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Suthar S, Mondal KC. Open shell versus closed shell bonding interaction in cyclopropane derivatives: EDA-NOCV analyses. J Comput Chem 2023; 44:2184-2211. [PMID: 37530758 DOI: 10.1002/jcc.27190] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2023] [Revised: 06/19/2023] [Accepted: 06/21/2023] [Indexed: 08/03/2023]
Abstract
Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent-Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the CC bond of the cyclopropane (splitting two bonds at a time out of three CC bonds) ring is preferred to form two dative covalent CC bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3 -rings. These corresponding QTAIM parameters of C3 -rings are quite different for CC single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal-olefin systems.
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Affiliation(s)
- Sonam Suthar
- Department of Chemistry, Indian Institute of Technology Madras, Chennai, India
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Tyler JL, Aggarwal VK. Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics. Chemistry 2023; 29:e202300008. [PMID: 36786481 PMCID: PMC10947034 DOI: 10.1002/chem.202300008] [Citation(s) in RCA: 15] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2023] [Revised: 02/11/2023] [Accepted: 02/14/2023] [Indexed: 02/15/2023]
Abstract
The use of metalated (aza)bicyclo[1.1.0]butanes in synthesis is currently experiencing a renaissance, as evidenced by the numerous reports in the last 5 years that have relied on such intermediates to undergo unique transformations or generate novel fragments. Since their discovery, these species have been demonstrated to participate in a wide range of reactions with carbon and heteroatom electrophiles, as well as metal complexes, to facilitate the rapid diversification of (aza)bicyclo[1.1.0]butane-containing compounds. Key to this is the relative acidity of the bridgehead C-H bonds which promotes facile deprotonation and subsequent functionalization of an unsubstituted position on the carbon framework via the intermediacy of a metalated (aza)bicyclo[1.1.0]butane. Additionally, the late-stage incorporation of deuterium atoms in strained fragments has led to the elucidation of numerous reaction mechanisms that involve strained bicycles. The continued investigation into the inimitable reactivity of metalated bicycles will cement their importance within the field of organometallic chemistry.
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Affiliation(s)
- Jasper L. Tyler
- School of ChemistryUniversity of BristolCantock's CloseBristolBS8 1TSUK
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Kelly CB, Milligan JA, Tilley LJ, Sodano TM. Bicyclobutanes: from curiosities to versatile reagents and covalent warheads. Chem Sci 2022; 13:11721-11737. [PMID: 36320907 PMCID: PMC9580472 DOI: 10.1039/d2sc03948f] [Citation(s) in RCA: 54] [Impact Index Per Article: 27.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2022] [Accepted: 08/24/2022] [Indexed: 09/16/2023] Open
Abstract
The unique chemistry of small, strained carbocyclic systems has long captivated organic chemists from a theoretical and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as laboratory curiosities. However, new approaches for preparing, functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.
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Affiliation(s)
- Christopher B Kelly
- Discovery Process Research, Janssen Research & Development LLC 1400 McKean Road, Spring House PA 19477 USA
| | - John A Milligan
- Department of Biological and Chemical Sciences, College of Life Sciences, Thomas Jefferson University 4201 Henry Avenue Philadelphia PA 19144 USA
| | - Leon J Tilley
- Department of Chemistry, Stonehill College 320 Washington Street Easton MA 02357 USA
| | - Taylor M Sodano
- Therapeutics Discovery, Janssen Research & Development LLC 1400 McKean Road, Spring House PA 19477 USA
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Abstract
The computational modeling of fullerenes plays a fundamental role in designing low-dimension carbon nanostructures. Nevertheless, the relative energies of fullerenes larger than C20 and C24 have not been comprehensively examined by means of highly accurate ab initio methods, for example, the CCSD(T) method. Here we report such an investigation for a diverse set of 29 C40 isomers. We calculate the energies of the C40 fullerenes using the G4(MP2) composite ab initio method, which approximates the CCSD(T) energy in conjunction with a triple-ζ-quality basis set (CCSD(T)/TZ). The CCSD(T)/TZ isomerization energies span 43.1-763.3 kJ mol-1. We find a linear correlation (R2 = 0.96) between the CCSD(T)/TZ isomerization energies and the fullerene pentagon signatures (P1 index), which reflect the strain associated with fused pentagon-pentagon rings. Using the reference CCSD(T)/TZ isomerization energies, we