Lewis Acid Catalyzed Synthesis of Poly(pyrazolyl)borate Ligands

Lewis acids catalyze the reaction of Li[MeBH3] and Na[BH4] with pyrazoles to yield poly(pyrazolyl)borates under mild conditions. The reaction of Li[MeBH3] with 2 equiv of 3-mesitylpyrazole (HpzMs) at 23 °C (2 days) affords Li[MeB(3-mesityl-pz)2H] (Li[MeBpMs]) and Li[MeB(3-mesityl-pz)(5-mesityl-pz)H] (Li[MeBpMs*]) in 88% yield (3:1 isomer ratio) in the presence of MeB(OiPr)2 (5 mol % vs HpzMs) but only 28% yield without this additive. Similar enhancements in the yield of Li[MeBpR] products are observed in the reaction of Li[MeBH3] with 3-tBu-pyrazole, 3,5-Me2-pyrazole (Hpz*), and 3,5-tBu2-pyrazole. The reaction of Li[MeBH3] with 3 equiv of HpzMs (THF, 60 °C, 2 days) in the presence of 3 mol % MeB(OiPr)2 affords Li[MeB(3-Ms-pz)3] (75%), but only Li[MeBpMs] and Li[MeBpMs*] without this additive. Similar results were observed for the reaction of Li[MeBH3] with 3-CF3-pyrazole. The reaction of Na[BH4] and 3 equiv of Hpz* (THF, 60 °C, 4 h) yields Na[B(3,5-Me2-pz)2H2] in 93% yield in the presence of BF3·Et2O (5 mol % vs Hpz*) but only 5% without this additive. Coordination of the pyrazole to the Lewis acid is expected to decrease the pKa of the pyrazole and increase the rate of B−H bond protonolysis

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of Ti^III and Ti^IV Tris(pyrazolyl)borate Complexes

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of TiIII and TiIV Tris(pyrazolyl)borate Complexe

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylene

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylen

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of Ti^III and Ti^IV Tris(pyrazolyl)borate Complexes

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of TiIII and TiIV Tris(pyrazolyl)borate Complexe

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of Ti^III and Ti^IV Tris(pyrazolyl)borate Complexes

Comparison of Olefin Polymerization Behavior of Catalysts Generated by MAO Activation of TiIII and TiIV Tris(pyrazolyl)borate Complexe

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylene

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylen

Copolymerization of Silyl Vinyl Ethers with Olefins by (α-diimine)PdR⁺

This paper reports that (α-diimine)PdMe+ catalyzes the copolymerization of olefins and silyl vinyl ethers. The reactions of (α-diimine)PdMe+ (α-diimine = (2,6-iPr2−C6H3)NCMe−CMeN(2,6-iPr2−C6H3)) with excess vinyl ethers CH2CHOR (1a−d:  R = tBu (a), SiMe3 (b), SiPh3 (c), Ph (d)) in CH2Cl2 at 20 °C afford polymers for 1a (rapidly) and 1b (slowly) but not for 1c or 1d. The structures of poly(1a,b) indicate a cationic polymerization mechanism. The reaction of (α-diimine)PdMe+ with 1−2 equiv of 1a−d proceeds by sequential CC π-complexation to form (α-diimine)PdMe(CH2CHOR)+ (2a−d), 1,2 insertion to form (α-diimine)Pd(CH2CHMeOR)+ (3a−d), reversible isomerization to (α-diimine)Pd(CMe2OR)+ (4a−d), β-OR elimination to generate (α-diimine)Pd(OR)(CH2CHMe)+ (not observed), and allylic C−H activation to yield (α-diimine)Pd(η3-C3H5)+ (5) and ROH. The reaction of (α-diimine)PdMe+ with 1-hexene/1b and 1-hexene/1c mixtures in CH2Cl2 at 20 °C affords copolymers containing up to 20 mol % silyl vinyl ether. The copolymers were purified to be free of any −[CH2CHOSiR3]n− homopolymer. The copolymer structures are similar to that of homopoly(1-hexene) generated under the same conditions. The major comonomer units are CH3CH(OSiR3)CH2−, CH2(OSiR3)CH2− and −CH2CH(OSiR3)CH2−. The 1-hexene/CH2CHOSiR3 copolymers can be desilylated to give 1-hexene/CH2CHOH copolymers. The results of control experiments argue against cationic and radical mechanisms for the copolymerization, and an insertion/chain-walking mechanism is proposed

Synthesis, <i>cis</i>/<i>trans</i> Isomerization, and Reactivity of Palladium Alkyl Complexes That Contain a Chelating N-Heterocyclic-Carbene Sulfonate Ligand

The chemistry of palladium alkyl complexes that incorporate the NHC-sulfonate ligand N-(2,6-iPr2-Ph)-N′-2-benzenesulfonate-NHC ([C-O]−, NHC = cyclo-CNCH2CH2N) is described. The reaction of {C-O}Ag (2) with Pd2(μ-Cl)2Me2(PPh3)2 affords cis-C,C-{C-O}PdMe(PPh3) (3, 79%). The reaction of 2 with Pd2(μ-Cl)2Me2(2,6-lutidine)2 at 25 °C in CH2Cl2 gives a 4/1 mixture of trans-C,C-{C-O}PdMe(2,6-lutidine) (4a) and cis-C,C-{C-O}PdMe(2,6-lutidine) (4b), which were isolated in 62% and 18% yield, respectively, by recrystallization. The NHC-sulfonate ligands bind in a κ2-C,O fashion in 3 and 4a,b. 4a isomerizes to 4b by dissociation of the Ar-SO3– unit to form a configurationally labile three-coordinate intermediate. This process is accelerated by hydrogen bond donors (CD3OD, lutidinium) and Lewis acids (B(C6F5)3) that can labilize the sulfonate group. 4a and 4b react with 1 equiv of B(C6F5)3 to yield the O-bound adduct cis-C,C-{C-O-B(C6F5)3}PdMe(2,6-lutidine) (5). 5 decomposes at 40 °C by C–C reductive elimination to afford (in the presence of pyridine to trap the B(C6F5)3) N-(2,6-iPr2-Ph)-N′-2-benzenesulfonate-imidazolium methyl (6). trans-C,C-4a reacts with CO and tBuNC to yield the insertion products {C-O}Pd{C(O)Me}(2,6-lutidine) (7) and {C-O}Pd{C(NtBu)Me}(tBuNC) (9), in which the acyl and iminoacyl ligands are cis to the NHC ligand, via stereospecific displacement of Ar-SO3– by the substrate followed by migratory insertion. The bis-isocyanide adduct {κ1-C-C-O}PdMe(tBuNC)2, in which the tBuNC ligands are trans, is an intermediate in the formation of 9. tBuNC reversibly displaces the Ar-SO3– ligand of 9 to form {κ1-C-C-O}Pd{C(NtBu)Me}(tBuNC)2 (10). In contract, cis-C,C-4a does not undergo net reaction with CO and reacts with tBuNC via reductive elimination to yield 6. Displacement of the ArSO3– ligand of cis-C,C-4b by potential substrates yields adducts in which the substrate and Me group are trans and insertion is not possible

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylene

Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylen