VAL), 1 (Scheme I)
was sy nthesised by co-polymerising both monomers
in 8 : 2 mole ratio in I-butanol using AIBN as th e
initiator.
The IR spectrum of 1 showed a promin ent stretch
for carbonyl of aza lactone at 18 14 cm' ) and carbonyl
of amide at 1646 cm·) . Molecul ar weight by viscosity
measurement was found to be Mv = 5.1 X 105 using
values of 9.59 x 10.3 and 0.65 for k and a respective ly
KOSHTI
~o
NH
A
+
el
al.: SYNTH ESIS OF Pd(O) PHOSPHIN E CATALYST
~
o " N
O~
853
AIBN
I-butanol
700 C
o
0
" N
l-i--
NH
A
0 '/
\
1
dry THF
700 C
1
2
NH
NH
~~P(Ph)
A
I
2
H
2
Scheme I
in THF at 30°C. 7 Unfortun ately, du e to broad signals,
the PMR spectrum was not of any help in ascertainin g
the mole rati o of 1 vis-a-vis th e feed rati o. However,
the relative mol e rati o of N-i so propyl ac rylamide to
vin yl azalactone co uld be eas ily determined by reacting azalactone units of 1 with excess of benzy lamin e
and after isol ati on of polymer bound benzy lamine, the
free amine was estimated by either UV-spectroscopy
or HPLC. Altern ati vely, the monomer ratio in the copol ymer can be determin ed by simple titrimetry after
opening th e aza lactone rin g by a di amine like N,Ndimeth ylaminopropyl di amin e (see ex perimental, sy nth esis of 1). Bascd on thi s anal ys is th e actu al mole
rati o of the monomers was foun n to be quite close to
th e feed rati o.
The copolymer 1 was th en reacted with amin opropy l diphenyl phos phine in dry TH F and polymer
bound phosphine 2 was purifi cd by pouring the reacti on mixture into hexa ne. 3 1P spectrum of thi s polymer
2 showed characteri sti c signals at 8 - 17 .1 and 34.3
co rrcspondin g to phos phine and phosphine ox ide
(- 5.0 %) respect ive ly. The poly meri c Pd(O) catalys t 3
was made by disso lving 2 in a suit able so lve nt and
th en cxchanging li ga nd with Pd (Oh(dbah using 2 : Pd
:: 4 : I. The colou r change frolll red to go ldcn yell ow
ind ica ted the li gand exc hange. The ca talys t 3 was th en
i 'olatcd by solvent prec ipi tati on. On dry ing, 3 1p spectrum of ye ll ow colo ured 3 showed charac teristi c sig-
nals at 8 19.41 and at 8 32.66 for phosphine bound to
Pd and phosphine oxide, respecti vely.
The purifi ed 3, thu s obtained, was used for hydrogenation of N-i sopropyl acrylamide in THF. The rate
of hydrogen uptake (measured by hydrogen filled
burette connected to reacti on fl ask) was found to be
sa me as th at observed with nonpolymeri c version
of homogeneous Wilkinson like Pd(O) catalyst
(Chart I).
The catalyst 3 was al so empl oyed for allyli c substi tuti on of all yl acetate by di eth yl amin e. In both reacti ons, th e sa me catalys t was recycled five times by
addin g fres h substrate at th e end of eac h cycle. Fi nall y, th e catalys t was isol ated by solvent prec ipitati on and th e recove red catalyst was reused for anoth er
fres h ex periment. The co nve rsions were qu antitat ive
and there was no noti ceable drop in catalyti c activ ity .
Hec k reacti on to produce p- meth oxycinnami c ac id
was conveni entl y done in aqueo us N-meth yl pyrrolidone (NMP) ( I : I) as medium and an inorgani c base.