examine the relationship between the percentage of exact Hartree-Fock (HF) exchange in hybrid density functional theory (DFT) methods and the pentagon-pentagon strain energies. We find that the performance of hybrid DFT methods deteriorates with the pentagon-pentagon strain energy. This deterioration in performance becomes more pronounced with the inclusion of high amounts of HF exchange. For example, for B3LYP (20% HF exchange), the root-mean-square deviation (RMSD) relative to G4(MP2) increases from 8.9 kJ mol-1 for the low-strain isomers (P1 = 11) to 18.0 kJ mol-1 for the high-strain isomers (P1 > 13). However, for BH&HLYP (50% HF exchange) the RMSD increases from 23.0 (P1 = 11) to 113.2 (P1 > 13) kJ mol-1. A similar trend is observed for the M06/M06-2X pair of functionals. Namely, for M06 (27% HF exchange) the RMSD increases from 0.8 (P1 = 11) to 21.0 (P1 > 13) kJ mol-1, whereas for M06-2X (54% HF exchange) the RMSD increases from 16.7 (P1 = 11) to 77.7 (P1 > 13) kJ mol-1. Overall, we find that the strain associated with pentagon adjacency is an inherently challenging problem for hybrid DFT methods involving high amounts of HF exchange and that there is an inverse relationship between the optimal percentage of HF exchange and the pentagon-pentagon strain energy. For example, for BLYP the optimal percentages of HF exchange are 13% (P1 = 11), 10% (P1 = 12), 7.5% (P1 = 13), and 6% (P1 > 13).
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Affiliation(s)
- Amir Karton
- School of Science and Technology, University of New England, Armidale, NSW 2351, Australia
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Jiang Y, Hu TD, Yu LY, Ding YH. A more effective catalysis of the CO 2 fixation with aziridines: computational screening of metal-substituted HKUST-1. NANOSCALE ADVANCES 2021; 3:4079-4088. [PMID: 36132833 PMCID: PMC9419783 DOI: 10.1039/d1na00150g] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2021] [Accepted: 06/03/2021] [Indexed: 05/12/2023]
Abstract
A vital issue for the fixation and conversion of CO2 into useful chemical products is to find effective catalysts. In this work, in order to develop more effective and diverse catalysts, we implemented the first computational screening study (at M06-2X//B3LYP level) on the cycloaddition of CO2 with aziridines under eighteen metal-substituted HKUST-1 MOFs and tetrabutylammonium bromide (TBAB) as a co-catalyst. For all considered catalytic systems, the ring-opening of aziridine is calculated to be the rate-determining step. Up to 11 M-HKUST-1 systems, i.e., Rh (31.87 kcal mol-1), Y (31.02), Sc (30.50), V (30.02), Tc (29.90), Cd (29.80), Ti (29.32), Mn (29.05), Zn (28.29), Fe (27.85) and Zr (25.09), possess lower ring-opening barrier heights than the original Cu-HKUST-1 (32.90), indicative of their superior catalytic ability to the original Cu-HKUST-1 in theory. With the lowest ring-opening barrier, Zr-HKUST-1 is strongly advocated for future synthetic and catalytic studies.
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Affiliation(s)
- Yan Jiang
- Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University Changchun 130023 P. R. China
| | - Tian-Ding Hu
- Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University Changchun 130023 P. R. China
| | - Li-Ying Yu
- Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University Changchun 130023 P. R. China
| | - Yi-Hong Ding
- Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University Changchun 130023 P. R. China
- Key Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and Materials Engineering, Wenzhou University Wenzhou 325035 P. R. China
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Abstract
AbstractBicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes are structurally unique compounds that exhibit diverse chemistry. Bicyclo[1.1.0]butane is a four-membered carbocycle with a bridging C(1)-C(3) bond and 1-azabicyclo[1.1.0]butane is an analog of bicyclo[1.1.0]butane featuring a nitrogen atom at one bridgehead. These structures are highly strained, allowing them to participate in a range of strain-releasing reactions which typically cleave the central, strained bond to deliver cyclobutanes or azetidines. However, despite these molecules being discovered in the 1950s and 1960s, and possessing a myriad of alluring chemical features, the chemistry and applications of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butanes remain underexplored. In the past 5 years, there has been a resurgent interest in their chemistry driven by the pharmaceutical industry’s increasing desire for new methods to access cyclobutanes and azetidines. This short review intends to provide a timely summary of the most recent developments in the chemistry of bicyclo[1.1.0]- and 1-azabicyclo[1.1.0]butane to highlight the diverse chemistry they can access, their value as synthetic precursors to cyclobutanes and azetidines, and to identify areas for future research.
