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DOI: 10.1039/B210358N (Paper) New J. Chem., 2003, 27, 178-183

Convergent dendron approach using a phenol triallyl building block: synthesis of a phenol dendron containing 27 allyl termini

Sylvain Nlate *a, Jean-Claude Blais b and Didier Astruc *a
aGroupe Nanosciences Moléculaires et Catalyse, LCOO (UMR CNRS N° 5802), Université Bordeaux I, 351 Cours de la Libération, Talence Cedex, France. E-mail: s.nlate@lcoo.u-bordeaux.fr; d.astruc@lcoo.u-bordeaux.fr
bLaboratoire de Chimie Structurale Organique Biologique (EP CNRS N° 103), Université Paris VI, 4 Place Jussieu, 75252, Paris, France. E-mail: blaisjc@moka.ccr.jussieu.fr

Received (in Montpellier, France) 14th October 2002 , Accepted 25th November 2002

First published on 13th December 2002

Abstract

A successful convergent dendron synthesis is achieved using a phenol triallyl (AB3) building block by a sequence of reactions consisting of hydrosilylation-phenol protection. The key feature is the hydrosilylation using dimethylchloromethylsilane that avoids the side dehydrohalogenation previously found with linear branches containing β-hydrogens. The advantage of the AB3 building block is the multiplication of the dendron branches by three at each generation, which allows the synthesis of the second-generation 27-allyl phenol dendron.

Since Vögtle' seminal report of the first iteration 25 years ago,1 dendrimer syntheses have been achieved using a large variety of strategies and frameworks, and numerous potential application are forecast.2–4 The divergent synthetic method has been the one used most because it is the easiest one to carry out, and also because it is the only one that can lead to large dendrimers even if defects cannot be discarded. The convergent method appeared in the early 90's with the reports of the groups of Frechet,5 Miller6 and Moore7 and has been recently reviewed.8 It is elegant, and its great advantages are that it can lead to pure dendrons that are bifunctional: they can be decorated on the periphery for a given function or property, and they still bear another functional group at the focal point for attachment onto a dendritic core, nanoparticle, polymer, surface, etc. Although dendrons have a limited size because their synthesis cannot be extended due to steric bulk around the focal point that needs be reactive, it is likely that they will play a key role in future nanotechnology involving molecular components. We have already shown that small dendrons bearing ferrocenylsilyl or amidoferrocenyl termini are efficient sensors for the dihydrogenophosphate anion when, as part of thiolate ligands, they are attached to gold nanoparticles.9 So far, Fréchet's dendrons (AB2 units) have been widely used in the literature, and AB3 units are relatively scarce.8 We now report another convergent dendron synthesis based on a phenoltriallyl building block up to the second generation, i.e. 27 allyl groups. This molecular brick has already been used to synthesize dendrimers, but these attempts of convergent syntheses of dendrons have so far been frustrated by side reactions.10 Indeed the synthesis involved the nucleophilic substitution of the iodo group of the iodoalkyl branches by the phenol triallyl, but dehydrohalogenation became dominant. The present strategy prevents this side reaction by removing β-hydrogens on the termini.

Experimental section

Trichloro-trisilicium aryl sulfide 2

A mixture of triallyl sulfide dendron 1 (0.50 g, 1.93 mmol) and SiHMe2(CH2Cl) (0.945 g, 8.706 mmol, 1.1 mL), in Et2O, was stirred for 48 h at ambient temperature. The solution was filtered on Celite, and the solvent was removed under vacuum. After column chromatography on silica gel with a mixture of pentane ∶ Et2O (95 ∶ 5) as eluent, 2 was obtained as a colorless oil (1.020 g, 1.74 mmol, 90%). Elemental analysis calcd for C26H49Si3Cl3S: H 8.45, C 53.44; found: H 8.35, C 53.00%. 1H NMR (CDCl3): δppm 7.21 (s, C6H4, 4H); 2.72 (s, ClCH2, 6H); 2.50 (s, SCH3, 3H); 1.60 (m, CH2, 6H); 1.06 (m, CH2, 6H); 0.57 (m, SiCH2, 6H); 0.05 (s, SiCH3, 18H). 13C NMR (CDCl3): δppm 143.3 (Cq, ArS); 140.0 (Cq, Ar); 127.4 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 30.2 (ClCH2); 17.5 (CH2CH2Si); 15.9 (SCH3); 14.2 (CH2Si); −4.0 (SiMe).

Triiodo-trisilicium aryl sulfide 3

A mixture of 2 (0.500 g, 0.855 mmol) and, NaI (1.77 g, 12.8 mmol), in 2-butanone, was stirred for 24 h at 80[thin space (1/6-em)]°C. After removal of the solvent under vacuum, the residue was extracted with Et2O (50[thin space (1/6-em)]×[thin space (1/6-em)]3 mL). This solution was washed with 3[thin space (1/6-em)]×[thin space (1/6-em)]50 mL of a saturated aqueous solution of Na2S2O3. The solvent was removed under vacuum, which gave 3 as a yellow oil (0.624 g, 0.726 mmol 85%). Elemental analysis calcd for C26H49Si3I3S: H 5.75, C 36.37; found: H 5.55, C 35.82%. 1H NMR (CDCl3): δppm 7.21 (s, C6H4, 4H); 2.50 (s, SCH3, 3H); 1.96 (s, ICH2); 1.60 (m, CH2, 6H); 1.06 (m, CH2, 6H); 0.60 (m, SiCH2, 6H); 0.09 (s, SiCH3, 18H). 13C NMR (CDCl3): δppm 143.1 (Cq, ArS); 140.6 (Cq, Ar); 127.5 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 17.5 (CH2CH2Si); 16.00 (SCH3); 15.7 (CH2Si); −3.39 (SiMe); −13.60 (CH2I).

9-allyl-trisilicium aryl sulfide 5

A mixture of triallylmethylphenol 4 (0.598 g, 2.620 mmol) and K2CO3 (0.413g, 2.990 mmol), in 20 mL DMF, was stirred for 30 min at ambient temperature. The triiodo-trisilicium aryl sulfide 3 (0.500 g, 0.582 mmol) in solution in 5 mL DMF was added, and the reaction mixture was stirred at 80[thin space (1/6-em)]°C 48 h. The solvent was removed under vacuum and the product was extracted with Et2O. The organic layer was washed several times with water in order to remove DMF, then dried over sodium sulfate, filtered and evaporated under vacuum. After column chromatography on silica gel using a mixture of pentane ∶ Et2O (9 ∶ 1) as the eluent, 5 was obtained as a colorless oil (0.410 g, 0.353 mmol, 60%). Elemental analysis calcd for C74H106Si3O3S: H 9.21, C 76.62; found: H 9.23, C 76.52%. MALDI TOF mass spectrum, m/z: 1160.04 [M]+ (calcd 1159.98). 1H NMR (CDCl3): δppm 7.19 (d, C6H4, 10H); 6.84 (d, C6H4, 6H); 5.54 (m, CH2[double bond, length half m-dash]CH, 9H); 5.00 (m, CH2[double bond, length half m-dash]CH, 18H); 3.49 (s, CH2O, 6H); 2.45 (s, SCH3, 3H); 2.43 (d, CH2[double bond, length half m-dash]CH–CH2, 18H); 1.66 (m broad, CH2, 6H); 1.20 (m broad, CH2, 6H); 0.61 (m broad, CH2, 6H); 0.05 (s, SiMe, 18H). 13C NMR (CDCl3): δppm 159.41 (Cq, ArO); 145.00 (Cq, ArS); 137. 25 (Cq, Ar); 134.78 (CH2[double bond, length half m-dash]CH); 134.62 (Cq, Ar); 127.46 (CHAr); 127.09 (CHAr); 117.43 (CH2[double bond, length half m-dash]CH); 114.64 (CHAr); 113.55 (CHAr); 60.15 (CH2O); 43.53 (Cq–CH2); 42.65 (Cq–CH2); 42.01 (CH2); 17.70 (CH2); 16.01 (SCH3); 14.64 (CH2); −4.56 (SiMe).

Trichloro-trisilicium phenol dendron 6

A mixture of triallyl dendron 4 (1 g, 4.380 mmol) and SiHMe2(CH2Cl) (2.279 g, 24.091 mmol, 1.2 mL), in Et2O, was stirred for 48 h at ambient temperature. The solution was filtered on Celite, and the solvent was removed under vacuum. After column chromatography on silica gel with a mixture of pentane ∶ Et2O (95 ∶ 5) as eluent, 6 was obtained as a colorless oil (2.209 g, 3.985 mmol, 91%). Elemental analysis calcd for C25H47Si3Cl3O: H 8.55, C 54.18; found: H 8.35, C 54.62%. 1H NMR (CDCl3): δppm 7.12 (d, C6H4, 2H); 6.76 (d, C6H4, 2H); 4.65 (s, OH, 1H); 2.72 (s, ClCH2, 6H); 1.60 (m, CH2, 6H); 1.06 (m, CH2, 6H); 0.57 (m, SiCH2, 6H); 0.05 (s, SiCH3, 18H). 13C NMR (CDCl3): δppm 155.0 (Cq, ArO); 140.0 (Cq, Ar); 127.5 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 30.5 (ClCH2); 17.5 (CH2CH2Si); 14.5 (CH2Si); −4.1 (SiMe).

Triiodo-trisilicium phenol dendron 7

A mixture of 6 (2.178 g, 3.930 mmol) and, NaI (2.279 g, 58.945 mmol), in 2-butanone, was stirred for 24 h at 80[thin space (1/6-em)]°C. After removal of the solvent under vacuum, the residue was extracted with Et2O (50[thin space (1/6-em)]×[thin space (1/6-em)]3 mL). This solution was washed with 3[thin space (1/6-em)]×[thin space (1/6-em)]50 mL of a saturated aqueous solution of Na2S2O3. The solvent was removed under vacuum, which gave 7 as a yellow oil (3.234 g, 3.903 mmol, 99%). Elemental analysis calcd for C25H47Si3I3O: H 5.72, C 36.24; found: H 5.64, C 37.01%. 1H NMR (CDCl3): δppm 7.12 (d, C6H4, 2H); 6.76 (d, C6H4, 2H); 4.76 (s, OH, 1H); 1.94 (s, ICH2, 6H); 1.60 (m, CH2, 6H); 1.05 (m, CH2, 6H); 0.58 (m, SiCH2, 6H); 0.07 (s, SiCH3, 18H). 13C NMR (CDCl3): δppm 155.1 (Cq, ArO); 140.6 (Cq, Ar); 127.5 (CHAr); 114.8 (CHAr); 43.1 (Cq–CH2); 41.9 (CH2); 17.5 (CH2CH2Si); 15.7 (CH2Si); −2.95 (SiMe); −13.1 (CH2I).

Triiodo-trisilicium phenol propionate 8

13.35 mL (1.776 g, 9.655 mmol) of a fresh solution of C2H5COI, prepared by slowly adding 2.4 mL C2H5COCl to 5 g of Me3SiI and 22.6 mL CH2Cl2, was added to a solution of 7 (2 g, 2.414 mmol) in 30 mL CH2Cl2. Then, NEt2Pri (0.030 g, 0.214 mmol) was also added. The reaction mixture was stirred for 16 h at ambient temperature. After removal of the solvent under vacuum, the residue was extracted with CH2Cl2 (3[thin space (1/6-em)]×[thin space (1/6-em)]40 ml). The organic solution was washed with a saturated aqueous solution of sodium bicarbonate, then with an aqueous solution saturated with Na2S2O3 and dried over sodium sulfate. After removing the solvent under vacuum, the protected phenol dendron 8 was obtained as an orange–yellow oil. Elemental analysis calcd for C28H51Si3I3O: H 5.92, C 36.72; found: H 5.72, C 36.21%. (1.943 g, 2.196 mmol, 91%). 1H NMR (CDCl3): δppm 7.24 (d, C6H4, 2H); 7.03 (d, C6H4, 2H); 2.59 (q, CH3CH2CO, 2H); 1.96 (s, ICH2, 6H); 1.65 (m, CH2, 6H); 1.05 (m, CH2, 6H); 0.60 (m, SiCH2, 6H); 0.08 (s, SiCH3, 18H). 13C NMR (CDCl3): δppm 175.8 (CO); 150.0 (Cq, ArO); 145.1 (Cq, Ar); 127.3 (CHAr); 120.8 (CHAr); 43.6 (Cq–CH2); 41.8 (CH2); 27.6 (CH2); 17.6 (CH2CH2Si); 15.7 (CH2Si); 9.1 (CH3CH2CO); −3.0 (SiMe); −13.1 (CH2I).

