NORMAL AND ANOMALOUS RING-OPENING OF 1-3-NORMAL-PENTAARYLCYCLOBUTENYLPALLADIUM COMPLEXES

Arylation (with NaBPh4 in acetone) of the cyclobutadienepalladium complex, [PdCl2To4)]2 (To=p-tolyl), gives the 1-3-η-cyclobutenyl complex [Pd(C4To4Ph)Cl]2 with the phenyl group entering stereospecifically endo to the metal. Ring opening occurs on reaction of the monomeric [Pd(C4To4Ph)X] (X=acac, S2CNR2) with ligands (in particular, PPhMe2) to give the σ-butadienyl complexes [Pd(C4To4Ph)X(PPhMe2)], where the 1-3-η-cyclobutenyl ligand has opened stereospecifically in the expected conrotatory manner. In contrast, the cyclobutenyl dithiocarbamates [Pd(C4To4Ph)(S2CNR2)] (17) undergo a spontaneous ring opening (2 days/20°C or 2 h/60°C) to give a mixture of the expected conrotatory ring-opened σ,π-butadienyl [Pd(C4To4Ph)(S2CNR2)] [20, p-tolyls E on C(3), C(4)] and the unexpected, formally disrotatory, isomer [21, p-tolyls Z on C(3), C(4)] in a ca. 40:60 ratio, as shown by an x-ray structure determination, HPLC, and NMR spectroscopy. Investigations to elicit the route by which the isomer 21 is formed are described. The reaction 17→20+21 is unimolecular and does not appear to involve ionic intermediates or to be photochemically initiated. Free cyclobutenyl radicals are easily formed from these complexes and a convenient way is by reaction of cyclobutenyl complexes with Ph2PCH2PPh2 (dppm); this also leads to the Pd(I) complex [Pd2Cl2(dppm)2]. Evidence is presented against free radicals participating in the ring opening, and in favor of their being intermediates in decomposition side reactions. The reaction does not appear to proceed by a radical chain mechanism either. A mechanism is proposed which involves the conrotatory ring opening of 17 to give the expected isomer (20) which then equilibrates with the unexpected isomer (21) via a metallocyclopentenyl intermediate in which C(3) can flip from one side to the other. © 1978, American Chemical Society. All rights reserved.