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

Abstract

Reaction of in situ generated {η 5 -[MeOC(O)]C 5 H 4 }Co(PPh 3 ) 2 with methyl 3-phenyl-2-propynoate followed by diphenylacetylene in refluxing toluene resulted in the formation of the cobalt sandwich compound {η 5 -[MeOC(O)]C 5 H 4 }Co{η 4 -C 4 Ph 3 [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 {η 5 -C 5 H 4 [C(O)OH]}Co{η 4 -C 4 Ph 3 [C(O)OH]} (2). The dicarboxylic acid 2 was converted to the novel bis(oxazolinyl) derivative [η 5 -(4-iPr-2-Ox)C 5 H 4 ]Co[η 4 -C 4 Ph 3 (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, (η 5 -RC 5 H 4 )Co(η 4 -C 4 Ph 3 R′) (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.

Functionalized 1,3‐Diaminotruxillic Acids by Pd‐Mediated C–H Activation and [2+2]‐Photocycloaddition of 5(4H)‐Oxazolones

“…Trinuclear complexes of stoichiometry [(C ^ N)Pd(µ‐OAc) 2 Pd(µ‐OAc) 2 Pd(C ^ N)], such as those shown in Scheme , are sometimes found in orthopalladation processes involving C–H bond activation, but these are quite rare. Such complexes have a symmetry that is compatible with the number of signals found in both the 1 H and 19 F NMR spectra of 2h and we therefore propose a trinuclear complex such as that shown in Scheme for the third compound in the mixture 2h .…”

Synthesis and Characterization of the Complete Series [(C5H5‐n Cln)Co(C4Ph4)] (n=1–5)

“…[12] An alternative "improved protocol for the synthesis" was therefore suggested, [9] that also allowed for large variations in the aryl moiety. The possibility to post-functionalize the phenyl rings was also reported [13,14] While the use of substituted cyclopentadienides (C 5 H 4 R) with R = CHO or COOMe allowed for a lot of post-functionalizations of the cyclopentadienyl ring, [1,5,6,8] it turned out much more difficult to post-functionalize the coordinated C 5 H 5 ring. [7] While mercuration with Hg(OAc) 2 /LiCl/ HClO 4 yielded mono-and/or dimercurated products [{C 5 H 5-n (HgCl) n }Co(C 4 Ph 4 )], [3,15,16] mercuration with Hg(OOCCF 3 ) 2 led to the formation of the pentamercurated [{C 5 (HgO 2 CCF 3 ) 5 } Co-(C 4 Ph 4 )].…”

Chemistry of the highly stable hindered cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) and its derivatives

“…The diester (compound 377) having ester units on both the Cp and Cb ring was converted to the chiral bisoxazoline ligand (compound 379) as shown in Scheme 77. An unprecedented C−H activation of one of the cyclobutadiene bound phenyl groups was observed on the reaction of 379 with Pd(OAc) 2 ,resulting in a palladacycle (compound 380) having both the oxazoline groups also bound to the palladium as well[182]. Synthesis of chiral 1,1' bis(oxazoline) and its palladium complex involving C-H activation[182] Scheme 78.…”

“…An unprecedented C−H activation of one of the cyclobutadiene bound phenyl groups was observed on the reaction of 379 with Pd(OAc) 2 ,resulting in a palladacycle (compound 380) having both the oxazoline groups also bound to the palladium as well[182]. Synthesis of chiral 1,1' bis(oxazoline) and its palladium complex involving C-H activation[182] Scheme 78. Synthesis of 385 and 386, having linear tri-palladium assembly bridged by four acetate groups[182] Analogous compounds 383 and 384, having an oxazoline group bound to the cyclobutadiene ring but having methyl or acetyl group on the Cp ring were also prepared.Reaction of these ligands with palladium acetate resulted in the formation of chiral complexes 385 and 386, consisting of a linear tri-palladium assembly bridged by four acetate groups and the ends by the cyclopalladated cobalt sandwich unit (Scheme 78).…”

“…Synthesis of chiral 1,1' bis(oxazoline) and its palladium complex involving C-H activation[182] Scheme 78. Synthesis of 385 and 386, having linear tri-palladium assembly bridged by four acetate groups[182] Analogous compounds 383 and 384, having an oxazoline group bound to the cyclobutadiene ring but having methyl or acetyl group on the Cp ring were also prepared.Reaction of these ligands with palladium acetate resulted in the formation of chiral complexes 385 and 386, consisting of a linear tri-palladium assembly bridged by four acetate groups and the ends by the cyclopalladated cobalt sandwich unit (Scheme 78). However, attempted Overman-Claisen rearrangement studies using these palladacyles as catalysts did not show any enantioselectivity when tried on trichloroacetimidates as substrates.Cyclobutadiene linked dimeric cobalt sandwich compounds of the type [( 5 -RCp)Co( 4 -C 4 Ph 3 )] 2 prepared by Elias and coworkers are also having varying substituents on both Cp and Cb rings, with substituent on the Cb ring being another [( 5 -RCp)Co( 4 -C 4 Ph 3 )] moiety having functional group on its Cp ring.…”