Normal and anomalous ring opening of 1-3-.eta.-pentaarylcyclobutenylpalladium complexes

Abstract

Arylation (with NaBPh4 in acetone) of the cyclobutadienepalladium complex, [PdC12(C4T04)]2 (To = p-tolyl), gives the 1 -3-~-cyclobutenyl complex [Pd(C4To4Ph)CI] 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 u-butadienyl complexes [Pd(C4To4Ph)X(PPhMez)], where the 1-3-~-cyclobutenyl ligand has opened stereospecifically in the expected conrotatory manner. In contrast, the cyclobutenyl dithiocarbamates [Pd(C4To4Ph)(SzCNRz)] (17) undergo a spontaneous ring opening (2 days/20 O C or 2 h/60 "C) to give a mixture of the expected conrotatory ringopened u,r-butadienyl [Pd(C4To4Ph)(SzCNR2)] [20, p-tolyls E on C(3), C(4)] and the unexpected, formally disrotatory, isomer [2l,p-tolyls 2 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 PhzPCH2PPh2 (dppm); this also leads to the Pd(1) complex [PdzC12(dpprn)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.Over the past few years considerable interest has developed in ring-opening and closing reactions in organometallic systems and the mechanisms by which they proceed. One of us has, in this context, shown that in the Pd(I1)-induced oligomerization of disubstituted acetylenes bearing bulky substituents the chief products are cyclobutadiene complexes (e.g., 3) which arise by a cyclization of intermediate u-butadienyl complexes (e.g., l);1,2 a presumed but so far unconfirmed further intermediate is a chlorocyclobutenyl complex (2). 1 \ 'PdCI 2t.BuCzCMe + PdCl,(PhCN), -t-Bu 1 t-Bu Me t-Bu 3 2 The opposite process, the opening of a cyclobutenyl complex, has been demonstrated by Powell et al. for the endo-alkoxytetraphenylcyclobutenylpalladium P-diketonate complex (4) to the a-butadienyl complex (5).3 This ring opening occurs p h y O E t 4 5 0002-7863/78/1500-4700$01 .OO/Ostereospecifically and in the conrotatory mode expected for a concerted thermally allowed reaction4 Powell et al. also noted that the exo-alkoxy isomer of 4 did not undergo ring opening, which they ascribed to a possible "steric inhibition between phenyl groups in the transition state of a conrotatory ring opening" of the exo-alkoxy...

Stereospecific Reaction of Molecular Halogens with Palladacyclopentadienes Containing Bidentate Nitrogen Ligands To Give 1,4-Dihalo-1,3-dienes via Palladium(IV) Intermediates

“…Although it was recently assumed that in double acetylene insertions the isomerization is intrinsic to the second insertion reaction, 62 also a stepwise isomerization of a dienyl fragment has been observed. 63 Such a stepwise isomerization was also observed for the (σ-butadienyl)palladium complexes containing trifluoromethyl substituents (7 to 8). It is not exactly clear in what respect the substituents determine the occurrence of any isomerization.…”

Intramolecular carbon-carbon vs. carbon-nitrogen bond formation in an organopalladium compound

“…(Despite several recrystallizations no better analyses could be obtained for this compound.) lH NMR (CDC13): 8.24-6.26 (m, 23 H, aromatic protons), 5.80 (dd, 1 H, HC1), 5.07 (m, 1 H, HC15), 4.48 and 4.32 (2 m, 2 H, H2Cl6), 2.87 and 2.73 (2 m, 2 H, H2C3), 2.29 (s, 3 H, NMe), 1.52 and 1.37 (2 m, 2 H, H2C2) (the assignment of the H resonances of 9 was made with the help of a 2D COSY NMR spectrum in CD2C12 at 400.13 MHz). Mass spectrum: m/z 530 (M+), 529 (M+ -H), 515 (M+ -Me).…”

Bis(diisopropylphosphinomethyl)amine Nickel(II) and Nickel(0) Complexes: Coordination Chemistry, Reactivity, and Catalytic Decarbonylative C–H Arylation of Benzoxazole

“…These resonances are assigned to the five methyl groups of the pmcb ring. Accordingly, in the 13 68 From the reaction of nickelocene with 2-butyne in the presence of methyllithium, Pasynkiewicz and co-workers isolated a product resulting from 3-fold insertion of the alkyne and subsequent cyclization. 69 The formation of tetraalkyl-cyclobutenyl nickel(II) complexes was also reported by Hiyama et al by reaction of two molecules of alkyne with aryl-hydrido complexes, which occur during the Nicatalyzed hydrofluoroarylation of alkynes.…”