Metal-mediated [2+3] cycloaddition of nitrones to palladium-bound isonitriles.

Abstract

The reactions between equimolar amounts of cis-[PdCl(2)(C[triple bond]NR)(2)] (R = cyclohexyl (Cy) 1, tBu 2, 2,6-Me(2)C(6)H(3) (Xyl) 3) and the acyclic nitrones O(+)N(R(2))=C(H)R(3) (R(2) = Me 5, R(2) = CH(2)Ph 6, R(3) = 4-MeC(6)H(4)) proceeded in C(6)H(6) at 5 degrees C for around 4 h and then the reaction systems were maintained at 20-25 degrees C for 20 h to provide the carbene complexes [PdCl(2){C(ONR(2)C(c)HR(3)) = N(d)R}(C[triple bond]NR)(C(c)-N(d))] (8-13) in good yields (54-70%). The latter species originated from the previously unreported metal-mediated [2+3] cycloaddition of nitrones to coordinated isonitriles. For the reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-3) and 5 or 6 performed in C(6)H(6) at 20-25 degrees C, the selectivity of the cycloaddition was lost, and the reactions gave mixtures of 8-13 and the complexes [PdCl(2){N(R(2))=CHR(3)}(C[triple bond]NR)] (14-19, ca. 75%) derived from intramolecular deoxygenation of the nitrones. The reactions of equimolar amounts of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-3 and R = 4-MeOC(6)H(4) 4) and the nonaromatic cyclic nitrone (-)O(+)N(a)=CHCH(2)CH(2)C(b)Me(2)(N(a)-C(b)) (7) in CHCl(3) at 5 degrees C for around 2 h led to the carbene species [PdCl(2){C(ON(a)CMe(2)CH(2)CH(2)C(b)H) = N(e)R}(C[triple bond]NR)(N(a)-C(b))(C(b)-N(e))] (21-24), isolated in yields of 78-92%. When the reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-4) and 7 (1:1 molar ratio) were performed in CHCl(3) at 20-25 degrees C for around 1 h, the carbenes 21-24 and the complexes [PdCl(2){N(f)CMe(2)CH(2)CH(2)C(g)H}(C[triple bond]NR)(N(f)-C(g))] (25-28, ca. 35%), derived from the deoxygenation of 7, were identified by ESI(+)-MS and (1)H NMR analysis (after redissolution in CDCl(3)). The reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-4) and 7 (1:2 molar ratio) in CHCl(3) at 5 degrees C furnished mixtures of [PdCl(2){C(ON(a)CMe(2)CH(2)CH(2)C(b)H)=N(e)R}(N(f)CMe(2)CH(2)CH(2)C(g)H)(N(a)-C(b))(C(b)-N(e))(N(f)-C(g))] (29-32; ca. 25-30%), 25-28 (ca. 10-15%), and [PdCl(2)(ON(a)CMe(2)CH(2)CH(2)C(b)H)(2)(N(a)-C(b))]. The same reactions at room temperature afforded the complex [PdCl(2)(N(f)CMe(2)CH(2)CH(2)C(g)H)(2)(N(f)-C(g))] (33; ca. 20 %) in a mixture with 29-32 (ca. 5-10%) and 25-28.