Co-reporter:Alexander J. Wagner, Dmitry Yu. Zubarev, Alán Aspuru-Guzik, and Donna G. Blackmond
ACS Central Science April 26, 2017 Volume 3(Issue 4) pp:322-322
Publication Date(Web):March 21, 2017
DOI:10.1021/acscentsci.7b00085
Chiral pentose sugars mediate the enantioselective synthesis of amino acid precursors, with the magnitude of the chiral induction dictated by a subtle cooperativity between sugar hydroxyl groups. Ribose and lyxose give opposite chiral preferences, and theoretical calculations reveal the pseudoenantiomeric nature of transition state structures from the two sugars. Prebiotically plausible mixtures of natural d-sugars lead to enantioenrichment of natural l-amino acid precursors. Temporal monitoring and kinetic modeling of the reaction reveal an unusual dynamic kinetic resolution that shifts toward an enantioselective pathway over time, providing an amplification mechanism for the transfer of chiral information. This work adds to growing evidence for synergy in the etiology of the single chirality of the two most important classes of biological molecules, the sugars that make up DNA and RNA and the amino acids that form proteins.
Co-reporter:Neil Hawbaker, Eric Wittgrove, Bianca Christensen, Neal Sach, and Donna G. Blackmond
Organic Process Research & Development 2016 Volume 20(Issue 2) pp:465-473
Publication Date(Web):February 25, 2015
DOI:10.1021/op500360w
Rapid reaction screening in flow systems may help to reduce the time and material required to optimize, scale-up, and implement a flow process. Compartmentalization using small fluorous plugs has been implemented in several commercial reactors as a means for running a large number of isolated reactions in series within a flow reactor. Dye tracking, visual mixing, and reactivity studies are used to better understand the factors controlling dispersion within a commercial reactor. The role of dispersion on reactivity is elucidated using model reactions, and an optimized method for performing high-throughput screening is proposed.
Co-reporter:Dr. Daniele Masseroni;Dr. Simone Mosca;Matthew P. Mower;Dr. Donna G. Blackmond;Dr. Julius Rebek Jr.
Angewandte Chemie 2016 Volume 128( Issue 29) pp:8430-8433
Publication Date(Web):
DOI:10.1002/ange.201602355
Abstract
The majority of reactions currently performed in the chemical industry take place in organic solvents, compounds that are generally derived from petrochemicals. To promote chemical processes in water, we examined the use of synthetic, deep water-soluble cavitands in the Staudinger reduction of long-chain aliphatic diazides (C8, C10, and C12). The diazide substrates are taken up by the cavitand in D2O in folded, dynamic conformations. The reduction of one azide group to an amine gives a complex in which the substrate is fixed in an unsymmetrical conformation, with the amine terminal exposed and the azide terminal deep and inaccessible within the cavitand. Accordingly, the reduction of the second azide group is inhibited, even with excess phosphine, and good yields of the monofunctionalized products are obtained. In contrast, the reduction of the free diazides in bulk solution yields diamine products.
Co-reporter:Dr. Daniele Masseroni;Dr. Simone Mosca;Matthew P. Mower;Dr. Donna G. Blackmond;Dr. Julius Rebek Jr.
Angewandte Chemie International Edition 2016 Volume 55( Issue 29) pp:8290-8293
Publication Date(Web):
DOI:10.1002/anie.201602355
Abstract
The majority of reactions currently performed in the chemical industry take place in organic solvents, compounds that are generally derived from petrochemicals. To promote chemical processes in water, we examined the use of synthetic, deep water-soluble cavitands in the Staudinger reduction of long-chain aliphatic diazides (C8, C10, and C12). The diazide substrates are taken up by the cavitand in D2O in folded, dynamic conformations. The reduction of one azide group to an amine gives a complex in which the substrate is fixed in an unsymmetrical conformation, with the amine terminal exposed and the azide terminal deep and inaccessible within the cavitand. Accordingly, the reduction of the second azide group is inhibited, even with excess phosphine, and good yields of the monofunctionalized products are obtained. In contrast, the reduction of the free diazides in bulk solution yields diamine products.
