Chemistry of metallacyclobutanones. Part 2. Synthesis and ring inversion of some highly puckered metallacyclobutan-3-one (slipped oxodimethylenemethane) complexes of palladium; crystal structures of 2,4-bis(methoxycarbonyl)-1,1-bis(triphenylphosphine)palladacyclobutan-3-one, 2,4-bis(methoxycarbonyl)-1,1-bis(triphenylarsine)palladacyclobutan-3-one, and 1,1-(2′,2″-bipyridyl)-2,4-bis(methoxycarbonyl)palladacyclobutan-3-one

Abstract

The reactions of the zerovalent palladium complex [Pd2(dba)3]·CHCl3(dba = dibenzylidene-acetone, PhCHCH–C(O)–CHCHPh), in the presence of a donor ligand L and dioxygen, with esters of 3-oxopentanedioic acid (RCH2COCH2R) in diethyl ether, afford, in high yield, the pallada-cyclobutan-3-one compounds [P[graphic omitted]HR)L2][R = CO2Me, L = PPh3, PMePh2, PMe2Ph, PEt3, or AsPh3, L2= 2,2′-bipyridyl (bipy); R = CO2Et, L = PPh3 or AsPh3, L2= bipy; R = CO2Prn, L2= bipy]. The compounds [P[graphic omitted]HR)L2](R = CO2Me or CO2Et, L = PPh3 or AsPh3) are also formed upon treatment of benzene solutions of [PdL4] with RCH2COCH2R in the presence of air. Treatment of the ketone MeO2CCH2COCH2CO2Me with either [Pd(O2)(PPh3)2] in benzene or [Pd(OCO3)(PPh3)2] in ethanol also affords [P[graphic omitted]H(CO2Me)}(PPh3)2]. Spectroscopic data (i.r. and n.m.r.) for the new compounds are reported, and discussed in relation to their structures, which were firmly established for [P[graphic omitted]H (CO2Me)}L2]·H2O [L = PPh3(3a), L = AsPh3(3e), and L2= bipy (3f)] by single-crystal X-ray diffraction studies. Crystals of (3a) are triclinic, space group P, Z= 2, in a unit cell with lattice parameters a= 11.995(5), b= 17.72(1), c= 10.30(1)A, α= 94.7(1), β= 110.8(1), and γ= 102.84(2)°. The structure was refined to R 0.055 (R′ 0.060) for 5 093 reflections having 7 ⩽ 2θ⩽ 54°(Mo-KαX-radiation) collected at room temperature. Crystals of (3e) are triclinic, space group P, Z= 2, in a unit cell with lattice parameters a= 12.219(6), b= 17.56(1), c= 10.44(1)A, α= 93.3(1), β= 110.8(1), and γ= 103.56(2)°. The structure was refined to R 0.043 (R′ 0.046) for 6 025 reflections having 7 ⩽ 2θ⩽ 54° collected at room temperature. Crystals of (3f) are monoclinic, space group P21/c, Z= 4, in a unit cell with lattice parameters a= 8.43(1), b= 14.594(6), c= 14.170(6)A and β= 92.1(1)°. The structure was refined to R 0.043 (R′ 0.032) for 4 000 reflections having 7 ⩽ 2θ⩽ 60° collected at room temperature. The three molecules contain a highly puckered palladacyclobutan-3-one ring [fold angles 53.3(2) for (3a), 52.3(4) for (3e), and 51.3(3)° for (3f)] with a weak transannular Pd–C bond [Pd–C(2) 2.389(7) for (3a), 2.384(6) for (3e), and 2.374(3)A for (3f)]. The molecules can be alternatively formulated with slipped η3-oxodimethylenemethane ligands. Variable-temperature 1H n.m.r. data for the compounds [P[graphic omitted]HR)L2](R = CO2Me, L = PPh3, PMePh2, PMe2Ph, PEt3, or AsPh3; L2= bipy) are interpreted in terms of inversion of the palladacyclobutan-3-one ring through a planar transition state.