After performin g fo ur cyc les, the catalyst was recovered by pouring the reacti on mixture in warm bri ne
solution (40°C) and reused . In another exa mple of
Hec k reacti on, sy nth esis of octyl p-methoxy cinnamate
was ca rri ed OLi t in MP al one. Ary lati on of octy l acrylate was do ne by iodoani sole using tri eth ylamine. After
five successful cycles th e isolati on of prod uct wa,'
acco mpli shed very conveni entl y by solven t extracti on
854
INDI AN J. CHEM., SEC B, APRIL 2004
o
OH
NMP. H,O. Na,CO,
90"C
l
~oH
o
¢O
CH3
~ I
I
'-----_P_N_I_PA_M_~~ P(Ph)2J4Pd(O)
(3)
¢'
NMP, 45"C
H
~N'(
I
H" NMP, RT.
o
~
o
O~
H
I
NMP, TEA, 90"C
o
O~
Chart I
(hexane). The catalytic activity of 3 in above mentioned Hec k reactio ns was found to be quite comparable with that of Pd(OMdbah and conversions by HPLC
analysis and/o r actual isolation of products were found
to be quantitative. In addition to chromatographic
co mpariso ns with authe nti c sa mpl es, PMR analys is
also co nfirmed the purity of prod ucts . Thus,
p -methoxycinnamic ac id and octy l methoxy cinnamate
were fo und to be exc lu sively trans stereo isomers.
C urre ntl y we are e ngaged in ex tending thi s methodo logy to sy nthes ise po lyme r suppo rted pall adacycles, chiral phosphines and polymer suppo rted catalyst usin g othe r tran sition meta ls especia ll y rh od ium
for ho mogeneo us cata ly sis.
In su mma ry, we have show n th at PNIPAM bound
Pd (O) co mpl exes ca n be recove red by e ither th erma l
o r so lve nt prec ipitat io n and he nce the product can be
iso lated eas il y. The ca ta lytic acti vity in ho moge neo us
sys te m is as good as th e ir lo w mo lec ul ar weight a na-
logues. And more impo rtantly , these po lyme r bound
phosphine pall adium complexes can be readil y sy nthes ised by making use of azalacto ne ring for functionalisation of po lymeric suppo rt.
Experimental Section
All reagents a nd solvents were obta ined fro m
commerc ia l sources and used w ithout puri fication
unl ess otherwise stated. Dry THF was prepared by
di stilling the so lve nt over be nzophe no ne and sodium.
Vinyl aza lactone was obtai ned from SNPE Chimi e,
France. IH a nd 13C NMR spectra were o bta ined o n a
Varian Unity p300 spectrometer. TMS was L1 sed for
inte rn al refere nce for PMR and 80% phosphoric ac id
was L1 sed as extern al refere nce for 31p NMR spec tra.
Infrared spec tra were repo rted as thin film s between
NaCi plates o r as pressed KBr p ~lIets usin g Pe rkin Elme r FT-IR spec trometer. UV-Vi s ib le spectra were
o bta ined usin g Varian Cary 50 spectrometer.
KOS HTI er al.: SYNTH ESIS O F Pd(O) PHOSPHIN E CATA LYST
Synthesis of copolymer poly (NIPAM-co- VAL)
1. N-Isopropyl acryl a mide (5.0 g, 44.25 mmo le) and
vinyl azal acto ne ( 1.54 g, 11 mmo le) were di sso lved in
t-butanol (80 mL) at 65°C unde r nitrogen. A solution
of AIBN (l00 mg) in i-buta nol (20 mL) was added to
thi s soluti o n. Th e reacti o n mi xture was then stirred at
70°C for 20 ho urs. Th e reacti o n mi xture was con centrated by re mov ing 50 % o f th e so lvent on a ro tary
evapo rato r. The conce ntrated so luti o n thu s obtained
was poured into stirred he xane (500 mL) to prec ipitate the copo lymer. The prec ipitated so lid polymer
was filtered and dri ed unde r hi g h vacuum to yi e ld 6. 3
g (96 %) o f co po lymer. IR (KBr): 1646, 18 14, 3299
I
c m- . IH NMR (3 00 MH z, CDCI 3 ) : 8 1.0 to 1.27 ,3. 13 ,
3.97 (broad sig na ls fo r po ly mer back bo ne and Nisopro pyl groups) and 2. 13 (broad tin y sig na l fo r dimethy l g roups of azalactone unit).