Co-reporter:Alexander J. Wagner and Donna G. Blackmond
ACS Central Science 2016 Volume 2(Issue 11) pp:775
Publication Date(Web):November 11, 2016
DOI:10.1021/acscentsci.6b00336
Co-reporter:Qixun Shi;Matthew P. Mower;Julius Rebek, Jr.
PNAS 2016 Volume 113 (Issue 33 ) pp:9199-9203
Publication Date(Web):2016-08-16
DOI:10.1073/pnas.1610006113
Water-soluble, deep cavitands serve as chaperones of long-chain diesters for their selective hydrolysis in aqueous solution.
The cavitands bind the diesters in rapidly exchanging, folded J-shape conformations that bury the hydrocarbon chain and expose
each ester group in turn to the aqueous medium. The acid hydrolyses in the presence of the cavitand result in enhanced yields
of monoacid monoester products. Product distributions indicate a two- to fourfold relative decrease in the hydrolysis rate
constant of the second ester caused by the confined space in the cavitand. The rate constant for the first acid hydrolysis
step is enhanced approximately 10-fold in the presence of the cavitand, compared with control reactions of the molecules in
bulk solution. Hydrolysis under basic conditions (saponification) with the cavitand gave >90% yields of the corresponding
monoesters. Under basic conditions the cavitand complex of the monoanion precipitates from solution and prevents further reaction.
Co-reporter:Alexander G. O'Brien, Oana R. Luca, Phil S. Baran and Donna G. Blackmond
Reaction Chemistry & Engineering 2016 vol. 1(Issue 1) pp:90-95
Publication Date(Web):08 Dec 2015
DOI:10.1039/C5RE00050E
An electrochemical cell coupled with a recycle loop through a transmission FTIR cell is employed in studies of two free radical organic reactions, the oxidation of allylic alcohols and the trifluoromethylation of heteroarenes. Rapid mixing through the recycle loop allows continuous monitoring of reaction progress. Electrochemical generation of free radicals allows their controlled mediation into the reaction mixture for more efficient reaction. Kinetic profiles provide mechanistic insight into reactions under electrochemical control.
Co-reporter:Matthew P. Mower
Journal of the American Chemical Society 2015 Volume 137(Issue 6) pp:2386-2391
Publication Date(Web):January 22, 2015
DOI:10.1021/ja512753v
Unusual sigmoidal kinetic profiles in the Machetti–De Sarlo base-catalyzed 1,3-dipolar cycloaddition of acrylamide to N-methylnitroacetamide are rationalized by detailed in situ kinetic analysis. A dual role is uncovered in which a substrate acts as a precursor to catalyze its own reaction. Such kinetic studies provide a general protocol for distinguishing among different mechanistic origins of induction periods in complex organic reactions.
Co-reporter:Donna G. Blackmond
Journal of the American Chemical Society 2015 Volume 137(Issue 34) pp:10852-10866
Publication Date(Web):August 18, 2015
DOI:10.1021/jacs.5b05841
The use of modern kinetic tools to obtain virtually continuous reaction progress data over the course of a catalytic reaction opens up a vista that provides mechanistic insights into both simple and complex catalytic networks. Reaction profiles offer a rate/concentration scan that tells the story of a batch reaction time course in a qualitative “fingerprinting” manner as well as in quantitative detail. Reaction progress experiments may be mathematically designed to elucidate catalytic rate laws from only a fraction of the number of experiments required in classical kinetic measurements. The information gained from kinetic profiles provides clues to direct further mechanistic analysis by other approaches. Examples from a variety of catalytic reactions spanning two decades of the author’s work help to delineate nuances on a central mechanistic theme.
