Zhaomin Hou

Recent developments in organolanthanide polymerization catalysts

This review describes recent advances in the synthesis and polymerization chemistry of organolanthanide complexes (including those of scandium and yttrium), with emphasis being placed on the complexes that show novel activity and selectivity in polymerization reactions.

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

Copper‐Catalyzed Direct Carboxylation of CH Bonds with Carbon Dioxide

The use of carbon dioxide (CO2) as a C1 building block for chemical synthesis has recently attracted much interest because of its abundance, low cost, nontoxicity, and high potential as a renewable source. However, the high thermodynamic stability and low reactivity of CO2 means that its use in C C bond-forming reactions usually requires organometallic reagents or other preactivated substrates such as organic halides (Scheme 1, routes a and b). Although the reaction of C H bonds with CO2 would be the most attractive and atom-economic route for the synthesis of carboxylic acids (Scheme 1, route c), the direct carboxylation of a C H bond with CO2 has remained a challenge. [4]

Novel polymerization catalysts and hydride clusters from rare-earth metal dialkyls

This Review gives an overview on recent progress in the synthesis and chemistry of rare-earth metal dialkyl complexes bearing monoanionic ancillary ligands, with an emphasis on novel polymerization catalysts. These structurally well-defined and highly reactive compounds are prepared either by alkane elimination reactions between trialkyl rare-earth complexes and acidic neutral ligands, or by the metathetical reactions of rare-earth trihalides with the alkali metal salts of the corresponding ligands. On treatment with an appropriate borate compound, the dialkyl complexes are converted into the corresponding cationic monoalkyl species, which serve as excellent catalysts for the polymerization and copolymerization of a variety of olefins to yield a series of new polymer materials that exhibit novel properties. Alternatively, hydrogenation of the dialkyl rare-earth complexes with H(2) affords a new class of rare-earth polyhydride complexes with unique features in terms of both their structure and reactivity.

Catalytic boracarboxylation of alkynes with diborane and carbon dioxide by an N-heterocyclic carbene copper catalyst.

By the use of an N-heterocyclic carbene copper(I) complex as a catalyst, the boracarboxylation of various alkynes (e.g., diaryl alkynes, aryl/alkyl alkynes, and phenylacetylene) with a diborane compound and carbon dioxide has been achieved for the first time, affording the α,β-unsaturated β-boralactone derivatives regio- and stereoselectively via a borylcupration/carboxylation cascade. Some important reaction intermediates were isolated and structurally characterized to clarify the reaction mechanism.

N-Heterocyclic carbene (NHC)–copper-catalysed transformations of carbon dioxide

This minireview gives an overview of the chemical transformations of carbon dioxide (CO2) catalysed by N-heterocyclic carbene (NHC)–copper complexes. NHC–copper complexes can serve as excellent catalysts for the carboxylation of various substrates with CO2 and the reduction of CO2 to CO or formic acid derivatives. In addition, NHC ligands enable the isolation of structurally characterisable key reaction intermediates, thus helping in understanding the mechanistic details of the catalytic processes. The related reactions catalysed by other metal complexes with NHC ligands are also briefly described.

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

Dinitrogen Cleavage and Hydrogenation by a Trinuclear Titanium Polyhydride Complex

Titanium Cleaver A century after its discovery, the Haber Bosch process is still used to produce ammonia from nitrogen for fertilizer. Nonetheless, the process requires high temperature and pressure, and chemists continue to look for synthetic analogs to microbial nitrogenase enzymes, which have managed to slice through the N2 triple bond under ambient conditions for millennia. Most efforts in this vein have relied on a boost from the reducing power of alkali metals. Shima et al. (p. 1549; see the Perspective by Fryzuk) instead explored the reactivity of a titanium hydride cluster, which cleanly slices through N2 at room temperature and incorporates the separated N atoms into its framework. Though ammonia was not produced, the system offers hope in the search for mild nitrogen reduction catalysts. The collective reactivity of three hydride-bridged titanium centers cleaves dinitrogen under mild conditions. [Also see Perspective by Fryzuk] Both the Haber-Bosch and biological ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong N≡N triple bond and forming an N–H bond. This has spurred investigations of the reactivity of molecular multimetallic hydrides with dinitrogen. We report here the reaction of a trinuclear titanium polyhydride complex with dinitrogen, which induces dinitrogen cleavage and partial hydrogenation at ambient temperature and pressure. By 1H and 15N nuclear magnetic resonance, x-ray crystallographic, and computational studies of some key reaction steps and products, we have determined that the dinitrogen (N2) reduction proceeds sequentially through scission of a N2 molecule bonded to three Ti atoms in a μ-η1:η2:η2-end-on-side-on fashion to give a μ2-N/μ3-N dinitrido species, followed by intramolecular hydrogen migration from Ti to the μ2-N nitrido unit.

Carboxylation of Alkylboranes by N-Heterocyclic Carbene Copper Catalysts: Synthesis of Carboxylic Acids from Terminal Alkenes and Carbon Dioxide†

Caught in the act: N-Heterocyclic carbene copper(I) complexes (1; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) serve as an excellent catalyst for the carboxylation of alkylboranes (2; R=alkyl) with CO(2) to afford a variety of functionalized carboxylic acids (3) in high yields. A novel copper methoxide/alkylborane adduct (A) and its subsequent CO(2) insertion product (B) have been isolated and shown to be true active catalyst species.

Rare-earth-catalyzed C-H bond addition of pyridines to olefins.

An efficient and general protocol for the ortho-alkylation of pyridines via C-H addition to olefins has been developed, using cationic half-sandwich rare-earth catalysts, which provides an atom-economical method for the synthesis of alkylated pyridine derivatives. A wide range of pyridine and olefin substrates including α-olefins, styrenes, and conjugated dienes are compatible with the catalysts.