The mo no mer mo le rati o of 8 : 2 was ascertained by
reacting a small porti o n o f copo lymer with N,Ndimethylarninopropyl di amine in THF. The pol yme r
was prec ipitated by adding reacti o n mi xture to hexane.
The prec ipitated polymer was purifi ed by di sso lving in
water and reprec ipitating by heating the aqueous
soluti on to 50°C. The white prec ipitated po lymer was
then dri ed under vacuum . It was fo und to have a mine
va lue of 77 when titrated w ith standard HC I so luti o n as
against amine value o f 80 for 8 : 2 copo lymer. Th e
mo lec ul ar weig ht (M v) of thi s copo ly mer was
determined to be 5. 1 x 10 5 by viscometry.7
Synthesis of aminopmpyl diphenyl phosphine.
Sodium metal ( 1.25g, 54 Imno le) was washed with
hexanes and cut into small pieces into a dry N2 flu shed
ro und bolto m fl ask. The sodium was sus pended in d ry,
degassed TH F (20 mL). C hl o ro diphenyl phosphin e
(3.0 g, 13.6 mmole) was d isso lved in TH F ( 10 mL) and
added to the sodi um . The red reacti o n mass was stirred
overni ght at room te mpe rature. C hl oropro pyl amine
hydrochl oride (884 mg, 6 .8 mmo le) was suspended in
dry TH F ( 10 mL) and coo led to -78°C. The red reacti on mi xture wa s diluted w ith d ry, degassed T HF
(10 mL) and tran sferred via canul a to the suspensio n.
T hi s reacti on mass was the n stirred for 90 minu tes and
th en the coo lin g bath was remo ved. T he reacti o n was
sti rred as the bath was all owed to wa rm to roo m te mpe rature. After stirring fo r 7 hr, the reacti o n was heated
at 50°C for 10 hI". !\ fter remova l or T H F, the reac ti on
was d il uted with ether (20 mL) and was hed tw iee with
bri ne so lutio n ( 10 mL). T he o rga ni c layer was extracted three timcs each with 10 mL IN H 2SO.j . T he
aq ueous layer \vas neutra li sed w ith 8N NaOI-l and then
855
ex tracted with ether (3 x 50 mL). The organi c ex tracts
were dri ed over sodium sulph ate and rotary evapo rated
under reduced pressure to affo rd 1.07 g (67 %) of a
yell o w o il. IH NMR (CDCI 3 , 300 MHz): 8 1.6 (m, 2H),
2.05 (m , 2H), 2 .5 (bs, 2H) , 2. 8 (t, 2 H), 7. 25 - 7.45 (m,
lOH). 3 1p NMR (CDCI 3 , 120 MH z): 8 - 16 (phosphine)
and 34 (oxide).
Synthesis of PNIPAM bound phosphine 2. T o a
stirred soluti o n o f po ly (NIPAM-co-VAL) 1 (2.6 g) in
dry THF (15 mL) unde r nitrogen, a min o propy l di phenyl phosphine ( 1.07 g, 4.4 m mo le) in dry TH F
(1 5 mL) was added under nitrogen. The mi xture was
stirred at 50°C fo r 4 hr. Th e po ly me r was the n
precipitated by po uring th e reacti o n mi xture into
hexa ne (100 mL). IR s pectroscopy showed co mpl ete
di sappearance of carbo nyl stretch of azalacto ne. It
was quickl y filte red a nd dri ed under hi g h vac uum to
g ive 3 .2 g o f co lo url ess so lid . It was sto red unde r
nitroge n for s ubseque nt use in preparati o n of
cata lysts. IR (KBr) 1646, 3239 c m· l . 'H N MR
(CDCb, 300 MH z) : Broad s ig nal s at ~ 0 .8 to 1. 1,
1.75 , 1.78 to 2 .09 , 3.94 and 7 .2 to 7 .59. 3 1p N MR
(C DC I), 120 MHz): broad sig nals at 8 -17 .1 a nd 34.3.
