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Christopher N. Bowman - One of the best experts on this subject based on the ideXlab platform.
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development of glassy step growth thiol vinyl sulfone polymer networks
Macromolecular Rapid Communications, 2014Co-Authors: Maciej Podgorski, Shunsuke Chatani, Christopher N. BowmanAbstract:Thermo-mechanical properties of neat phosphine-catalyzed thiol-Michael networks fabricated in a controlled manner are reported, and a comparison between thiol-acrylate and thiol-vinyl sulfone step growth networks is performed. When highly reactive vinyl sulfone monomers were used as Michael acceptors, glassy polymer networks were obtained with glass transition temperatures ranging from 30-80 °C. Also, the effect of side-chain functionality on the mechanical properties of thiol-vinyl sulfone networks was investigated. It was found that the inclusion of thiourethane functionalities, aryl structures, and most importantly the elimination of interchain ester linkages in the networks significantly elevated the network's glass transition temperature as compared to neat ester-based thiol-Michael networks. Keywords: thiol-Michael addition, step growth polymerization, temporal control, dynamic mechanical analysis 1. Introduction The base- or nucleophile-mediated thiol-Michael addition, unlike the more traditional radical thiol-acrylate reaction, proceeds without any acrylate homopolymerization, and leads to a pure thioether product. As it is a step-growth process, this approach results in uniform as well as low shrinkage and shrinkage stress network polymers when multifunctional monomers are reacted.[1-4] However, the majority of thiol-Michael cross-linking reactions yield elastomeric materials with low glass transition temperatures that fail quickly when subjected to larger mechanical stresses, mainly due to flexible thioether bonds formed in the reaction product. This behavior limits these materials in regards to their potential implementation in applications that require high toughness and hardness at ambient conditions. As such, thiol-Michael addition networks have been most widely implemented in hydrogel synthesis for biomedical applications to date.[5-11] Other areas where thiol-Michael cross-linking reactions have been used include applications such as mechanophotopaterning[12, 13], microfluidics[14], nano-, or microgel synthesis[15, 16] and dual-cure systems with potential applications in optical, shape memory, and impression materials.[17] Nearly all of these examples utilize thiol-acrylate reactions since a multitude of multifunctional acrylates are commercially available as substrates, and the thiol-acrylate reaction is facile to employ. One of the less frequently considered options, but also excellent Michael acceptors, are vinyl sulfone-containing monomers, which have been proven to be highly efficient reactants, e.g. in thiol-Michael hydrogel synthesis based on vinyl sulfone-functionalized PEG precursors.[18] In a comparative study of the relative reactivities of vinyl Sulfones and acrylates, the former were shown to exhibit much higher reaction rates which was attributed to the greater electron deficiency of the vinyl sulfone. Besides, an impressive selectivity was observed in a stoichiometric mixture of thiol, vinyl sulfone and acrylate where strongly preferential reaction with the vinyl sulfone was observed.[19] Further, unlike the acrylate thioether ester, the thioether sulfone is hydrolytically stable, and the presence of polar sulfone groups should facilitate electrostatic interactions between dipoles, which is expected to positively affect the toughness and other mechanical properties. There are few examples describing the properties of thiol-vinyl sulfone step growth networks with increased content of sulfone groups, particularly in dense networks rather than hydrogels.[20] Therefore, the motivation for this study was to assess the relative importance of the sulfone characteristics on the polymer network properties as compared to those of similarly cross-linked thiol-acrylates. Incorporating an initiating system composed of a nucleophile-acid pair, that enables temporal control over the reaction between thiols and electron-deficient vinyls, we synthesized novel network polymers from multifunctional thiols and vinyl sulfone monomers. Since there is only one commercially available (in a gram scale) difunctional vinyl sulfone, i.e. divinyl sulfone (DVS), herein we also present a method to synthesize other vinyl Sulfones of higher functionality either in thiol-Michael or oxa-Michael reactions. The new vinyl monomers, DVS, and acrylates of similar structural design were used to fabricate network polymers, which were subsequently evaluated for their viscoelastic behavior. Further, thiol-vinyl sulfone networks incorporating pendant functionalities were also prepared, and the effect of substituent groups on the network properties was assessed.
