New hybrid inorganic-organic polymers containing cyclophosphazenes as pendant groups: Cyclophosphazene ligands containing hydrazone linkages and their conversion to polymers

Article abstract of DOI:10.1139/v02-099

The reaction of the cyclotriphosphazene N₃P₃Cl₅[O-C₆H₄-p-C ₆H₄-p-CH=CH₂] (2) with 10 equiv of N-methylhydrazine proceeds in a regio-specific manner to afford the multi-functional hydrazide N₃P₃[N(Me)NH₂]₅[O-C₆H ₄-p-C₆H₄-p-CH=CH₂] (3). Condensation of 3 with o-hydroxy benzaldehyde or pyridine-2-carboxaldehyde affords the corresponding hydrazones N₃P₃[N(Me)N=CH-C₆H₄-o-OH] ₅[O-C₆H₄-p-C₆H₄-p-CH= CH₂] (4) and N₃P₃[N(Me)N=CH-C₆H₄N] ₅[O-C₆H₄-p-C₆H₄-p-CH= CH₂] (5), respectively. These hydrazones can be homopolymerized to afford polymers 6 and 7 containing multi-site coordinating cyclophosphazenes as pendant groups.

Full text of DOI:10.1139/v02-099

1415
New hybrid inorganic-organic polymers containing
cyclophosphazenes as pendant groups:
Cyclophosphazene ligands containing hydrazone
linkages and their conversion to polymers
Vadapalli Chandrasekhar, Venkatasubbaiah Krishnan, Arunachalampillai Athimoolam,
and Gurusamy Thangavelu Senthil Andavan
Abstract: The reaction of the cyclotriphosphazene N3P3Cl5[O-C6H4-p-C6H4-p-CH=CH2] (2) with 10 equiv of
N-methylhydrazine proceeds in a regio-specific manner to afford the multi-functional hydrazide N3P3[N(Me)NH2]5[O-
C6H4-p-C6H4-p-CH=CH2] (3). Condensation of 3 with o-hydroxy benzaldehyde or pyridine-2-carboxaldehyde affords
the corresponding hydrazones N3P3[N(Me)N=CH-C6H4-o-OH]5[O-C6H4-p-C6H4-p-CH=CH2] (4) and N3P3[N(Me)N=CH-
C6H4N]5[O-C6H4-p-C6H4-p-CH=CH2] (5), respectively. These hydrazones can be homopolymerized to afford polymers 6
and 7 containing multi-site coordinating cyclophosphazenes as pendant groups.
Key words: cyclophosphazene, hydrazone, pendant polymers, hybrid polymers, polymeric ligands.
Résumé : La réaction du cyclotriphosphazène N3P3Cl5[O-C6H4-p-C6H4-p-CH=CH2] (2) avec dix équivalents de
N-méthylhydrazine se produit d’une façon régiospécifique et conduit à la formation de l’hydrazide multifonctionnel
N3P3[N(Me)NH2]5[O-C6H4-p-C6H4-p-CH=CH2] (3). Les condensations du composé 3 avec l’o-hydroxybenzaldéhyde ou
la pyridine-2-carboxaldéhyde fournissent respectivement les hydrazones correspondantes N3P3[N(Me)N=CH-C6H4-
o-OH]5[O-C6H4-p-C6H4-p-CH=CH2] (4) et N3P3[N(Me)N=CH-C6H4N]5[O-C6H4-p-C6H4-p-CH=CH2] (5). Ces hydrazones
peuvent être soumises à des homopolymérisations qui conduisent aux polymères 6 et 7 portant cyclophosphazènes à
plusieurs sites de coordination comme groupes pendants.
Mots clés : cyclophosphazène, hydrazone, polymères pendants, polymères hybrides, ligands polymériques.
[Traduit par la Rédaction] Chandrasekhar et al. 1420
Introduction
volving the reactive P—Cl bonds of polydichloro-
phosphazene ([N=PCl2]n) (2, 3). The latter itself is prepared
from a ring opening polymerization of N3P3Cl6, or by a con-
densation reaction involving either Cl3P=NSiMe3 (4) or
Cl3P=NP(O)Cl2 (5, 6).
Among the inorganic polymers, polyphosphazenes
([N=PR2]n) occupy a prominent place (1). These constitute
the largest family of inorganic polymers with over 800 rep-
resentative examples.
