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)Å, 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-K_αX-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)Å, 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)Å 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)Å 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.