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Relative reactivity and selectivity of vinyl Sulfones and acrylates towards the thiol–Michael addition reaction and polymerization
Polymer Chemistry, 2013Co-Authors: Shunsuke Chatani, Devatha P. Nair, Christopher N. BowmanAbstract:The reactivity, selectivity and kinetics of vinyl Sulfones and acrylates in base and nucleophile-catalyzed thiol–Michael addition reactions were examined in detail in this study. The vinyl Sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (
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relative reactivity and selectivity of vinyl Sulfones and acrylates towards the thiol michael addition reaction and polymerization
Polymer Chemistry, 2013Co-Authors: Shunsuke Chatani, Devatha P. Nair, Christopher N. BowmanAbstract:The reactivity, selectivity and kinetics of vinyl Sulfones and acrylates in base and nucleophile-catalyzed thiol–Michael addition reactions were examined in detail in this study. The vinyl Sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (<10%) of HA, which demonstrates the high selectivity of vinyl Sulfones over acrylates. The reaction rate of EVS with HT was approximately 7 times higher than that of HA. A detailed study of the kinetics of the nucleophile-catalyzed thiol–Michael addition reaction was carried out, and it was shown that the delay observed in the initial stages of the nucleophile-catalyzed thiol–Michael addition reaction is due to the relatively slow attack of the nucleophiles on the vinyl. The presence of protic species other than thiols in the reaction mixture has also been shown to significantly impede the reaction rate, and in extreme cases, has been shown to inhibit the Michael addition reaction. These results provided a better understanding of the conditions under which the thiol–Michael addition reaction can or cannot be considered as a click reaction. Finally, the high reaction selectivity of vinyl Sulfones over acrylates via thiol–Michael addition reaction in ternary systems is used to control gelation behavior in crosslinked polymer networks formed by thiol–Michael addition reactions.
Carmen Nájera - One of the best experts on this subject based on the ideXlab platform.
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3 5 bis trifluoromethyl phenyl Sulfones for the highly stereoselective julia kocienski synthesis of α β unsaturated esters and weinreb amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gomezbengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones for the Highly Stereoselective Julia–Kocienski Synthesis of α,β‐Unsaturated Esters and Weinreb Amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gómez-bengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones in the Julia–Kocienski Olefination – Application to the Synthesis of Tri‐ and Tetrasubstituted Olefins
European Journal of Organic Chemistry, 2006Co-Authors: Diego A. Alonso, Mónica Fuensanta, Carmen NájeraAbstract:3,5-Bis(trifluoromethyl)phenyl (BTFP) Sulfones 8a–d are successfully employed in the modified Julia olefination reaction with carbonyl compounds employing phosphazene base P4-tBu at room temp. in THF, affording tri- and tetrasubstituted olefins in good yields. The Julia–Kocienski olefination between primary alkyl BTFP Sulfones 8a,b and aromatic and aliphatic ketones affords the corresponding trisubstituted alkenes in good yields and low stereoselectivities. On the other hand, higher yields and stereoselectivities are obtained in the synthesis of trisubstituted olefins through the other approach, the coupling of secondary alkyl BTFP Sulfones 8c,d with aliphatic, aromatic and α,β-unsaturated aldehydes. For the first time, tetrasubstituted olefins are synthesized by means of the Julia–Kocienski protocol when the isopropyl BTFP sulfone 8c reacts with aliphatic and aromatic ketones, employing P4-tBu as base at THF reflux. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
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Synthesis of β- and γ-hydroxy Sulfones by regioselective opening of β,γ-epoxy Sulfones
Tetrahedron, 2001Co-Authors: Carmen Nájera, José M. SansanoAbstract:Abstract β,γ-Epoxy Sulfones 1 derived from allylic Sulfones react regio-selectively with organomagnesium compounds in the presence or not of catalytic amounts of copper(I) bromide to afford β-hydroxy Sulfones 2 or γ-tosylated allylic alcoholates 5 respectively. The Michael type addition of Grignard reagents to intermediates 5 in the presence of catalytic amount of copper( l ) bromide yields γ-hydroxy Sulfones 6 . The PCC oxidation of β- and γ-hydroxy Sulfones give β- and γ-oxo Sulfones 10 and 11 respectively. In the case of γ-OXO Sulfones their treatment with DBU affords α-substituted, α, β- unsaturated carbonyl compounds.
Shunsuke Chatani - One of the best experts on this subject based on the ideXlab platform.
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development of glassy step growth thiol vinyl sulfone polymer networks
Macromolecular Rapid Communications, 2014Co-Authors: Maciej Podgorski, Shunsuke Chatani, Christopher N. BowmanAbstract:Thermo-mechanical properties of neat phosphine-catalyzed thiol-Michael networks fabricated in a controlled manner are reported, and a comparison between thiol-acrylate and thiol-vinyl sulfone step growth networks is performed. When highly reactive vinyl sulfone monomers were used as Michael acceptors, glassy polymer networks were obtained with glass transition temperatures ranging from 30-80 °C. Also, the effect of side-chain functionality on the mechanical properties of thiol-vinyl sulfone networks was investigated. It was found that the inclusion of thiourethane functionalities, aryl structures, and most importantly the elimination of interchain ester linkages in the networks significantly elevated the network's glass transition temperature as compared to neat ester-based thiol-Michael networks. Keywords: thiol-Michael addition, step growth polymerization, temporal control, dynamic mechanical analysis 1. Introduction The base- or nucleophile-mediated thiol-Michael addition, unlike the more traditional radical thiol-acrylate reaction, proceeds without any acrylate homopolymerization, and leads to a pure thioether product. As it is a step-growth process, this approach results in uniform as well as low shrinkage and shrinkage stress network polymers when multifunctional monomers are reacted.[1-4] However, the majority of thiol-Michael cross-linking reactions yield elastomeric materials with low glass transition temperatures that fail quickly when subjected to larger mechanical stresses, mainly due to flexible thioether bonds formed in the reaction product. This behavior limits these materials in regards to their potential implementation in applications that require high toughness and hardness at ambient conditions. As such, thiol-Michael addition networks have been most widely implemented in hydrogel synthesis for biomedical applications to date.[5-11] Other areas where thiol-Michael cross-linking reactions have been used include applications such as mechanophotopaterning[12, 13], microfluidics[14], nano-, or microgel synthesis[15, 16] and dual-cure systems with potential applications in optical, shape memory, and impression materials.[17] Nearly all of these examples utilize thiol-acrylate reactions since a multitude of multifunctional acrylates are commercially available as substrates, and the thiol-acrylate reaction is facile to employ. One of the less frequently considered options, but also excellent Michael acceptors, are vinyl sulfone-containing monomers, which have been proven to be highly efficient reactants, e.g. in thiol-Michael hydrogel synthesis based on vinyl sulfone-functionalized PEG precursors.[18] In a comparative study of the relative reactivities of vinyl Sulfones and acrylates, the former were shown to exhibit much higher reaction rates which was attributed to the greater electron deficiency of the vinyl sulfone. Besides, an impressive selectivity was observed in a stoichiometric mixture of thiol, vinyl sulfone and acrylate where strongly preferential reaction with the vinyl sulfone was observed.[19] Further, unlike the acrylate thioether ester, the thioether sulfone is hydrolytically stable, and the presence of polar sulfone groups should facilitate electrostatic interactions between dipoles, which is expected to positively affect the toughness and other mechanical properties. There are few examples describing the properties of thiol-vinyl sulfone step growth networks with increased content of sulfone groups, particularly in dense networks rather than hydrogels.[20] Therefore, the motivation for this study was to assess the relative importance of the sulfone characteristics on the polymer network properties as compared to those of similarly cross-linked thiol-acrylates. Incorporating an initiating system composed of a nucleophile-acid pair, that enables temporal control over the reaction between thiols and electron-deficient vinyls, we synthesized novel network polymers from multifunctional thiols and vinyl sulfone monomers. Since there is only one commercially available (in a gram scale) difunctional vinyl sulfone, i.e. divinyl sulfone (DVS), herein we also present a method to synthesize other vinyl Sulfones of higher functionality either in thiol-Michael or oxa-Michael reactions. The new vinyl monomers, DVS, and acrylates of similar structural design were used to fabricate network polymers, which were subsequently evaluated for their viscoelastic behavior. Further, thiol-vinyl sulfone networks incorporating pendant functionalities were also prepared, and the effect of substituent groups on the network properties was assessed.
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Relative reactivity and selectivity of vinyl Sulfones and acrylates towards the thiol–Michael addition reaction and polymerization
Polymer Chemistry, 2013Co-Authors: Shunsuke Chatani, Devatha P. Nair, Christopher N. BowmanAbstract:The reactivity, selectivity and kinetics of vinyl Sulfones and acrylates in base and nucleophile-catalyzed thiol–Michael addition reactions were examined in detail in this study. The vinyl Sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (
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relative reactivity and selectivity of vinyl Sulfones and acrylates towards the thiol michael addition reaction and polymerization
Polymer Chemistry, 2013Co-Authors: Shunsuke Chatani, Devatha P. Nair, Christopher N. BowmanAbstract:The reactivity, selectivity and kinetics of vinyl Sulfones and acrylates in base and nucleophile-catalyzed thiol–Michael addition reactions were examined in detail in this study. The vinyl Sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (<10%) of HA, which demonstrates the high selectivity of vinyl Sulfones over acrylates. The reaction rate of EVS with HT was approximately 7 times higher than that of HA. A detailed study of the kinetics of the nucleophile-catalyzed thiol–Michael addition reaction was carried out, and it was shown that the delay observed in the initial stages of the nucleophile-catalyzed thiol–Michael addition reaction is due to the relatively slow attack of the nucleophiles on the vinyl. The presence of protic species other than thiols in the reaction mixture has also been shown to significantly impede the reaction rate, and in extreme cases, has been shown to inhibit the Michael addition reaction. These results provided a better understanding of the conditions under which the thiol–Michael addition reaction can or cannot be considered as a click reaction. Finally, the high reaction selectivity of vinyl Sulfones over acrylates via thiol–Michael addition reaction in ternary systems is used to control gelation behavior in crosslinked polymer networks formed by thiol–Michael addition reactions.
