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Dendrimers Arborols and Cascade Molecules Breakthrough into Generations of New Materials.

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Dendrimers, Arborols, and Cascade Molecules :
Breakthrough into Generations of New Materials
By Hans-Bernhard Mekelburger, Wilfried Jaworek, and Fritz Vogtle*
Over a decade ago dendritic (Greek: branched) molecules
began to attract ever increasing attention in organic,
supramolecular, and polymer chemistry, and recently also in
coordination chemistry. The initial impetus can be traced to
our cascadelike synthesis of noncyclic, branched polyamines
in 1978 (Scheme I).[’] Thereafter the research groups of
1
2
‘CN
NC)
4
3
Scheme 1 . The first cascadelike synthesis of branched polyamines [l]
Denkewalter, Newkome, Tomalia, and others expanded this
theme considerably (Scheme 2).12] The extremely branched
molecules are mainly synthesized from identical building
blocks that contain branching sites, and often have a variety
of functional groups on the periphery. These dendrimerst3I
are constructed in stages in repeatable synthesis steps (repetitive synthesis strategy“]). Each reaction cycle creates a new
“generation”. In the schemes we have highlighted the first,
third, and fifth generation in red; the “core” (K) that is
usually in the center of the molecule and the even generations
are not highlighted (see in particular 27 in Scheme 6). The
divergent synthesis method (construction, as it were, from
the center outwards) is distinguished from the convergent
method in which larger fragments are built up first and then
coupled to the core; both methods have their strengths and
their weaknesses.[41
[*I
Prof. Dr. F. Vogtle. H.-B. Mekelburger
Institut fur Orgdnische Chemie und Biochemie der Universitdt
Gerhard-Domagk-Strasse 1 , D-W-5300 Bonn 1 (FRG)
Dr. W. Jaworek
Textar GmbH Grnndsatzentwicklung/EPN
A n g e u . C h r m I n f . Ed. Engl. 1992, 31, N o . 12
0 VCH
Scheme 2. Three examples of dendrimers [2]: polylysine 6 (Denkewalter et at.),
arbor017 (Newkome et al.), and polyamidoamine 8 (Tomalia et aL).
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1) SOCI,
*
;HO
$$-F
O
3) HCOOH
OH
14
DCCl
2) HCOOH
HO
‘OH
‘OH
1) SWr2
OCHZPh
OCHZPh
Scheme 4. Preparation of the dendrimer 17 containing 36 carboxy groups
2) U A ,
OCH2Ph
3) H2. W / C
Scheme 3. Divergent synthesis of arborols with an exclusively hydrocarbon
skeleton [7].
1572
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The pronounced branching provides the higher generations with an increasingly three-dimensional structure that is
characterized by a growing number of cavities within the
molecule. One aim in the design of new dendrimers is therefore the formation of niches tailormade for specific guests in
order to generate host/guest relationships and aggregates.
The dendrimers became even more attractive after it was
shown that their surfaces displayed fractal (self-imaging) features. This contribution explores the most important developments in this field.
Newkome et al. reported the synthesis of a r b ~ r o l s [with
~]
saturated hydrocarbon skeletons. They have the characteristics of globular micelles (Scheme 3).”’ Treatment of the core
unit 9 with the anion of the alkyne 10 yielded the dodecahydroxy compound 11 after cleavage of the protecting groups
and hydrogenation of the triple bond. After bromination
with thionyl bromide the reaction sequence was repeated.
Like its lower analogue 11, the 36-fold alcohol 12 is very
soluble in alcohol, but only slightly soluble in chloroform or
water. Finally this second generation polyalcohol was oxidized with ruthenium tetroxide to form hexatriacontacarboxylic acid 13, from which the water-soluble ammonium
and tetramethylammonium derivatives were made. Both
these dendrimers accept nonpolar molecules (for example,
diphenylhexatriene, chlorotetracycline, or pinacyanol chloride) as guests in the niches within their lipophilic framework. They do not aggregate and can therefore be considered “unimolecular micelles”.[sl
0570-0833192jr2f2-f5?2S 3.50+ ,2510
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Newkome also approached the preparation of micellar
dendrimers with a hydrophilic surface and a compact
lipophilic interior from the core building block adamantanetetracarboxylic acid (14, Scheme 4), lengthening the
chains through the amide bonds.[g1Each amine molecule 15
has three tevt-butyl ester groups. Their hydrolysis yielded the
dodecacarboxylic acid 16, which was again treated with the
amine 15. Another hydrolysis afforded the water-soluble second generation dendrimer 17 containing 36 carboxyl groups
at its periphery.
