Eugene Y.-X. Chen

Coordination Polymerization of Polar Vinyl Monomers by Single-Site Metal Catalysts

1.3. Scope of Review 5161 2. Methacrylate Polymerization 5161 2.1. Lanthanide Complexes 5161 2.1.1. Nonbridged Lanthanocenes 5161 2.1.2. ansa-Lanthanocenes 5164 2.1.3. Half-Lanthanocenes 5166 2.1.4. Non-lanthanocenes 5166 2.2. Group 4 Metallocenes 5170 2.2.1. Nonbridged Catalysts 5170 2.2.2. ansa-C2v-Ligated Catalysts 5173 2.2.3. ansa-C2-Ligated Catalysts 5173 2.2.4. ansa-C1-Ligated Catalysts 5176 2.2.5. ansa-Cs-Ligated Catalysts 5177 2.2.6. Constrained Geometry Catalysts 5178 2.2.7. Half-Metallocene Catalysts 5180 2.2.8. Supported Catalysts 5180 2.3. Other Metallocene Catalysts 5180 2.4. Nonmetallocene Catalysts 5181 2.4.1. Group 1 and 2 Catalysts 5181 2.4.2. Group 13 Catalysts 5183 2.4.3. Group 14 Catalysts 5186 2.4.4. Transition-Metal Catalysts 5187 3. Acrylate Polymerization 5188 3.1. Lanthanocenes 5188 3.2. Group 4 Metallocenes 5189 3.3. Nonmetallocenes 5190 4. Acrylamide and Methacrylamide Polymerization 5191 4.1. Acrylamides 5191 4.2. Methacrylamides 5192 4.3. Asymmetric Polymerization 5193 5. Acrylonitrile and Vinyl Ketone Polymerization 5196 5.1. Acrylonitrile 5196 5.2. Vinyl Ketones 5196 6. Copolymerization 5197 6.1. Polar-Nonpolar Block Copolymers 5197 6.2. Polar-Nonpolar Random Copolymers 5199 6.3. Polar-Polar Copolymers 5204 7. Ion-Pairing Polymerization 5206 8. Summary and Outlook 5208 9. Acknowledgments 5208 10. References 5208

Alane‐Based Classical and Frustrated Lewis Pairs in Polymer Synthesis: Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High‐Molecular‐Weight Polymers

The seminal works by the research groups of Stephan and Erker uncovered the concept of “frustrated lewis pairs” (FLPs) to describe pairs formed by a sterically encumbered borane Lewis acid (most commonly B(C6F5)3) and a base (e.g. (tBu)3P), in which these two components are sterically precluded from forming classical donor/acceptor adducts Instead, the unquenched, opposite reactivity of FLPs can carry out unusual reactions or reactions that were previously known to be possible only by transition-metal complexes, such as activation of H2, CO2, N2O, B H bonds, alkenes and alkynes, as well as catalytic hydrogenation. The direct use of FLPs in polymer synthesis and/or in polymerization catalysis is currently still missing from this list. Our research group reported in 2002 that strongly acidic, sterically encumbered alane Al(C6F5)3 [5] and bulky 2,6-di-tertbutyl pyridine, which do not form a classical acid/base adduct when the unsolvated alane is used, work cooperatively to activate and break arene C H bonds when the toluene/alane adduct, C7H8·Al(C6F5)3, [6] is used, or when toluene is added to the above-mentioned mixture (Scheme 1). Preceding the current term FLP, this result was an early example of the unusual reactivity of FLPs relative to what was generally recognized. Since then, our interest has continued to focus on utilizing the high Lewis acidity of Al(C6F5)3 and, more importantly in many cases, the unique catalytic feature of the active species derived from this alane, for the polymerization of functionalized alkenes. Reported herein is a significant development in this continuing effort: Al(C6F5)3based Lewis pairs rapidly polymerize polar vinyl monomers at room temperature (Scheme 2), including methyl methacrylate (MMA) and the naturally renewable methylene butyr-

Conjugate-addition organopolymerization: rapid production of acrylic bioplastics by N-heterocyclic carbenes.

