Benzyl bispidine as an efficient replacement for (-)-sparteine in ring opening polymerisation

Article abstract of DOI:10.1039/c2sc22053a

The synthesis and application of a dibenzyl-functionalized bispidine, in combination with 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) co-catalyst, has been demonstrated to be an excellent catalyst for the controlled ring-opening polymerisation (ROP) of lactide and cyclic carbonate monomers. Notably, the polymerisation proceeds with negligible transesterification or epimerisation, with the polymerisation of stereopure l-lactide affording highly crystalline poly(lactide) with a T_m of 156 °C. ROP of racemic lactide results in the observation of a modest degree of stereocontrol such that the probability of isotactic enchainment, P_m = 0.74. Comparison of a range of alternative hydrogen bond donor co-catalysts revealed that TU displayed the highest polymerisation rates in combination with the dibenzyl-functionalized bispidine.

Full text of DOI:10.1039/c2sc22053a

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Benzyl bispidine as an efficient replacement for
(ꢀ)-sparteine in ring opening polymerisation†
Cite this: Chem. Sci., 2013, 4, 1092
Richard Todd,a Gabriel Rubio,ab Daniel J. Hall,a Sarah Tempelaara
a
*
and Andrew P. Dove
The synthesis and application of a dibenzyl-functionalized bispidine, in combination with 1-(3,5-
bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) co-catalyst, has been demonstrated to be an
excellent catalyst for the controlled ring-opening polymerisation (ROP) of lactide and cyclic carbonate
monomers. Notably, the polymerisation proceeds with negligible transesterification or epimerisation,
with the polymerisation of stereopure L-lactide affording highly crystalline poly(lactide) with a Tm of 156
ꢁC. ROP of racemic lactide results in the observation of a modest degree of stereocontrol such that the
probability of isotactic enchainment, Pm ¼ 0.74. Comparison of a range of alternative hydrogen bond
donor co-catalysts revealed that TU displayed the highest polymerisation rates in combination with the
dibenzyl-functionalized bispidine.
Received 22nd November 2012
Accepted 12th December 2012
DOI: 10.1039/c2sc22053a
www.rsc.org/chemicalscience
reported to proceed with an almost absence of trans-
esterication and epimerization side reactions, and thus
Introduction
The last decade has led to many advances in organocatalytic
ring-opening polymerisation (ROP) of several cyclic monomers,
most commonly including esters and carbonates.1–8 On account
of their facile access, commercial availability and efficient
nature, organic catalysts have become a valuable addition to
this eld. While several simple species have been reported to
mediate these processes, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), triazabicyclodecene (TBD) and N-heterocyclic carbenes
are amongst the most active,9–14 the bifunctional catalyst system
containing both a thiourea and tertiary amine (Fig. 1), has been
provides a critical method for the production of precisely
dened polymers. The high selectivity towards polymerisation
in preference to deleterious side reactions is attributed to the
supramolecular recognition (and concurrent activation) of the
cyclic ester monomers in preference to the linear esters by the
thiourea moiety.15,16 Critically however, it was demonstrated
that the thiourea and the amine need not be conjoined and in
turn this has led to the discovery of more active analogues, most
notably the combination of 1-(3,5-bis(triuoromethyl)phenyl)-3-
cyclohexylthiourea (TU) and (ꢀ)-sparteine, a naturally occurring
alkaloid, which nds the optimum balance between activity and
selectivity.16 The increased rate (25 fold increase) can be
assigned to the two tertiary amines of (ꢀ)-sparteine being forced
into close proximity as a consequence of the xed rigid back-
bone with the lone pairs being oriented towards each other in
such a way to promote hydrogen bonding. Concurrently, the
weakly basic nature of (ꢀ)-sparteine (pKa ¼ 21.66),17 maintains
the high selectivity towards ROP vs. transesterication and
epimerisation. This concept has since been extended to a wide
range of hydrogen bond acceptors and donors that maintain the
simultaneous activation of both the carbonyl group of the
monomer and the initiating/propagating alcohol.18–22
Fig. 1 Ring opening-polymerisation catalysts.
In addition to its uses in ROP, (ꢀ)-sparteine has a number of
other uses such as a chiral ligand in asymmetric deprotonations,
substitutions and metalations.23–26 Despite this widespread use
and utility, (ꢀ)-sparteine has become increasingly difficult to
acquire through commercial sources. Several potential routes to
obtain (ꢀ)-sparteine exist including isolation from the abundant
Scotch Broom (Cytisus scoparius),27 or total synthesis via a
multistep route that has varying overall yields.28,29 Furthermore,
aDepartment of Chemistry, University of Warwick, Coventry, CV4 7AL, UK. E-mail: a.p.
dove@warwick.ac.uk; Tel: +44 (0)24 7652 4107
b
´
`
University of Barcelona, Martı i Franques Street, 1 08028, Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, polymer tacticity analysis, thermal analysis and MALDI-ToF
spectra. See DOI: 10.1039/c2sc22053a
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the derivation of synthetic replacements may enable more highly aer ash column chromatography using a selective gradient
active and stereoselective analogues to be realised.
solvent system.
These challenges to obtaining (ꢀ)-sparteine have led to the
The ROP of L-lactide, LA, using 10 mol% TU and 5 mol% 2,
investigation of synthetically simple analogues. Aided by was studied in CDCl3 ([LA]0 ¼ 0.7 M) at ambient temperature
computational modelling, Hedrick and co-workers recently using 1-phenylethanol as the initiator at a monomer-to-initiator
identied several commercially available tertiary amines that ratio of 50 ([M]0/[I]0 ¼ 50). Monitoring the polymerisation by 1H
˚
displayed similar nitrogen–nitrogen spacing (ꢂ3 A) and lone NMR spectroscopy revealed that it had reached 87% monomer
pair orientations to (ꢀ)-sparteine.30 Of the tertiary amines conversion aer only 81 min. Analysis of the resultant polymer
selected, tris[2-dimethylamino)ethyl]amine (Me6TREN) and by gel-permeation chromatography (GPC) revealed a number
1,4,7-trimethyl-1,4,7-triazacyclononane (TACN), in combination average molecular weight (Mn) of 13.4 kg molꢀ1 with a narrow
with TU, proved the most effective co-catalysts in the ROP of dispersity (ÐM ¼ 1.08). Under the same conditions, polymeri-
lactide. While TACN displayed comparable polymerization sation of L-LA with (ꢀ)-sparteine and Me6TREN in place of 2
kinetics to (ꢀ)-sparteine, transesterication was noted to occur reached 93% and 92% monomer conversion aer 66 and 450
rapidly upon complete conversion of the monomer. Application min respectively with similar molecular parameters for the
of Me6TREN led to better polymerisation control albeit at a resulting PLA observed by GPC analysis (Table 1). To more
slightly longer polymerisation time.
precisely compare catalyst reactivities, kinetic studies of LA ROP
We hypothesised that closer structural analogue to for each catalyst system were investigated in triplicate (Fig. 2).
a
(ꢀ)-sparteine would lead to more comparable behaviour in ROP, First order kinetic plots of ln([M]0/[M]t) against time reveal a
and hence targeted the preparation of analogous bispidines. linear correlation, with apparent rate constants (Kapp) calcu-
Herein we report the improved synthesis of a dibenzyl-functional lated as 1.35 hꢀ1, 2.53 hꢀ1 and 0.38 hꢀ1 for 2, (ꢀ)-sparteine and
bispidine, chosen for its identical backbone and similar basicity Me6TREN in combination with TU respectively. Under these
(pKa ¼ 21.25) to (ꢀ)-sparteine in addition to its apparent ease of conditions, while the bispidine catalyses lactide ROP at a lower
synthesis,17 and explore its activity in the ROP of lactide, rate than (ꢀ)-sparteine, the polymerisation occurs almost four
comparing directly to that of (ꢀ)-sparteine and Me6TREN. times faster then when Me6TREN is applied.
