AN ALTERNATIVE SYNTHETIC APPROACH TOWARD DENDRITIC MACROMOLECULES : NOVEL BENZENE-CORE DENDRIMERS VIA ALKYNE CYCLOTRIMERIZATION

We report an alternative method for the convergent1 synthesis of dendrimers in which the dendrimer core is generated from a dendritic precursor by an alkyne cyclotrimerization reaction.2 In this method, convergent dendrons1,3 attached to an acetylenic moiety are cyclized in a [2 + 2 + 2] cycloaddition process. The cycloaddition can be mediated by various transition-metal complexes and exhibits a high degree of chemoselectivity toward triple bonds, rendering the reaction tolerant of many functional groups.2 If carried out with a difunctionalized dendritic alkyne, this reaction affords a benzene moiety surrounded by six dendrons as depicted in Scheme 1. A related approach was used by Duchene and Vogtle4 for the synthesis of a dodecafunctionalized hostmolecule. An acetylenic system consisting of benzylic ethers of 2-butyne1,4-diol was chosen for commercial availability as well as its synthetic compatibility. In particular, this linkage prevents the occurrence of a Claisen rearrangement.5 The substituted alkynes 1a-d were obtained by the Williamson ether coupling of 2-butyne-1,4-diol with the appropriate polybenzyl ether-type1 dendritic bromide (Scheme 2). The trimerization reaction of 1a-d was carried out in refluxing toluene using dicobalt octacarbonyl6 as the catalyst to afford the novel structures 2a-d. To our knowledge, this represents the first time that such large and precisely defined macromolecules (MW ≈ 10 000 for the third generation) have been successfully prepared by a cyclotrimerization reaction. As expected, the time required to complete the trimerization reaction increased with generation (Table 1), while the yield decreased as a result of steric crowding around the nascent core. Because of the nature of the transformation, the reaction is extremely clean, and no partially reacted products can be formed. Therefore, aside from recovered starting materials, compounds 2a-d were the only products isolated after reaction. As a result, their purification by column chromatography was greatly facilitated. The catalyst, dicobalt octacarbonyl, applied in amounts of 5 mol % or less, is fairly robust and easy to handle since it operates in a variety of different solvents.7 Benzene-core dendrimers 2a-d have been fully characterized by a variety of spectroscopic techniques. Both the MALDI-TOF mass spectra and the size-exclusion chromatography traces (Figure 1) of the dendrimers confirm their monodispersity and high purity. To probe their solution dynamics, NMR relaxation time (T1) measurements were performed. Herein, a correlation of the relaxation behavior of the spectroscopic probe, i.e., the proton, with its local environment can be used to gain information about the relative density distribution within the macromolecule.8 Because of the high spectral resolution allowing clear observation of the different layers (Scheme 3) of the structure, this approach was used to gain information about the entire dendrimer framework. As shown in Figure 2, the T1 values for the terminal benzylic protons (e) are almost constant while a slight decrease in T1 is observed for the exterior phenyl protons (f). This suggests that there is no change in steric congestion at the periphery of the dendrimer as the generation increases. The relaxation times for the successive layers within the dendrimer decrease from the core to the periphery, suggesting a radial increase in density of the macromolecule. This finding fits the simplified model of de (1) (a) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (b) Frechet, J. M. J.; Jiang, Y.; Hawker, C. J.; Philippides, A. E. Proc. IUPAC Int. Symp., Macromol. 1989, 19. (2) For recent reviews on transition-metal-mediated [2 + 2 + 2]cycloadditions, see: (a) Fruhauf, H.-W. Chem. ReV. 1997, 97, 523. (b) Lautens, M.; Klute, W.; Tam, W. Chem. ReV. 1996, 96, 49. (c) Grotjahn, D. B. In ComprehensiVe Organometallic Chemistry II; Abel, E. A., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 12, pp 741-770. (d) Melikyan, G. G.; Nicholas, K. M. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, 1995; pp 99-138. (e) Schore, N. E. In ComprehensiVe Organic Synthesis; Trost, B. M., Flemming, I., Eds.; Pergamon: Oxford, 1991; Vol. 5, pp 1129-1162. (3) (a) Hawker, C. J.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 1990, 1010. (b) Miller, T. M.; Neenan, T. X. Chem. Mater. 1990, 2, 346. (c) Wooley, K. L.; Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4252. (d) Kwock, E. W.; Miller, T. M.; Neenan, T. X. Chem. Mater. 1991, 3, 775. (e) Leon, J. W.; Kawa, M.; Frechet, J. M. J. J. Am. Chem. Soc. 1996, 118, 8847. (f) Jayaraman, M.; Frechet, J. M. J. J. Am. Chem. Soc. 1998, 120, 12996-12997. (g) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Dendritic Molecules: Concepts, Synthesis, PerspectiVes; VCH: Weinheim, 1996; Chapter 5 and references therein. (4) Duchene, K. H.; Vogtle, F. Synthesis 1986, 659. (5) Olsman, H.; Graveland, A.; Arens, J. F. Recl. TraV. Chim. Pays-Bas 1964, 83, 301. (6) (a) Kaufman, R. J.; Sidhu, R. S. J. Org. Chem. 1982, 47, 4941. (b) Hubel, W.; Hoogzand, C. Chem. Ber. 1960, 93, 103. (7) A cobalt-catalyzed cyclotrimerization has recently been performed in aqueous solution: Sigman, M. S.; Fatland, A. W.; Eaton, B. E. J. Am. Chem. Soc. 1998, 120, 5130. (8) (a) Tomoyose, Y.; Jiang, D.-L.; Jin, R.-H.; Aida, T.; Yamashita, T.; Horie, K. Macromolecules 1996, 29, 5236. (b) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (c) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895. Scheme 1