Half-titanocenes (CpCH 2 CH 2 O)TiCl 2 (1) and (CpCH 2 CH 2 OCH 3 )TiCl 3 (2), (Cp ) cyclopentadienyl), activated by methylaluminoxane (MAO), have been tested in the polymerization of styrene, p-methylstyrene, and p-chlorostyrene. Both catalysts produce syndiotactic polystyrene and poly-p-methylstyrene in the whole range of explored temperatures and concentrations. In the polymerization of p-chlorostyrene, however, catalyst 1 affords a syndiotactic polymer only at the low temperature and the high monomer concentrations; and catalyst 2 gives only an atactic product. The results have been compared to those obtained in the presence of the catalytic system CpTiCl 3 (3)/MAO and have been discussed considering the possible oxidation states of the titanium species and the geometries of the active sites.
Treatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)-AlR 2 (R ) Me (1); R ) Et (2); R ) iBu (3)) in high yield. In the solid state, complexes 1-3 were isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp ligand κ 2 -coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl rings. The investigation of the solution structure of 1-3 by means of VT 1 H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy (∆H # ) for racemization processes (10.1 ( 0.2 kcal‚mol -1 (1); 11.6 (0.3 kcal‚mol -1 (2); 14.1 ( 0.2 kcal‚mol -1 (3)) was dependent on steric hindrance at the aluminum center. Reaction of 1 or 2 with B(C 6 F 5 ) 3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [(bpzmp)AlR] + [RB(C 6 F 5 ) 3 ] -(R ) Me (4); R ) Et ( 5)), in which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic compounds (B(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ]) proceeds by net β-H abstraction from the isobutyl group, forming [(bpzmp)AliBu] + [HB(C 6 F 5 ) 3 ] -(6) along with isobutene; in contrast, reaction with the Bro ¨nsted acid [HNMe 2 Ph][B(C 6 F 5 ) 4 ] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ringopening polymerization of -caprolactone ( -CL), producing high-molecular-weight polymers. The 1 H NMR monitoring of the reaction between 4 and -CL in 1:1 molar ratio showed that the initiation involves monomer insertion into the Al-O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe] + -[MeB(C 6 F 5 ) 4 ] -(bpzphe ) 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate (8). The growth of the polymer chain occurs through continuous insertions of the monomer in the Al-alkoxide bond.
New ruthenium Grubbs' and Hoveyda-Grubbs' second generation catalysts bearing N-alkyl/N-isopropylphenyl N-heterocyclic carbene (NHC) ligands with syn or anti backbone configuration were obtained and compared in model olefin metathesis reactions. Different catalytic efficiencies were observed depending on the size of the N-alkyl group (methyl or cyclohexyl) and on the backbone configuration. The presence of an N-cyclohexyl substituent determined the most significant reactivity differences between catalysts with syn or anti phenyl groups on the backbone. In particular, anti catalysts proved highly efficient, especially in the ring-closing metathesis (RCM) of encumbered diolefins, while syn catalysts showed low efficiency in the RCM of less hindered diolefins. This peculiar behavior, rationalized through DFT studies, was found to be related to the high propensity of these catalysts to give nonproductive metathesis events. Enantiopure anti catalysts were also tested in asymmetric metathesis reactions, where moderate enantioselectivities were observed. The steric and electronic properties of unsymmetrical NHCs with the N-cyclohexyl group were then evaluated using the corresponding rhodium complexes. While steric factors proved unimportant for both syn and anti NHCs, a major electron-donating character was found for the unsymmetrical NHC with anti phenyl substituents on the backbone.
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