triaminopyrimidine (2) for Rh(I)-catalyzed orthoalkyl and
orthoalkenylation,7 in which a rhodium complex can be
immobilized as an insoluble solid after the reaction and thus
separated from products for repeated use.
Scheme 2. Synthesis of 1a
The barbiturate (BA) and 2,4,6-triaminopyrimidine (TP)
motifs are known to form large and stable supramolecular
assemblies through six hydrogen bondings per molecule.8
There may be two possible modes of aggregation, an infinite
tape and a discrete rosette-like form. Thus, we anticipated
that the introduction of a phosphinyl group on BA should
lead to supramolecular assemblies (Scheme 1), which can
a Reagents and conditions: (i) NaH, DMF, 60 °C 8 h; (ii) excess
urea and NaH, DMF, 100 °C 24 h; (iii) Cl3SiH, PhSiH3, 60 °C, 4 h.
of acetophenone with tert-butylethylene (4a) was carried out
at 150 °C for 2 h in 1,4-dioxane (Scheme 3).
Scheme 1. Barbiturate-Substituted Phosphine Ligand (1) and
Its Possible Interaction with Triaminopyrimidine (2)
Scheme 3. Orthoalkylation Using the SAS Catalytic System
During the reaction, the solution was homogeneous
because a hydrogen-bonding network cannot be established
at the high reaction temperature (Figure 1a,b), but a pale
yellow solid precipitate was formed at room temperature
upon addition of n-pentane after the reaction (Figure 1c).9
The formation of the solid precipitate can be attributed to
the hydrogen-bonded self-assembly of 1 and 2, including the
Rh(I) metal with phosphorus coordination of 1, not to a
simple solidified mixture. This speculation is clearly sup-
ported by comparing preliminary DSC traces between each
component and their mixture having the same composition
in the catalytic reaction (Figure 2).
The DSC trace of the mixture of 1 and 2 exhibited a new
endothermic signal (Figure 2c) without showing the identical
endothermic signals of each component (Figure 2a,b).10 In
the case of the Rh(I)-containing mixture of 1, 2, and 5, we
could not observe any significant endothermic peak up to
240 °C (Figure 2d). Considering the theoretical coordination
number of square planar Rh(I) (CN ) 4), the structure of
the mixture of 1, 2, and 5 might be a highly cross-linked
polymeric one in which the hydrogen-bonded linear polymers
serve dual roles as a catalyst support and a ligand for the
transition metals.
The ligand 1 was prepared successfully in several steps:
a nucleophilic addition of decyl diethylmalonate on the
4-benzylbromide group of triphenylphosphine oxide, fol-
lowed by barbiturate formation with urea, and subsequent
deoxygenation of phosphine oxide (Scheme 2).
(8) (a) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspec-
tiVes; Verlag Chemie: Weinheim, Germany, 1995; Chapter 9, p 139. (b)
Lehn, J. M.; Mascal, M.; Decian, A.; Fischer, J. J. Chem. Soc., Chem.
Commun. 1990, 479.
(9) For selective precipitation of the homogeneous catalyst using PEG
support, see: (a) Yao, Q. Angew. Chem., Int. Ed. 2000, 39, 3896. For
autoprecipitation of the catalyst after the reaction, see: (b) Dioumaev, V.
K.; Bullock, R. M. Nature 2003, 424, 530. For the self-supported
heterogeneous catalysts, see: (c) Bianchini, C.; Farnetti, E.; Graziani, M.;
Kaspar, J.; Vizza, F. J. Am. Chem. Soc. 1993, 115, 1753. (d) Dorta, R.;
Shimon, L.; Milstein, D. J. Organomet. Chem. 2004, 689, 751. (e) Wang,
X.; Ding, K. J. Am. Chem. Soc. 2004, 126, 10524.
Using the SAS catalytic system of barbiturate 1 bearing a
phosphinyl group and pyrimidine 2 in the presence of 5 mol
% of 5 as a catalyst, the orthoalkylation of benzylimine 3a
(7) (a) Jun, C.-H.; Moon, C. W.; Hong, J.-B.; Lim, S.-G.; Chung, K.-
Y.; Kim, Y.-H. Chem.sEur. J. 2002, 8, 485. (b) Jun, C.-H.; Hong, J.-B.;
Kim, Y.-H.; Chung, K.-Y. Angew. Chem., Int. Ed. 2000, 39, 3440. (c) Lim,
S.-G.; Lee, J.-H.; Moon, C. W.; Hong, J.-B.; Jun, C.-H. Org. Lett. 2003, 5,
2759. (d) Lim, S.-G.; Ahn, J.-A.; Jun, C.-H. Org. Lett. 2004, 6, 4687.
(10) (a) Bauer, T.; Thomann, R.; Mu¨lhaupt, R. Macromolecules 1998,
31, 7651. (b) Fuchs, K.; Bauer, T.; Thomann, R.; Wang, C.; Friedrich, C.;
Mu¨lhaupt, R. Macromolecules 1999, 32, 8404.
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