Reactions of cyclopalladated complexes with boronic acids

Abstract

Six known C(sp2),N and C(sp3),N cyclopalladated complexes derived from N,N-dimethylbenzylamine, 4,4-dimethyl-2-phenyl-2-oxazoline, 2-tert-butyl-2,2-dimethyl-2-oxazoline, O-methyloxime of d-camphor, 8-methylquinoline, and N,N-dimethylhydrazone of d-camphor were synthesized and used in CC bond formation reactions with aryl, benzyl and allylboronic acids or esters. Two protocols for a successful CC coupling were developed; both involve the use of a base and the conversion of dimeric cyclopalladated complexes to the mononuclear derivatives with PPh3 as an auxiliary ligand. The C(sp2)–C(sp2) bond formation was achieved by reacting mononuclear complexes of N,N-dimethylbenzylamine and 4,4-dimethyl-2-phenyl-2-oxazoline with ArB(OH)2 in acetone at 60 °C in the presence of Cs2CO3. Reactions of (i) C(sp3),N complexes PPh3-6, PPh3-8 and PPh3-10 with ArB(OH)2 (Ar = Ph, p-NO2C6H4, p-MeOC6H4, 8-quinolyl and 3-pyridyl) and (ii) C(sp2),N oxazoline-derived complex PPh3-4 with pinacol esters of allyl- and benzylboronic acids occurred in a refluxing 4:1 mixture of dioxane-water in the presence of K3PO4 and afforded C(sp2)–C(sp3) coupling products in 46–89% yield. Desirable compounds with an C(sp3)–C(sp3) bond were isolated in 17–67% yield in the reactions of pinacol ester of benzylboronic acid with C(sp3),N complexes PPh3-6 and PPh3-8. The stereoselectivity of the transformation was investigated using reactions of p-NO2C6H4B(OH)2 with two diastereomeric complexes, (1S,2S,4R,Z)-12 and (1S,2R,4R,Z)-12, which were obtained from N,N-dimethylhydrazone of d-camphor, and had different absolute configurations of the chiral center attached to the metal. Both reactions yielded the same isomer, (1R,3R,4R,Z)-1,1-dimethyl-2-[1,7,7-trimethyl-(4-nitrophenyl)bicyclo[2.2.1]heptan-2-ylidene]hydrazine, as the major product. The X-ray crystallographic study of this product proved the R absolute configuration of the new chiral center suggested using 1H NMR data.