Masayoshi Nishiura

Carboxylation of Organoboronic Esters Catalyzed by N‐Heterocyclic Carbene Copper(I) Complexes

Carbon dioxide (CO2) is an attractive, cheap, and nontoxic C1 source. However, because of its high thermodynamic stability and low reactivity, the use of CO2 as a C1 source for C C bond formation usually requires highly nucleophilic organometallic reagents, such as alkyllithium compounds and Grignard reagents. Less nucleophilic organoboron compounds, though easily available, usually do not react with CO2. Recently, transition-metal-catalyzed addition of carbon nucleophiles to CO2 has attracted much attention. [2,3] In this context, Iwasawa and co-workers have reported the catalytic carboxylation of aryland alkenylboronic esters with CO2 in the presence of a rhodium(I) compound and additives. This reaction is potentially useful for the synthesis of functionalized carboxylic acid derivatives because of the easy availability of various functionalized organoboronic esters. Unfortunately, however, the Rh catalyst systems showed only limited tolerance toward functional groups. Although carbonyl and cyano groups survived the reaction conditions, more reactive functional moieties, such as bromo, iodo, and vinyl groups, seemed intolerant. Moreover, little information about the active catalyst species and the reaction mechanism was available because of the complexity of the catalyst systems. These difficulties have limited the application scope of the Rh catalyst systems. The search for new catalysts for more efficient, selective CO2 transformation as well as the clarification of the catalytic process is therefore of interest and importance. We report herein an excellent N-heterocyclic carbene copper(I) catalyst system for the carboxylation of aryland alkenylboronic esters with CO2. This Cu catalyst system not only showed higher functional-group tolerance, but could also afford structurally characterizable active catalyst species, thus offering unprecedented insight into the mechanistic aspects of the catalytic process. Copper complexes bearing N-heterocyclic carbene (NHC) ligands have been reported to act as efficient catalysts for the transformation of various carbonyl compounds, such as conjugate reduction of a,b-unsaturated carbonyl compounds, hydrosilylation of ketones, and also for the reduction of CO2. [7] In addition, many copper compounds have also been reported to promote nucleophilic addition of organoboron compounds to electrophiles, such as a,b-unsaturated carbonyls and allylic carbonates. These results encouraged us to examine the carboxylation of organoboronic esters with CO2 by use of N-heterocyclic carbene copper complexes as catalysts. At first we examined the reaction of 4methoxyphenylboronic acid 2,2-dimethyl-1,3-propanediol ester (1a) with CO2 using N-heterocyclic carbene copper species generated in situ from CuCl, IPr·HCland tBuOK. In

Isoprene Polymerization with Yttrium Amidinate Catalysts: Switching the Regio‐ and Stereoselectivity by Addition of AlMe3

The preparation of polymers with desired microstructures and properties by controlling the regioand stereoselectivity of olefin polymerization is an important research area. Approaches toward this goal have, to date, mainly involved modifying the ancillary ligands of metal catalysts. The use of chelating amidinate ligands as an alternative to cyclopentadienyl ligands in the development of rare-earth-metal (Group 3 and lanthanide) based polymerization catalysts has received considerable attention. Although the majority of amidinate-containing rare-earth-metal complexes reported to date contain two or three amidinate ligands, recent work by Hessen and co-workers has demonstrated that benzamidinates with bulky substituents at the nitrogen atoms, such as N,N’-bis(2,6-diisopropylphenyl)benzamidinate [PhC(NC6H4iPr2-2,6)2] , can serve as excellent ancillary ligands for a series of mono(amidinate)/dialkyl or cationic mono(amidinate)/alkyl rare-earth-metal complexes. However, despite the extensive interest in using amidinate rareearth-metal complexes as polymerization catalysts, the polymerization chemistry reported to date for these complexes has been limited mainly to that of ethylene and polar monomers; studies on the polymerization of higher olefins remain scarce. In particular, the use of an amidinateligated rare-earth-metal catalyst for the polymerization of a conjugated diene, such as isoprene, has not been reported to date. We recently found that cationic rare-earth alkyl complexes can serve as excellent catalysts for the polymerization and copolymerization of various olefins. During these studies, we became interested in the polymerization of isoprene by cationic rare-earth alkyl complexes bearing a single amidinate ligand, and report herein that the amidinateligated yttrium complex [(NCN)Y(o-CH2C6H4NMe2)2] (1; NCN = PhC(NC6H4iPr2-2,6)2) is a unique catalyst precursor for the polymerization of isoprene. Thus, complex 1 shows extremely high activity and excellent 3,4-isospecificity for the polymerization of isoprene in the presence of one equivalent of [Ph3C][B(C6F5)4]. More remarkably, however, the regioand stereoselectivity of this catalyst system can be switched from 3,4-isospecific to 1,4-cis selective simply by adding an alkylaluminum compound, such as AlMe3. Although the polymerization of isoprene by various catalyst systems has been studied extensively, 5] such a dramatic switching of the regioand stereoselectivity is, to our knowledge, unprecedented. Isolation of the heterotrinuclear Y/Al complex [(NCN)Y{(m-Me)2AlMe2}2] (2) from the reaction of 1 with AlMe3 and its performance in the polymerization of isoprene are also described. Treatment of the tris(aminobenzyl)yttrium complex [Y(oCH2C6H4NMe2)3] [7] with one equivalent of the amidine ligand N,N’-bis(2,6-diisopropylphenyl)benzamidine (NCNH) in THF or toluene at room temperature overnight affords the corresponding mono(amidinate) bis(aminobenzyl) complex 1 in 85% yield (Scheme 1). The reaction can be completed in 3 h if it is carried out at 70 8C. Complex 1 was fully characterized by H and C NMR spectroscopy, elemental analysis, and X-ray crystallography (Figure 1). The NCN unit in 1 is bonded to the Y center through its two N atoms, as observed in other amidinate complexes. The two aminobenzyl groups are bonded to the Yatom in a chelating fashion through both the N atom and the benzyl carbon atom. Intramolecular coordination of the amino group means that complex 1 does not possess a THF co-ligand, in contrast with the THF-containing CH2SiMe3 analogue [(NCN )Y(CH2SiMe3)2(thf)]. [2d] Complex 1 is slightly soluble in hexane but highly soluble in toluene and THF. The neutral complex 1 does not catalyze the polymerization of isoprene but it becomes extremely active in the presence of one equivalent of [Ph3C][B(C6F5)4], whereby it converts 750 equivalents of isoprene quantitatively into polyisoprene in 2 min at room temperature. This reaction proceeds with high 3,4-regioselectivity (91%) and some degree of isotacticity (mm 50%) (Table 1, entry 3). When the polymerization is carried out at low temperature ( 10 8C), an even higher regioand stereoselectivity is

