Alkene and carbon monoxide insertion reactions of nitrogen-coordinated monoorganopalladium(II) complexes: The stepwise construction of alternating copolymers of CO and alkenes on a palladium center

The stepwise synthesis of a palladium(II) bod N-N = N,N,”,”-tetramethylethylenediamine (tmeda) or 2,2’-bipyridyl (bpy)). The bpy-coordinated acetyl complexes (4a-c) show an unexpected isomerization reaction of the groups around the metal center for which thermodynamic parameters have been determined. A mechanism for the isomerization is proposed. The ionic complexes [Pd(COMe)(MeCN)(N-N)]OTf (5, 6; OTf = trifluoromethanesulfonate), obtained in situ from reaction of 2a-c and 4a-c with AgOTf, react with a number of alkenes. Nonstrained alkenes do insert, but decomposition (probably involving B-elimination) is fast. Alkenes containing potential donor groups do not insert at all. Only in the case of strained alkenes like norbomene and its derivatives were the insertion products isolated and fully characterized by I3C NMR, IR, and X-ray analysis. Crystals of the dicyclopentadiene insertion product [Pd(C1$I12COMe)(bpy)]OTf (8c) are monoclinic, space group P21/n, a = 8.3086(4) A, b = 15.3894(6) A, c = 18.1479(9) A, B = 99.98(1)’, V = 2285.36(19) A3, Z = 4, R = 0.042 (wR = 0.034). Here, the alkene insertion has occurred in cis,exo fashion on the norbomene-type alkene moiety, indicating this reaction to be both stereoand chemoselective. The molecular structure shows that the acyl group is coordinated to the metal. Products resulting from a second CO insertion into the Pd-C bond of 8a, i.e. PdX(COC7HloCOMe)(bpy) (X = C1 (12a) or I (12b)), were formed in the presence of a large excess of a sodium halide salt. Reaction of 12a,b with AgOTf and norbomene gave the alkyl complex [P~(C~HI~COC~HI~COM~)(~~~)]OT~ (Ua). Norbomadiene was inserted similarly. Attempted recrystallization of the insertion product of 5-norbomene-endu2,3-dicarboxylic anhydride (13c) gave the alkyl complex 8a. This is the first example of a reversible alkene insertion reaction on an isolated palladium complex. Addition of CO and excess NaI (20 equiv) to 13a gave the neutral acyl complex P~I(COC~H~~COC~HI~COM~)(~~~) (14), the first isolated and fully characterized CO/alkene oligomer still connected to the palladium(I1) catalytic site. Crystals are triclinic, space group P i with a = 10.128(2) A, b = 11.655(2) A, c = 13.633(2) A, a = 109.89(1)’, /3 = 100.58(1)0, y = 92.92(2)’, V = 1476.3(4) A3, Z = 2 and R = 0.044 (wR = 0.057). Norbomadiene also reacts with 2 equiv. of 5 or 6 to give the homoligated dinuclear complexes [(N-N)P~((C~H~)(COM~)~-~,~}P~(N-N)](OT~)Z with -N = tmeda (9) or bpy (10). Crystals of 9 are orthorhombic, space group P212121, a = 11.176(3) A, b = 11.362(1) A, c = 30.394(3) A, V = 3859(1) A3, Z = 4, wR2 = 0.124, R = 0.049. In this complex a double alkene insertion reaction has taken place involving both alkene functionalities of a dialkene: 9 is symmetrically substituted with the Pd atoms at opposite (trans) positions.