9-Allyl-trisilicium phenol dendron 9

A mixture of triallylmethylphenol 9 (0.680 g, 2.978 mmol) and, K2CO3 (0.417 g, 2.978 mmol), in 20 mL DMF, was stirred for 30 min at ambient temperature. The protected triiodo-trisilicium phenol propionate 8 (0.440 g, 0.497 mmol) in solution in 5 mL DMF was added, and the reaction mixture was stirred at 80[thin space (1/6-em)]°C for 48 h. Then 0.360 g K2CO3 and 0.600 mL water were added, and the reaction mixture was stirred at 80[thin space (1/6-em)]°C for 24 h. The solvent was removed under vacuum and the product extracted with Et2O. The organic layer was washed several times with water in order to remove DMF, then dried over sodium sulfate, filtered and evaporated under vacuum. After column chromatography on silica gel using a mixture of pentane ∶ Et2O (9 ∶ 1) as the eluent, 9 was obtained as a colorless oil (0.380 g, 0.336 mmol, 68%). Elemental analysis calcd for C73H104Si3O4: H 9.28, C 77.60; found: H 9.31, C 77.09%. MALDI TOF mass spectrum, m/z: 1151.77 [M[thin space (1/6-em)]+[thin space (1/6-em)]Na]+ (calcd 1152.87). 1H NMR (CDCl3): δppm 7.20 (d, C6H4, 6H); 7.09 (d, C6H4, 2H); 6.87 (d, C6H4, 6H); 6.67 (d, C6H4, 2H); 5.56 (m, CH2[double bond, length half m-dash]CH, 9H); 5.03 (m, CH2[double bond, length half m-dash]CH, 18H); 4.62 (s, OH, 1H); 3.52 (s, CH2O, 6H); 2.43 (d, CH2[double bond, length half m-dash]CH–CH2, 18H); 1.61 (m broad, CH2, 6H); 1.14 (m broad, CH2, 6H); 0.60 (m broad, CH2, 6H); 0.06 (s, SiMe, 18H). 13C NMR (CDCl3): δppm 159.38 (Cq, ArO); 152.84 (Cq, ArO); 139.77 (Cq, Ar); 137.17 (Cq, Ar); 134.74 (CH2[double bond, length half m-dash]CH); 127.51 (CHAr); 127.40 (CHAr); 117.40 (CH2[double bond, length half m-dash]CH); 114.65 (CHAr); 113.52 (CHAr); 60.08 (CH2O); 43.13 (Cq–CH2); 42.60 (Cq–CH2); 41.96 (CH2); 17.63 (CH2); 14.58 (CH2); −4.61 (SiMe).

27-Allyl-9-siliciumphenol dendron 10

A mixture of 9-allyl-3-silicium phenol dendron 9 (0.500 g, 0.442 mmol) and, K2CO3 (0.070 g, 0.500 mmol), in 10 mL DMF, was stirred for 30 min. at ambient temperature. The protected tri-iododendron 8 (0.087 g, 0.098 mmol) in solution in 3 mL DMF was added, and the reaction mixture was stirred at 80[thin space (1/6-em)]°C 72 h. Then, 0.052 g K2CO3 and 0.090 mL water were added, and the reaction mixture was stirred at 80[thin space (1/6-em)]°C for 48 h. The solvent was removed under vacuum and the product was extracted with Et2O. The organic layer was washed several times with water in order to remove DMF, then dried over sodium sulfate, filtered and evaporated under vacuum. After chromatography on silica gel using a mixture of pentane ∶ Et2O (9 ∶ 1) as the eluent, 10 was obtained as a colorless oil (0.169 g, 0.044 mmol, 45%). Elemental analysis calcd for C244H356Si12O13: H 9.36, C 76.43; found: H 9.54, C 75.72. MALDI TOF mass spectrum, m/z: 3856.37 [M[thin space (1/6-em)]+[thin space (1/6-em)]Na]+ (calcd 3857.51). 1H NMR (CDCl3): δppm 7.20 (d, C6H4, 24H); 7.01 (d, C6H4, 2H); 6.88 (d, C6H4, 24H); 6.67 (d, C6H4, 2H); 5.55 (m, CH2[double bond, length half m-dash]CH, 27H); 5.02 (m, CH2[double bond, length half m-dash]CH, 54H); 4.67 (s, OH, 1H); 3.51 (s, CH2O, 6H); 2.43 (d, CH2[double bond, length half m-dash]CH–CH2, 54H); 1.63 (m broad, CH2, 18H); 1.15 (m broad, CH2, 18H); 0.61 (m broad, CH2, 18H); 0.06 (s, SiMe, 54H). 13C NMR (CDCl3): δppm 159.27 (Cq, ArO); 159.00 (Cq, ArO); 139.08 (Cq, Ar); 137.07 (Cq, Ar); 134.63 (CH2[double bond, length half m-dash]CH); 127.30 (CHAr); 127.05 (CHAr); 117.32 (CH2[double bond, length half m-dash]CH); 113.40 (CHAr); 113.31 (CHAr); 60.02 (CH2O); 42.92 (Cq–CH2); 42.49 (Cq–CH2); 41.86 (CH2); 17.54 (CH2) 14.43 (CH2); −4.69 (SiMe).

