In Situ Generation of Carbenes:  A General and Versatile Platform for Organocatalytic Living Polymerization

Nyce, Gregory W.; Glauser, Thierry; Connor, Eric F.; Möck, Andreas; Waymouth, Robert M.; Hedrick, James L.

doi:10.1021/ja021084+

Cited by 310 publications

(219 citation statements)

References 67 publications

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“…The catalysts reported include various tertiary phosphines, 45 (dimethylamino)-pyridine, 11 and n-heterocyclic carbene complexes, 3,46 all functioning as Lewis bases. In these reactions, the catalyst facilitates the opening of the ring.…”

Section: Introductionmentioning

confidence: 99%

A Recoverable, Metal-Free Catalyst for the Green Polymerization of ε-Caprolactone

Wilson

Jones

2004

Macromolecules

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n-Propylsulfonic acid-functionalized porous and nonporous silica materials are evaluated in the ring-opening polymerization of -caprolactone. All catalysts allow for the controlled polymerization of the monomer, producing polymers with controlled molecular weights and narrow polydispersities. Polymerization rates are low, with site-time yields generally 1-3 orders of magnitude lower than metalbased systems. The catalysts are easily recovered from the polymerization solution after use and are shown to contain significant residual adsorbed polymer. Solvent extraction techniques are useful for removing most of the polymer, although the extracted solids are not effective catalysts in recycle experiments under the conditions used here. These new materials represent a green alternative to traditional metal-based catalysts, as they are recoverable and leave no metal residues in the polymer.

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Section: Introductionmentioning

confidence: 99%

A Recoverable, Metal-Free Catalyst for the Green Polymerization of ε-Caprolactone

Wilson

Jones

2004

Macromolecules

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“…This is commonly brought about by the use of metal catalyst precursors, although organic catalysis is possible by employing a nucleophilic base such as 4-dimethyl aminopyridine or an N-heterocyclic carbene with alcohol initiators (2)(3)(4)(5). It is now generally accepted that metal-alkoxide species are the key reactive intermediates in the propagation steps and, furthermore, that these, too, play a role in the competing side reactions of transesterification and chain transfer (6).…”

mentioning

confidence: 99%

Cyclic esters and cyclodepsipeptides derived from lactide and 2,5-morpholinediones

Chisholm

Gallucci

Yin

2006

Proc. Natl. Acad. Sci. U.S.A.

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The reaction between Bu n Li in benzene and the solid polystyrene support PS-C 6H4CH2NH2 leads to a lithiated species that can be represented as PS-C 6H4CH2NHLi(LiBu)x, where x ϳ 4, which is active in the ring-opening of the cyclic esters L-lactide, rac-lactide, and 2,5-morpholinediones. With Ϸ10 eq of these monomeric six-membered rings and with heating, cyclic esters (MeCHC ( dynamic combinatorial library ͉ ring enlarging T he ring-opening polymerization of cyclic esters such as lactides (LA) is a well established procedure for the production of polyesters (1). This is commonly brought about by the use of metal catalyst precursors, although organic catalysis is possible by employing a nucleophilic base such as 4-dimethyl aminopyridine or an N-heterocyclic carbene with alcohol initiators (2-5). It is now generally accepted that metal-alkoxide species are the key reactive intermediates in the propagation steps and, furthermore, that these, too, play a role in the competing side reactions of transesterification and chain transfer (6). These reactions are collectively shown in Scheme 1.Transesterification is normally a deleterious side reaction because it leads to a loss of control of molecular mass in a living polymerization and, because in a stereoselective ring-opening polymerization, transesterification scrambles the stereocenters along a chain. As shown in Scheme 1, transesterification can occur by a bimolecular mechanism or a reaction involving a single chain. The latter reaction, intrachain transesterification leads to cycles. Previously, we found that the polymerization of lactide by Ph 2 SnX 2 initiators, where X ϭ OPr i or NMe 2 , gave different ratios of chains to cycles with the cycles being significantly more prominent for X ϭ NMe 2 . This apparently arises because of the favorable chelation of the SnOCHMeC(O)NMe 2 groups (7).Prompted by these observations, we set out to explore a synthetic route for cyclic oligoesters derived from LA and we describe herein our findings. It is worthy of mention that others have synthesized cyclic esters and carbonates principally as a route for the formation of polyesters and polycarbonates (8). Brunelle and coworkers (9) at GE Corporate Research andDevelopment specifically developed high-yielding syntheses of cyclic esters derived from 1,4-aromatic acyl chlorides and diols. These ringed compounds melt to yield low viscous media, which by a thermo-neutral ring-opening polymerization give polyesters with no elimination products. This chemistry forms the technology platform of Cyclics Corporation. 2. Kricheldorf and coworkers (10, 11) have used principally tin(IV) diolates as initiators for the ring-opening polymerization and the reaction with dicarboxylic acid chlorides to cleave the cyclic esters. 3. Hodge and coworkers (12) used a solid support and step growth polymerization to produce a polyester chain followed by treatment with a metal catalyst, most often Bu n 2 SnO, to affect intrachain transesterification and release of the cyclic esters to the solution phase. 4....

