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Levandowski BJ, Raines RT, Houk KN. Hyperconjugative π → σ* CF Interactions Stabilize the Enol Form of Perfluorinated Cyclic Keto-Enol Systems. J Org Chem 2019; 84:6432-6436. [PMID: 30990687 DOI: 10.1021/acs.joc.9b00825] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Lindner and Lemal showed that perfluorination of keto-enol systems significantly shifts the equilibrium toward the enol tautomer. Quantum mechanical calculations now reveal that the shift in equilibrium is the result of the stabilization of the enol tautomer by hyperconjugative π → σ*CF interactions and the destabilization of the keto tautomer by the electron withdrawal induced by the neighboring fluorine atoms. The preference for the enol tautomer further increases in smaller perfluorinated cyclic keto-enol systems. This trend is in contrast to the nonfluorinated compounds, where the enol is strongly disfavored in the smaller rings. The fluoro effect overrides the effect of the ring size that controls the equilibria in nonfluorinated compounds. The increased overlap of the enol π bond with the σ*CF orbitals of the allylic C-F bonds results in the increased preference for the enol tautomer in smaller perfluorinated keto-enol systems. We show here why the effect is much greater than in 3,3-difluorocyclooctyne.
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Affiliation(s)
- Brian J Levandowski
- Department of Chemistry and Biochemistry , University of California , Los Angeles , California 90095 , United States.,Department of Chemistry , Massachusetts Institute of Technology , Cambridge , Massachusetts 02139 , United States
| | - Ronald T Raines
- Department of Chemistry , Massachusetts Institute of Technology , Cambridge , Massachusetts 02139 , United States
| | - K N Houk
- Department of Chemistry and Biochemistry , University of California , Los Angeles , California 90095 , United States
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Harris T, Alabugin IV. Strain and stereoelectronics in cycloalkyne click chemistry. MENDELEEV COMMUNICATIONS 2019. [DOI: 10.1016/j.mencom.2019.05.001] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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Francisco CB, Fernandes CS, de Melo UZ, Rittner R, Gauze GF, Basso EA. Strong hyperconjugative interactions limit solvent and substituent influence on conformational equilibrium: the case of cis-2-halocyclohexylamines. Beilstein J Org Chem 2019; 15:818-829. [PMID: 31019574 PMCID: PMC6466766 DOI: 10.3762/bjoc.15.79] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2019] [Accepted: 03/20/2019] [Indexed: 01/17/2023] Open
Abstract
The presence of strong stereoelectronic interactions involving the substituents in cis-2-substituted cyclohexanes may lead to results different from those expected. In this work, we studied the conformational behavior of cis-2-fluoro- (F), cis-2-chloro- (Cl), cis-2-bromo- (Br) and cis-2-iodocyclohexylamine (I) by dynamic NMR and theoretical calculations. The experimental data pointed to an equilibrium strongly shifted toward the ea conformer (equatorial amine group and axial halogen), with populations greater than 90% for F, Cl and Br in both dichloromethane-d2 and methanol-d4. Theoretical calculations (M06-2X/6-311++G(2df,2p)) were in agreement with the experimental, with no influence of the solvent or the halogen on the equilibrium. A principal component analysis of natural bond orbital energies pointed to the σ*C–X and σC–H orbitals and the halogen lone pairs (LPX) as the most significant for the hyperconjugative interactions that influenced the equilibrium. The σC–H → σ*C–X hyperconjugation and the interactions involving the LPX counterbalance each other, explaining the non-influence of the halogen on the conformational equilibrium. These interactions are responsible for the strong preference for the ea conformer in cis-2-halocyclohexylamines, being strong enough to restrain the shift in the equilibrium due to other factors such as steric repulsion or solvent effects.
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Affiliation(s)
- Camila B Francisco
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | | | - Ulisses Z de Melo
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | - Roberto Rittner
- Chemistry Institute, University of Campinas, 6154, Campinas 13083-970, Brazil
| | - Gisele F Gauze
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | - Ernani A Basso
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
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dos Passos Gomes G, Wimmer A, Smith JM, König B, Alabugin IV. CO2 or SO2: Should It Stay, or Should It Go? J Org Chem 2019; 84:6232-6243. [DOI: 10.1021/acs.joc.9b00503] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Affiliation(s)
- Gabriel dos Passos Gomes
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32309, United States
| | - Alexander Wimmer
- Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, Regensburg 93040, Germany
| | - Joel M. Smith
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32309, United States
| | - Burkhard König
- Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, Regensburg 93040, Germany
| | - Igor V. Alabugin
- Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32309, United States
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Alkynes as Synthetic Equivalents of Ketones and Aldehydes: A Hidden Entry into Carbonyl Chemistry. Molecules 2019; 24:molecules24061036. [PMID: 30875972 PMCID: PMC6471418 DOI: 10.3390/molecules24061036] [Citation(s) in RCA: 40] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2019] [Revised: 03/08/2019] [Accepted: 03/11/2019] [Indexed: 11/16/2022] Open
Abstract
The high energy packed in alkyne functional group makes alkyne reactions highly thermodynamically favorable and generally irreversible. Furthermore, the presence of two orthogonal π-bonds that can be manipulated separately enables flexible synthetic cascades stemming from alkynes. Behind these "obvious" traits, there are other more subtle, often concealed aspects of this functional group's appeal. This review is focused on yet another interesting but underappreciated alkyne feature: the fact that the CC alkyne unit has the same oxidation state as the -CH2C(O)- unit of a typical carbonyl compound. Thus, "classic carbonyl chemistry" can be accessed through alkynes, and new transformations can be engineered by unmasking the hidden carbonyl nature of alkynes. The goal of this review is to illustrate the advantages of using alkynes as an entry point to carbonyl reactions while highlighting reports from the literature where, sometimes without full appreciation, the concept of using alkynes as a hidden entry into carbonyl chemistry has been applied.