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Affiliation(s)
- Alexander Fawcett
- Department of Chemistry, University of California, Berkeley, CA 94720, USA
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Turkowska J, Durka J, Gryko D. Strain release – an old tool for new transformations. Chem Commun (Camb) 2020; 56:5718-5734. [DOI: 10.1039/d0cc01771j] [Citation(s) in RCA: 40] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
This Feature Article provides an overview of research advances in the chemistry of spring-loaded molecules, focusing mainly on strain-release transformations.
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Affiliation(s)
- Joanna Turkowska
- Institute of Organic Chemistry Polish Academy of Sciences
- Warsaw 01-224
- Poland
| | - Jakub Durka
- Institute of Organic Chemistry Polish Academy of Sciences
- Warsaw 01-224
- Poland
- Department of Chemistry
- Warsaw University of Technology
| | - Dorota Gryko
- Institute of Organic Chemistry Polish Academy of Sciences
- Warsaw 01-224
- Poland
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Mo Y, Zhang H, Su P, Jarowski PD, Wu W. Intramolecular multi-bond strain: the unrecognized side of the dichotomy of conjugated systems. Chem Sci 2016; 7:5872-5878. [PMID: 30034728 PMCID: PMC6024246 DOI: 10.1039/c6sc00454g] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2016] [Accepted: 05/19/2016] [Indexed: 11/29/2022] Open
Abstract
Apart from the more familiar π-conjugation, there is also significant π–π repulsion which is a kind of unrecognized intramolecular strain and can be quantified with the linear B4H2 model system.
Electron conjugation stabilizes unsaturated systems and diminishes the differences among bond distances. Experimentally, Kistiakowsky and coworkers first measured and noticed the difference between the hydrogenation heats of carbon–carbon double bonds in conjugated systems. For instance, the hydrogenation heat of butadiene is 57.1 kcal mol–1, which is less than two times that of the hydrogenation heat of 1-butene (30.3 kcal mol–1), and the difference (3.5 kcal mol–1) is the extra stabilization due to the resonance between two double bonds in the former, and is referred to as the experimental resonance energy. Following Kistiakowsky's definition, Rogers et al. studied the stepwise hydrogenation of 1,3-butadiyne and concluded that there is no conjugation stabilization in this molecule. This claim received objections instantly, but Rogers and coworkers further showed the destabilizing conjugation in 2,3-butanedione and cyanogen. Within resonance theory, the conjugation energy is derived “by subtracting the actual energy of the molecule in question from that of the most stable contributing structure.” The notable difference between the experimental and theoretical resonance energies lies in that the former needs other real reference molecules while the latter does not. Here we propose and validate a new concept, intramolecular multi-bond strain, which refers to the repulsion among π bonds. The π–π repulsion, which is contributed to by both Pauli exchange and electrostatic interaction, is quantified with the B4H2 model system (16.9 kcal mol–1), and is compared with the σ–σ repulsion in B2H4 (7.7 kcal mol–1). The significance of the π–π repulsion can be demonstrated by the much longer carbon–nitrogen bond in nitrobenzene (1.486 Å) than in aniline (1.407 Å), the very long and weak nitrogen–nitrogen bond (1.756 Å) in dinitrogen tetroxide, and the instability of long polyynes. This new concept successfully reconciles the discrepancy between experimental and theoretical conjugation energies. However, we maintain that by definition, electron conjugation must be stabilizing.
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Affiliation(s)
- Yirong Mo
- The State Key Laboratory of Physical Chemistry of Solid Surfaces , Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China.,Department of Chemistry , Western Michigan University , Kalamazoo , Michigan 49008 , USA .
| | - Huaiyu Zhang
- The State Key Laboratory of Physical Chemistry of Solid Surfaces , Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China.,Department of Chemistry , Western Michigan University , Kalamazoo , Michigan 49008 , USA .
| | - Peifeng Su
- The State Key Laboratory of Physical Chemistry of Solid Surfaces , Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China
| | - Peter D Jarowski
- University of Surrey , Advanced Technology Institute , Guildford , GU2 7XH , UK .
| | - Wei Wu
- The State Key Laboratory of Physical Chemistry of Solid Surfaces , Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and College of Chemistry and Chemical Engineering , Xiamen University , Xiamen , Fujian 361005 , China.,iChEM , Xiamen University , Xiamen , Fujian 361005 , China .
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Keyvani ZA, Shahbazian S, Zahedi M. Tracing the Fingerprint of Chemical Bonds within the Electron Densities of Hydrocarbons: A Comparative Analysis of the Optimized and the Promolecule Densities. Chemphyschem 2016; 17:3260-3268. [DOI: 10.1002/cphc.201600632] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2016] [Indexed: 11/10/2022]
Affiliation(s)
- Zahra Alimohammadi Keyvani
- Faculty of Chemistry, Department of Pure Chemistry; Shahid Beheshti University, G. C.; Tehran 19839 Evin, P.O. Box 19395-4716 Iran), Tel/Fax: (98) 2122-431-661
| | - Shant Shahbazian
- Faculty of Chemistry, Department of Pure Chemistry; Shahid Beheshti University, G. C.; Tehran 19839 Evin, P.O. Box 19395-4716 Iran), Tel/Fax: (98) 2122-431-661
| | - Mansour Zahedi
- Faculty of Chemistry, Department of Pure Chemistry; Shahid Beheshti University, G. C.; Tehran 19839 Evin, P.O. Box 19395-4716 Iran), Tel/Fax: (98) 2122-431-661
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Karton A, Schreiner PR, Martin JML. Heats of formation of platonic hydrocarbon cages by means of high-level thermochemical procedures. J Comput Chem 2015; 37:49-58. [DOI: 10.1002/jcc.23963] [Citation(s) in RCA: 48] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2015] [Revised: 04/25/2015] [Accepted: 05/11/2015] [Indexed: 12/24/2022]
Affiliation(s)
- Amir Karton
- School of Chemistry and Biochemistry; The University of Western Australia; Perth Western 6009 Australia
| | - Peter R. Schreiner
- Institute of Organic Chemistry; Justus-Liebig University; Heinrich-Buff-Ring 58 35392 Giessen Germany
| | - Jan M. L. Martin
- Department of Organic Chemistry; Weizmann Institute of Science; Reḥovot Israel
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Walczak MAA, Krainz T, Wipf P. Ring-strain-enabled reaction discovery: new heterocycles from bicyclo[1.1.0]butanes. Acc Chem Res 2015; 48:1149-58. [PMID: 25775119 DOI: 10.1021/ar500437h] [Citation(s) in RCA: 101] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Mechanistically as well as synthetically, bicyclo[1.1.0]butanes represent one of the most fascinating classes of organic compounds. They offer a unique blend of compact size (four carbon atoms), high reactivity (strain energy of 66 kcal/mol), and mechanistic pathway diversity that can be harvested for the rapid assembly of complex scaffolds. The C(1)-C(3) bond combines the electronic features of both σ and π bonds with facile homolytic and heterolytic bond dissociation properties and thereby readily engages pericyclic, transition-metal-mediated, nucleophilic, and electrophilic pathways as well as radical acceptor and donor substrates. Despite this multifaceted reaction profile and recent advances in the preparation of bicylo[1.1.0]butanes, the current portfolio of synthetic applications is still limited compared with those of cyclopropanes and cyclobutanes. In this Account, we describe our work over the past decade on the exploration of substituent effects on the ring strain and the reactivity of bicyclo[1.1.0]butanes, particularly in the context of metal-mediated processes. We first describe Rh(I)-catalyzed cycloisomerization reactions of N-allyl amines to give pyrrolidine and azepine heterocycles. The regioselectivity of the C,C-bond insertion/ring-opening step in these reactions is controlled by the phosphine ligand. After metal carbene formation, an intramolecular cyclopropanation adds a second fused ring system. A proposed mechanism