Diego A. Alonso - One of the best experts on this subject based on the ideXlab platform.
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3 5 bis trifluoromethyl phenyl Sulfones for the highly stereoselective julia kocienski synthesis of α β unsaturated esters and weinreb amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gomezbengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones for the Highly Stereoselective Julia–Kocienski Synthesis of α,β‐Unsaturated Esters and Weinreb Amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gómez-bengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones in the Julia–Kocienski Olefination – Application to the Synthesis of Tri‐ and Tetrasubstituted Olefins
European Journal of Organic Chemistry, 2006Co-Authors: Diego A. Alonso, Mónica Fuensanta, Carmen NájeraAbstract:3,5-Bis(trifluoromethyl)phenyl (BTFP) Sulfones 8a–d are successfully employed in the modified Julia olefination reaction with carbonyl compounds employing phosphazene base P4-tBu at room temp. in THF, affording tri- and tetrasubstituted olefins in good yields. The Julia–Kocienski olefination between primary alkyl BTFP Sulfones 8a,b and aromatic and aliphatic ketones affords the corresponding trisubstituted alkenes in good yields and low stereoselectivities. On the other hand, higher yields and stereoselectivities are obtained in the synthesis of trisubstituted olefins through the other approach, the coupling of secondary alkyl BTFP Sulfones 8c,d with aliphatic, aromatic and α,β-unsaturated aldehydes. For the first time, tetrasubstituted olefins are synthesized by means of the Julia–Kocienski protocol when the isopropyl BTFP sulfone 8c reacts with aliphatic and aromatic ketones, employing P4-tBu as base at THF reflux. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
Mónica Fuensanta - One of the best experts on this subject based on the ideXlab platform.
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3 5 bis trifluoromethyl phenyl Sulfones for the highly stereoselective julia kocienski synthesis of α β unsaturated esters and weinreb amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gomezbengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones for the Highly Stereoselective Julia–Kocienski Synthesis of α,β‐Unsaturated Esters and Weinreb Amides
European Journal of Organic Chemistry, 2008Co-Authors: Diego A. Alonso, Mónica Fuensanta, Enrique Gómez-bengoa, Carmen NájeraAbstract:The 3,5-bis(trifluoromethyl)phenyl (BTFP) Sulfones tert-butyl α-(BTFPsulfonyl)acetate (4) and Weinreb α-(BTFPsulfonyl)acetamide (5) have successfully been employed in the Julia–Kocienski olefination of aldehydes with K2CO3 as the base at 120 °C in DMF under solid/liquid phase-transfer catalysis conditions to afford α,β-unsaturated esters and Weinreb amides, respectively. The corresponding products were obtained in good yields and with high E stereoselectivities (E/Z up to >99:1), especially in the case of the amides. A detailed computational study of the Julia–Kocienski olefination with BTFP sulfone 4 was carried out and confirmed the existence of an equilibrium in the initial addition of the sulfone enolate to the aldehyde and, in contrast to other proposed mechanisms, a non-concerted final elimination of SO2 and 3,5-bis(trifluoromethyl)phenoxide. A plausible explanation for the high E diastereoselectivity observed in the reaction has been suggested based on kinetic considerations at spirocyclic TS2 and thermodynamic factors during the elimination after TS2. ESI-MS studies carried out during the olefination reaction of benzaldehyde with BTFP sulfone 4 were used to characterize the sulfone enolate and the intermediate assumed for the reaction mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
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3,5‐Bis(trifluoromethyl)phenyl Sulfones in the Julia–Kocienski Olefination – Application to the Synthesis of Tri‐ and Tetrasubstituted Olefins
European Journal of Organic Chemistry, 2006Co-Authors: Diego A. Alonso, Mónica Fuensanta, Carmen NájeraAbstract:3,5-Bis(trifluoromethyl)phenyl (BTFP) Sulfones 8a–d are successfully employed in the modified Julia olefination reaction with carbonyl compounds employing phosphazene base P4-tBu at room temp. in THF, affording tri- and tetrasubstituted olefins in good yields. The Julia–Kocienski olefination between primary alkyl BTFP Sulfones 8a,b and aromatic and aliphatic ketones affords the corresponding trisubstituted alkenes in good yields and low stereoselectivities. On the other hand, higher yields and stereoselectivities are obtained in the synthesis of trisubstituted olefins through the other approach, the coupling of secondary alkyl BTFP Sulfones 8c,d with aliphatic, aromatic and α,β-unsaturated aldehydes. For the first time, tetrasubstituted olefins are synthesized by means of the Julia–Kocienski protocol when the isopropyl BTFP sulfone 8c reacts with aliphatic and aromatic ketones, employing P4-tBu as base at THF reflux. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