Co-reporter:Yining Ji; R. Erik Plata; Christopher S. Regens; Michael Hay; Michael Schmidt; Thomas Razler; Yuping Qiu; Peng Geng; Yi Hsiao; Thorsten Rosner; Martin D. Eastgate
Journal of the American Chemical Society 2015 Volume 137(Issue 41) pp:13272-13281
Publication Date(Web):September 29, 2015
DOI:10.1021/jacs.5b01913
Kinetic, spectroscopic, crystallographic, and computational studies probing a Pd-catalyzed C–H arylation reaction reveal that mono-oxidation of the bis-phosphine ligand is critical for the formation of the active catalyst. The bis-phosphine mono-oxide is shown to be a hemilabile, bidentate ligand for palladium. Isolation of the oxidative addition adduct, with structural elucidation by X-ray analysis, showed that the mono-oxide was catalytically competent, giving the same reaction rate in the productive reaction as the Pd(II)/xantphos precursor. A dual role for the carboxylate base in both catalyst activation and reaction turnover was demonstrated, along with the inhibiting effect of excess phosphine ligand. The generality of the role of phosphine mono-oxide complexes in Pd-catalyzed coupling processes is discussed.
Co-reporter:Yining Ji, Tamas Benkovics, Gregory L. Beutner, Chris Sfouggatakis, Martin D. Eastgate, and Donna G. Blackmond
The Journal of Organic Chemistry 2015 Volume 80(Issue 3) pp:1696-1702
Publication Date(Web):January 6, 2015
DOI:10.1021/jo502641d
The Achmatowicz rearrangement is a powerful method for the construction of pyranones from simple furan derivatives. Here, we describe the development of improved reaction conditions and an interrogation into the fate of the metal center during this interesting transformation. The reaction to form the synthetically important lactol, 6-hydroxy-2H-pyran-3(6H)-one (3), proceeds cleanly in the presence of tert-butyl hydroperoxide (TBHP, 2) using low loadings of VO(OiPr)3 as catalyst. The nonaqueous conditions developed herein allow for easy isolation of product 3 and synthetically important derivatives, a key advantage of this new protocol. Detailed experimental, spectroscopic, and kinetic studies along with kinetic modeling of the catalytic cycle support a positive-order dependence in both furfurol and TBHP concentrations, first-order dependence in catalyst (VO(OiPr)3), and a negative dependence on the 2-methyl-2-propanol (4) concentration. 51V-NMR spectroscopic studies revealed that 2-methyl-2-propanol (4) competes with substrates for binding to the metal center, rationalizing its inhibitory effect.
Co-reporter:Ryan D. Baxter, Yong Liang, Xin Hong, Timothy A. Brown, Richard N. Zare, K. N. Houk, Phil S. Baran, and Donna G. Blackmond
ACS Central Science 2015 Volume 1(Issue 8) pp:456
Publication Date(Web):November 2, 2015
DOI:10.1021/acscentsci.5b00332
Kinetic, spectroscopic, and computational studies of radical C–H arylations highlight the interplay between chemical and physical rate processes in these multiphase reactions. Anomalous concentration dependences observed here may be reconciled by considering the role of phase transfer processes that mediate concentrations in each phase. In addition, understanding interactions through phase boundaries enables their use in optimization of reaction performance.
Co-reporter:Dragos Gherase, Devin Conroy, Omar K. Matar, and Donna G. Blackmond
Crystal Growth & Design 2014 Volume 14(Issue 3) pp:928-937
Publication Date(Web):February 3, 2014
DOI:10.1021/cg401072d
Experimental studies help to deconvolute the driving forces for crystal growth during attrition-enhanced deracemization, demonstrating an interplay between crystal size and crystal number in the emergence of homochirality. A semiempirical population balance model is presented based on considerations of the solubility driving force, as outlined by the Gibbs–Thomson rule, and a frequency factor based on the total interfacial surface area between solid crystals and the solution phase.
Co-reporter:Dr. Alexer G. O'Brien;Dr. Akinobu Maruyama;Dr. Yasuhide Inokuma; Makoto Fujita; Phil S. Baran; Donna G. Blackmond
Angewandte Chemie International Edition 2014 Volume 53( Issue 44) pp:11868-11871
Publication Date(Web):
DOI:10.1002/anie.201407948
Abstract
Electrochemical reactions are shown to be effective for the CH functionalization of a number of heterocyclic substrates that are recalcitrant to conventional peroxide radical initiation conditions. Monitoring reaction progress under electrochemical conditions provides mechanistic insight into the CH functionalization of a series of heterocycles of interest in medicinal chemistry.
Co-reporter:Dr. Jordi Burés;Dr. Paul Dingwall;Dr. Alan Armstrong;Dr. Donna G. Blackmond
Angewandte Chemie International Edition 2014 Volume 53( Issue 33) pp:8700-8704
Publication Date(Web):
DOI:10.1002/anie.201404327
Abstract
An unusual solvent-induced inversion of the sense of enantioselectivity observed in the α-selenylation of aldehydes catalyzed by a diphenylprolinol silyl ether catalyst is correlated to the presence of intermediates formed subsequent to the highly selective CSe bond-forming step in the catalytic cycle. This work provides support for a mechanistic concept for enamine catalysis and includes a general role for “downstream intermediates” in selectivity outcomes in organocatalysis.
Co-reporter:Fionn O’Hara, Donna G. Blackmond, and Phil S. Baran
Journal of the American Chemical Society 2013 Volume 135(Issue 32) pp:12122-12134
Publication Date(Web):July 17, 2013
DOI:10.1021/ja406223k
Radical addition processes can be ideally suited for the direct functionalization of heteroaromatic bases, yet these processes are only sparsely used due to the perception of poor or unreliable control of regiochemistry. A systematic investigation of factors affecting the regiochemistry of radical functionalization of heterocycles using alkylsulfinate salts revealed that certain types of substituents exert consistent and additive effects on the regioselectivity of substitution. This allowed us to establish guidelines for predicting regioselectivity on complex π-deficient heteroarenes, including pyridines, pyrimidines, pyridazines, and pyrazines. Since the relative contribution from opposing directing factors was dependent on solvent and pH, it was sometimes possible to tune the regiochemistry to a desired result by modifying reaction conditions. This methodology was applied to the direct, regioselective introduction of isopropyl groups into complex, biologically active molecules, such as diflufenican (44) and nevirapine (45).
Co-reporter:Ryan D. Baxter, Donna G. Blackmond
Tetrahedron 2013 69(27–28) pp: 5604-5608
Publication Date(Web):
DOI:10.1016/j.tet.2013.04.007
Co-reporter:Jason E. Hein and Donna G. Blackmond
Accounts of Chemical Research 2012 Volume 45(Issue 12) pp:2045
Publication Date(Web):February 22, 2012
DOI:10.1021/ar200316n
The process of delineating the origins of the chemistry of life starts with the consideration of the molecules that might have existed on prebiotic earth and extends to the discussion of potential mechanisms for assembly of these molecules into informational polymers capable of self-replication and transmittance of genetic information. At some point along this pathway, the property of single chirality emerges as the hallmark of the amino acids and sugars present in biological molecules. In the 20th century, researchers developed abstract mathematical theses for the origin of biomolecular homochirality from a presumably racemic collection of prebiotic molecules. Before the end of that century, experimental findings corroborated a number of basic features of these theoretical models, but these studies involved chemical systems without direct prebiotic relevance. Currently researchers are examining prebiotically plausible conditions that couple chemical and physical processes leading to single chirality of sugars and amino acids with subsequent chemical reactions that enhance molecular complexity. While these studies have been conducted for the most part in the context of the RNA World hypothesis, the experimental findings remain relevant to a “metabolism first” model for the origin of life.To many chemists interested in chembiogenesis, the synthesis of activated pyrimidine ribonucleotides under potentially prebiotic conditions by Sutherland’s group provided a landmark demonstration of what Eschenmoser has described as “an intrinsic structural propinquity” between certain elementary chemical structures and modern biological molecules. Even while some synthetic issues for plausible prebiotic construction of RNA remain unsolved, our work has focused on coupling these synthetic advances with concepts for the evolution of biomlolecular homochirality. Drawing on our own findings as well as those from others, we present an intriguing “chicken or egg” scenario for the emergence of single chirality of sugars and amino acids. Our work incorporates both chemical and physical phenomena that allow for the amplification of a small initial imbalance of either sugars by amino acids or amino acid by sugars, suggesting that an enantioenriched chiral pool of one type of molecule could lead to a similarly enantioenriched pool of the other.