Recently the same author reported on poly(amidoa1coho1)s with two spherical head groups joined to a central C-C
triple bond through alkyl chains." O1 These dumbbell-shaped
arborols form rodlike structures constructed by helical or
scissorlike stacking, which can in turn cluster together to
form aggregates of higher order. This property is reflected in
the macroscopic tendency to form thermally reversible
aqueous gels.
Frechet et
employed the convergent method to synthesize dendritic polyethers up to the sixth generation. Bromide 18, which is at the periphery of the dendrimer and
contains two benzyl ether units, was treated with 3,5-dihydroxybenzyl alcohol 19, a monomeric building block
(Scheme 5). On reaction with tetrabromomethane and
triphenylphosphane in excess, another bromide (20) was
formed that could be extended with the monomeric building
block 19 to give the second generation. At the end of the
synthesis the reaction of the wedge-shaped bromides (e.g. 20.
21, 22) with the trifunctional core unit afforded spherical
polyethers such as 24. In a variation of this synthesis, even
unsymmetrically substituted polyethers could be obtained.'"' The attraction of the convergent method is the
small number of molecules that are involved in the reaction
steps to give each successive generation. The large excesses of
reagents can be avoided, which results in good yields. However, for higher generations the yields decrease because of
increasing steric hindrance at the reacting functional groups.
The use of more flexible monomers (e.g. 25) should therefore
provide better yields. After convergent synthesis and cleavage of the benzylic protecting groups, Frechet et al. obtained
the sixfold phenol 26 (Scheme 6) that was used as extended
core for a divergent procedure in a reaction with the bromide
of the third generation (22) to give the spherical dendrimer
(27).["]
Miller, Neenan et al.[l3] also used the convergent method
to prepare aromatic hydrocarbon dendrimers. Aryl boronic
+
18
3
22
I
Ph-0.
HO
')
HO%OH
t
2) CBr+ PPh3
Ph
HO
')
HO%OH
I
2) C B r 4 PPhJ
Ph
Scheme 5. Convergent synthesis of polyether dendrimers [4].
A n g m Clipm Int Ed Engl 1992, 31. N o 12
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Scheme 6. Convergent-divergent synthesis of larger polyethers through incorporation of more flexible building blocks [12].
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acids were coupled to the building block 3,5-dibromo-ltrimethylsilylbenzene in a Suzuki reaction, and the
aryltrimethylsilane was subsequently transformed into an
aryl boronic acid again. This procedure yielded poly( 1,3,5phenylene)s after the final coupling with the core molecule
1,3,5-tribromobenzene. The hydrocarbon of the third generation, containing 46 benzene rings, was obtained in a combined convergent-divergent synthesis. These dendrimers,
with the exception of the sparingly soluble first generation,
dissolve in THF, toluene, and chloroform and are thermally
stable.
Only a short while ago Moore et al.
succeeded in
preparing particularly stiff dendrimers constructed from
phenylacetylene building blocks. Using the palladium-catalyzed Heck reaction in a convergent synthesis, they obtained
the largest pure hydrocarbon yet isolated (molecular mass
over 14 kDa). The incorporation of numerous tert-butyl
groups on the periphery of the molecule ensured an excellent
solubility in organic solvents.
Masurnune et aL1’51 reported the synthesis of silicon dendrimers[l6] up to the third generation. The repetitive sequence consists of two steps: the terminal Si-H groups are
first transformed catalytically into Si-OH groups, and then
the new generation is produced in a reaction with chlorosilane building blocks. The surface of these dendrimers is
therefore covered with many Si-H groups. Their facile transformations make other functional groups readily accessible,
which enables the modulation of the physical properties of
these dendrimers.
Dendrilners having charges within the cascade structure
were described by Engel et al. These polyphosphonium~”]
and polyammon,um salts[i 81 contain tris@-methoxymethylphenyl)phosphane and triethanolamine as monomeric building blocks, respectively. Whereas the ammonium dendrimers
are soluble in alcohols and particularly in water, the phosphonium dendrimers display somewhat different behaviour.
To the second generation they are soluble in organic solvents
and slightly soluble in water; however, from the third
generation the solubility is very limited.