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“…Nevertheless, while NHCs are excellent transesterification catalysts, the convenience of these species as catalysts is compromised by their sensitivity to air and moisture, necessitating airsensitive techniques. To this end, we have focused on strategies for generating the reactive N-heterocyclic carbenes in situ, [20] including the deprotonation of imidazolium salts (1), [21] or the thermolysis of adducts (2). [22] Inspired by some of the original work of Arduengo, Wanzlick, Lappert and Bredereck, [23,24,25] we reasoned that the commercially available compounds, tert-butoxybis-(dimethylamino)methane (3a, also known as Brederecks reagent), and tris(dimethylamino)methane (4), [24,25] are reminiscent of other protected forms of Nheterocyclic carbenes [20,22] and might be expected to show similar reactivity.…”

mentioning

confidence: 99%

Bredereck's Reagent Revisited: Latent Anionic Ring‐Opening Polymerization and Transesterification Reactions

Csihony¹,

Beaudette²,

Sentman³

et al. 2004

Adv Synth Catal

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The ring-opening polymerization of lactide with commercially available Bredereck-type reagents in the presence or absence of alcohol initiators was carried out affording polylactide with controlled molecular weight and narrow polydispersities. An anionic mechanism involving heterolytic cleavage to alkoxides is proposed, where these reagents function as latent anionic initiators for the ring-opening polymerization of lactide.Keywords: anionic polymerization; Brederecks reagent; carbene; organic catalysis; polylactide There has been a recent resurgence of interest in the development of organocatalytic reactions. Catalysis with simple organic molecules can provide an attractive alternative to organometallic catalysis, particularly in situations where residual metals or ligands compromise the purity or end-use of the product. Representative recent examples include MacMillans elegant studies of highly enantioselective organocatalytic inter-and intramolecular Diels-Alders reactions, 1,3-dipolar cycloadditions, 1,4-conjugate Friedel-Crafts additions as well as the alkylation of indoles. [1,2,3] Proline has proven to be an efficient organic catalyst for a variety of reactions such as the Mannich reaction to generate precursors to b-lactams, [3] and several groups have reported effective nonenzymatic catalysts for the kinetic resolution of secondary alcohols using both "planar-chiral" phosphines [4] and amine catalyst frameworks. [2,5] Miller has devised several new functional peptides that catalyze the kinetic resolution of selected secondary alcohols [6] as well as a variety of other transformations. [7] Other noteworthy examples of organic catalysts include those of Fuji, [8] Spivey, [9] Oriyama, [10] Vedejs, [11] Breslow, [12] Enders, [13] and Rovis [14] as well as extensive studies of biocatalysts.[15]We were recently challenged to develop effective organic polymerization reactions in an effort to develop new materials for microelectronics, where residual metals must be avoided. During our initial survey of a variety of organic catalysts for the ring-opening polymerization (ROP) of cyclic esters we showed that nucleophilic catalysts such as tertiary amines, [16] phosphines [17] and stabilized singlet carbenes were effective polymerization catalysts for strained cyclic esters.[18] Of these, the N-heterocyclic carbenes (NHCs) were by far the most active. Extensions of this work demonstrated that NHCs are also potent transesterification catalysts [19] for a variety of esters and alcohols. Nevertheless, while NHCs are excellent transesterification catalysts, the convenience of these species as catalysts is compromised by their sensitivity to air and moisture, necessitating airsensitive techniques. To this end, we have focused on strategies for generating the reactive N-heterocyclic carbenes in situ, [20] including the deprotonation of imidazolium salts (1), [21] or the thermolysis of adducts (2). [22] Inspired by some of the original work of Arduengo, Wanzlick, Lappert and Bredereck, [23,24,25] we reasoned that...

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In Situ Generation of Carbenes: A General and Versatile Platform for Organocatalytic Living Polymerization

Cited by 310 publications

References 67 publications

A Recoverable, Metal-Free Catalyst for the Green Polymerization of ε-Caprolactone

A Recoverable, Metal-Free Catalyst for the Green Polymerization of ε-Caprolactone

Cyclic esters and cyclodepsipeptides derived from lactide and 2,5-morpholinediones

Bredereck's Reagent Revisited: Latent Anionic Ring‐Opening Polymerization and Transesterification Reactions

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