Co-reporter:Ryan D. Baxter ; David Sale ; Keary M. Engle ; Jin-Quan Yu
Journal of the American Chemical Society 2012 Volume 134(Issue 10) pp:4600-4606
Publication Date(Web):February 10, 2012
DOI:10.1021/ja207634t
Detailed kinetic studies and novel graphical manipulations of reaction progress data in Pd(II)-catalyzed olefinations in the presence of mono-N-protected amino acid ligands reveal anomalous concentration dependences (zero order in o-CF3-phenylacetic acid concentration, zero order in oxygen pressure, and negative orders in both olefin and product concentrations), leaving the catalyst concentration as the sole positive driving force in the reaction. NMR spectroscopic studies support the proposal that rate inhibition by the olefinic substrate and product is caused by formation of reversible off-cycle reservoirs that remove catalyst from the active cycle. NMR studies comparing the interaction between the catalyst and substrate in the presence and absence of the ligand suggest that weak coordination of the ligand to Pd prevents formation of an inactive mixed acetate species. A fuller understanding of these features may lead to the design of more efficient Pd(II) catalysts for this potentially powerful C–H functionalization reaction.
Co-reporter:Jordi Burés ; Alan Armstrong
Journal of the American Chemical Society 2012 Volume 134(Issue 15) pp:6741-6750
Publication Date(Web):March 27, 2012
DOI:10.1021/ja300415t
Detailed mechanistic study of two reactions catalyzed by diarylprolinol ether catalysts, the conjugate addition of aldehydes to nitro-olefins and the α-chlorination of aldehydes, leads to the proposal that the stereochemical outcome in these cases is not determined by the transition state of the step in which the stereogenic center is formed from enamine attack on the electrophile but instead is correlated with the relative stability and reactivity of diastereomeric intermediates downstream in the catalytic cycle. This combination of kinetic and thermodynamic factors illustrates a remarkable Curtin–Hammett scenario that can result in either an enhancement or an erosion of the selectivity that would be predicted by the transition state for enamine attack on the electrophile. Evidence is offered to suggest that this concept may represent a general phenomenon for pyrrolidine-based catalysts lacking an acidic directing proton. Implications for catalyst and reaction design are discussed.
Co-reporter:Jason E. Hein ; Blessing Huynh Cao ; Cristobal Viedma ; Richard M. Kellogg
Journal of the American Chemical Society 2012 Volume 134(Issue 30) pp:12629-12636
Publication Date(Web):July 10, 2012
DOI:10.1021/ja303566g
Insights into the mechanism of attrition-enhanced deracemization and resolution of solid enantiomorphic chiral compounds are obtained by crystal size and solubility measurements and by isotopic labeling experiments. Together these results help to deconvolute the various chemical and physical rate processes contributing to the phenomenon. Crystal size measurements highlight a distinct correlation between the stochastic, transient growth of crystals and the emergence of a single solid enantiomorph under attrition conditions. The rapid mass transfer of molecules between the solution and solid phases under attrition is demonstrated, and the concept of a crystal-size-induced solubility driving force is exploited to overcome the stochastic nature of the crystal growth and dissolution processes. Extension to non-racemizing conditions provides a novel methodology for chiral resolution. Implications both for practical chiral separations and for the origin of biological homochirality are discussed.