Synthesis of [PNIPAM-P(Phhl,Pd(O) 3. A so luti o n of 2 ( 1.0 g) in dry THF (25 mL) was prepared
under nitrogen. T o thi s stirred so luti o n, a so luti o n of
Pd(OM dbah (125 mg) in dry THF (25 mL) was added
usin g a do ubl e tipped needl e under positiv e nitrogen
press ure. Within 5 minutes res ulting so luti o n c hanged
fro m dark purple to go lden yell ow. The so luti o n was
stirred fo r additi o na l 15 minutes. T o thi s so lu tion ,
hexa ne (l 00 mL) was add ed a nd 3 was prec ipitated as
go lde n so lid . Th e supe rn ate nt laye r was the n re moved
leav in g the po ly mer bo und cata lyst 3 ready fo r use in
e ithe r wate r, mi xed so lvent system o r pl ain organic
so lve nts s uch as ace to nitril e, T HF and NM P. 3 1p
NMR (C DCI ), 120 MH z): 0 19.4 1 and 32.66.
Hydrogenation of' N -isopmpyl acrylamidc:
P,·eparation of N -isopropyl propylamid e. T he prec ipitated catal yst 3 (200 mg) was red isso lved in NMP
( 10 mL) and th e reac tio n fl ask was ev acuated . The reacti o n fl ask was Fill ed with hyd rogen thro ug h a bu rette
that was co nnected w ith two two-way va lves. After
perfo rm ing evacuatio n and fillin g of reactio n fla sk with
hyd rogen three ti mes, N- isopropy l acrylami de ( 100 mg,
0.88 mmo le) in NM P ( 10 mL) was added thro ugh the
septum using a syrin ge. T his was immed iate ly fo llowed by evacuati o n and filling of hydrogen sequence
was perFormed two times and the reacti o n was contin ued to stir at room temperature under hydrogen. After
856
INDIAN J. CHEM ., SEC B, APRIL 2004
completion of hydrogen uptake, the reaction mass was
stirred for additional one hour under hydrogen . A small
sample drawn by a syringe was used to do TLC to confirm the completion of reaction . The conversions were
ascertained by GC (Capillary column: DB-I; Column
temp. prog.: 80°C isothermal; Injector temp.: 250°C;
Detector temp.: 300°C). The retention time of Nisopropyl propylamjde was checked with authentic material. After ascel1aining disappearance of N-isopropyl
acrylamide either by TLC or GC a fresh instalment of
starting materi al was added to initiate the next cyc le.
The reaction mixture was concentrated to remove most
of ethanol and poured into hexane to precipitate the
polymeric catalyst. The supernatent layer was concentrated to get N-isopropy l propylamjde as wh ite needles
(ex tremely hygroscopic). It sublimed at 66°C (0.5 mm
Hg).8 IH NMR (CDCl 3, 300 MHz): 0 1.07 to 1.31 (one
triplet and one doublet overlapping, 9H), 2.11 to 2.18
(q, 2H, J = 7.8 Hz), 3.89 to 3.98 (p, ] H, J = 6.63 Hz).
