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Solid-phase organik syntheses - Burdges K.

Burdges K. Solid-phase organik syntheses - John Wiley & Sons, 2000. - 283 p.
ISBN 0-471-22824-9
Download (direct link): phaseorganicsynthesis2000.pdf
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Solid-phase methods have also been used to generate similar para-substituted phenylacetylene oligomers.13 In these syntheses, complete generation of the target hexadecamer was shown to be viable for both solution-and solid-phase methods. However, the solid-phase method generated the target in higher yield (79% vs. 24% overall by solution phase), and the purification involved only one chromatographic step.
The potential of solid-phase methods for meta-substituted phenylacetylene oligomers was further investigated utilizing monomers with four different side chains (Figure 4.6).15 Monomers A and were synthesized on and cleaved from the support. As seen in Table 4.2, data for the brominated and trimethylsilylacetylated versions of the monomers demonstrate that the coupling and cleavage reactions proceed in high yield and
R
A 0-(n-C6H13)
C02-(n-C6Hl3)
'Bu D CN
for phenylacetylene monomers.
R
Figure 4.6. Side-chain variations
4.4. ILLUSTRATIVE APPLICATIONS 131
TABLE 4.2. Yields and Purities of Monomers A and and Heterooligomers A-D Formed on a Solid Phase
R
A 0-(n-C6H13)
C02-(n-C6Hi3)
ClBu
DCN
Monomer Yield (%) Purity0 (%)
I-A-Br 98 98
I-A-TMS 94 97
I-B-Br 90 95
I-B-TMS 98 94
^Monitored by HPLC analyses.
purity. Heterooligomers were also synthesized via a stepwise addition to the solid support giving dimers and hexamers in moderate yield and high purity by high-performance liquid chromatography (HPLC; Table 4.3).
Another method for preparation of higher oligomers takes advantage of site isolation on a solid phase.6 It proceeds via alternate addition of a pair of symmetrical difunctional monomers in a stepwise addition obviating the need for deprotection steps (Scheme 6). The sequence is extended one unit at a time, unlike solution approaches that tend to involve growth from both ends. This method has not yet been fully investigated, so it is difficult to speculate about its effectiveness or to foresee potential problems.
TABLE 4.3. Oligomers of Monomers A-C
Oligomer Yield (%) Purity (%)
I-A-A-TMS 84 95
I-A-B-TMS 88 94
I-B-A-TMS 76 91
I-B-B-TMS 75 96
I-B3-A3-TMS 48 92
I-(D-C)3-TMS 58 98
132
SOLID-PHASE SYNTHESIS OF SEQUENCE-SPECIFIC PHENYLACETYLENE OLIGOMERS
R .A, Qsr
Scheme 6.
Br
Br
*Bu
lBU
Bu
couple
cleave
Scheme 7.
4.4. ILLUSTRATIVE APPLICATIONS 133
TABLE 4.4. Solution- and Solid-Phase Synthesis of Dendrimers
Generation Dendrimer Solution-Phase Solid-Phase Yield
Yield (%) (%)
0 I-M3-(tBu)4 88 85
1 -7-0)8 82 80
2 I_M15-(lBu)16 84 78
3 -31-(1)32 81 77
4 I-M63-(lBu)64 85 68
4.4.2. Dendrimers
Branched phenylacetylene monomers can be used to construct dendrimers on supports. This was accomplished using a triazene tether as a focal point, the reaction triad outlined above, and an AB2 monomer, as seen in Scheme 7.16 As in similar solution-phase convergent dendrimer syntheses, the first step was to prime the periphery of the dendrimer with r-butyl groups by di-coupling of a triazene-tethered dibromoaryl monomer with (di-^-butyl-phenyl)acetylene. In this and all subsequent coupling steps, an excess amount of the monodendron acetylene was used to drive the reaction to completion. The reagents and catalysts were washed away and the excess monodendron was recovered. At the end of the first step, the tri-aryl dendron I-M3-(Y-Bu)4 was cleaved from the support (M represents the dendritic monomer unit). Two equivalents of this product were then coupled with the triazene-tethered di-acetylene aryl monomer to produce the heptaaryl dendron I-M7-(Y-Bu)8.
The process shown above was repeated for up to four cycles and the data obtained for these syntheses are shown in Table 4.4. A significant amount of monocoupling occurred during the fourth generation, and all attempts to drive this reaction to completion failed. This was likely due to congestion of the growing dendrimer within the resin cavities. Consistent with this, the fourth-generation dendrimer was obtained when a lower resin loading was used. Overall, dendrons up to the third generation can be made in gram quantities. Yields were slightly lower than for comparable solution-phase approaches, but purification was much easier.
4.4.3. Hyperbranched Polymers
A hyperbranched polymerization from solid support was developed as a one-pot synthesis for branched polymers that mimic dendrimers (Scheme
SOLID-PHASE SYNTHESIS OF SEQUENCE-SPECIFIC PHENYLACETYLENE OLIGOMERS
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