A remarkable feature of poly-
Another class of closely related polymers is those that
contain a cyclophosphazene unit as a pendant group (7–15).
In contrast to polyphosphazenes, which contain an inorganic
backbone and (mostly) organic substituents, the pendant
polymers contain an organic backbone and intact cyclo-
phosphazene groups as the substituents. Although these
polymers have received much less attention than polyphos-
phazenes, in principle these systems also possess a similar
potential in terms of easy tunability of polymer property and
function. The pendant polymers are obtained by polymeriz-
ing cyclophosphazenes containing vinyl groups (Scheme 1).
phosphazenes is that the property and function of each
polymer can be readily fine tuned by varying the nature of
the “R” group on the phosphorus atom. Thus, polymers with
diametrically opposite properties such as water solubility or
hydrophobicity, or with varied applications such as bio-inert
materials or polymer electrolytes, are readily assembled by
choosing an appropriate substituent on the phosphorus atom
(1). Another versatility of polyphosphazenes is that unlike
organic polymers, the structural diversity can be introduced
by a macromolecular nucleophilic substitution reaction in-
Pioneering studies by Allen and co-workers (7–11) have
revealed that the presence of an appropriate spacer group
“Z” and (or) the presence of electron releasing substituents
on the olefinic moiety facilitate the homopolymerization of
the monomer by minimizing the ꢀ-electron withdrawing ef-
fect of the cyclophosphazene group (11). The utility of this
approach for the preparation of useful polymers has been
demonstrated by Inoue and co-workers (13–15). They have
found that polymers containing etheroxy-substituted
cyclophosphazenes readily form complexes with lithium
Received 23 February 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 11 October 2002.
Dedicated to Professor Tristram Chivers for his contributions
to Canadian chemistry.
V. Chandrasekhar,1 V. Krishnan, A. Athimoolam, and
G.T.S. Andavan. Department of Chemistry, Indian Institute
of Technology, Kanpur – 208 016, India.
1Corresponding author (e-mail: vc@iitk.ac.in).
Can. J. Chem. 80: 1415–1420 (2002)
DOI: 10.1139/V02-099
© 2002 NRC Canada

1416
Can. J. Chem. Vol. 80, 2002
Scheme 1.
solvent was evaporated in vacuo to afford 3. It was purified,
first by adding n-hexane to a solution of 3 in chloroform,
and secondly by recrystallization from a mixture of chloro-
form and n-hexane (1:1). Yield: 1.82 g (81.9%); mp 62°C.
1H NMR (CDCl3) ꢁ: 7.47–7.22 (m, 8H, aromatic), 6.75 (dd,
1H, J = 17.6 and 10.8 Hz, HRC=CH2), 5.78 (d, 1H, J =
17.5 Hz, trans-HRC=CHH ), 5.27 (d, 1H, J = 10.8 Hz,
cis-HRC=CHH), 3.20 (broad, 10H, -NH2), 3.06, 2.85, and
2.68 (d, 15H, J = 11.5, 11.0, and 10.8 Hz, -N(Me)).
31P NMR (CDCl3) ꢁ: 29.4 (d, P(N(Me)NH2)2), 22.4 (t,
2JP-N-P = 40.4 Hz, P(OR)N(Me)NH2). FAB-MS m/z: 555
([M]+). Anal. calcd. for C19H36N13OP3 (555.49): C 41.1,
H 6.5, N 32.8; found: C 40.9, H 6.8, N 32.5.
salts, which function as novel polymer electrolytes for lith-
ium ion transport. We have been intrigued by the possibility
of extending this approach to prepare new types of poly-
meric ligands that can bind to transition metal ions. Such
ligands would also be of contemporary interest, in view of
the importance of polymeric materials in organic synthesis,
in the form of solid-phase inert supports or in the form of re-
agents and catalysts (16–21). Herein, we describe the syn-
thesis and utility of a multi-functional cyclophosphazene
monomer (N3P3[N(Me)NH2]5[O-C6H4-p-C6H4-p-CH=CH2])
towards building new polymeric ligand systems.