Allylic substitution of allyl acetate, preparation
of allyl diethylamine. The precipitated catalyst (3)
(200 mg) was redisso lved in fresh NMP (10 mL)
under nitrogen. Through the septum, a mixture of di ethylamine (300 mg, 4 .1 mmole) and allyl acetate
(200 mg, 2.0 mmole) in dry NMP ( 10 mL) was introduced into the reaction flas k and the stirring was continued at 45°C for 12 hr. Gas chromatography analysis indicated complete consumption of a ll yl acetate
and formation of allyl diethylamin e. The same catalyst was used for five more cyc les o f 2.0 mmole sca le
of substrate. The catalyst was iso lated from the product by so lvent (hexane) precipitation. After removal
of catalyst, the so lvent was rotary evaporated to yield
the tertiary amin e as co lourless liquid, b. p. ] 10°C
(Lit. 9 b. p. ] 10°C). IH NMR (CDCl 3 , 300 MHz): 0 1.0
(t, 6H, J = 7.2 Hz), 2.53 (q, 4H, J = 6.9 Hz), 5. 10 to
5.29 (m, 2H), 5.83 to 5.92 (m, I H).
Heck reaction, preparation of 2-ethyl hexyl pmethoxycinnamate. The PNIPAM supported catalyst
3 (200 mg) was redi sso lved in NMP ( 10 mL) under
nitrogen. To thi s stirred so luti on, a mixture containing
iodoani so le (300 mg, 1.28 mmol e), tri ethylamine (5] 8
mg, 5.13 mmole) and 2-ethy l hexy l acrylate (236 mg,
1.28 mmole) in dry NMP (10 mL) was added and stirring was continued under nitrogen blanket at 90°C for
12 hr. HPLC analysis (Co lumn : OmniSpher C 18;
Mobile Phase: 90 : 10 :: Methanol : Water; Flow Rate
: ] .0 ml/min.; UV detection at: 280 nm) indicated
total di sappearance of iodoani so le and formation of
2-ethyl hexy l methoxyci nnamate. After every 12 hr
the same catalyst was used for additional four cycles
and the conversions were complete based on HPLC.
At the end of fourth cycle the reacti o n mixture was
extracted with hexane (l00 mL). Removal of hexa ne
on rotary evaporator afforded near quantitative recovery of 2-ethyl hexyl methoxycinn amate. Purity by
HPLC was found to be > 99.0 %. IH NMR (CDCI 3 ,
300 MHz): 0 0.88 to 0.94 (two overlapping tripl ets,
6H), 1.3] to 1.46 (m, 8H), 1.61 to 1.66 (m, ]H), 3.83
(s, 3H), 4.] 1 (d, 2H, J = 4.2 Hz), 6.30 (d, IH, J = 15.9
Hz), 6.89 (d, 2H, J = 8.7 Hz, further split due to meta
coup ling with coupling constants of 2.2 and 3.0 Hz),
7.48 (d, 2H, J = 8.7 Hz, further split d ue to meta coupling with J = 2.2 Hz), 7.63 (d, ] H, J = 15.9 Hz).
Heck reaction, preparation of p-methoxycinnamic acid. To a stirred solution of catalyst 3 (200 mg)
in aqueous NMP (l : I, 20 ml) under nitrogen, sod ium
carbonate (544 mg, 5.13 mmol e), iodoanisole (300 mg,
1.28 mmole) and acry lic acid (92.3 mg, 1.28 mmole)
were added. The reaction system was thoroughl y
purged with nitrogen and stirring was continued at
90°C for 12 hr. HPLC analysis at th is stage indicated
complete conversion of iodoani sole to p- methoxyinnamic acid. After five cycles with the sa me catalyst
the product was iso lated by phase separating 3 by pouring the reaction mixture into brine and warming it. Th e
alkaline supernatent layer was acidi fied and the product
was isolated as pale yellow solid, m.p. 172°C (Lit. IO m.
p. 172°C). IH NMR (CDCI 3, 300 MHz): 83 .84 (s, 3H),
6.30 (d, IH, J = 16.0 Hz), 6.93 (d, 2H, J = 9.0 Hz),
7.50 (d, 2H, J = 9.0 Hz), 7.75 (d, IH , J = 16.0 Hz).
Acknowledgement
Mrs. Nutan Agadi of RSIC.
thanked for the NMR spectra.
LIT, Mumbai
IS
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