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Although their activity is often inferior to that of other systems, the use of vanadium-based catalysts in homogeneous Ziegler-Natta polymerizations allows the preparation of high-molecular-weight polymers with narrow molecular-weight distributions, ethene/alpha-olefin copolymers with high alpha-olefin incorporation, and syndiotactic polypropene. The main reason for the low activity of these catalysts is their deactivation during catalysis by reduction of active vanadium species to low-valent, less active or inactive species. We here present an up-to-date review of this area with particular emphasis on the attempts to improve catalyst performance and stability by the use of additives or ancillary ligands.

The stepwise synthesis of a palladium(II) bo&d altemating oligomer of CO and strained alkenes is described together with the factors that allow full control of each successive step. The sequence starts with acetylpalladium-(11) complexes PdX(COMe)(N-N) (2,4), (X = Cl(a), Br(b), or I(c); 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 stereo-and 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~H I~C O C~H I~C O M~) (~~~) ] O T~ (Ua). Norbomadiene was inserted similarly. Attempted recrystallization of the insertion product of 5-norbomene-endu-2,3-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 ( C O C~H~~C O C~H I~C O M~) (~~~) (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-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 ...

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A new class of well-defined neutral mono-, and dicationic ruthenium(II) complexes containing a neutral terdentate donor system [C 5 H 3 N(CH 2 E) 2-2,6] (E) PPh 2 (PNP) or NMe 2 (NN′N)) has been found effective as catalyst precursor in N-(cyclo)alkylation reactions of aromatic amines with diols Y(CH 2 CH 2 OH) 2 (Y) CH 2 , NR). With these catalysts, N-phenylpiperidine is synthesized from aniline and 1,5-pentanediol in 85% yield (at 180°C for 24 h in 1,4-dioxane). With neutral RuCl 2 (NN′N)-(PPh 3) as a catalyst precursor, aniline can be selectively N-monoalkylated with diols of the type HO(CH 2) n OH (n) 4-6, 10) to give N-(n-hydroxyalkyl)anilines in 40-75% yield. To our knowledge, this represents the first useful catalytic route to this type of compounds. The new catalysts can also be used in the synthesis of arylpiperazines. For example, N-phenyl-N′-methylpiperazine is obtained from aniline and MeN(CH 2 CH 2 OH) 2 in yields up to 34%. N-[m-(Trifluoromethyl)phenyl]-N′-methylpiperazine, TFMPMP, is successfully produced from m-(trifluoromethyl)aniline and MeN-(CH 2 CH 2 OH) 2 in 44% yield using monocationic [RuOTf(NN′N)(PPh 3)]OTf as the catalyst precursor. A mechanism for the N-(cyclo)alkylation reaction is proposed.