Organocatalysis using N-heterocyclic carbenes (NHCs) has attracted growing interest because of its unique reactivity and selectivity in many different types of organic reactions. The utility of NHC-mediated reactions has also been expanded to polymer synthesis, predominantly through the ring-opening polymerization (ROP) of cyclic monomers such as cyclic esters, epoxides, cyclic siloxanes, and N-carboxyl anhydrides. NHC-catalyzed step-growth polymerization has been reported as well. Polymerization of a,b-unsaturated esters (acrylics) such as methyl methacrylate (MMA) has also been recently realized through the classic group-transfer polymerization (GTP) initiated by silyl ketene acetals using NHCs as alternative nucleophilic catalysts to activate the acetal initiator. In addition, such acrylic monomers can be rapidly polymerized by frustrated Lewis pairs (FLPs) consisting of bulky NHC bases, such as the Arduengo carbenes 1,3-di-tert-butylimidazolin-2-ylidene (ItBu) and 1,3-di-mesityl-butyl-imidazolin-2-ylidene (IMes), and the strongly acidic, sterically encumbered, perfluoroaryl alane, via the proposed zwitterionic imidazolium enolaluminate intermediates. When using such Arduengo NHCs alone, no MMA conversion was observed in either toluene or THF. In contrast, the Enders triazolylidene carbene TPT (1,3,4triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene), which was estimated to be 10 times less nucleophilic than the imidazolylidene carbene (IMes), catalyzes tail-to-tail dimerization (umpolung) of MMA and other methacrylate substrates, whereas the common imidazolylidene carbenes are ineffective. As can be seen clearly from the above overview, although the NHC-mediated ROP of cyclic monomers has been highly successful, the NHC-mediated conjugate-addition polymerization of functionalized alkenes such as acrylics still requires the use of a nucleophilic initiator in the case of GTP or a Lewis acid catalyst in the case of the FLP polymerization. To the best of our knowledge, efficient conjugate-addition polymerization of such monomers directly by NHCs (i.e., in the absence of any other initiator or catalyst components) has not been previously achieved. Communicated herein is the first, rapid conjugate-addition polymerization of a,b-unsaturated esters by NHCs alone, with the most active catalyst achieving quantitative monomer conversion in less than one minute. Among the NHC catalysts investigated (Figure 1), the most nucleophilic NHC (ItBu) directly polymerizes a large excess (e.g., 800 equiv) of renewable methylene butyrolactone monomers, including naturally occurring amethylene-g-butyrolactone (MBL) and plant biomassderived g-methyl-a-methylene-g-butyrolactone (MMBL), at room temperature into mediumor high-molecular-weight polymers in less than one minute, thus giving a high turnover frequency (TOF) of greater than 4.8 10 h . The rate of the polymerization is strongly affected by the relative nucleophilicity of the NHC catalysts employed herein (Figure 1), and there exists a remarkable selectivity of the NHC for substrate structures, thus leading to three different modes of reaction involving acrylics. We hypothesized that even though NHCs showed no activity in the polymerization of the most common acrylic monomer MMA in toluene or THF, 11] they could directly polymerize renewable the monomers MBL and MMBL because such a-methylene-g-butyrolactones exhibit greater reactivity in chain-growth polymerization than typical alkyl methacrylates (e.g., MMA) and this activity is attributable to the presence of both the nearly planar five-membered lactone ring (which provides resonance stabilization for the active species) and the higher energy exocyclic C=C double bond (as a result of the ring strain and the fixed s-cis conformation). Indeed, with a monomer to initiator ([M]/[NHC]) ratio of 200, rather fast polymerization of MMBL by ItBu (0.5 mol%) was observed even in toluene at room temperature, thus achieving 93 and 99% conversions in 30 minutes and 1 hour, respectively (entry 1, Table 1). The polymerization was heterogeneous, thereby producing poly-g-methyl-a-methylene-gbutyrolactone (PMMBL) with a number-average molecular weight (Mn) of 58.4 kg mol 1 at 93% conversion, which is about three times higher than the calculated Mn. The less nucleophilic IMes showed noticeably lower activity (entry 2), whereas the least nucleophilic NHC of this series, TPT, exhibited the lowest activity (entry 3). The polymerization by ItBu in THF was still heterogeneous and slower; it yielded PMMBL with a considerably lower Mn of 23.0 kg mol 1 (polydispersity index, PDI = 1.69, entry 4). A similar trend Figure 1. Rapid polymerization of the renewable MMBL by ItBu in DMF at room temperature and correlation of the polymerization activity of NHCs with their relative nucleophilicity.

Diesel and Alkane Fuels From Biomass by Organocatalysis and Metal–Acid Tandem Catalysis

Combo deal: Biomass furaldehydes are upgraded into oxygenated diesel and high-quality C10-12 linear alkane fuels. The first of two steps involves solvent-free self-condensation (Umpolung) through organocatalysis using an N-heterocyclic carbene (NHC), yielding C10 -C12 furoin intermediates. In the metal-acid tandem catalysis step, in water, the furoin intermediates are converted into oxygenated biodiesel by hydrogenation, etherification or esterification; or into premium alkane jet fuels by hydrodeoxygenation.

