Highly screw-sense selective polymerization of 1,2-diisocyano-3,6-di-p-tolylbenzene initiated by optically active binaphthylpalladium(II) complexes

Full text of DOI:10.1021/ja961818g

9188
J. Am. Chem. Soc. 1996, 118, 9188-9189
Scheme 1
Highly Screw-Sense Selective Polymerization of
1,2-Diisocyano-3,6-di-p-tolylbenzene Initiated by
Optically Active Binaphthylpalladium(II)
Complexes
Yoshihiko Ito,* Takafumi Ohara, Ryoto Shima, and
Michinori Suginome
Department of Synthetic Chemistry and
Biological Chemistry
Graduate School of Engineering,
Kyoto UniVersity, Kyoto 606-01, Japan
ReceiVed May 30, 1996
3-yl]palladium(II) complexes B′QPd and BQPd in good yields
Synthesis of new polymers, which are featured by their
secondary structures as well as main chain structure, is desired
for directing toward functional materials. Synthetic construction
of polymers with well-defined stable helices has been a
challenging subject, since helical structures are characteristic
of important biopolymers such as DNA and peptides.1 We
reported a new synthesis of helical poly[2,3-(1,4-diazanaphtha-
lene)]species (i.e., poly(2,3-quinoxaline)) by methylpalladium-
(II)-catalyzed polymerization of 1,2-diisocyanobenzenes.2 The
aromatizing polymerization proceeds with successive insertion
of the two isocyano groups into the palladium-carbon bond of
the growing [poly(quinoxalinyl)]palladium(II) complexes, which
are isolable. Although a methylpalladium(II) complex bearing
bis((S)-2-methylbutyl)phenylphosphine as a chiral ligand failed
to induce screw-sense selection in the polymerization of 1,2-
diisocyano-3,6-di-p-tolylbenzene, the ligands allowed us to
separate right- and left-handed helices in the form of diaster-
eomeric [penta(quinoxalinyl)]palladium(II) complexes.3 Of
interest is that the polymerization of 1,2-diisocyanobenzene
derivatives with the [penta(quinoxalinyl)]palladium(II) com-
plexes of pure screw-sense thus isolated proceeded with
retention of their screw-sense. Herein, we disclose highly
screw-sense selective polymerization of 1,2-diisocyano-3,6-di-
p-tolylbenzenes initiated by enantiomerically pure binaphth-
ylpalladium(II) complexes, whose substituents on the binaphthyl
groups crucially affected the selectivity. A stable and rigid
helical structure of the growing [oligo(quinoxalinyl)]palladium-
(II) complex was revealed by an X-ray analysis.
as yellow crystalline solids.6
In the presence of a catalytic amount of B′QPd, polymeri-
zation of 2 (39 equiv) in THF proceeded at room temperature
to give, after quenching the reaction with NaBH4, polymer
B′QpH (Mn ) 5.85 × 103, 62% yield),7 which exhibited a CD
spectrum with a Cotton effect characteristic of the optically
active, helical poly(2,3-quinoxaline)species (Scheme 1, Figure
1). The oligomerization of 2 (5 equiv based on the palladium
catalyst) permitted us to separate and isolate [penta(quinoxali-
nyl)]palladium(II) complex B′Q5Pd in 31% yield as reddish
crystals from a mixture of the corresponding oligomers (Scheme
Enantiomerically pure catalysts for the polymerization of the
1,2-diisocyanobenzene were prepared from (p)-(S)-2-iodo-2′-
methoxy-1,1′-binaphthyl 1a and (m)-(S)-2-iodo-1,1′-binaphthyl
1b with the axial chirality (eq 1).4 Oxidative addition of 1 onto
the palladium(0) complex generated in situ from (cyclopenta-
dienyl)(π-allyl)palladium(II) and 3 equiv of dimethylphe-
nylphosphine gave binaphthylpalladium(II) iodide complexes,5
which were then reacted with 1,2-diisocyano-3,6-di-p-tolylben-
zene 2 to afford the corresponding [2-(2-binaphthyl)quinoxalin-
1
1).8 A 400 MHz H NMR spectrum of the complex exhibited
the distinct ten methyl singlets of the tolyl groups and one
methyl singlet of the methoxynaphthyl group.
An X-ray crystal analysis of B′Q5Pd revealed a right-handed
helical structure in which the two adjacent quinoxaline rings
Q(1)-Q(2), Q(2)-Q(3), Q(3)-Q(4), and Q(4)-Q(5) were
arranged at the dihedral angles of +151°, +148°, +148°, and
+171°, respectively, with an iodo(dimethylphenylphosphine)-
palladium terminus intramolecularly coordinated by a nitrogen
atom of the fourth quinoxaline Q(4) (Figure 2).9 The helical
(1) (a) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of
Organic Compounds; Wiley: New York, 1994; pp 1060-1071. (b)
Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349-372 and references
therein.
(2) Ito, Y.; Ihara, E.; Murakami, M.; Shiro, M. J. Am. Chem. Soc. 1990,
112, 6446.
(3) (a) Ito, Y.; Ihara, M.; Murakami, M. Angew. Chem., Int. Ed. Engl.
1992, 31, 1509. (b) Ito, Y.; Ihara, E.; Murakami, M.; Sisido, M.
Macromolecules 1992, 25, 6810. (c) Ito, Y.; Kojima, Y.; Murakami, M.
Tetrahedron Lett. 1993, 51, 8279. (d) Ito, Y.; Kojima, Y.; Suginome, M.;
Murakami, M. Heterocycles 1996, 42, 597-615.
(4) The notation (p) or (m) (plus or minus, respectively) represents the
sense of the chirality of binaphthyl groups without considering their
substituents. Hattori, T.; Hotta, H.; Suzuki, T.; Miyano, S. Bull. Chem. Soc.
Jpn. 1993, 66, 613-622.
(5) For the generation of phosphine-palladium(0) complexes from
(cyclopentadienyl)(π-allyl)palladium(II), see: Bennett, M. A.; Chiraratva-
tana, C.; Robertson, G. B.; Tooptakong, U. Organometallics 1988, 7, 1403-
1409 and references therein.
(6) No diastereoisomerism was observed for BQPd and B′QPd at room
temperature. Enantiomerically pure B′QPd was confirmed by HPLC analysis
(a Chiralcel OD-H column) of the corresponding methyl ester, prepared by
the reaction of B′QPd with CO (20 atm) in MeOH at 100 °C.
(7) Molecular weights were estimated by analytical GPC using poly-
(styrene) species as standard. According to the GPC estimation, penta
quinoxaline B′Q5H (fw ) 1826) prepared by the reaction of B′Q5Pd
with NaBH4 was calculated to have lower molecular weights (Mn ) 1.28
× 103).
(8) The mixture contained tetra-, penta-, and [hexa(quinoxalinyl)]-
palladium complexes, each of which may consist of both of the right- and
left-handed helices. In fact, when the mixture was treated with NaBH4 to
remove the palladium moiety, two diastereomeric hexamers, which exhibited
CD spectra corresponding to opposite screw-sense, were isolated by GPC
and subsequent HPLC purification (see supporting information).
S0002-7863(96)01818-5 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 38, 1996 9189
Figure 3. Possible orientations of the binaphthyl terminus of the
optically active polymers B′QpH0 and BQpH.
remarkably strong CD spectrum between 250 and 370 nm
(Figure 1). This finding may suggest that the palladium(II)
complex B′Q5Pd promotes the polymerization of 2 with
retention of its preorganized screw-sense resulting in highly
pure, right-handed helical structure. On the basis of the intensity
of the CD spectrum of B′QpH0, the palladium(II) complex
catalyst B′QPd, which is not yet helically preorganized, induced
the polymerization of 2 only in low screw-sense selectivity.
Figure 1. CD spectra of the optically active poly(2,3-quinoxaline)
species B′QpH, B′QpH0, and BQpH.
It should be noted that the highly screw-sense selective
polymerization of 2 was promoted by the palladium(II) complex
catalyst BQPd, in which the chiral binaphthyl group has no
substituent (eq 2). A polymerization of 2 (39 equiv) was
catalyzed at room temperature by BQPd to afford, after
quenching the reaction with NaBH4, poly(2,3-quinoxaline)-
species BQpH (Mn ) 5.81 × 103) with a CD spectrum bearing
ca. 80% intensity of B′QpH0 (Figure 1).
The CD spectrum of BQpH, which exhibits Cotton effect of
the same sign as B′QpH0, indicates that the initiator BQPd
induces the formation of the right-handed helix with high screw-
sense selectivity. Plausible arrangements of the binaphthyl
terminus of the polymer chains are illustrated on the basis of
the X-ray structure of B′Q5Pd (Figure 3). It is noted that the
benzene rings of the terminal naphthyl groups (C5′-C10′) are
oppositely oriented in polymers B′QpH0 and BQpH.
In conclusion, highly screw-sense selective polymerization
and oligomerization of the 1,2-diisocyanobenzene were devel-
oped by use of the optically active palladium initiator with
simple chiral auxiliary, leading to practical preparation of new
and stable helical molecules.
Figure 2. Molecular Structure of B′Q5Pd. ORTEP drawing (left; 30%
probability), a side view (top right; tolyl groups are omitted), and a
top view (bottom right; binaphthyl, tolyl, Me2PhP, and iodo groups
are omitted).
structure once formed was stable with no change of CD
spectrum even on heating in benzene at 80 °C for 24 h.
The stable palladium(II) complex B′Q5Pd was still active and
induced the polymerization of 2 at room temperature, producing
poly(2,3-quinoxaline)species B′QpH0 (Mn ) 7.18 × 103) after
reduction with NaBH4 (Scheme 1). The polymers showed a
(9) Crystal data for B′Q5Pd: crystal size 0.30 × 0.20 × 0.50 mm;
monoclinic, space group P21 (No. 4), Z ) 2; a ) 12.853 (4), b ) 40.251-
(9), c ) 12.486(3) Å; â ) 117.36(2)°; V ) 5737(2) Å3, Fcalcd ) 1.27 g/cm3;
µ ) 40.22 cm-1; max 2θ ) 125° (Cu KR, λ ) 1.54178 Å, graphite
monochromator, ω/2θ scan, T ) 293 K); 10431 reflections measured, 9746
independent, 8803 included in the refinement, Lorentzian polarization; direct
methods, anisotropical refinement for non-hydrogen atoms by full-matrix
least-squares against |F| with program package CrystanG (Mac Science),
1376 parameters; R ) 0.070, Rw ) 0.091. Molecular packing diagram is
shown in the supporting information.
Supporting Information Available: Detailed experimental pro-
cedures, characterization of the new compounds, and molecular packing
diagram, tables of final atomic coordinates, thermal parameters, bond
distances, and bond angles for compound B′Q5Pd (17 pages). See any
current masthead page for ordering and Internet access instructions.
JA961818G