Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding

6-Methyl-2-butylureidopyrimidone dimerizes via four hydrogen bonds in the solid state as well as in CHCl3 soln. via a donor-donor-acceptor-acceptor (DDAA) array of hydrogen bonding sites in the 4[1H]-pyrimidinone tautomer. An intramol. hydrogen bond from the pyrimidine NH group to the urea oxygen atom preorganizes the mols. for dimerization. The dimerization const. of the dimer in CHCl3 exceeds 106 M-1. In CHCl3 contg. DMSO, the dimer is in equil. with the monomeric 6[1H]-pyrimidinone tautomer. In 6-phenyl-2-butylureidopyrimidone, the 4[1H]-pyrimidinone tautomer coexists with the pyrimidin-4-ol form, which dimerizes with similar high dimerization consts. via four hydrogen bonds in a DADA array. The latter tautomer predominates in derivs. with electroneg. 6-substituents, like 6-nitrophenyl- and 6-trifluoromethyl-2-butylureidopyrimidone. Due to its simple prepn. and high dimerization const., the ureidopyrimidone functionality is a useful building block for supramol. chem