Selective Reduction of CO2 to CH4 by Tandem Hydrosilylation with Mixed Al/B Catalysts

This contribution reports the first example of highly selective reduction of CO2 into CH4 via tandem hydrosilylation with mixed main-group organo-Lewis acid (LA) catalysts [Al(C6F5)3 + B(C6F5)3] {[Al] + [B]}. As shown by this comprehensive experimental and computational study, in this unique tandem catalytic process, [Al] effectively mediates the first step of the overall reduction cycle, namely the fixation of CO2 into HCOOSiEt3 (1) via the LA-mediated C═O activation, while [B] is incapable of promoting the same transformation. On the other hand, [B] is shown to be an excellent catalyst for the subsequent reduction steps 2-4, namely the hydrosilylation of the more basic intermediates [1 to H2C(OSiEt3)2 (2) to H3COSiEt3 (3) and finally to CH4] through the frustrated Lewis pair (FLP)-type Si-H activation. Hence, with the required combination of [Al] and [B], a highly selective hydrosilylative reduction of CO2 system has been developed, achieving high CH4 production yield up to 94%. The remarkably different catalytic behaviors between [Al] and [B] are attributed to the higher overall Lewis acidity of [Al] derived from two conflicting factors (electronic and steric effects), which renders the higher tendency of [Al] to form stable [Al]-substrate (intermediate) adducts with CO2 as well as subsequent intermediates 1, 2, and 3. Overall, the roles of [Al] and [B] are not only complementary but also synergistic in the total reduction of CO2, which render both [Al]-mediated first reduction step and [B]-mediated subsequent steps catalytic.

Elusive Silane–Alane Complex [SiH⋅⋅⋅Al]: Isolation, Characterization, and Multifaceted Frustrated Lewis Pair Type Catalysis†

The super acidity of the unsolvated Al(C6F5)3 enabled isolation of the elusive silane-alane complex [Si-H⋅⋅⋅Al], which was structurally characterized by spectroscopic and X-ray diffraction methods. The Janus-like nature of this adduct, coupled with strong silane activation, effects multifaceted frustrated-Lewis-pair-type catalysis. When compared with the silane-borane system, the silane-alane system offers unique features or clear advantages in the four types of catalytic transformations examined in this study, including: ligand redistribution of tertiary silanes into secondary and quaternary silanes, polymerization of conjugated polar alkenes, hydrosilylation of unactivated alkenes, and hydrodefluorination of fluoroalkanes.

Chromium(0) nanoparticles as effective catalyst for the conversion of glucose into 5-hydroxymethylfurfural.

Its nano: Small and uniform chromium nanoparticles, either preformed or generated in situ, effectively catalyze the conversion of glucose into 5-hydroxymethyl furfural. The results compare favorably with those achieved by using a catalyst system based on divalent CrCl(2) in ionic liquids (ILs). In addition, the chromium nanoparticles are found in the CrCl(2)/IL system, and the implications of their presence in that system is investigated.

Probing site cooperativity of frustrated phosphine/borane Lewis pairs by a polymerization study.

The first highly active phosphine (P)/borane (B) Lewis pair polymerization is promoted unexpectedly by P-B adducts. The P and B site cooperativity is essential for achieving effective polymerization, as shown by this study examining the reactivity of a library of P/B Lewis pairs toward polymerization of a renewable acrylic monomer.

Protic compound mediated living cross-chain-transfer polymerization of rac-lactide: synthesis of isotactic (crystalline)–heterotactic (amorphous) stereomultiblock polylactide

A highly efficient strategy for one-pot synthesis of programmable, crystalline-amorphous stereomultiblock PLA from rac-lactide.

Organocatalysis in biorefining for biomass conversion and upgrading

Organocatalysis using small-molecule organic compounds as catalysts has risen to prominence in organic synthesis and polymer synthesis. However, its application in biorefining for catalytic biomass conversion and upgrading into sustainable chemicals, materials, and biofuels has come to light only recently. This emergence of applying organocatalysis for biorefining has not only broadened the scope of organocatalysis and offered metal-free “greener” alternatives for biomass conversion and upgrading, it has also showed some unique activity and selectivity in such transformations as compared to metal-mediated processes. This review captures highlights of this emerging area by focusing on utilization of organocatalytic means for catalytic conversions of cellulose, glucose and fructose, upgrading of furaldehydes, and organocatalytic polymerization of biomass feedstocks.