Palladium(0)-catalysed allylation of uracils and thiouracils. Influence of the solvent on the regioselectivity of the allylation
Uracil and 5-substituted uracils are monoallylated at N-1 in H2O- CH3CN with the catalytic systemPd(OAc)2/P(C6H4-m -SO3Na)3 (or tppts) although performing the reaction in H2O/THF with the system Pd2(dba)3/dppb leads to diallylations at N-1 + N-3.2-Thiouracil, 5-methyl-2-thiouracil (2- thiothymine) and 6-methyl-2-thiouracil are monoallylated at sulfur in H2O/CH3CN with the catalytic system Pd(OAc)2/P(C6H4-m-SO3Na)3 (or tppts). Performing the reactions in H2O/THF with the system Pd2(dba)3/dppb leads to diallylations at N-1 + N-3 of 2-thiouracil and 2-thiothymine whereas 6-methyl-2-thiouracil is diallylated at S + N-3.
Palladium(0)-catalyzed allylation of uracils and 2-thiouracils drastic effect on an aqueous reaction medium on the regioselectivity
Palladium(0)-catalyzed allylation of uracils in DMSO takes place at N-1 and at N-3. In sharp contrast, regiospecific allylation at N-1 is achieved in an organic-aqueous medium in the presence of palladium(II) acetate and the water-soluble sulfonated triphenylphosphine P(C6H4-m-SO3Na)3 (tppts). Similar reactions with 2-thiouracils are also drastically dependent on the solvent, taking place at N-1 and N-3 (in dioxane) or at sulfur (organic-aqueous medium). Probably reactions in the organic aqueous medium are kinetically controlled whereas allylation in DMSO or refluxing dioxane are thermodynamically controlled.
Palladium-catalyzed allylation of pyrimidine-2,4-diones (uracils) and of 6-membered heterocyclic ambident sulfur nucleophiles
Pd(O)-Catalyzed allylation of six-membered ambident heterocycles bearing NH-CO, NH-CS and CH2-CO moieties obey the regioselectivity rules: C>O; N>O; S>N; NH-CO>NH-CS. Uracil and 5-methyluracil (thymine) do not show regioselectivity (N-1 = N-3) whereas 6-methyluracil is regioselectively allylated at N-3 (N-3>N-1).
Moreno-Manas,Pleixats,Villarroya
p. 1457 - 1464
(2007/10/02)
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