Recently Shinkai et al.“ 91 reported the first arborols incorporating crown ethers. After the divergent method failed to
achieve the desired goal, a dendrimer of the second generation was synthesized stepwise in a convergent process
through amide bonds. Apart from aromatic building blocks
it contains nine diaza[l8]crown-6 units. The oily substance
should be able to bind alkali metal ions selectively, but this
has not yet been demonstrated.
Polynuclear transition metal complexes of a dendritic nature were first described by Balzani et al.;r201their building
blocks are not linked exclusively by covalent bonds but also
by metal-hgand bonds (Scheme 7). In a divergent synthesis
strategy ruthenium(r1) ions form complexes with ligands of
the 2,3-bis(2-pyridyI)pyrazine type and are therefore deliberately placed a definite distance apart. The complex 28 containing 22 metal centers can be oxidized and shows luminescence effects.
We prepared dendrimers with large monomeric units in a
divergent synthesis (Scheme 8).12 Readily soluble dendrimers were formed up to the third generation (29). This
general synthesis can be extended to other core units and to
higher generations. Further synthetic goals are “functional
Aiigew. Cliem. h i . Ed. Engl. 1992, 31. N o . 12
I
Scheme 7. Dendritic metal complex 28 containing 22 ruthenium ions [20].
dendrimers” with properties characteristic, for example, of
dyes, COmpleXing agents for metals, or liquid crystals. For
instance, a dendrimer containing six azobenzene groups undergoes reversible switching processes on irradiation with
light Of a
wave1ength.‘2*1
Scheme 8. Aromatic-aliphatic dendrimer 29 containing 24 carboxy groups
[21].
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All these examples from the past two years show that
dendrimers (or cascade molecules) are a topical field of research 1231 because their hierarchical architecture promises
interesting properties and applications (the micelles are only
one example). Nevertheless, the properties of strongly
branched molecules are largely unknown and await discovery. In this connection the chemical reactions between multifunctional dendrimers, for example, may be mentioned, as
well as dendrimers with many chiral elements, which might
display chiroselective recognition.
Above all, the synthesis of large, branched molecules bordering on polymers challenges synthetic and analytical
chemists alike. A large number of reaction centers or considerable steric congestion hinders the synthesis of
monodisperse molecules. The classical methods of analysis
in organic chemistry are sometimes pushed to their limits.
NMR spectra and elemental analyses are becoming increasingly less informative, and mass spectrometry of very heavy
molecules also is not without problems. Likewise the nomenclature of cascade molecules according to the common rules
has its difficulties. The names become extremely long and the
fundamental structure of the molecule is not immediately
clear from them. And then the effort of naming a dendrimer
of the second generation is out of all proportion. Newkome
et al.[241therefore recently proposed a new system that reflects the structure in the sense that it names the molecule
from the core towards the periphery.[25]Furthermore, the
class of compound becomes clear, since the names begin with
“Z-Cascade”, where Z is the number of functional groups on
the periphery.
In conclusion, these dendritic compounds cross the
boundaries of classical organic chemistry and as new materials will penetrate deeper into the topical fields of “nanostructures”, supermolecules, and polymers in the future. Even
now the dendrimers form a bridge to “soft materials”, as de
Gennes called polymers, tensides, liquid crystals, and colloids in his Nobel
German version: Angew. Chem. 1992, 104, 1609
[l] E. Buhleier. W Wehner, F. Vogtle, Synthesis 1978. 155-158; F. Vogtle, E.
Weber, Angew. Chem. 1979, Yl, 813-837; Angew. Chem. I n [ . Ed. Engl.
1979, 18. 753; since then the reduction method has been improved: R.
Moors. F. Vogtle. unpublished.
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[2] Review with many related references: D. A. Tomalia, A. M. Naylor, W. A.
Goddard 111, Angew. Chem. 1990, 102, 119-157; Angew. Chem. Inl. Ed.
Engl. 1990,29, 138-175.
[ 3 ] The term “starburst dendrimers” was favored by D. A. Tomalia. The first
word refers to the explosive propagation of the “arms” of the molecule,
and the second emphasizes the dendritic (branched). oligomeric nature of
this species.
141 C. J. Hawker, J. M. J. Frechet, J. Am. Chem. Sor. 1990, 112, 7638-7647;
see also I. Gitsov, K. L. Wooley, J. M. J. Frechet, Angew. Chem. 1992,104,
1282-1285; Angew. Chem. Int. Ed. Engl. 1992, 31, 1200-1202.