Preparation of N3P3[N(Me)N=CH-C6H4-o-OH]5[O-C6H4-
p-C6H4-p-CH=CH2] (4)
To a solution of 3 (1.11 g, 2.00 mmol) in ethanol (50 mL)
was added a solution of o-hydroxybenzaldehyde (1.46 g,
12.00 mmol) in ethanol (50 mL). The reaction mixture was
heated under reflux for 17 h. It was then allowed to cool to
room temperature and the solvent removed from it in vacuo
to obtain 4. Compound 4 was dissolved in acetonitrile
(10 mL) and an excess of n-hexane was added to it, to allow
1
re-precipitation. Yield: 1.39 g (64.5%); mp 117°C. H NMR
(CDCl3) ꢁ: 11.28, 11.09, and 11.03 (s, 5H, -OH), 7.64–7.51,
7.35–7.18, 7.13–6.99, and 6.82–6.75 (m, 28H, aromatic,
N=CH), 6.72 (dd, 1H, J = 17.5 and 10.5 Hz, HRC=CH2);
5.78 (d, 1H, J = 17.5 Hz, trans-HRC=CHH), 5.27 (d, 1H,
J = 10.5 Hz, cis-HRC=CHH), 3.22, 3.14, and 2.99 (d, 15H,
J = 8.8, 8.8, and 8.5 Hz, N(Me)). 31P NMR (CDCl3) ꢁ: 16.4
(d, P(N(Me)N=CH-C6H4-o-OH)2), 14.0 (t, 2JP-N-P = 58.2 Hz,
P(OR)N(Me)N=CH-C6H4-o-OH). FAB-MS m/z: 1076
([M]+). Anal. calcd. for C54H56N13O6P3 (1076.02): C
60.3, H 5.2, N 16.9; found: C 60.6, H 5.0, N 16.5.
Experimental section
General
Solvents and other general reagents used in this work
were purified according to standard procedures.
4-Hydroxy-4>-vinylbiphenyl was prepared according to the
reported procedure (13). 4-Hydroxy biphenyl (SD Fine
Chemicals, India), o-hydroxybenzaldehyde (Fluka, Switzer-
land), and pyridine-2-carboxaldehyde (Fluka, Switzerland)
were used as received. Hexachlorocyclotriphosphazene (Nip-
pon Soda, Japan) was recrystallized from n-hexane before
use. N3P3Cl5(O-C6H4-p-C6H4-p-CH=CH2) was prepared ac-
cording to the literature procedure (13, 22).
Preparation of N3P3[N(Me)N=CH-C6H4N]5[O-C6H4-p-
C6H4-p-CH=CH2] (5)
To a solution of 3 (1.11 g, 2.00 mmol) in ethanol (50 mL)
was added a solution of pyridine-2-carboxaldehyde (1.28 g,
12.00 mmol) in ethanol (50 mL). The reaction mixture was
heated under reflux for 17 h. After allowing the reaction
mixture to cool to room temperature, solvent was removed
from it under vacuum to obtain 5. Compound 5 was
re-precipitated using acetonitrile as the polar solvent and
n-hexane as the non-polar solvent. Yield: 1.34 g (66.9%);
mp. 99°C. 1H NMR (CDCl3) ꢁ: 8.53–7.09 (m, 33H, aromatic
and N=CH), 6.74 (dd, 1H, J = 17.7 and 11.0 Hz,
HRC=CH2), 5.78 (d, 1H, J = 17.8 Hz, trans-HRC=CHH),
and 5.27 (d, 1H, J = 11.0 Hz, cis-HRC=CHH), 3.38, 3.28,
and 3.14 (d, 15H, J = 8.3, 8.3, and 8.3 Hz, N(Me)). 31P NMR
(CDCl3) ꢁ: 18.1 (d, P(N(Me)N=CH-C6H4N)2), 13.7 (t,
2JP-N-P = 56.6 Hz, P(OR)N(Me)N=CH-C6H4N). FAB-MS
m/z: 1001 ([M]+). Anal. calcd. for C49H51N18OP3 (1000.97):
C 58.8, H 5.1, N 25.2; found: C 58.5, H 5.7, N 25.5.
Instrumentation
1H and 31P NMR spectra were recorded on a JEOL-JNM
LAMBDA 400 model spectrometer operating at 400.0 and
161.7 MHz, respectively. The chemical shifts are reported
with respect to internal tetramethylsilane (1H) and external
85% H3PO4 (31P). FAB-MS were recorded on a JEOL SX
102/DA 6000 mass spectrometer using xenon (6 kV, 10 mA)
as the FAB gas. TGA were recorded on a PerkinElmer Pyris
6 TGA model in a nitrogen atmosphere at a heating rate of
20°C min–1. DSC were recorded on a PerkinElmer Pyris 6
DSC model in a nitrogen atmosphere at a heating rate of
10°C min–1. Dilute solution viscosity studies were done on a
Schott-Gerate viscometer using an Ubbelohde viscometer
with a capillary pore size of 0.645 mm.