Co-reporter:Jordi Burés, Alan Armstrong and Donna G. Blackmond
Chemical Science 2012 vol. 3(Issue 4) pp:1273-1277
Publication Date(Web):26 Jan 2012
DOI:10.1039/C2SC01082H
The formation of enamines between aldehydes with α-stereocenters and pyrrolidine-based catalysts that lack an acidic proton is examined by kinetic and spectroscopic studies. The reaction exhibits “kinetic stereospecificity” in that each enantiomer of the aldehyde initially reacts to form a specific enamine stereoisomer, prior to thermodynamic equilibration of the E and Z enamines. For the case of prolinate catalysts, each of the stereoisomeric enamines is correlated with a specific stereoisomeric oxazolidinone. The reactions of E and Z enamines with electrophiles such as DEAD lead to products of opposite stereochemistry. The product enantioselectivity observed depends on the extent to which the E and Z enamines are pre-equilibrated prior to reaction with the electrophile. General implications for selectivity in organocatalytic reactions are discussed.
Co-reporter:Dr. Timo Gehring;Dr. Michela Quaranta;Dr. Barbara Odell; Donna G. Blackmond;Dr. John M. Brown
Angewandte Chemie International Edition 2012 Volume 51( Issue 38) pp:9539-9542
Publication Date(Web):
DOI:10.1002/anie.201203398
Co-reporter:Dr. Timo Gehring;Dr. Michela Quaranta;Dr. Barbara Odell; Donna G. Blackmond;Dr. John M. Brown
Angewandte Chemie 2012 Volume 124( Issue 38) pp:9677-9680
Publication Date(Web):
DOI:10.1002/ange.201203398
Co-reporter:Jordi Burés ; Alan Armstrong
Journal of the American Chemical Society 2011 Volume 133(Issue 23) pp:8822-8825
Publication Date(Web):May 15, 2011
DOI:10.1021/ja203660r
Kinetic studies of the conjugate addition of propanal to nitrostyrene catalyzed by diarylprolinol ethers reveal that formation of the product iminium species is rate-determining and is promoted by both the reaction product and acid additives. The beneficial role of a dominant cyclobutane intermediate in maintaining high stereoselectivity is highlighted. This mechanistic understanding led to the design of highly productive reaction protocols.
Co-reporter:Jason E. Hein, Alan Armstrong, and Donna G. Blackmond
Organic Letters 2011 Volume 13(Issue 16) pp:4300-4303
Publication Date(Web):July 15, 2011
DOI:10.1021/ol201639z
Deconvolution of the role of off-cycle species from the desired catalytic cycle leads to an optimized protocol for the prolinate-catalyzed amination of aldehydes. The scope of complex reaction networks will be greatly broadened by understanding ancillary rate processes that influence the productive catalytic pathway.
Co-reporter:Antonio C. Ferretti, Colin Brennan, Donna G. Blackmond
Inorganica Chimica Acta 2011 Volume 369(Issue 1) pp:292-295
Publication Date(Web):15 April 2011
DOI:10.1016/j.ica.2011.01.018
Reversible partitioning of a portion of the catalyst away from the active cycle provides a mechanistic rationalization of the anomalous reaction orders observed in the presence of additives. The concept of stable reservoir species leads to a practical protocol for maintaining a robust catalyst system that may be particularly useful for development of reactions under harsh conditions likely to lead to catalyst deactivation and incomplete turnover. Probing Pd-catalyzed amination reactions in the presence of competitive binding events reveals the key role that reservoir species connected reversibly to the catalytic cycle can play.Graphical abstractAnomalous reaction orders observed in Pd-catalyzed amination are rationalized by the reversible partitioning of the catalyst into a stable, off-cycle reservoir. A practical protocol for maintaining a robust catalyst system under harsh conditions is presented. Competitive binding events are used as a mechanistic probe.Research highlights► Anomalous reaction orders observed in Pd-catalyzed amination are rationalized by the reversible partitioning of the catalyst into a stable, off-cycle reservoir. ► A practical protocol for maintaining a robust catalyst system under harsh conditions is presented. ► Competitive binding events are used as a mechanistic probe.