Extension of the application of this species with a range of co-
catalysts and other monomer feedstocks is also demonstrated.
Table 1 Comparison of PLLA synthesised using various tertiary amines
Results and discussion
Bispidine synthesis and lactide ROP
Conversionb
(%)
Time
(min)
Mn
Kapp
c
Catalysta
(g molꢀ1
)
ÐM
(hꢀ1
)
c
The synthesis of benzyl bispidine has been reported previously
by two different synthetic routes. While one route involves a
double Mannich reaction of 1-benzyl-4-piperidone followed by a
Wolff–Kishner reduction with hydrazine,31 on account of the
broader substrate versatility, we preferred to apply the alterna-
tive four step synthetic route reported by Gogoll et al. starting
from dimethyl malonate, 3 (Scheme 1).17,32 Following the
(ꢀ)-Sparteine
2
Me6TREN
93
87
92
66
81
450
14 900
13 400
12 100
1.08
1.08
1.09
2.53
1.35
0.38
a
Conditions: 1 ml of CDCl3 at room temperature; [L-LA] ¼ 0.7 M; 1-
phenylethanol as initiator; 5 mol% tertiary amine; 10 mol% of TU;
[M]0/[I]0 ¼ 50. Determined by 1H NMR spectroscopy. Determined
b
c
by GPC analysis in CHCl3.
Knoevenagel condensation of
3 with paraformaldehyde,
reduction of the resulting adduct using LiAlH4 yielded the tetra-
alcohol species, 5. Iodination of 5 as previously reported proved
to be problematic on large scales and hence bromination with
phosphorous tribromide was undertaken.33 The crude product
could be readily puried by passing through a silica plug to
obtain 6b as a white solid in 61% yield. Reaction of 6b with
excess benzyl amine enabled the isolation of pure bispidine, 2,
Scheme 1 Benzyl bispidine synthesis. (a) Paraformaldehyde, KOH, 95 ꢁC; (b)
LiAlH4, THF; (c) 6a: I2, Pred, 120 ꢁC; 6b: PBr3, 100 ꢁC; (d) benzylamine, toluene,
reflux.
Fig. 2 Semi-log kinetic plot for the ROP of L-LA using TU in combination with a
tertiary amine cocatalyst. (Red ¼ (ꢀ)-sparteine; blue ¼ 2; green ¼ Me6TREN).
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Table 2 Variation in [M]0/[I]0 with the 2–TU binary catalyst systema
The controlled nature of the benzyl bispidine-catalysed
polymerisation was further veried by the linear correlation
between Mn values obtained by GPC and the monomer
Conversionb (%)
Time (min)
Mnc(g molꢀ1
)
ÐM
c
[M]/[I]
1
conversion determined by H NMR spectroscopy (Fig. 3a) and
10
20
50
100
250
90
94
92
89
90
21
43
75
137
366
3280
6050
13 200
26 150
61 600
1.18
1.09
1.08
1.06
1.03
the [M]/[I] ratio (Fig. 3b). The resultant PLAs all displayed
narrow dispersities (1.18–1.03, Table 2) which indicates that
minimal transesterication is occurring. Further analysis of the
resultant polymers by matrix-assisted laser desorption ionisa-
tion time of ight mass spectrometry (MALDI-ToF MS) revealed
only single distributions with spacings of 144 m/z between
neighbouring peaks (Fig. 4). The absence of a second distribu-
tion with spacings of 72 m/z is consistent with the absence of
transesterication side reactions.
a
Conditions: CDCl3; room temperature; [L-LA] ¼ 0.7 M; 1-phenylethanol
as initiator; 5 mol% of 2; 10 mol% of TU. Determined by 1H NMR
b
spectroscopy. c Determined by GPC analysis in CHCl3.