[5] D. Farin, D. Avnir, Angew. Chem. 1991, 103, 1409-1410; Angetv. Chem.
I n [ . Ed. Engl. 1991, 30, 1379.
[6] The name arborol, coined by G. R. Newkome, is a composite term derived
from “arbor” (tree) and “alcohol”.
[7] G. R. Newkome, C. N. Moorefield, G. R. Baker, A. L. Johnson, R. K.
Behera, Angew. Chem. 1991,103,1205-1207; Angen. Chem. I n t . Ed. Engl.
1991, 30, 1176-1178.
[S] G. R. Newkome, C. N. Moorefield, G. R. Baker, M. J. Saunders, S . H.
Grossman, Angew. Chem. 1991, 103, 1207-1209; Angen. Chem. Inr. Ed.
Engl. 1991, 30, 1178-1180.
[9] G. R. Newkome, A. Nayak, R. K. Behera. C. N. Moorefield, G. R. Baker,
J. Org. Chem. 1992,57, 358-362.
1101 G. R. Newkome, C. N. Moorefield, G. R. Baker, R. K. Behera, G. H.
Escamillia, M. J. Saunders, Angew. Chem. 1992, 104, 901 -903; Angew,
Chem. I n t . Ed. Engi. 1992, 31, 917.
1111 K. L. Wooley, C. J. Hawker, J. M. J. Frkchet, J. Chem. SOC.Perkin Trans.
11991, 1059-1076.
[12] K. L. Wooley, C. J. Hawker, J. M. J. Frechet, J. Am. Chem. Sor. 1991, 113,
4252-4261.
1131 T. M. Miller. T. X . Neenan, R. Zayas. H. E. Blair, J. Am. Chem. Soc. 1992,
114, 1018-1025.
1141 Z. Xu, J. S. Moore, Angew. Chem. 1993, 105, in press; Angew. Chrm. Int.
Ed. Engl. 1993, 32, in press.
[15] H. Uchida, Y. Kabe, K. Yoshino, A. Kawamata, T. Tsumuraya, S. Masamune, J. Am. Chem. SOC.1990, 112, 7077-7079.
(161 Another synthesis of silicon dendrimers by polymerization: L. J. Mathias,
T. W. Carothers, J. Am. Chem. SOC.1991, 113, 4043-4044.
[17] K. Rengan, R. Engel, L Chem. SOC.Perkin Truns. 1 1991, 987-990.
(181 K. Rengan, R. Engel, J. Chem. SOC.Chem. Commun. 1992, 757-758.
1191 T. Nagasaki, M. Ukon, S. Arimori, S. Shinkai, J. Chem. Soc. Chem. Commun. 1992, 608-610.
[201 S. Serroni, G. Denti, S. Campagna, A. Juris, M. Ciano, V. Balzani, Angew.
Chem. 1992,104,1540-1542; Angew. Chem. Int. Ed. Engl. 1992,31,1493-1495.
1211 H:B. Mekelburger, F. Vogtle, Suprumoiecuiur Chemistry, in press.
1221 H.-B. Mekelburger, F. Vogtle, unpublished.
(231 Other syntheses of dendrimers: a ) A. Higuchi, H. Indda. T. Kobata, Y.
Shirota. Adv. Muter. 1991, 3, 549-550; E. C. Constable, A . M . W. C.
Thompson, J. Chem. SOC.Chem. Commun. 1992,617-619; F. Moulines, B.
Gloaguen, D. Astruc, Angew. Chem. 1992, 104.452-454: Angew. Chem.
I n t . Ed. Engl. 1992, 31, 458; Y H. Kim, J. Am. Chem. SOC.1992. 114,
4947-4948; A. W. vdn der Made, P. W N. M. van Leeuwen, J. Chem. SOC.
Chem. Commun. 1992, 1400-1401.
[241 G. R. Newkome, G. R. Baker, J. K. Young, personal communication.
[251 Cf. the related nomenclature of open-chain oligoethers (podands): E.
Weber, F. Vogtle, Inorg. Chim. Acra 1980, 45, L65-L67.
[261 P.-G. de Gennes, Angew Chem. 1992, 104, 856-859; Angetv. Chem. Int.
Ed. Engl. 1992. 3/, 842
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