3.01 Alkylations of Enols and Enolates

Stoltz, Brian M.; Bennett, Nathan B.; Duquette, Douglas C.; Goldberg, Alexander F. G.; Liu, Y.; Loewinger, M. B.; Reeves, Corey M.

doi:10.1016/b978-0-08-097742-3.00301-3

Cited by 40 publications

(41 citation statements)

References 449 publications

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“…Analysis of the 13 C NMR of the resulting low molecular weight poly(lactide) (M n ~ 2700 Da) was consistent with a 2,2-dimethyl methyl acetate endgroup resulting from acylation at the carbon center of the methyl isobutyrate enolate initiator (Figure S17–S19, Supporting Information). These results are consistent with literature examples of carbon alkylation of methyl isobutyrate, 25–28 and indicate that ester enolates initiate lactide polymerization by a Claisen-type condensation at the carbon of the initiating enolate.…”

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confidence: 92%

Cyclopropenimine Superbases: Competitive Initiation Processes in Lactide Polymerization

et al. 2015

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Cyclopropenimine superbases were employed to catalyze the ring-opening polymerization of lactide. Polymerization occurred readily in the presence and absence of alcohol initiators. Polymerizations in the absence of alcohol initiators revealed a competitive initiation mechanism involving deprotonation of lactide by the cyclopropenimine to generate an enolate. NMR and MALDI-TOF analysis of the poly(lactides) generated from cyclopropenimines in the absence of alcohol initiators showed acylated lactide and hydroxyl endgroups. Model studies and comparative experiments with guanidine and phosphazene catalysts revealed the subtle influence of the nature of the superbase on competitive initiation processes.Organocatalysis has proven a versatile strategy for ring-opening polymerization reactions. [1][2][3][4][5] Organic molecules catalyze ring-opening polymerization (ROP) by a variety of mechanisms that are distinct from those of metal alkoxide initiators; the mild conditions and high functional group tolerance of many of these organic catalysts have provided new opportunities for macromolecular synthesis and design. 1,2 Superbases 6 such as Nheterocyclic carbenes (NHCs), guanidines, amidines, isothioureas, and phosphazenes have proven especially effective as organic catalysts for ROP; in the presence of alcohol initiators these catalysts can hydrogen bond to the alcohol or chain-ends to activate them for the ringopening of strained monomers (Scheme 1, illustrated for lactide). 2 At low initiator (alcohol) concentrations, competitive initiation mechanisms can occur and can be probed by carrying out polymerizations in the absence of alcohols. 2,7 In the absence of alcohol initiators, Nheterocyclic carbenes, 8,9 amidines, 10 and isothioureas 11 mediate zwitterionic ring-opening polymerization reactions by a nucleophilic 12 mechanism; this latter strategy has proven useful for generating cyclic macromolecules. 9 Herein, we describe ring-opening polymerization reactions with another class of potent neutral bases derived from bis(dialkylamino)-cyclopropenimines. [13][14][15] Lambert recently showed that these compounds have comparable basicity to phosphazenes, are readily *Corresponding Author: waymouth@stanford.edu. ASSOCIATED CONTENT NMR Spectra, GPC traces, MALDI and ESI-MS of polymers and model studies. This material is available free of charge via the Internet at http://pubs.acs.org HHS Public AccessThree achiral cyclopropenimines bearing N-alkyl substituents (Table 1, right inset) were prepared. [13][14][15] Polymerizations of lactide initiated with 1-pyrenebutanol in the presence of catalytic amounts of cyclopropenimine 1 proceeded rapidly with greater than 85% conversion in 30 seconds (Table S1, Supporting Information). Molecular weights up to 13 kDa were obtained by controlling the monomer to alcohol ratio. The molecular weight distributions ranged from 1.2-1.4 and generally increased over the course of the reaction. These data, coupled with the observations that polymeric ions corresponding to both odd and ...

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confidence: 92%

Cyclopropenimine Superbases: Competitive Initiation Processes in Lactide Polymerization

et al. 2015

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“…The Cr(salen) complex 97 (Figure 7) is able to selectively react with one of the rapidly equilibrating geometric isomers and leads to the enantioselective construction of quaternary carbon stereocenters. 100,101 …”

Section: Dynamic Kinetic Resolutionsmentioning

confidence: 99%

Advances in Stereoconvergent Catalysis from 2005 to 2015: Transition-Metal-Mediated Stereoablative Reactions, Dynamic Kinetic Resolutions, and Dynamic Kinetic Asymmetric Transformations

et al. 2017

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An important subset of asymmetric synthesis is dynamic kinetic resolution, dynamic kinetic asymmetric processes and stereoablative transformations. Initially, only enzymes were known to catalyze dynamic kinetic processes but recently various synthetic catalysts have been developed. This review summarizes major advances in non-enzymatic, transition metal promoted dynamic asymmetric transformations reported between 2005 and 2015.

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“…Although S N 2 reactions of non-fluorinated ketone enolates with sp 3 -hybridized electrophiles are a fundamental transformation for accessing α-substituted ketone derivatives, 19 alkylation reactions of α,α-difluoroketone enolates with sp 3 -based electrophiles have been restricted by two problems. First, access to the desired terminal α,α-difluoroketone enolates is limited, because chemoselective deprotonation of α,α-difluoromethyl ketones preferentially generates internal non-fluorinated ketone enolates under both thermodynamic and kinetic conditions (Scheme 6B).…”

Section: Pd-catalyzed Decarboxylative Alkylation Of αα-Difluoroketmentioning

confidence: 99%

Metal-Catalyzed Decarboxylative Fluoroalkylation Reactions

Ambler

Yang

Altman

2016

Synlett

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Metal-catalyzed decarboxylative fluoroalkylation reactions enable the conversion of simple O-based substrates into biologically relevant fluorinated analogs. Herein, we present decarboxylative methods that facilitate the synthesis of trifluoromethyl- and difluoroketone-containing products. We highlight key mechanistic aspects that are critical for efficient catalysis, and that inspired our thinking while developing the reactions.

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3.01 Alkylations of Enols and Enolates

Cited by 40 publications

References 449 publications

Cyclopropenimine Superbases: Competitive Initiation Processes in Lactide Polymerization

Cyclopropenimine Superbases: Competitive Initiation Processes in Lactide Polymerization

Advances in Stereoconvergent Catalysis from 2005 to 2015: Transition-Metal-Mediated Stereoablative Reactions, Dynamic Kinetic Resolutions, and Dynamic Kinetic Asymmetric Transformations

Metal-Catalyzed Decarboxylative Fluoroalkylation Reactions

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