Results and discussion

Dendritic growth has been carried out in parallel using the arylsulfide triallyl 1 and the phenol triallyl 4 that is an AB3 unit for dendritic or hyperbranched polymer construction. The model 1 serves to test the divergent reaction of the allyl termini since it does not need protection. The compounds 1 and 4 have been reported, and are accessible by perallylation of the CpFe+ complexes of the aryl ether and aryl sulfide respectively10b,11 (Scheme 1).
scheme, filename = b210358n-s1.gif
Scheme 1

The growing strategy is based on the hydrosilylation of the allyl branches using chloromethyldimethylsilane that has already been successfully used by Seyferth for dendrimer synthesis.12 The chloromethyl termini are transformed into iodomethyl by reaction with sodium iodide in refluxing butanone, then nucleophilic substitution by phenol triallyl leads to the next generation. With 1, these reactions can be carried out in a straightforward way without any protection (Scheme 2).


scheme, filename = b210358n-s2.gif
Scheme 2

With the phenol triallyl brick, it is of course necessary to protect the phenol group during the dendron construction, i.e. this synthesis is now a real convergent strategy for the construction of a dendron that is ultimately deprotected either to react on a protected dendron or onto another framework. Thus, after introduction of the iodo groups on the branches, the phenol function is protected by the propionate before synthesis of the first generation nona-allyl dendron (Scheme 3). The protection allows the condensation of the phenol triallyl AB3 unit, and later, of the phenol nona-allyl AB9 first-generation dendron. This convergent synthesis involving protection–deprotection has been carried out until the synthesis of the 27-allyl phenol that has been obtained in its deprotected form, i.e. as the second-generation dendron (Scheme 4). The first generation 9-allyl and second generation 27-allyl dendrons have been characterized by their standard spectroscopic and analytical data, but also especially by the prominent molecular peak in the MALDI TOF mass spectrum. In these mass spectra (see Fig. 1), impurities due to unfinished reactions are detected but are present only in relatively small amounts. The side peak at 3814.32 Dalton that is close to the molecular peak results from a small impurity corresponding to the phenoltriallyl AB3 unit 4 in which one allyl group is missing (unfinished synthesis during the CpFe+-induced allylation reaction). The second side peak at 3557.27 Dalton corresponds to a dendron in which the ninth tripod is missing (again uncompleted introduction of the phenoltriallyl group 4). The third side peak at 3515.36 Dalton corresponds to the combination of both impurities (unfinished triallylation and unfinished introduction of the phenoltriallyl group 4). The peak at 1277.77 Dalton corresponds to exactly one third of the molecular peak, and is thus attributed to this molecular peak with a tricationic charge (m/z with z[thin space (1/6-em)]=[thin space (1/6-em)]3).


scheme, filename = b210358n-s3.gif
Scheme 3

scheme, filename = b210358n-s4.gif
Scheme 4

MALDI-TOF mass spectrum of the 27-allyl dendron 10 (molecular peaks [M + Na]+ at 3856.37).
Fig. 1 MALDI-TOF mass spectrum of the 27-allyl dendron 10 (molecular peaks [M[thin space (1/6-em)]+[thin space (1/6-em)]Na]+ at 3856.37).

This successful synthesis of the 27-allyl dendron phenol contrasts with the preceding attempt using a triiodoalkyl phenol bearing hydrogens in the position β relative to the iodo group. In that case, nucleophilic substitution proceeded correctly for two of the branches because it was faster than dehydrohalogenation. A rather pure 19-allyl phenol dendron was obtained (as shown in Scheme 5), however, the third nucleophilic substitution was considerably slowed down due to the steric bulk of the two other nearby covalently attached dendrons. Thus, dehydrohalogenation of the third iodoalkyl branch proceeded quantitatively (Scheme 5). With the new triiodoalkyl dendron, the lack of β positions inhibits the dehydrohalogenation reaction.


scheme, filename = b210358n-s5.gif
Scheme 5

Conclusion

The arylsulfide 1 and phenol derivative 4 are easily available in relatively good yields in only three steps from p-chlorotoluene via CpFe+-mediated synthesis. Thus, the chemistry reported here is easily carried out and should prove useful. The arylsulfide is a good model for the convergent synthesis from 4. It is also a good model for the synthesis of aryl thiol dendrons that could bind gold nanoparticles and surfaces. The convergent synthesis of the 27-allyl phenol dendron from 4 was successful and proceeds in relatively good yields. Its advantage is that it multiplies the number of branches by three at each generation, allowing a very rapid growth up to 27 allyl branches in the second-generation dendron.

Acknowledgements

We are grateful to the Institut Universitaire de France (IUF), the Centre National de la Recherche Scientifique (CNRS) and the Universities Bordeaux I and Paris VI for financial support.

References

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