Trimethylsilyl (TMS)-protected alkynes serve as selective alkyne cross coupling partners in Ti-catalyzed [2+2+1] pyrrole synthesis. Reactions of TMS-protected alkynes with internal alkynes and azobenzene catalyzed by Ti imido catalysts yield pentasubstituted 2-TMS-pyrroles with greater than 90% selectivity over the other 9 possible pyrrole products. The steric and electronic effects of the TMS group have both been identified to play key roles in this highly selective pyrrole synthesis. This strategy provides a convenient methodology to synthesize multisubstituted pyrroles as well as an entrypoint into further pyrrole diversification through facile modification of the resulting 2-silylpyrrole products, as demonstrated through a short formal synthesis of the marine natural product Lamellarin R.

Transient dative substrate-Ti interactions have been found to play a key role in controlling the regioselectivity of alkyne insertion and [2+2] cycloaddition in Ti-catalyzed [2+2+1] pyrrole synthesis and Ti-catalyzed alkyne hydroamination. TMS-protected alkynes with pendent Lewis basic groups can invert the regioselectivity of TMS-protected alkyne insertion, leading to the selective formation of highly substituted 3-TMS pyrroles. The competency of various potential directing groups was investigated, and it was found that the directing-group effect can be tuned by modifying the catalyst Lewis acidity, the directing-group basicity, or the directing-group tether length. Dative directing-group effects are unexplored with Ti catalysts, and this study demonstrates the potential power of dative substrate-Ti interactions in tuning selectivity.

The effect of proximal Zn halides on Ni-catalyzed ethylene polymerization is reported in this work. A series of (NON)NiLX (NON = 2,6-bis-((2,6-diisopropylphenyl)imino)methyl phenoxide; LX = methallyl or L = py, X = tolyl, 2-4) ethylene polymerization precatalysts have been synthesized, as well as a heterobimetallic Ni/Zn complex, (NON)Ni(CH)ZnBr (5). Each precatalyst could be activated (or promoted) by ZnX (X = Cl, Br, Et) to polymerize ethylene. In situ recruitment of ZnX by the free imine binding pocket of the NON complexes results in the generation of heterobimetallic active species that produce lower M polyethylene than monometallic controls. Room temperature ZnX-promoted polymerizations with these catalysts resulted in bimodal M distributions that result from different catalyst speciation: "dangling" imine-ligated ZnX species yield higher M polymer while N,O-chelated ZnX species yield lower M polymer. Running polymerizations at higher temperature yields in only lower M polymer resulting from exclusive formation of the thermodynamically favored N,O chelated Ni/Zn heterobimetallic. DFT calculations indicate that this bridging bimetallic complex undergoes β-H elimination more facilely than monometallic Ni analogues, resulting in lower molecular weight polymers.

A series of mono-and bimetallic Ni alkyl complexes of a β-oxo-δ-diimine (BODDI) ligand are reported. The monometallic complexes have a second binding pocket, of which the free "arm" can exist as either an enamine (e.g., 8, BODEI, β-oxo-δ-enamineiminato) or imine (e.g., 3, BODII, β-oxo-δ-imineiminato) tautomer. The identity of the tautomer in the secondary Ni coordination sphere has a significant effect on ethylene polymerization behavior: the enamine tautomer, which hydrogen bonds to the central O atom and is in conjugation with the N,O backbone chelate, is significantly more electron rich and yields a much lower molecular weight polymer than the imine tautomer, which rotates away from Ni to a distal position and has little effect on polymerization. Deprotonation of the second binding pocket with M(HMDS) (M = Li, Na, K) yields the Ni-alkali metal heterobimetallic complexes 3Li, 3Na, and 3K. The deprotonated alkali metal enamides display ethylene polymerization behavior similar to the neutral imine complex because the enamide arm can also distally rotate to minimize interaction with the Ni coordination sphere upon activation. ■ INTRODUCTIONLate transition metal catalysts have been widely employed in ethylene polymerization catalysis and polar comonomer ethylene copolymerization catalysis due to their functional group tolerance. 1 The majority of functional comonomer polymerization reactions have utilized either Brookhart-type α-diimine 2 or Drent-type phosphine sulfonate 3 ligands, indicating an opportunity for continued catalysis advancement through the design of new ligand sets with new metal−ligand interactions.Akin to this, recent progress has seen several elegant examples that utilize secondary coordination sphere interactions 4 to affect various aspects of ethylene homo-and copolymerization catalysis. For example, Jordan and Bazan have each demonstrated that coordination of exogenous Lewis acids to ligands on group 10 polymerization catalysts can drastically impact polymerization activities, molecular weight distributions, and comonomer incorporation. 5 Similarly, Do recently reported on the effects of installing alkali metal cations into the secondary coordination sphere of phenoxyiminato Ni ethylene polymerization catalysts. 6 Through prudent alkali metal choice, Do was able to observe up to 20-fold increases in catalytic rates as well as significant enhancement of polymer molecular weight and branching. Transition metal bimetallic effects have also been observed; for example, Agapie reported the copolymerization of ethylene and amino olefins catalyzed by homobimetallic dinickel complexes. 7 Tethered heteroatoms usually inhibit olefin polymerization through the formation of stable chelate rings, but addition of a bulky second metal site in the secondary coordination sphere of Ni prevents stable chelate formation. Other bimetallic Fe, Ni, Cu, and early transition metal systems have shown similar cooperative effects, where addition of a second metal impacts the overall molecular weight, activity, and comonom...

A strategy to synthesize branched polyketoesters from the carbonylative polymerization of bifunctional α,ω-alkenols such as 10-undecen-1-ol is presented. This strategy hinges on the competitive application of two related catalytic manifolds, alternating alkene/CO copolymerization, and alkene hydroesterification, which share a common metal acyl intermediate. Small molecule model studies of cationic Pd-catalyzed alkene carbonylation in the presence of alcohols demonstrate that the relative rates of ketone formation (through alternating alkene/CO insertion) and ester formation (through metal acyl alcoholysis) can be tuned across a wide range through judicious bis(phosphine) ligand design. Carbonylative polymerization of 10-undecen-1-ol with a (dppp(3,5-CF 3 ) 4 )Pd(OTs) 2 catalyst (dppp(3,5-CF 3 ) 4 = 1,3-bis[bis[3,5-bis(trifluoromethyl)phenyl]-phosphino]propane) led to the formation of high molecular weight polyketoesters with intermediate dispersity (M n > 20,000 g/mol, D̵ = 2.6) and a ketone/ester microstructure ratio of approximately 1:2. In these polymerization reactions, deploying electron-deficient bis(phosphines) to suppress deleterious alkene isomerization was the key to accessing the high molecular weight polymer. Further, terpolymerization reactions of 1-hexene/10-undecen-1-ol/CO or 1-fluoro-10-undecene/10-undecen-1-ol/CO by (dppp(3,5-CF 3 ) 4 )Pd(OTs) 2 were also successful. This proof of concept polymerization unlocks access to tunable polymer microstructures without extensive postpolymerization treatment.