Preparation of N3P3[N(Me)NH2]5[O-C6H4-p-C6H4-p-CH=CH2]
(3)
N-Methylhydrazine (2.03 g, 44.00 mmol) was dissolved in
chloroform (40 mL) and to this was added a solution of 2
(2.03 g, 4.00 mmol) in chloroform. The reaction mixture
was stirred at room temperature for 24 h. The N-methyl-
hydrazine hydrochloride that formed was filtered and the
Preparation of (6)
Compound 4 (0.79 g, 7.4 × 10–1 mmol) was dissolved in
1,2-dichloroethane (10 mL) along with AIBN (2% by
weight). The contents were purged with oxygen-free
nitrogen for 30 min, and the reaction mixture was then
© 2002 NRC Canada

Chandrasekhar et al.
1417
Scheme 2.
Scheme 3.
Scheme 4.
Results and discussion
heated at 80°C for 24 h. It was then allowed to come to room
temperature and poured into an excess of n-hexane to afford
6 as a powder. This was further purified by repeated
re-precipitations (dichloromethane as the solvent and n-
hexane as the non-solvent). Pure 6 was obtained as a white
solid. It was dried in vacuum (10–3 torr (1 torr = 133.322 kPa))
Synthesis and spectroscopy of the cyclophosphazene
derivatives 3 to 5
The cyclophosphazene N3P3Cl5[O-C6H4-p-C6H4-p-CH=CH2]
(2) has been shown previously by Inoue and co-workers (13
to 15) and by us (22) to be an excellent monomer in terms of
its ease of preparation, hydrolytic stability, and ready
polymerizability. It is readily prepared by the reaction of
N3P3Cl6 (1) with 4-hydroxy-4>-vinyl biphenyl in the pres-
ence of triethylamine as the hydrogen chloride scavenger
(Scheme 2).
The presence of five reactive P—Cl bonds allows 2 to be
used as a precursor for further reactions with nucleophiles.
Accordingly the reaction of 2 with 10 equiv of N-methyl-
hydrazine results in the complete substitution of the chlorine
atoms, affording N3P3[N(Me)NH2]5[O-C6H4-p-C6H4-p-CH=CH2]
(3). in an excellent yield (Scheme 3). This reaction is
regio-specific; the N-methyl end of the difunctional reagent
is found to exclusively attach to the phosphorus atom. Such
behavior has also been noted earlier in the reactions of
N-methylhydrazine with acyclic phosphorus(V) halides, such
as P(O)Cl3 or PhP(O)Cl2 (23–26). The most important fea-
ture of the reaction of 2 with N-methylhydrazine is that the
product (3) still contains five reactive groups in the form of
peripheral -NH2 units. Also, the polymerizable vinyl group
remains intact in the conversion of 2 to 3.
1
at 40°C for 6 h. Yield: 0.48 g (60.7%). H NMR (CDCl3) ꢁ:
11.27, 11.10, and 11.03 (s, 5H, OH), 7.10 (m(broad),
28H, aromatic and N=CH), 3.10 (m(broad), 15H, N(Me)),
1.25 (m, 3H, CH-CH2). 31P NMR (CDCl3) ꢁ: 16.4 (d,
2
P(N(Me)N=CH-C6H4-o-OH)2), 13.9 (t, JP-N-P = 58.3 Hz,
P(OR)N(Me)N=CH-C6H4-o-OH). Intrinsic viscosity:
1.29 cm3 g–1 (benzene), indicating a molecular weight of ap-
proximately 15 000 (22).
Preparation of (7)
The polymerization of 5 into 7 was carried out by using a
similar procedure as those outlined above. The quantities
used in this preparation are: 5 (0.74 g, 7.4 × 10–1 mmol).
Yield: 0.40 g (54%). 1H NMR (CDCl3) ꢁ: 8.48–7.10
(m(broad), 33H, aromatic and N=CH), 3.2 (m(broad), 15H,
N(Me)), 1.26 (m, 3H, CH-CH2). 31P NMR (CDCl3) ꢁ: 18.06
2
(d, P(N(Me)N=CH-C6H4N)2), 13.7 (t, JP-N-P
= 54.9 Hz,
P(OR)N(Me)N=CH-C6H4N). Intrinsic viscosity: 1.35 cm3 g–1
(benzene), indicating a molecular weight of approximately
15 000 (22).