Co-reporter:Michela Quaranta ; Timo Gehring ; Barbara Odell ; John M. Brown
Journal of the American Chemical Society 2010 Volume 132(Issue 43) pp:15104-15107
Publication Date(Web):October 13, 2010
DOI:10.1021/ja103204w
Observations of an intriguing inverse temperature dependence on reaction rate and a profound induction period in the Soai autocatalytic reaction are reported along with detailed kinetic and NMR investigations of the product alkoxide at low temperatures, leading to the suggestion that the active catalyst is derived in situ from the tetrameric ground state.
Co-reporter:Donna G. Blackmond ; Antonio Moran ; Matthew Hughes ;Alan Armstrong
Journal of the American Chemical Society 2010 Volume 132(Issue 22) pp:7598-7599
Publication Date(Web):May 13, 2010
DOI:10.1021/ja102718x
An intriguing reversal in product enantioselectivity accompanied by a change in the kinetic profile is observed in the α-amination of aldehydes catalyzed by proline in the presence of organic bases. Implications for the prevailing stereochemical models for proline and related aminocatalytic transformations are discussed.
Co-reporter:Donna G. Blackmond
Tetrahedron: Asymmetry 2010 Volume 21(11–12) pp:1630-1634
Publication Date(Web):23 June 2010
DOI:10.1016/j.tetasy.2010.03.034
The Kagan MLn models developed for rationalizing non-linear effects of catalyst enantiopurity have become a valuable mechanistic tool for probing complex asymmetric catalytic reactions. This work demonstrates how these models also provide clues about reactivity that may be used for further evidence to test a mechanistic hypothesis. Special considerations for probing non-linear effects in asymmetric synthesis using stoichiometric chiral auxiliaries and in asymmetric autocatalysis are highlighted in comparison with asymmetric catalysis.
Co-reporter:Cristobal Viedma Dr.;BastiaanJ.V. Verkuijl;JoséE. Ortiz;Trinidad deTorres;RichardM. Kellogg ;DonnaG. Blackmond Dr.
Chemistry - A European Journal 2010 Volume 16( Issue 16) pp:4932-4937
Publication Date(Web):
DOI:10.1002/chem.200902983
Abstract
Solution-phase racemization drives the evolution of single chirality in the solid phase by the “chiral amnesia” process first described by Viedma. The current investigations lay the basis for a better understanding of the mechanism of the solid-phase deracemization by uncoupling the chemical rate processes associated with the interconversion of enantiomers in the solution phase from the physical processes associated with solution–solid phase transfer via dissolution and reaccretion of molecules onto crystals. In addition, the enantiomer concentration profiles presented in this work, together with an analytical treatment of the racemization process in the presence of excess enantiopure solid, unequivocally reconfirm the validity of the Meyerhoffer double solubility rule for systems under solution racemization conditions.
Co-reporter:Jordi Burés, Alan Armstrong and Donna G. Blackmond
Chemical Science (2010-Present) 2012 - vol. 3(Issue 4) pp:NaN1277-1277
Publication Date(Web):2012/01/26
DOI:10.1039/C2SC01082H
The formation of enamines between aldehydes with α-stereocenters and pyrrolidine-based catalysts that lack an acidic proton is examined by kinetic and spectroscopic studies. The reaction exhibits “kinetic stereospecificity” in that each enantiomer of the aldehyde initially reacts to form a specific enamine stereoisomer, prior to thermodynamic equilibration of the E and Z enamines. For the case of prolinate catalysts, each of the stereoisomeric enamines is correlated with a specific stereoisomeric oxazolidinone. The reactions of E and Z enamines with electrophiles such as DEAD lead to products of opposite stereochemistry. The product enantioselectivity observed depends on the extent to which the E and Z enamines are pre-equilibrated prior to reaction with the electrophile. General implications for selectivity in organocatalytic reactions are discussed.