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Gryn'ova G, Corminboeuf C. Topology-Driven Single-Molecule Conductance of Carbon Nanothreads. J Phys Chem Lett 2019; 10:825-830. [PMID: 30668127 DOI: 10.1021/acs.jpclett.8b03556] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Highly conductive single-molecule junctions typically involve π-conjugated molecular bridges, whose frontier molecular orbital energy levels can be fine-tuned to best match the Fermi level of the leads. Fully saturated wires, e.g., alkanes, are typically thought of as insulating rather than highly conductive. However, in this work, we demonstrate in silico that significant zero-bias conductance can be achieved in such systems by means of topology. Specifically, caged saturated hydrocarbons offering multiple σ-conductance channels afford transmission far beyond what could be expected based upon conventional superposition laws, particularly if these pathways are composed entirely from quaternary carbon atoms. Computed conductance of molecular bridges based on carbon nanothreads, e.g., polytwistane, is not only of appreciable magnitude; it also shows a very slow decay with increasing nanogap, similarly to the case of π-conjugated wires. These findings offer a way to manipulate the transport properties of molecular systems by means of their topology, alternatively to the traditionally invoked electronic structure.
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Affiliation(s)
- Ganna Gryn'ova
- Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering , École Polytechnique Fédérale de Lausanne , CH-1015 Lausanne , Switzerland
| | - Clémence Corminboeuf
- Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering , École Polytechnique Fédérale de Lausanne , CH-1015 Lausanne , Switzerland
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Yoshidomi S, Abe M. 1,2-Diazacyclopentane-3,5-diyl Diradicals: Electronic Structure and Reactivity. J Am Chem Soc 2019; 141:3920-3933. [DOI: 10.1021/jacs.8b12254] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
| | - Manabu Abe
- JST-CREST, K’s Gobancho 6F, 7, Gobancho, Chiyoda-ku, Tokyo 102-0075, Japan
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Gomes GDP, Loginova Y, Vatsadze SZ, Alabugin IV. Isonitriles as Stereoelectronic Chameleons: The Donor-Acceptor Dichotomy in Radical Additions. J Am Chem Soc 2018; 140:14272-14288. [PMID: 30270623 DOI: 10.1021/jacs.8b08513] [Citation(s) in RCA: 40] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Radical addition to isonitriles (isocyanides) starts and continues all the way to the transition state (TS) mostly as a simple addition to a polarized π-bond. Only after the TS has been passed, the spin density moves to the α-carbon to form the imidoyl radical, the hallmark intermediate of the 1,1-addition-mediated cascades. Addition of alkyl, aryl, heteroatom-substituted, and heteroatom-centered radicals reveals a number of electronic, supramolecular, and conformational effects potentially useful for the practical control of isonitrile-mediated radical cascade transformations. Addition of alkyl radicals reveals two stereoelectronic preferences. First, the radical attack aligns the incipient C···C bond with the aromatic π-system. Second, one of the C-H/C-C bonds at the radical carbon eclipses the isonitrile N-C bond. Combination of these stereoelectronic preferences with entropic penalty explains why the least exergonic reaction (addition of the t-Bu radical) is also the fastest. Heteroatomic radicals reveal further unusual trends. In particular, the Sn radical addition to the PhNC is much faster than addition of the other group IV radicals, despite forming the weakest bond. This combination of kinetic and thermodynamic properties is ideal for applications in control of radical reactivity via dynamic covalent chemistry and may be responsible for the historically broad utility of Sn radicals ("the tyranny of tin"). In addition to polarity and low steric hindrance, radical attack at the relatively strong π-bond of isonitriles is assisted by "chameleonic" supramolecular interactions of the radical center with both the isonitrile π*-system and lone pair. These interactions are yet another manifestation of supramolecular control of radical chemistry.
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Affiliation(s)
- Gabriel Dos Passos Gomes
- Department of Chemistry and Biochemistry , Florida State University , Tallahassee , Florida 32309 , United States
| | - Yulia Loginova
- Department of Organic Chemistry, Faculty of Chemistry , Lomonosov Moscow State University , 1-3 Leninskiye Gory , Moscow 119991 , Russia
| | - Sergey Z Vatsadze
- Department of Organic Chemistry, Faculty of Chemistry , Lomonosov Moscow State University , 1-3 Leninskiye Gory , Moscow 119991 , Russia
| | - Igor V Alabugin
- Department of Chemistry and Biochemistry , Florida State University , Tallahassee , Florida 32309 , United States
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