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.