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Can. J. Chem. Vol. 80, 2002
1
Fig. 1. H NMR spectrum of compound 4.
The condensation of 3 with 5 equiv of o-hydroxyben-
zaldehyde proceeds in boiling ethanol to afford the hydra-
zone 4. Similarly condensation of 3 with pyridine-2-
carboxaldehyde affords the hydrazone 5 (Scheme 4). Both of
these cyclophosphazene derivatives contain a multi-site co-
ordination environment in the form of the imino nitrogens,
the cyclophosphazene ring nitrogen atoms, and the phenoxy
oxygen (or the pyridyl nitrogen). The FAB-MS of 3, 4, and 5
show prominent parent ion peaks at 555, 1076, and 1001 re-
1
spectively. The H NMR spectrum of 3 shows the presence
of the terminal -NH2 groups as indicated by the appearance
of a broad resonance at 3.2 ppm. This signal disappears
upon the conversion of 3 into the hydrazones 4 and 5. Inter-
estingly, in the case of 4, three hydroxy signals are seen as
sharp singlets at 11.21, 11.01, and 10.95 ppm, respectively.
The CH=N protons of 4 resonate at 7.55, 7.50, and
7.35 ppm. The vinyl protons appear as an AMX multiplet
for all the three compounds 3, 4, and 5. A representative
1H NMR spectrum for compound 4 is given in Fig. 1. The
presence (absence) of second order virtual coupling effects
are helpful in the assignment of individual N-CH3 chemical
shifts. As can be seen from Fig. 2, in each of the compounds
3, 4, and 5, three types of N-CH3 resonances are expected.
The most downfield N-CH3 resonance is readily assigned to
=P(OR)[N(CH3)N=CHR>], because of the chemical shift, as
well as the absence of virtual coupling effects. The other two
N-CH3 resonances (Types B and C) show pronounced virtual
coupling; these resonances are seen as virtual triplets with
an intense central line. B and C themselves can be distin-
guished from each other. Thus the more shielded signal (C)
is assigned as arising from the substituent flanked by the
aryloxy group. Such effects have been well demonstrated in
other situations among cyclophosphazenes (27).
1
Fig. 2. Virtual coupling effects in the N-Me region of the H NMR of 4.
© 2002 NRC Canada

Chandrasekhar et al.
1419
Scheme 5.
Table 1. 31P NMR data for compounds 2–7.
polymers have similar degradation profiles. An initial de-
composition leads to a char yield of approximately 68.5% at
400°C for 6 and 61.6% for 7. Four further gradual decompo-
sitions occur. Even at 800°C, however, the char yields are
fairly high (49.6% (6) and 45.9% (7)). Differential scanning
calorimetric studies reveal that polymers 6 and 7 have high
glass transition temperatures (Tg) viz., 139.4 and 119.6°C,
respectively.
ꢁ P(OR)(R)
ꢁ P(R2)
2JP-N-P
Hz
Compound
vA
vX
2
3
4
5
6
7
12.3
22.4
14.0
13.7
13.9
13.7
22.5
29.4
16.4
18.1
16.4
18.1
58.2
40.4
58.2
56.6
58.3
54.9
Conclusion
In conclusion, we have prepared new cyclophosphazene
ligands containing hydrazone linkages and have successfully
converted them into soluble and thermally stable polymeric
systems, retaining the ligand framework as pendant groups.
The metalation studies of these ligands are being pursued.
The 31P NMR spectra of 3, 4, and 5 are of the AX2 type.
The chemical shift and coupling constant data for compounds
2–7 are summarized in Table 1. An interesting observation is
that the conversion of the hydrazide 3 to the hydrazones 4
and 5 is accompanied by an upfield shift of the signals,
2
along with a concomitant increase in the JP-N-P values.
Acknowledgments
Homopolymerization of 4 and 5
We are thankful to The Council of Scientific and Indus-
trial Research India (CSIR) for funding, including CSIR
senior research fellowships to VK, AM, and GTS.
