CO/Ethene Copolymerization at Zirconocene Centers?

The alternating copolymerization of CO and ethene at the zirconocene centers Cp2Zr, Me2SiCp2Zr, and rac-Me2SiInd2Zr was investigated by DFT methods. CO coordinates much more strongly than ethene but has a rather high insertion barrier. Thus, propagation is slowed dramatically but the growing chains will not necessarily all incorporate CO. Secondary alkyls insert CO more efficiently than primary alkyls. After the first CO insertion, subsequent olefin and CO insertions will alternate. Olefin insertion will be very slow at high CO pressures, but at low [CO] the olefin and CO insertion barriers are comparable and are lower than that of the first CO insertion. Use of CO as a quenching and chain-counting method appears to be safe, provided a high pressure of CO is employed and the quenched reaction is not worked up at low temperature

Theoretical Study of the Reaction of Alkyllithium with Pyridylphosphines

The reaction of pyridylphosphines with alkyllithium reagents has been investigated using ab initio methods. In earlier experimental studies, it was found that reaction of RLi with R‘PPy2 produced a mixture of RPy, R‘Py, (R)(Py)PLi, and (R‘)(Py)PLi; the reaction was assumed to involve “reductive elimination” from an (R)(R‘)(Py)2P- Li+ intermediate. Our calculations show that the R for R‘ exchange does indeed involve such a species, although it is a transition state rather than an intermediate. Formation of phosphide, however, proceeds by direct attack of the alkyllithium on carbon C2 of a pyridyl group and is more properly designated as a nucleophilic substitution at carbon. Coordination of the pyridyl groups to lithium appears to be important in both reactions

A Second Transition State for Chain Transfer to Monomer in Olefin Polymerization Promoted by Group 4 Metal Catalysts

β-Hydrogen transfer (BHT) to monomer is the dominant chain termination pathway for olefin polymerization promoted by group 4 metal catalysts. The transition state (TSA) for BHT studied in earlier work is characterized by a strong metal−hydrogen interaction. Our theoretical study of a series of homogeneous single-site polymerization catalysts reveals the existence of a second transition state (TSC), competitive with TSA, which has no direct metal−hydrogen interaction and strongly resembles that for the main-group metal aluminum. The balance between the two reaction paths is sensitive to choice of metal and ligand structure

Binuclear Oxidative Addition of Aryl Halides

Reaction of LCoCH2SiMe3 (L = 2,6-bis[2,6-dimethylphenyliminoethyl]pyridine) with H2 produces LCo(N2), presumably via intermediate LCoH. Reaction of LCo(N2) (prepared in this way or via reaction of LCoCl2 with Na/Hg) with aryl halides ArX (X = Cl, Br, I) produces LCoAr and LCoX in a ratio depending on the nature of Ar and X. For X = Cl, the reaction is slowest but also produces the largest amount of LCoAr. Electron-withdrawing substituents both accelerate the reaction and improve the yield of LCoAr. Computational studies support a radical mechanism for this reaction, involving displacement of N2 to give LCo(XAr) followed by loss of the Ar radical, which then binds to a second Co(0) moiety

Ethene Polymerization at Cationic Aluminum Amidinate and Neutral Aluminum Alkyl. A Theoretical Study

The effect of substituent variation on olefin insertion and chain transfer in cationic aluminum amidinate alkyls [R1C(NR2)2AlEt]+ was studied by theoretical methods. Introduction of bulky substituents at C (t-Bu) and N (i-Pr) favors insertion more than chain transfer, but the system still keeps a clear preference for chain transfer, and even the full system [t-BuC(Ni-Pr)2AlEt]+ is predicted not to polymerize ethene. Changing to a neutral analogue (as in H2C(NH)2AlEt) and relieving the geometric constraints (in Me2AlEt) favor insertion even more, so that trialkylaluminum is finally predicted to have a clear preference for oligomerization

DFT Study of Pd(PMe₃)/NMe₃-Catalyzed Butadiene Telomerization of Methanol

A detailed DFT study of Pd(PMe3)/NMe3-catalyzed butadiene telomerization of methanol predicts that the rate- and selectivity-determining step of the catalytic cycle is the external nucleophilic attack of methoxide on the π-allyl-Pd fragment of the cationic (PMe3)Pd(η3:η2-octadienyl)+ (3a). A crucial factor affecting the regio- and chemoselectivity appears to be the equilibrium between the chelated 3a and the dechelated species (PMe3)Pd(η3-octadienyl)(L′)+ (L′ = butadiene 3x/3y, or PMe3 3p): 3a is predicted to convert highly selectively to the linear telomer 8-methoxy-1,6-octadiene, whereas dechelated 3x/3y and 3p should be the major sources of both the branched telomer 3-methoxy-1,7-octadiene and byproduct 1,3,7-octatriene (via a novel β-hydrogen elimination mechanism). Insights into base effects were gained by comparing LiOMe and NMe3 as cocatalysts

A Measure for σ-Donor and π-Acceptor Properties of Diiminepyridine-Type Ligands

Relative metal−ligand binding strengths for the ZnCl2 and low-spin CoIMe fragments and a variety of diiminepyridine (DIP)-like ligands have been calculated at the DFT level. The assumption of a linear energy relation ΔEstab(F,L) = αFσL + βFπL for the relative binding energy of fragment F to ligand L was used to derive scales for ligand parameters σL and πL representing the σ-donor and π-acceptor qualities of these ligands. The results show that DIP ligands in general are only fair σ-donors but exceptionally good π-acceptors, being eclipsed only by their bis(diazo)pyridine analogues and bis(carbene)pyridine variations. Bis(phosphinimine)pyridines are much poorer π-acceptors than DIP, but are comparable in σ-donor capacity. Introduction of substituents at the pyridine or N-aryl rings of DIP results in changes that are much smaller than the above-mentioned replacement of the ligand side arms. The analysis method also puts metal fragments on a scale for Lewis acidity (αF) and π-basicity (βF). For the series of fragments ScCl2−CuCl2 (all high-spin), results indicate that Lewis acidity increases almost monotonously; π-basicity decreases from ScCl2 to MnCl2 (where it vanishes), is significant again for FeCl2, but negligible for CoCl2−CuCl2

DFT Study of Pd(PMe₃)/NMe₃-Catalyzed Butadiene Telomerization of Methanol

A detailed DFT study of Pd(PMe3)/NMe3-catalyzed butadiene telomerization of methanol predicts that the rate- and selectivity-determining step of the catalytic cycle is the external nucleophilic attack of methoxide on the π-allyl-Pd fragment of the cationic (PMe3)Pd(η3:η2-octadienyl)+ (3a). A crucial factor affecting the regio- and chemoselectivity appears to be the equilibrium between the chelated 3a and the dechelated species (PMe3)Pd(η3-octadienyl)(L′)+ (L′ = butadiene 3x/3y, or PMe3 3p): 3a is predicted to convert highly selectively to the linear telomer 8-methoxy-1,6-octadiene, whereas dechelated 3x/3y and 3p should be the major sources of both the branched telomer 3-methoxy-1,7-octadiene and byproduct 1,3,7-octatriene (via a novel β-hydrogen elimination mechanism). Insights into base effects were gained by comparing LiOMe and NMe3 as cocatalysts

Binuclear Oxidative Addition of Aryl Halides

Reaction of LCoCH2SiMe3 (L = 2,6-bis[2,6-dimethylphenyliminoethyl]pyridine) with H2 produces LCo(N2), presumably via intermediate LCoH. Reaction of LCo(N2) (prepared in this way or via reaction of LCoCl2 with Na/Hg) with aryl halides ArX (X = Cl, Br, I) produces LCoAr and LCoX in a ratio depending on the nature of Ar and X. For X = Cl, the reaction is slowest but also produces the largest amount of LCoAr. Electron-withdrawing substituents both accelerate the reaction and improve the yield of LCoAr. Computational studies support a radical mechanism for this reaction, involving displacement of N2 to give LCo(XAr) followed by loss of the Ar radical, which then binds to a second Co(0) moiety

Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHTA) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHTB path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHTA path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHTA into a two-step path (BHTC), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHTB path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14-e metal alkyls) either BHTA or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHTB may be relevant for some Hf complexes of intermediate crowding. The BHTB path is expected to be more important for systems that are less unsaturated (16-e transition metal alkyls; 6-e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic