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

Abstract

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...