Variation of the hydrogen bond donor co-catalyst
Thermal analysis of PLLAs with a degree of polymerisation
(DP) of 100 synthesised using TU with both 2 and (ꢀ)-sparteine
observed a melting temperature, Tm, of 156 and 158 ꢁC
respectively. These match the previously reported melting
points for isotactic PLLA prepared using (ꢀ)-sparteine.34,35 In
combination with the observation of a singlet resonance in both
Several hydrogen bond donor alternatives to the thiourea co-
catalyst (TU) have been reported over recent years that are
successfully used in conjunction with (ꢀ)-sparteine (Fig. 5).18–22
In order to assess the efficiency of these co-catalysts in
1
the homonuclear decoupled H NMR and 13C NMR spectra of
PLLA synthesised in this manner, these observations conrm
the absence of stereo errors, and hence an absence of epimer-
isation, with identical levels resulting from both (ꢀ)-sparteine
and 2 catalysed polymerisations.36–39
ROP of rac-lactide was undertaken to determine if any ster-
eoselectivity would be observed using the catalyst combination
of TU and 2. Despite the absence of stereogenic centres in 2,
unlike (ꢀ)-sparteine, analysis of a DP100 poly(rac-lactide) by
homonuclear decoupled 1H NMR spectroscopy enabled the
calculation of the probability of isotactic enchainment, Pm, to
be 0.74, clearly demonstrating a preference to produce isotactic
PLA from the racemic monomer mixture. This is comparable to
that observed for rac-LA ROP with (ꢀ)-sparteine–TU which
resulted in a Pm ¼ 0.74 (literature value: Pm ¼ 0.77).16
Fig. 4 MALDI-ToF MS spectrum of PLLA synthesised using the 2–TU binary
catalyst system.
Fig. 3 (a) Number-average molecular weight (Mn) against % monomer conversion and (b) number-average molecular weight (Mn) and dispersity (ÐM) against initial
monomer-to-initiator ratio ([M]0/[I]0) for the ROP of L-LA using TU and 2.
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Fig. 5 Hydrogen bond donor co-catalysts.
conjunction with the dibenzyl-functional bispidine, 2, in
comparison to the initially reported system, we studied their
activity for the ROP of L-lactide.
The ROP of L-lactide, LA, using 5 mol% 2 with 10 mol%
hydrogen bond donor co-catalyst was again studied in CDCl3
([LA]0 ¼ 0.7 M) at ambient temperature using 1-phenylethanol
as the initiator at a monomer-to-initiator ratio of 50 ([M]0/[I]0 ¼
50). The limited solubility of 11 in chloroform required its
Fig. 6 Kinetic plots resulting from variation in hydrogen bond donor.
application at the lower molar ratio of 5 mol% (in preference to Notably however, co-catalyst 12 resulted in PLLA with lower
the standard 10 mol%). To provide an accurate comparison, molecular weight than predicted from the [M]0/[I]0 ratio.
polymerisations using co-catalyst TU were also undertaken at 5 MALDI-ToF MS analysis of this polymer revealed two distinct
mol% loading. Kinetic studies for each catalyst system were distributions, both displaying the expected 144 m/z spacing for
investigated in triplicate by monitoring monomer conversion the monomer repeat unit. A molecular weight difference of ꢂ40
against time using 1H NMR spectroscopy. First order kinetic m/z was observed between the distributions. One of these
plots of ln([M]0/[M]t) against time (Fig. 6) enabled the determi- distributions revealed values that correspond to sodium-
nation of Kapp values (Table 3). The tested catalysts demon- charged polymer chains initiated from 1-phenylethanol as
strated a broad range of activities, with Kapp values ranging from expected, however the other distribution showed values
1.35 hꢀ1 to 0.03 hꢀ1. While TU at 10 mol% loading resulted in revealed values that correspond to sodium-charged polymer
the most rapid polymerisation, the alternative co-catalysts were chains initiated from the co-catalyst 12, which possibly
observed to display much lower Kapp values such that the second contributes to the increased observed rate of polymerisation. In
most active co-catalyst is m-triuoromethylphenol, 12 with a the case of 9, in which two CF3 substituents are present on the
Kapp approximately an order of magnitude lower than that phenol ring, co-catalyst initiation was not observed which
observed for TU. Notably the amide catalyst 11 (5 mol% loading) suggests that the phenol in 9 is appropriately activated to only
remained highly active with the measured Kapp value approxi- undertake hydrogen bonding and not serve as a source of
mately 6 times lower than that TU at the reduced 5 mol% initiation in the polymerisation.
