Modular Construction of Dendritic Carbosilanes. Organization of Dendrimer Connectivity around Bifunctional Precursors That Are Adapted for Sequential Convergent and Divergent Propagative Steps

Article abstract of DOI:10.1021/ol000038g

(Matrix Presented) R_dend = -CH₂(CH₂)₂Si(Me)[CH₂(CH ₂)₂Si(Me)(CH₂CH=CH₂) ₂]₂ Regiospecific hydrosilylation of 1-bromo-4-(prop-2-enyl)benzene offers an efficient route to molecular building block precursors that can accommodate sequential divergent and convergent steps for dendritic extension, establishing a modular methodology for assembly and organization of connectivity used for synthesis of modified carbosilane dendrimers including 14.

Full text of DOI:10.1021/ol000038g

ORGANIC
LETTERS
2000
Vol. 2, No. 11
1549-1552
Modular Construction of Dendritic
Carbosilanes. Organization of Dendrimer
Connectivity around Bifunctional
Precursors That Are Adapted for
Sequential Convergent and Divergent
Propagative Steps
Miguel Angel Casado and Stephen R. Stobart*
Department of Chemistry, UniVersity of Victoria, British Columbia, Canada V8W 2Y2
stobartz@uVVm.uVic.ca
Received February 21, 2000
ABSTRACT
Regiospecific hydrosilylation of 1-bromo-4-(prop-2-enyl)benzene offers an efficient route to molecular building block precursors that can
accommodate sequential divergent and convergent steps for dendritic extension, establishing a modular methodology for assembly and
organization of connectivity used for synthesis of modified carbosilane dendrimers including 14.
Dendrimer chemistry has been elevated from curiosity to
celebrity stature in little more than a decade of intense
activity that has been led by Tomalia,1 Frechet,2 Newkome,3
and their co-workers, who have drawn on ideas originally
put into practice by Vo¨gtle et al.4 for “cascade” synthesis of
noncyclic polyaza compounds. The archetypal topology, in
which a regularly hyperbranched substructure radiates from
a core atom to a dense homofunctional array of peripheral
sites, is now the focus for a remarkable range of novel
technological ideas that may be brought within reach by
imaginative adaptation of a dendrimer periphery, through
compartmentalization of unique chemistry at the dendrimer
core, or developing core-periphery cooperative effects.4b,5
(3) (a) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Behera, R.
K.; Escamilla, G. H.; Saunders: M. J. Angew. Chem., Int. Ed. Engl. 1992,
31, 917. (b) Newkome, G. R.; Yao, Z. Q.; Baker, G. R.; Gupta, V. K.;
Russo, P. S.; Saunders: M. J. J. Am. Chem. Soc. 1986, 108, 849. (c)
Newkome, G. R.; Weis, C. D.; Moorefield, C. N.; Baker, G. R.; Childs, B.
J.; Epperson, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 307. (d) Newkome,
G. R.; Gu¨ther, R.; Moorefield, C. N.; Cardullo, F.; Echegoyen, L.; Pe´rez-
Cordero, E.; Luftmann, H. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2023.
(e) Newkome, G. R.; Patri, A. K.; God´ınez, L. A. Chem. Eur. J. 1999, 5,
1445.
(4) (a) Buhleier, E.; Wehner, W.; Vo¨gtle, F. Synthesis 1978, 155. (b)
Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1999, 38, 884.
(5) (a) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (b)
Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665.
(c) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl.
1999, 38, 1422.
(1) (a) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, C.;
Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (b)
Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, C.; Martin, S.;
Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466. (c) Yin,
R.; Zhu, Y.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 2678. (d) Miller,
L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A. J. Am. Chem. Soc. 1997,
119, 1005.
(2) (a) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 1998, 120, 10990. (b) Wooley, K. L.; Hawker, C. J.; Fre´chet,
J. M. J. J. Am. Chem. Soc. 1993, 115, 11496. (c) Piotti, M. E.; Rivera, F.,
Jr.; Bond, R.; Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1999,
121, 9471. (d) Jayaraman, M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1998,
120, 12996. (e) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J.
Am. Chem. Soc. 1998, 120, 10990.
10.1021/ol000038g CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/11/2000

Dendrimer assembly relies on a reiterative procedure, usually
using a divergent6 routine that develops the characteristic
shell heirarchy outward from a focal atom, or alternatively
through convergent7 strategy whereby the peripheral structure
is preorganized by successive coupling of branched subunits
into monodendron blocks that are brought together by
attachment to a multifunctional core fragment. We have
become interested in how these independent growth regimes
might be managed together, to construct a dendritic frame-
work from a building block that would become incorporated
as an internal skeletal spacer rather than at the dendrimer
periphery or its core, i.e., so that the aggregate product is
built up from a nucleus that can act as a template to structure
or compartmentalize its internal volume. We illustrate this
idea by showing how a simple bifunctional substrate can be
modified to permit consecutive divergent and convergent
steps,6,7 thereby establishing a modular program for linking
up independent dendritic blocks.
Scheme 1
Dendritic carbosilanes,8 which are of special interest as a
group because they are chemically inert and are fluid at high
molecular weight, are assembled by Si-C bond formation
that uses the silicon atoms as branch points to develop a
paraffin-like network. These materials are currently acces-
sible only by divergent synthesis (Grignard alkenyl group-
transfer to Si alternating with platinum-catalyzed alkene
hydrosilylation), although convergent coupling has recently
been used to attach dendritic carbosilane fragments onto a
core subunit through heteroatom (CPh-O-C) links as well
as to construct carbosiloxane analogues.9 The new approach
to functionalized carbosilane monodendrons described below
follows a logic that could be suited by adaptation to assembly
of other dendrimer systems, and uses as its starting point
two key reagents that are both obtained in high yields from
a common source. Thus, the para-substituted bromobenzenes
BrC6H4(CH2)3SiMexCl3-x (x ) 1 or 2) may be converted
either to the precursor BrC6H4(CH2)3SiMe(CH2CH:CH2)2, 1
(97%), for initially bifurcate dendritic growth from Si
followed by metalation (affording a dendritic aryl carbanion)
to allow convergent C-C coupling onto the periphery of a
dendritic carbosilane or to the silane BrC6H4(CH2)3SiHMe2,
2 (78%), that can be incorporated as a peripheral group by
hydrosilylation of a per(allyl-terminated) dendritic carbosi-
lane then used for divergent aryl-aryl coupling, e.g., to the
core bromophenyl fragment of a monodendron derived from
1. Repetition of this routine (i.e., incorporating 2 then linking
to the latter of a subunit derived from 1) establishes a
reiterative sequence to build complex hybrid assemblies.
We have observed that, while platinum-catalyzed hydrosi-
lylation of commercially available p-bromostryrene is not
completely10 regioselective, 1-bromo-4-(prop-2-enyl)benzene
may be obtained as the major product (80% purified yield)
if the monometalation of p-dibromobenzene under Grignard
conditions followed by nickel-catalyzed coupling11 to 3-bro-
mopropene is carefully controlled and that hydrosilylation
(6) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 138.
(7) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.
(8) (a) Krska, S. W.; Seyferth, D. J. Am. Chem. Soc. 1998, 120, 3604.
(b) van der Made, A. W.; van Leeuwen, P. W. N. M. J. Chem. Soc., Chem.
Commun. 1992, 1400. (c) Lorenz, K.; Frey, H.; Stu¨hn, B.; Mu¨lhaupt, R.
Macromolecules 1997, 30, 6860. (d) Lorenz, K.; Mu¨lhaupt, R.; Frey, H.;
Rapp, U.; Mayer-Posner, F. J. Macromolecules 1995, 28, 6657. (e) Chai,
M.; Pi, Z.; Tessier, C.; Rinaldi, P. L. J. Am. Chem. Soc. 1999, 121, 273. (f)
Seyferth, D.; Son, D. Y.; Rheingold, A. L.; Ostrander, R. L. Organometallics
1994, 13, 2682. (g) Roovers, J.; Toporowski, P. M.; Zhou, L.-L. Polym.
Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1992, 33, 182. (h) Zhou, L.-
L.; Roovers, J. Macromolecules 1993, 26, 963. (i) Roovers, J.; Zhou, L.-
L.; Toporowski, P. M.; van der Zwan, M.; Iatrou, H.; Hadjichristidis, N.
Macromolecules 1993, 26, 4324. (j) Hovestad, N. J.; Eggeling, E. B.;
Heidebuchel, E. H.; Jastrzebski, J. T.; Kragl, U.; Keim, W.; Vogt, D.; van
Koten, G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1655. (k) Knapen, J. W.
J.; van der Made, A. W.; van Leeuwen, P. W. N. M.; de Wilde, J. C.;
Grove, D.; van Koten, G. Nature 1994, 372, 659.
(10) (a) Under the same conditions that afford the chlorosilyl precursors
of 1 and 2 as pure γ-silyl isomers, the hydrosilylated products obtained
from p-bromostyrene were consistently found to be regioisomeric mixtures
(1H, 13C, 29Si NMR) containing at least 20% (depending on the silane) of
the enantiomeric R-silyl isomer. (b) Lithiation of a carbosilane monodendron
synthesized from p-bromostyrene has nevertheless recently been used as a
means for attaching carbosilane units at the phosphorus(III) centers in
ferrocenylphosphines: Oosterom, G. E.; van Haaren, R. J.; Reek, J. N. H.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem. Soc., Chem. Commun.
1999, 1119.
(9) (a) Frey, H.; Burgath, A.; Lutz, M.; Speck, A. L.; van Koten, G.
Chem. Eur. J. 1999, 5, 2191. (b) Gossage, R. A.; Mun˜oz-Mart´ınez, E.; van
Koten, G. Tetrahedron Lett. 1998, 39, 2397. (c) Morikawa, A.; Kakimoto,
M.; Imai, Y. Macromolecules 1992, 25, 3247.
1550
Org. Lett., Vol. 2, No. 11, 2000

of this alkene using HSiMexCl3-x (x ) 1 or 2) is regiospecific.
The pure linear products so obtained react with CH2CH:CH2-
MgBr or LiAlH4 to afford12 1 or 2 directly. These new
reagents satisfy two imperatives for use in further synthe-
sis: (a) they do not contain Si-CPh bonds, which can
undergo facile solvolysis; (b) while metalation or boronation
of the Br-CPh bond is facile, this crucial functionality does
not need to be protected during dendritic carbosilane expan-
sion. Conversion of 1 to the aryl Grignard reagent BrMgC6H4-
(CH2)3SiMe(CH2CH:CH2)2, 3, followed by treatment with
B(OMe)3 then13 HCl yields (HO)2BC6H4(CH2)3SiMe(CH2-
CH:CH2)2, 4, which undergoes palladium-catalyzed (Suzuki)
coupling with further 1 to afford the biphenyl (CH2:CHCH2)2-
MeSi(CH2)3C6H4-C6H4(CH2)3SiMe(CH2CH:CH2)2 (5), or
with 4,4′-diiodobiphenyl to give the corresponding tetra-
phenyl (CH2:CHCH2)2MeSi(CH2)3C6H4-(C6H4)2-C6H4(CH2)3-
SiMe(CH2CH:CH2)2 (6), isolated in 82 and 86% yield,
respectively.
Application of reagents 1 and 2 to modular carbosilane
dendrimer synthesis is illustrated as follows (Scheme 1).
Conversion of 1 to 3 followed by addition of the latter to
Si(CH2CH2CH2SiMe2Cl)4 (7) affords Si(CH2CH2CH2SiMe2-
C6H4CH2CH2CH2SiMe(CH2CH:CH2)2)4 (8), Pt-catalyzed ad-
dition of silane 2 to the simple substrate Si(CH2CH:CH2)4
yields Si(CH2CH2CH2SiMe2CH2CH2CH2C6H4Br)4 (9), while
treatment of the latter with 4/Pd(PPh3)4 gives Si(CH2CH2-
CH2SiMe2CH2CH2CH2C6H4-C6H4CH2CH2CH2SiMe(CH2-
CH:CH2)2)4 (10), all prototypal transformations that use and
generate dendrimer [G-1] analogues in 80% yield or better.
However, 1 can be extended by two reiterative hydrosilyl-
ation cycles to a [G-2] monodendron (11) that can be cleanly
metalated (Mg) and then added to 7, so attaching 4 [G-2]
units to a [G-1] core to yield a product of composition
C252H476Si33, (12), a colorless viscous oil (83%) with M
4433.2. Likewise, boronation of 11 to form its derivative 13
followed by coupling to 9 affords (see Scheme 2) analogue
(11) Hayashi, T.; Konishi, M.; Yokota, K.-I.; Kumada, M. J. Chem. Soc.,
Chem. Commun. 1981, 313.
(12) All compounds have been characterized by multinuclear NMR
spectroscopy (1H, 13C, 29Si), elemental analysis, MALDI-TOF spectrometry
and Gel Permeation Chromatography. Data for selected compounds are as
follows. 1: 1H NMR (CDCl3, δ) 7.38, 7.03 (d, 2H, C6H4), 5.74 (m, 2H,
CH), 4.85 (m, 4H, dCH2), 2.56 (t, 2H, CH2), 1.59 (m, 2H, CH2), 1.53 (d,
4H, CH2), 0.60 (m, 2H, CH2), 0.03 (s, 3H, CH3); 13C{1H} NMR (CDCl3,
δ) 141.4 (C6H4), 134.6 (CH), 131.3, 130.2, 119.4 (C6H4), 113.2 (dCH2),
39.2, 25.6, 21.3, 12.8 (CH2), -5.8 (CH3); 29Si{1H} (CDCl3, δ) 1.02;
MALDI-TOF (m/z, M + Ag+) calcd 431, found 431. Anal. Calcd for C16H23-
BrSi: C, 59.47; H, 7.17. Found: C, 59.95; H, 7.16. 2: 1H NMR (CDCl3,
δ) 7.37, 7.03 (d, 2H, C6H4), 3.84 (m, 1H, SiH), 2.57 (t, 2H, CH2), 1.63,
0.59 (m, 2H, CH2), 0.05 (d, 6H, CH3); 13C{1H} NMR (CDCl3, δ) 141.5,
131.3, 130.3, 119.4 (C6H4), 38.8, 26.3, 13.8 (CH2), -4.5 (CH3); 29Si{1H}
(CDCl3, δ) -13.05. Anal. Calcd for C11H17BrSi: C, 51.36; H, 6.66.
Found: C, 51.82; H, 6.72. 8: 1H NMR (CDCl3, δ) 7.46, 7.18 (d, 8H, C6H4),
5.80 (m, 8H, CH), 4.85 (m, 16H, dCH2), 2.62 (t, 8H, CH2), 1.64 (m, 8H,
CH2), 1.58 (d, 16H, CH2), 1.36 (m, 8H, CH2), 0.70 (m, 8H, CH2), 0.56 (m,
8H, CH2), 0.25 (s, 24H, CH3), -0.05 (s, 12H, CH3); 13C{1H} NMR (CDCl3,
δ) 143.2, 138.6 (C6H4), 134.8 (s, CH), 133.9, 128.3 (C6H4), 113.2 (s, )CH2),
40.0, 25.9, 21.3, 21.2, 18.6, 17.5, 14.3 (CH2), -2.8, -5.8 (CH3); 29Si{1H}
(CDCl3, δ) 1.02 (4 Si), 0.78 (1 Si), -4.16 (4 Si); MALDI-TOF (m/z, M +
Ag+) calcd 1151, found 1151. Anal. Calcd for C84H140Si9: C, 71.92; H,
10.06. Found: C, 72.27; H, 9.88. 9: 1H NMR (CDCl3, δ) 7.40, 7.03 (d,
8H, C6H4), 2.60 (t, 8H, CH2), 1.62 (m, 8H, CH2), 1.40 (m, 8H, CH2), 0.59
(m, 24H, CH2), 0.02 (d, 24H, CH3); 13C NMR (CDCl3, δ) 141.6, 131.3,
130.2, 119.3 (C6H4), 39.3, 26.0, 20.2, 18.6, 17.5, 15.2 (CH2), -3.3 (CH3);
29Si{1H} (CDCl3, δ) 0.60 (1Si), 1.70 (4Si). Anal. Calcd for C56H88Br4Si5:
C, 55.07; H, 7.26. Found: C, 56.30; H, 7.42. 10: 1H NMR (CDCl3, δ)
7.47, 7.20 (d, 16H, C6H4), 5.80 (m, 8H, CH), 4.85 (m, 16H, dCH2), 2.60
(t, 16H, CH2), 1.65 (m, 16H, CH2), 1.58 (d, 16H, CH2), 1.40 (m, 8H, CH2),
0.70 (m, 8H, CH2), 0.60 (m, 24H, CH2), 0.20 (s, 12H, CH3), 0.00 (s, 24H,
CH3); 13C{1H} NMR (CDCl3, δ) 141.6, 141.3, 138.6, 138.5 (C6H4), 134.7
(s, CH), 128.8, 126.8 (C6H4), 113.2 (s, )CH2), 39.7, 39.5, 26.2, 25.8, 21.3,
20.3, 18.6, 17.6, 15.5, 13.0 (s, CH2), -3.2, - 5.8 (s, CH3); 29Si{1H} (CDCl3,
δ) 8.38 (4 Si), 12.09 (5 Si); MALDI-TOF (m/z, M + Ag+) calcd 1983,
found 1985. Anal. Calcd for C120H180Si9: C, 76.85; H, 9.67. Found: C,
76.29; H, 9.59. 11: 1H NMR (CDCl3, δ) 7.38, 7.02 (d, 2H, C6H4), 5.77
(m, 8H, CH), 4.84 (m, 16H, dCH2), 2.55 (t, 2H, CH2), 1.54 (m, 18H, CH2),
1.40 (m, 12H, CH2), 0.75 (m, 26H, CH2), -0.02 (s, 15H, CH3), -0.10 (s,
6H, CH3); 13C{1H} NMR (CDCl3, δ) 141.6 (C6H4), 134.8 (CH), 131.3,
130.2, 119.3 (C6H4), 113.1 (s, dCH2), 39.4 (s, CH2), 26.0, 21.8, 18.9, 18.8,
18.5, 18.3, 17.9, 17.6, 17.4, 13.8 (CH2), -5.0, -5.3, -5.7 (CH3); 29Si{1H}
(CDCl3, δ) 1.85 (1 Si), 1.199 (2 Si), 0.43 (4 Si); MALDI-TOF (m/z, M +
Ag+) calcd 1189, found 1188. Anal. Calcd for C58H107Si7: C, 64.45; H,
9.98. Found: C, 64.06; H, 9.68. 12: 1H NMR (CDCl3, δ) 7.42, 7.18 (d,
8H, C6H4), 5.80 (m, 32H, CH), 4.90 (m, 64H, dCH2), 2.62 (t, 8H, CH2),
1.60 (m, 72H, CH2), 1.35 (d, 64H, CH2), 1.05 (m, 8H, CH2), 0.95 (m, 8H,
CH2), 0.62 (m, 96H, CH2), 0.28 (s, 24H, CH3), 0.10 (s, 24H, CH3), -0.05
(s, 48H, CH3), -0.10 (s, 12 H, CH3); 13C{1H} NMR (CDCl3, δ) 143.2,
138.6 (C6H4), 134.8 (CH), 133.9, 128.3 (C6H4), 113.1 (s, dCH2), 21.5,
18.8-17.5 (CH2), 1.0, -2.8, -5.0, -5.7 (CH3); 29Si{1H} (CDCl3, δ) 1.71
(4 Si), 1.19 (8 Si), 0.88 (1 Si), 0.43 (16 Si), -4.11 (4 Si); MALDI-TOF
(m/z, M + Ag+) calcd 4541, found 4545. Anal. Calcd for C252H476Si33: C,
68.27; H, 10.82. Found: C, 67.44; H, 10.32. 14: 1H NMR (CDCl3, δ)
7.47, 7.20 (d, 16H, C6H4), 5.80 (m, 32H, CH), 4.85 (m, 48H, dCH2), 2.60
(t, 16H, CH2), 1.65 (m, 16H, CH2), 1.58 (d, 64H, CH2), 1.40 (m, 32H,
CH2), 0.60 (m, 134H, CH2), 0.05 (s, 24H, CH3), -0.03 (s, 48H, CH3), -0.06
(s, 12H, CH3), -0.10 (s, 24 H, CH3); 13C{1H} NMR (CDCl3, δ) 141.6,
141.3, 138.6, 138.5 (C6H4), 134.7 (CH), 128.8, 126.8 (C6H4), 113.2 ()CH2),
39.7, 39.5, 26.2, 25.8, 21.3, 20.3, 18.6, 17.6, 15.5, 13.0 (CH2), -3.2, -5.8
(CH3); 29Si{1H} (CDCl3, δ) 1.69 (4 Si), 1. 03 (5 Si), 0.29 (24 Si); MALDI-
Scheme 2
14 of 12, C288H516Si33 (M 4905.9; 57% after chromatographic
purification) which remains fluid at ambient temperature. The
rodlike biphenyl spacers introduced by the aryl-aryl cou-
pling step into the skeletons of 10 and 14 may also be
incorporated directly by using 4,4′-dibromobiphenyl as a
precursor to analogues of reagents 1 and 2, prototypes 8 and
TOF (m/z, M + Ag+) calcd 5014, found 5019. Anal. Calcd for C288H516
Si33: C, 70.51; H, 10.60. Found: C, 70.70; H, 10.45.
-
(13) (a) Miller, R. B.; Dugar, S. Organometallics 1984, 3, 1261. (b)
Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
Org. Lett., Vol. 2, No. 11, 2000
1551

Figure 1. Plots of log Mn vs retention time for carbosilanes: (, compound 1 and its [G-1], [G-2] monodendron homologues; 2, compound
8 and its [G-1], [G-2] dendrimer homologues; b compound 10 and its [G-1], [G-2] dendrimer homologues; 9, bromobiphenyl analogue of
bromophenyl monodendron 1 and its [G-1], [G-2] homologues.
9, and monodendron 11, while metalation of the latter using
BuLi (rather than Mg) may be used to join dendritic
carbosilane blocks to other heteroatoms including O, Sn, Hg,
and10b P.
radius, and larger peripheral surface area than those of
carbosilane analogues obtained by divergent growth. Related
accessible target structures include those based on compound
6 in which tetraphenyl, triphenyl, and biphenyl spacers are
linked together through a dendritic skeleton.
Most significantly, the homologues to [G-2] for each
family (including in particular modular components such as
9 and 11) can be obtained as monodisperse, analytically pure
materials: plots of GPC retention volume vs log Mn (Figure
1) are linear even for the core-functionalized [G-0] (i.e., 1),
[G-1], and [G-2] (i.e., 11) series of bromophenyl monoden-
drons. Efficient convergent coupling of the latter, e.g., to
give 12 or 14, lays the foundation for a versatile, reiterative
methodology that can be used to make defect-free assemblies
with increased internal volume, expanded hydrodynamic
Acknowledgment. Funding for this research from the
NSERC, Canada, is gratefully acknowledged. We thank Ms.
A. Barriscale for her help with certain synthetic operations
and J. Roovers (NRC-ICPET, Ottawa) for his assistance with
GPC chromatography.
OL000038G
1552
Org. Lett., Vol. 2, No. 11, 2000