Dinuclear Pentacoordinated Organoyttrium Biphenolates as Initiators for the Synthesis of High-Molecular Weight Isotactic Poly(2-vinylpyridine).

Dinuclear organoyttrium biphenolato complexes have been synthesized for the first time starting from Y(CH2SiMe3)3(THF)2 and readily available biphenols with bulky substituents at the 3,3'-positions. XRD analysis revealed that these complexes are formed as homochiral dimers even though racemic biphenols were employed. Such dinuclear penta-coordinated yttrium complexes were evaluated as initiators in the polymerization of 2-vinylpyridine (2-VP). High activity and excellent isotacticity (mm > 99%) were observed. DSC analysis of poly(2-VP) showed a Tm = 221 °C, which is the highest value reported to date. Attempts to improve the low initiation efficiency of the present system were made by co-use with several organo-main-group metal reagents. Specifically, good molecular weight control within the high Mn region was achieved by the addition of Et3Al (Al/Y = 10).

Random copolymerization of epsilon-caprolactone with lactide using a homosalen-Al complex.

The bulky substituents of homosalen complexes decelerate the ring-opening polymerization of racemic lactide (LA). The substituent effects provide the first catalysis for the random copolymerization of epsilon-caprolactone (CL) with LA (CL/LA = 1:1). The copolymerization of CL with LA (CL/LA = 1:1) by the iPr(3)Si-substituted homosalen-Al complex 2 affords the practically random copolymer in a controlled manner. The reactivity ratios, average sequence lengths of CL and LA during the copolymerization, abundance ratios of the triad caproyl sequences, and T(g) value of the obtained copolymer indicate that the copolymer was random with a somewhat alternative tendency (r(CL)r(LA) = 0.80).

An efficient protocol of iridium-catalyzed allylic substitution reaction and its application to polymer synthesis: complementary regio- and stereoselective allylation polycondensation via Ir and Pd catalyses.

An efficient protocol of the Ir-catalyzed allylic substitution reaction is reported using N,O-bis(trimethylsilyl)acetamide as a base in the presence of nBu4NF as a cocatalyst. The reaction completely proceeded under very mild conditions, and a branched allylated compound that is not easy to access via the Tsuji-Trost reaction can be synthesized. The reaction system is practical enough to be applicable for polymer syntheses. The Ir- and Pd-catalyzed allylation polycondensations generally show complementary regio- and stereoselectivities. The Ir-catalyzed reaction is versatile, and a mixed dual regioselectivity such as a branched-linear selectivity on each electrophile can also be achieved.

Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes.

Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymerization (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH(2)=N,N'-bis(salicylidene)ethylene-1,2-diamine; homosalenH(2)=N,N'-bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepared in situ are pure enough to apply to the polymerizations without purification. The crystal structures of the key complexes are elucidated by X-ray diffraction, which confirms that they are chiral. However, analysis of the (1)H and (13)C NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in solution at 25 degrees C and that the complexes are achiral under the polymerization conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-dimethyl substituents in the backbone (ArCH==NCH(2)CMe(2)CH(2)N==CHAr), whereas tBuMe(2)Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (P(meso)=0.9(8); T(m)=210 degrees C). The ratio of the rate constants in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymerization, which is the most attractive in industry, although with some loss of stereoselectivity at high temperature, and the afforded polymer shows a higher melting temperature (P(meso)=0.9(2), T(m) up to 189 degrees C) than that of homochiral poly(L-lactide) (T(m)=162-180 degrees C). The "livingness" of the bulk polymerization at 130 degrees C is maintained even at a high conversion (97-98 %) and for an extended polymerization time (1-2 h).

Homogeneous two-component polycondensation without strict stoichiometric balance via the Tsuji-Trost reaction: remote control of two reaction sites by catalysis.

Carothers and Flory established the fundamental principles of homogeneous two-component polycondensation reactions by which a variety of commonly used polymers such as polyesters and polyamides (nylons) are synthesized even now. One of the most critical factors to achieve a high degree of polymerization is a strict adherence of stoichiometric balance of the two components. An imbalanced ratio of the two starting materials leads to lower degree of polymerization and, consequently, polymers of poor quality. Here we report the details of a strategy, validated by appropriate experiments, which enables an efficient homogeneous two-component polycondensation via palladium-catalyzed allylic substitution (the Tsuji-Trost) reaction without strict stoichiometric balance.

Heterodimerization of olefins. 1. Hydrovinylation reactions of olefins that are amenable to asymmetric catalysis.

Through a systematic examination of ligand and counterion effects, new protocols for a nearly quantitative and highly selective codimerization of ethylene and various functionalized vinylarenes have been discovered. In a typical reaction, 4-bromostyrene and ethylene undergo codimerization in the presence of 0.0035 equiv each of [(allyl)NiBr]2, triphenylphosphine, and AgOTf in CH2Cl2 at -56 degrees C to give 3-(4-bromophenyl)-1-butene in >98% yield and selectivity. Corresponding reactions with [(allyl)PdX]2 are much less efficient and less selective and may require further optimization before a viable system can be identified. Another useful protocol that gives comparable yield and selectivity involves the use of a single-component catalyst prepared from allyl 2-diphenylphosphinobenzoate, Ni(COD)2, and (C6F5)3B. Recognition of a synergistic relationship between a chiral hemilabile ligand (for example, (R)-2-methoxy-2'-diphenylphosphino-1,1'-binaphthyl, MOP) and a highly dissociated counteranion (BARF or SbF6) in an enantioselective version of the Ni-catalyzed reaction raises the prospects of developing a practical route for the synthesis of 3-arylbutenes. Several pharmaceutically relevant compounds, including widely used 2-arylpropionic acids, can be synthesized from these key intermediates. This reaction appears to be quite general. Synthesis of several new 2-diphenylphosphino-1,1-binaphthyl derivatives, prepared to probe the effect of hemilabile coordination on the efficiency and selectivity of the reaction, are also described.

Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes: exploration of a chain-end control mechanism.

Stereoselective polymerization of racemic lactide (rac-LA) was examined using Al-achiral ligand complexes. By introduction of substituents in aromatic rings of Schiff base ligands, a higher selectivity was obtained without any chiral auxiliaries in the catalyst via a chain-end control mechanism. The T(m) values (T(m) 170-192 degrees C) were comparable to or higher than that of homochiral polymer, poly(L-LA) (T(m) 162 degrees C), and a thermally more stable polylactide than poly(L-LA) was prepared from rac-LA.