Polymerization of aryl isocyanides possessing bulky substituents at an ortho position initiated by organorhodium complexes

Article abstract of DOI:10.1021/om0005809

Organorhodium complexes, prepared from the reaction of rhodium chloride with PPh₃ and LiR, initiate the polymerization of aryl isocyanides at an ortho position in the presence of PPh₃, to provide polyisocyanides. The organometallic complexes catalyze the polymerization of isocyanides. The molecular structure displays a square planar geometry around the rhodium atom, with one triphenylphosphine ligand.

Full text of DOI:10.1021/om0005809

Organometallics 2000, 19, 4669-4671
4669
P olym er iza tion of Ar yl Isocya n id es P ossessin g Bu lk y
Su bstitu en ts a t a n or th o P osition In itia ted by
Or ga n or h od iu m Com p lexes
Mari Yamamoto, Kiyotaka Onitsuka, and Shigetoshi Takahashi*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, J apan
Received J uly 7, 2000
Summary: The organorhodium(I) complexes Rh(R)(nbd)-
(tert-butyl isocyanide).8 However, ortho-substituted aryl
(PPh3) (R ) Me2C6H3, 2,4,6-Pri C6H2, 2-PhC6H4; nbd )
isocyanides did not polymerize by µ-ethynediyl Pd-Pt
dinuclear complexes. In this communication, we have
developed the novel polymerization of aryl isocyanides
possessing bulky substituents at an ortho position
initiated by well-defined organorhodium complexes.
The organorhodium complex Rh(2,6-Me2C6H3)(nbd)-
(PPh3) (1a ; nbd ) 2,5-norbornadiene) was prepared by
the reaction of [Rh(nbd)Cl]2 with PPh3 and Li(2,6-
Me2C6H3) in THF in 96% yield (eq 1).9,10 Complex 1a
3
2,5-norbornadiene), prepared from the reaction of [Rh-
(nbd)Cl]2 with PPh3 and LiR, effectively initiate the
polymerization of aryl isocyanides possessing bulky
substituents at an ortho position in the presence of PPh3
to provide polyisocyanides in good yields.
In recent years much interest has been focused on the
synthesis of polyisocyanides, since some of them show
unique properties originating from a helical conforma-
tion of their main chain.1 Although it has been known
that some organometallic complexes catalyze the po-
lymerization of isocyanides,2 living polymerization sys-
tems using group 10 metal complexes were recently
found. Organopalladium(II) complexes promote the liv-
ing polymerization of 1,2-diisocyanoarenes,3 while (η3-
allyl)nickel complexes are active for the living polym-
erization of aliphatic isocyanides.4 Previously we have
shown the living polymerization of aryl isocyanides
initiated by µ-ethynediyl Pd-Pt dinuclear complexes.5
Although some poly(aryl isocyanide)s adopt a stable
helical conformation in solution, the conformational
property of the polymer main chain is strongly affected
by the steric effect of substituents on aromatic rings.6,7
Therefore, the polymers of aryl isocyanides possessing
bulky substituents at an ortho position are attractive
since they may have a rigid helical structure like poly-
was fully characterized by spectral analyses as well as
X-ray crystallography.11 The molecular structure pre-
sented in Figure 1 displays a square-planar geometry
around the rhodium atom with one triphenylphosphine
ligand, in sharp contrast to that of the analogous
acetylide complex Rh(CtCPh)(nbd)(PPh3)2 (1d ), which
adopts a trigonal-bipyramidal structure having two
triphenylphosphine ligands.9 The plane of the 2,6-xylyl
group is perpendicular with regard to the coordination
plane of rhodium to minimize the steric repulsion
between the 2,6-xylyl group and triphenylphosphine.
(1) For reviews, see: (a) Millich, F. Chem. Rev. 1972, 72, 101. (b)
Millich, F. Adv. Polym. Sci. 1975, 19, 117. (c) Drenth, W.; Nolte, R. J .
M. Acc. Chem. Res. 1979, 12, 30. (d) Nolte, R. J . M. Chem. Soc. Rev.
1994, 23, 11.