loading. Polymerisation using co-catalyst 7 displayed extremely
low reaction rate under these conditions, with <5% monomer produced using 2/8 as co-catalysts for the ROP of L-LA revealed a
conversion being observed aer one week.
number of overlapping quartets in the range of 5.10–5.25 ppm,
Interestingly, 1H NMR spectroscopic analysis of the polymer
In each case, analysis of the molecular parameters of the consistent with the epimerisation of the monomer during the
resultant polymers revealed low dispersity values which polymerisation. Despite the previous report of the ROP of L-LA
suggests that the polymerisations were well controlled (Table 3). using (ꢀ)-sparteine/8 proceeding in the absence of
Table 3 Variation in hydrogen bond donor co-catalyst
Conversionc (%)
Time (min)
Mnd (g molꢀ1
)
ÐM
Kapp (hꢀ1
)
d
Co-Catalyst
[M]/[I]
TUa
TUb
8a
50
50
50
50
50
50
50
86
90
86
91
86
92
93
75
350
13 500
12 750
8450
1.08
1.07
1.06
1.11
1.08
1.08
1.20
1.354 ꢃ 0.053
0.391 ꢃ 0.027
0.103 ꢃ 0.001
0.044 ꢃ 0.002
0.029 ꢃ 0.001
0.063 ꢃ 0.007
0.138 ꢃ 0.007
1220
3240
4200
2710
1470
9a
9090
10a
11b
12a
10 300
11 900
4010
a
Conditions: 1 ml of CDCl3 at room temperature; [L-LA] ¼ 0.7 M; 5 mol% 2; 10 mol% co-catalyst; 1-phenylethanol as initiator. b Conditions: 1 ml of
CDCl3 at room temperature; [L-LA] ¼ 0.7 M; 5 mol% 2; 5 mol% co-catalyst; 1-phenylethanol as initiator. Determined by 1H NMR spectroscopy.
c
d
Determined by GPC analysis in CHCl3.
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Table 4 Comparison of catalysts for the ROP of cyclic carbonates
Monomer
Catalyst
[M]0/[I]0
Conversionc (%)
Time (min)
Mn (g molꢀ1
)
ÐM
TMCa
TMCa
MACb
MACb
MACb
(ꢀ)-Sparteine
50
50
20
20
20
92
90
90
90
89
4280
4750
400
410
45 days
9100d
9100d
6400e
6200e
6100e
1.03d
1.03d
1.11e
1.10e
1.13e
2
(ꢀ)-Sparteine
2
Me6TREN
a
Conditions: 0.5 ml of CDCl3 at room temperature; [TMC] ¼ 2.0 M; 5 mol% tertiary amine; 5 mol% TU; benzyl alcohol as initiator. b Conditions: 0.5
ml of CDCl3 at room temperature; [MAC] ¼ 0.5 M; 5 mol% tertiary amine; 10 mol% TU; benzyl alcohol as initiator. Determined by 1H NMR
c
spectroscopy. d Determined by GPC analysis in CHCl3. e Determined by GPC analysis in DMF.
1
epimerisation,22 in our hands, comparable H NMR spectra of ROP of lactide, producing well controlled polymers in the
the resultant polymers were also obtained for this system.
absence of observable transesterication and epimerisation.