The compounds 4 and 5 can be readily homopolymerized
in dichloroethane by using AIBN as the initiator to afford
polymers 6 and 7 (Scheme 5). These polymers are obtained
in their pure form by several re-precipitations, by dissolving
in dichloroethane and using n-hexane as a non-polar solvent.
The final dried products are air-stable powders soluble in a
variety of organic solvents such as dichloromethane, tetra-
hydrofuran, etc. Dilute solution viscosity studies on 6 and 7
reveal that they have intrinsic viscosities of 1.29 and
1.35 cm3 g–1, respectively, as measured in benzene. This
suggests that these polymers have an MW of approximately
15 000 (22).
References
1. J.E. Mark, H.R. Allcock, and R. West. Inorganic polymers.
Prentice Hall, New Jersey. 1992.
2. H.R. Allcock. Acc. Chem. Res. 12, 351 (1979).
3. H.R. Allcock. Chem. Eng. News, 22, (1985).
4. C.H. Honeyman, I. Manners, C.T. Morrissey, and H.R. Allcock.
J. Am. Chem. Soc. 117, 7035 (1995).
5. G. D’Halluin, R. De Jaeger, J.P. Chambrette, and P. Potin.
1H NMR spectra of 6 and 7 show the complete disappear-
ance of the AMX multiplet due to the vinyl protons and a
broadening of other signals. The 31P NMR spectra of 6 and 7
are of the AX2 type, and the signal positions are unchanged
vis-à-vis 4 and 5. This strongly suggests that the cyclo-
phosphazene moiety remains intact and does not suffer any
degradation during the polymerization reaction.
Macromolecules, 25, 1254 (1992).
6. R.D. Jaeger and P. Potin. Phosphorous Sulfur Silicon Relat.
Elem. 76, 223 (1993).
7. C.W. Allen and R.P. Bright. Macromolecules, 19, 571 (1986).
8. C.W. Allen. Stud. Inorg. Chem. 14, 171 (1992).
9. C.W. Allen. Trends. Polym. Sci. 2, 342 (1994).
10. C.W. Allen. MGCN Main Group Chem News, 6, 6 (1998).
11. C.W. Allen, E.D. Brown, and S.D. Worley. Inorg. Chem. 39,
810 (2000).
Thermogravimetric analysis of the polymers 6 and 7 re-
veal that these are fairly robust polymers. Both of these
© 2002 NRC Canada

1420
Can. J. Chem. Vol. 80, 2002
12. H.R. Allcock, W.R. Laredo, E.C. Kellam III, and R.V. Morford.
Macromolecules, 34, 787 (2001).
20. S. Itsuno, K. Watanabe, T. Koizumu, and K. Ito. React. Polym.
24, 219 (1995).
13. K. Inoue, M. Takagi, M. Nakano, H. Nakamura, and T. Tanigaki.
Makromol. Chem. Rapid Commun. 9, 345 (1988).
14. K. Inoue, M. Nakano, M. Takagi, and T. Tanigaki. Macro-
molecules, 22, 1530 (1989).
21. F. Minutolo and J.A. Katzenellenbogen. Angew. Chem. Int.
Ed. Engl. 38, 1617 (1999).
22. I.I. Selvaraj and V. Chandrasekhar. Polymer, 38, 3617 (1997).
23. J.P. Majoral, R. Kraemer, J. Navech, and F. Mathis. Tetrahe-
dron, 32, 2633 (1976).
15. K. Inoue, M. Nakano, M. Takagi, and T. Tanigaki. J. Am.
Chem. Soc. 113, 7609 (1991).
24. A.M. Caminade and J.P. Majoral. Chem. Rev. 94, 1183 (1994).
25. A.M. Caminade and J.P. Majoral. Synlett, 1019 (1996).
26. K.V. Katti, V.S. Reddy, and P.R. Singh. Chem. Soc. Rev. 24,
97 (1995).
16. G.W. Kabalka and R.M. Pagni. Tetrahedron, 53, 7999 (1997).
17. S.J. Shuttleworth, S.M. Allin, and P.K. Sharma. Synthesis,
1217 (1997).
18. S. Kobayashi and S. Nagayama. J. Am. Chem. Soc. 120, 2985
(1998).
27. S.S. Krishnamurthy and M. Woods. Ann. Rep. NMR
Spectrosc. 19, 175 (1987).
19. R.B. Merrifield. Angew. Chem. Int. Ed. Engl. 24, 799 (1985).
© 2002 NRC Canada