(2) (a) Yamamoto, Y.; Takizawa, T.; Hagihara, N. Nippon Kagaku
Zasshi 1966, 87, 1355. (b) Otsuka, S.; Nakamura, A.; Yoshida, T. J .
Am. Chem. Soc. 1969, 91, 5932. (c) Yamamoto, Y.; Yamazaki, H. Inorg.
Chem. 1977, 16, 3182.
(3) (a) Ito, Y.; Ihara, E.; Murakami, M.; Shiro, M. J . Am. Chem. Soc.
1990, 112, 6446. (b) Ito, Y.; Ihara, E.; Murakami, M. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 1509. (c) Ito, Y.; Ohara, T.; Shima, R.;
Suginome, M. J . Am. Chem. Soc. 1996, 118, 9188. (d) Ito, Y.; Miyake,
T.; Hatano. S.; Shima, R.; Ohara, T.; Suginome, M. J . Am. Chem. Soc.
1998, 120, 11880.
Other rhodium complexes, Rh(2,4,6-Pri C6H2)(nbd)-
3
(4) (a) Deming, T. J .; Novak, B. M. Macromolecules 1991, 24, 6043.
(b) Deming, T. J .; Novak, B. M. J . Am. Chem. Soc. 1992, 114, 4400. (c)
Deming, T. J .; Novak, B. M. J . Am. Chem. Soc. 1992, 114, 7926. (d)
Deming, T. J .; Novak, B. M. J . Am. Chem. Soc. 1993, 115, 9101.
(5) (a) Onitsuka, K.; J oh, T.; Takahashi, S. Angew. Chem., Int. Ed.
Engl. 1992, 31, 851. (b) Onitsuka, K.; Yanai, K.; Takei, F.; J oh, T.;
Takahashi, S. Organometallics 1994, 13, 3862.
(6) (a) Takei, F.; Yanai, K.; Onitsuka, K.; Takahashi, S. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1554. (b) Takei, F.; Onitsuka, K.;
Takahashi, S. Polym. J . 1999, 31, 1029. (c) Takei, F.; Yanai, K.;
Onitsuka, K.; Takahashi, S. Chem. Eur. J . 2000, 6, 983.
(7) (a) Kamer, P. C. J .; Cleij, M. C.; Nolte, R. J . M.; Harada, T.;
Hezemans, A. M. F.; Drenth, W. J . Am. Chem. Soc. 1988, 110, 1581.
(b) Kamer, P. C. J .; Nolte, R. J . M.; Drenth, W. J . Am. Chem. Soc.
1988, 110, 6818.
(8) (a) Nolte, R. J . M.; van Beijnen, A. J . M.; Drenth, W. J . Am.
Chem. Soc. 1974, 96, 5932. (b) van Beijnen, A. J . M.; Notle, R. J . M.;
Drenth, W.; Hezemans, A. M. F. Tetrahedron 1976, 32, 2017.
(9) (a) Kishimoto, Y.; Eckerle, P.; Miyatake, T.; Ikariya, T.; Noyori,
R. J . Am. Chem. Soc. 1994, 116, 12131. (b) Kishimoto, Y.; Eckerle, P.;
Miyatake, T.; Kainosho, M.; Ono, A.; Ikariya, T.; Noyori, R. J . Am.
Chem. Soc. 1999, 121, 12035.
(10) Misumi, Y.; Masuda, T. Macromolecules 1998, 31, 7572.
(11) Crystallographic data for 1a : formula C33H32PRh, fw ) 562.50,
monoclinic, space group P21/c (No. 14), a ) 9.617(2) Å, b ) 18.513(4)
Å, c ) 15.755(7) Å, â ) 105.87(2)°, V ) 2697(1) Å3, Z ) 4, dcalcd ) 1.385
g cm-3, -50 °C, ω-2θ scan, 6° < 2θ < 55°, µ(Mo KR) ) 7.10 cm-1, R
(Rw) ) 0.029 (0.051) for 316 parameters against 5304 reflections with
I > 3.0σ(I) out of 6195 unique reflections by the full-matrix least-
squares method, GOF ) 1.36.