Cationic Scandium Allyl Complexes Bearing Mono(cyclopentadienyl) Ligands: Synthesis, Novel Structural Variety, and Olefin-Polymerization Catalysis

The one-pot salt-metathesis reaction of ScCl(3), cyclopentadienyl lithium salts, and allylmagnesium chlorides afforded with ease the corresponding base-free half-sandwich scandium di(eta(3)-allyl) complexes [(C(5)Me(4)SiMe(3))Sc(C(3)H(5))(2)] (1 a), [(C(5)Me(5))Sc(C(3)H(5))(2)] (1 b), and [(C(5)Me(5))Sc(2-MeC(3)H(4))(2)] (1 c) in high yields. Reaction of 1 a with 1 equivalent of [PhNMe(2)H][B(C(6)F(5))(4)] in toluene gave rapidly the N,N-dimethylaniline-coordinated cationic mono(eta(3)-allyl) complex [(C(5)Me(4)SiMe(3))Sc(eta(3)-C(3)H(5))(eta(6)-PhNMe(2))][B(C(6)F(5))(4)] (2). The similar reaction of 1 a with [Ph(3)C][B(C(6)F(5))(4)] yielded the analogous toluene-separated ion pair [(C(5)Me(4)SiMe(3))Sc(eta(3)-C(3)H(5))(eta(6)-PhMe)][B(C(6)F(5))(4)] (3). When [PhNMe(2)H][BPh(4)] was treated with 1 a, the contact ion pair [(C(5)Me(4)SiMe(3))Sc(eta(3)-C(3)H(5))( mu,eta(6)-Ph)BPh(3)] (4) was obtained. Recrystallization of 2, 3, and 4 in THF yielded the corresponding thf-separated ion pair complexes [(C(5)Me(4)SiMe(3))Sc(eta(3)-C(3)H(5))(thf)(2)][B(C(6)F(5))(4)] (5) and [(C(5)Me(4)SiMe(3))Sc(eta(3)-C(3)H(5))(thf)(2)][BPh(4)] (6). The N,N-dimethylaniline-coordinated cationic scandium allyl complex 2 and the toluene-coordinated analogue 3 showed high activity (activity: 3>2) toward the polymerization and copolymerization of isoprene and norbornene to afford random copolymers with a broad range of isoprene content (33-86 mol %). The tight ion pair 4 and the thf-coordinated complexes 5 and 6 showed no activity under the same conditions. These results offer unprecedented insight into the structure-activity relationship of a cationic metal polymerization-catalyst system.

Cationic scandium aminobenzyl complexes. Synthesis, structure and unprecedented catalysis of copolymerization of 1-hexene and dicyclopentadiene

A structurally well-defined THF-free cationic half-sandwich scandium aminobenzyl complex serves as a novel catalyst for the first copolymerization of 1-hexene with dicyclopentadiene to give the random copolymers with a wide range of 1-hexene contents (32-70 mol%) unavailable previously.