Additionally the ROP of rac-lactide revealed similar stereo-
selectivities for 2 and (ꢀ)-sparteine, with a Pm of 0.74 calculated
for both catalyst systems. Optimisation of the co-catalyst
ROP of cyclic carbonates
To further investigate the versatility of the 2–TU binary catalyst
system, polymerisations were undertaken of the 6-membered
cyclic carbonates, trimethylene carbonate (TMC) and 5-methyl-
5-allyloxycarbonyl-1,3-dioxan-2-one (MAC) of which the
(ꢀ)-sparteine–TU system has previously demonstrated well
controlled homopolymerisation.40–42 Polymerisations were
carried out under conditions published in literature, using both
2 and (ꢀ)-sparteine in combination with TU as catalysts with a
target [M]0/[I]0 ¼ 50 for TMC and 20 for MAC.
revealed
that
1-(3,5-bis(triuoromethyl)phenyl)-3-cyclo-
hexylthiourea (TU) resulted in the highest observed polymeri-
sation rates. In addition, the ROP of cyclic carbonates with 2
was explored achieving rates almost identical to those observed
using (ꢀ)-sparteine whilst the control over the molecular weight
and dispersity was maintained. In conclusion, benzyl bispidine
has proven to be a versatile catalyst and an excellent replace-
ment for (ꢀ)-sparteine.
The ROP of TMC was performed using 5 mol% of 2 or
Acknowledgements
1
(ꢀ)-sparteine with 5 mol% TU. Aer ca. 70 h H NMR spectro-
scopic analysis of the polymerisation mixtures revealed monomer
conversions >90% (Table 4). GPC analysis of the resultant pol-
y(TMC)s showed similar Mn values resulted from polymerisations
using both catalysts with narrow dispersities observed. MALDI-
ToF analysis of the poly(TMC) synthesised using 2–TU revealed a
single distribution having the desired spacing of 102 m/z. No
secondary distributions were observed in the MALDI-ToF spectra.
The polymerisations of MAC were undertaken using 5 mol%
of 2 or (ꢀ)-sparteine with 10 mol% TU, as recently reported.41
Targeting a DP of 20, polymerisations catalysed by TU in
combination with 2 or (ꢀ)-sparteine achieved 90% monomer
conversion aer 410 and 400 min respectively. Notably these
results demonstrate the comparable activity of both bispidine co-
catalysts for the ROP of cyclic carbonates. In both cases, GPC
analysis revealed that the molecular weights and dispersities for
the resulting poly(MAC) were within error and MALDI-ToF MS
analysis revealed only the desired single distribution with the
predicted spacing of 200 m/z that corresponds to the molecular
weight of the repeat unit. Interestingly, the application of 10 mol
% TU with 5 mol% Me6TREN revealed a slower polymerization
such that 89% conversion was recorded aer 45 days. GPC
analysis of the resultant polymer revealed molecular weights and
dispersities in close agreement to (ꢀ)-sparteine and 2.
The Research Councils UK (RCUK) are acknowledged for
funding a fellowship to A.P.D. We gratefully acknowledge
nancial support from EPSRC for funding a DTA studentship
(R.T.) and a postdoctoral fellowship to D.J.H. (EP/H034420/1) as
well as the purchase of the Bruker Ultraex MALDI-ToF MS
instrument. The ERASMUS programme is acknowledged for
funding to support G.R. The GPC and thermal analysis equip-
ment used in this research was obtained through Birmingham
Science City: Innovative Uses for Advanced Materials in the
Modern World (West Midlands Centre for Advanced Materials
Project 2), with support from Advantage West Midlands (AWM)
and part funded by the European Regional Development Fund
(ERDF). Purac are gratefully acknowledged for the kind dona-
tion of lactide monomers, Aldrich Materials Science are
thanked for donation of starting materials and Dr Ivan Prokes is
thanked for assistance with NMR spectroscopic measurements.
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