10.1021/om0005809 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 10/07/2000

4670 Organometallics, Vol. 19, No. 23, 2000
Communications
Ta ble 1. P olym er iza tion of or th o-Su bstitu ted Ar yl
Isocya n id es by Or ga n or h od iu m Com p lexesa
ArNC/ PPh3/ conv
yield 10-3
×
Mw/
Mn
d
d
run init ArNC
Rh
Rh
(%)b polym (%)c Mn
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1b
1b
1b
1c
2a
2a
2a
2a
2b
2a
2b
2c
2a
2a
2a
2a
2b
25
50
75
100
50
50
50
50
25
50
10
10
10
10
10
10
10
10
10
10
20
20
10
100 3a -25
100 3a -50
100 3a -75
83
80
99
4.7 1.12
9.3 1.19
14.1 1.26
17.5 1.47
22.6 1.34
8.9 1.29
15.2 1.27
16.6 1.25
5.2 1.13
9.5 1.26
14.5 1.36
16.6 1.51
16.6 1.35
96 3a -100 92
100 3b-50
100 3c-50
100 3d -50
100 3e-50
100 3f-25
100 3f-50
100 3f-75
96 3f-100
100 3g-50
83
82
80
98
65
90
95
84
82
10 1c
11 1c
12 1c
13 1c
75
100
50
a
Conditions: [Rh]0 ) 5 mM, 20 °C, THF. Abbreviations:
init ) initiator; conv ) conversion; polym ) polymer. b Determined
by GPC on the basis of the internal standard (naphthalene).
c Isolated yield of the resulting polymers after reprecipitation with
d
methanol. Determined by GPC using polystyrene standards.
cm-1, which is characteristic of ν(CdN) of poly(aryl
isocyanide).5 In the 13C{1H} NMR spectrum of 3a , a
relatively sharp resonance due to the imino carbons of
the polymer backbone was observed at δ 155.1 (half-
bandwidth 33 Hz), indicating that 3a has a high
stereoregularity on the nitrogen atoms of the imino
groups.13 The 1H NMR spectrum of 3a in CDCl3 dis-
played a singlet signal at δ 2.35 assignable to methyl
protons of the polymer end derived from the 2,6-xylyl
ligand of 1a . These results suggest that the polymeri-
zation might proceed via multiple and successive inser-
tion of 2a into the Rh-C bond of 1a . Unfortunately, no
information on the other chain end was obtained by
spectral analyses of 3a .
F igu r e 1. ORTEP drawing of 1a . Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles
(deg) are as follows: Rh(1)-P(1) ) 2.3046(6), Rh(1)-C(1)
) 2.062(2), Rh(1)-C(9) ) 2.146(3), Rh(1)-C(10) ) 2.167-
(3), Rh(1)-C(14) ) 2.193(3), Rh(1)-C(15) ) 2.207(3); P(1)-
Rh(1)-C(1) ) 93.43(6), P(1)-Rh(1)-C(14) ) 100.2(1), P(1)-
Rh(1)-C(15) ) 100.94(9), C(1)-Rh(1)-C(9) ) 96.2(1),
C(1)-Rh(1)-C(10) ) 95.9(1).
(PPh3) (1b) and Rh(2-PhC6H4)(nbd)(PPh3) (1c), were
also prepared by a reaction similar to that of 1a in good
yields.
In the polymerization of 2a without PPh3, a polymer
along with a significant amount of oligomers was
produced in a moderate yield, suggesting that addition
of PPh3 is essential for smooth polymerization. When
PCy3, P(OPh)3, or Ph2P(CH2)3PPh2 was added to the
reaction system instead of PPh3, the polymerization was
remarkably depressed. The present polymerization of
2a also proceeded in toluene and DMF to produce 3a in
good yields, while the reaction in CH2Cl2 resulted in
partial polymerization to give a polymer with a low
molecular weight. It should be noted that NiCl2‚6H2O,
which is a representative catalyst for the polymerization
of isocyanides, including ortho-substituted aryl isocya-
nides such as o-tolyl and 2,6-dichlorophenyl isocya-
nides,8 was not effective for the polymerization of 2a .
Polymerization of 2-((trimethylsilyl)ethynyl)phenyl
isocyanide (2a )12 by complex 1a in the presence of PPh3
in THF at 20 °C for 2 h resulted in the formation of a
yellow-brown polymer (3a ) in an almost quantitative
yield (eq 2). Polymer 3a was soluble in common organic
solvents such as benzene, ether, and dichloromethane
and purified by reprecipitation with methanol. The IR
spectrum of polymer 3a exhibited an absorption at 1613
The representative results of the polymerization are
shown in Table 1. Complexes 1b,c also initiated the
polymerization of 2a to give polymers in good yields.
Reactions of 2-tert-butyl-4-((octyloxy)carbonyl)phenyl
and 2-tert-butyl-4-(octyloxy)phenyl isocyanides (2b,c)
under similar conditions gave high-molecular-weight
polymers. The Mn values of the resulting polymers
agreed with the calculated ones based on the initial ratio
of the monomer to the initiator, suggesting that the
(12) (a) Onitsuka, K.; Segawa, M.; Takahashi, S. Organometallics
1998, 17, 4335. (b) Rainier, J . D.; Kennedy, A. R.; Chase. E. Tetrahe-
dron Lett. 1999, 40, 6325. (c) Suginome, M.; Fukuda, T.; Ito, Y. Org.
Lett. 1999, 1, 1977.
(13) Green, M. M.; Gross, R. A.; Schilling, F. C.; Zero, K.; Crosby,
C., III. Macromolecules 1988, 21, 1839.

Communications
Organometallics, Vol. 19, No. 23, 2000 4671
1
molecular weight of the resulting polymer can be
controlled in this reaction. The polydispersity indexes
of the resulting polymers were appreciably small but
slightly increased with an increase in the molecular
weight. Since block copolymers were not obtained by
addition of second monomers after consumption of first
monomers, the present polymerization does not have a
living nature and the active species at the chain end
would slowly decompose with the progress of the po-
lymerization.
The rhodium complexes 1a -c prepared here are
effective to the polymerization of aryl isocyanides with
a bulky substituent at an ortho position because 4-(pro-
poxycarbonyl)phenyl and 2-n-butylphenyl isocyanides
did not give high-molecular-weight polymers. The or-
ganic group on Rh is crucial for an activity of initiator,
since the polymerization of 2a by Rh(CtCPh)(nbd)-
(PPh3)2 (1d ),9 Rh(Me)(nbd)(PPh3)2 (1e),14 and Rh(Ph)-
(nbd)(PPh3) (1f)15 produced a mixture of oligomers and
polymers. It should be noted that the integral ratio
between the signals at δ 8.74 attributed to the aromatic
protons in the repeating unit and the methyl signal
(δ 2.35) of the xylyl group at the initiating chain end in
the H NMR spectra of 3a is in good agreement with
the initial 2a /1a ratio. Therefore, the initiator efficiency
of 1a would be virtually quantitative and the molecular
1
weights of 3a determined by the H NMR spectrum are
consistent with the calculated ones.
Polymerization of isocyanides by group 9 metal com-
plexes is quite rare, while extensive studies using group
10 metal complexes have been made.1 The reaction
presented here may be attractive as the first example
of the polymerization of bulky aryl isocyanides. Further
studies, including the scope and limitation of the present
polymerization as well as reaction mechanism, are now
in progress.
Ack n ow led gm en t. This work was supported by
Grant-in-Aid for Scientific Research from the Ministry
of Education, Science, Sports and Culture. We thank
The Materiel Analysis Center, ISIR, Osaka University,
for the support of spectroscopic measurements, X-ray
crystallography, and microanalyses.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and tables of X-ray crystallographic data for
complex 1a . This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Schrock, R. R.; Osborn, J . A. J . Am. Chem. Soc. 1976, 98, 2134.
(15) Takesada, M.; Yamazaki, H.; Hagihara, N. Nippon Kagaku
Zasshi 1968, 89, 1122.
OM0005809