New Chiral Palladacycles from an Unprecedented Cyclopalladation of Cyclobutadiene-Bound Phenyl Groups of Cobalt Sandwich Compounds

Reaction of in situ generated {η5-[MeOC­(O)]­C5H4}­Co­(PPh3)2 with methyl 3-phenyl-2-propynoate followed by diphenylacetylene in refluxing toluene resulted in the formation of the cobalt sandwich compound {η5-[MeOC­(O)]­C5H4}­Co­{η4-C4Ph3[C­(O)­OMe]} (1), having methyl ester units on both the cyclopentadienyl (Cp) and cyclobutadiene (Cb) rings. Hydrolysis of the ester groups using aqueous KOH resulted in the dicarboxylic acid {η5-C5H4[C­(O)­OH]}­Co­{η4-C4Ph3[C­(O)­OH]} (2). The dicarboxylic acid 2 was converted to the novel bis­(oxazolinyl) derivative [η5-(4-iPr-2-Ox)­C5H4]­Co­[η4-C4Ph3(4-iPr-2-Ox)] (3; Ox = oxazolinyl) by its reaction with oxalyl chloride, (S)-2-amino-3-methyl-1-butanol, triethylamine, and mesyl chloride. Reaction of the chiral bis­(oxazoline) ligand 3 with Pd­(OAc)2 in acetic acid at 60 °C resulted in the formation of the novel chiral palladacycle 4, resulting from an unprecedented cyclopalladation involving one of the cyclobutadiene-bound phenyl groups. Palladacycle 4 has two trans-oriented oxazolinyl units and one of the phenyl groups of the cyclobutadiene ring bound to the palladium center along with an acetyl group. The reaction of KX (X = Br, I) with 4 in acetone–water medium yielded the bromo- and iodo-derived chiral palladacycles 5a,b. Compounds analogous to 3 having a chiral oxazolinyl unit on the Cb ring and a methyl or acetyl unit on the Cp ring, (η5-RC5H4)­Co­(η4-C4Ph3R′) (R′ = oxazolinyl; R = acetyl (8a), methyl (8b)) were also prepared and characterized. Analogous reactions of 8a,b with 1.5 equiv of Pd­(OAc)2 in acetic acid at 60 °C gave the chiral palladacycles 9a,b. Analysis of these compounds indicated that, in contrast to the monomeric chiral palladacycle obtained from the reaction of 3, compounds 9a,b are molecules with a unique linear tetraacetate-bridged tripalladium core having the two cyclopalladated cobalt sandwich units at the periphery. Structural analysis of 9a indicated that the molecule possesses the same type of seven-membered palladacycle observed in the cases of 4 and 5a,b.