rationalizes why rhodium(I) complexes with monodentate ligands favor five-membered heterocycles, as opposed to Rh(I)-bidentate ligand catalysts, which rearrange N-allyl amines to seven-membered heterocycles. The scope of Rh(I)-catalyzed cycloisomerization reactions was extended to allyl ethers, which provide a mixture of five- and seven-membered cyclic ethers regardless of the nature of the phosphine additive and Rh(I) precatalyst. The chemical diversity of these cycloisomerization products was further expanded by a consecutive one-pot metathesis reaction. Rh(I)-catalyzed cycloisomerizations of propargyl amides, ethers, and electron-deficient bicyclo[1.1.0]butanes diverged mechanistically and often led to a significant number of decomposition products. In these cases, Pt(II) emerged as a superior, more alkynophilic late transition metal with its own mechanistic peculiarities. While monosubstituted bicyclo[1.1.0]butanes led to the formation of tetrahydropyridines, 1,3-disubstituted and electron-deficient bicyclo[1.1.0]butanes reacted distinctly differently with Pt(II) and ultimately provided a complementary set of nitrogen- and oxygen-containing cyclic scaffolds. The metal-catalyzed ring transformations of bicyclo[1.1.0]butanes presented herein suggest additional strategies for new reaction discoveries that can access a wide variety of novel cyclic frameworks from relatively simple starting materials. In addition, these case studies highlight the considerable potential for future applications in natural products, medicinal, and diversity-oriented synthesis based on the wealth of mechanistic pathways available to these strained small-ring carbocycles.
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Affiliation(s)
- Maciej A. A. Walczak
- Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States
| | - Tanja Krainz
- Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States
| | - Peter Wipf
- Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States
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Damrauer R. Interactions of Tetrahedrane and Tetrasilatetrahedrane with CH2and SiH2: A Computational Study. Organometallics 2010. [DOI: 10.1021/om100193f] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Affiliation(s)
- Robert Damrauer
- Chemistry Department, Denver Campus, University of Colorado Denver, Box 194, P.O. Box 173364, Denver, Colorado 80217-3364
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Wu W, Ma B, I-Chia Wu J, Schleyer P, Mo Y. Is Cyclopropane Really the σ-Aromatic Paradigm? Chemistry 2009; 15:9730-6. [DOI: 10.1002/chem.200900586] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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14
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Wheeler SE, Houk KN, Schleyer PVR, Allen WD. A hierarchy of homodesmotic reactions for thermochemistry. J Am Chem Soc 2009; 131:2547-60. [PMID: 19182999 PMCID: PMC2711007 DOI: 10.1021/ja805843n] [Citation(s) in RCA: 394] [Impact Index Per Article: 26.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of "homodesmotic" reactions are not equivalent. New definitions are introduced, and a consistent hierarchy of reaction classes (RC1-RC5) for hydrocarbons is constructed: isogyric (RC1) superset of isodesmic (RC2) superset of hypohomodesmotic (RC3) superset of homodesmotic (RC4) superset of hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol(-1) across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions are demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 +/- 0.4 kcal mol(-1)), 1,3,5,7-octatetrayne (217.5 +/- 0.6 kcal mol(-1)), the larger polyynes C(10)H(2) through C(26)H(2), and an infinite acetylenic carbon chain.
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Affiliation(s)
- Steven E Wheeler
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA.
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Dobrowolski MA, Cyrański MK, Merner BL, Bodwell GJ, Wu JI, Schleyer PVR. Interplay of pi-electron delocalization and strain in [n](2,7)pyrenophanes. J Org Chem 2008; 73:8001-9. [PMID: 18798674 DOI: 10.1021/jo8014159] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The geometries of a series of [n](2,7)pyrenophanes (n = 6-12) were optimized at the B3LYP/6-311G** DFT level. The X-ray crystal structures determined for the [9](2,7)- and [10](2,7)pyrenophanes agreed excellently with the computed structures. The degree of nonplanarity of the pyrene moiety depends on the number of CH2 groups in the aliphatic bridge and, as analyzed theoretically, influences the strain energy and the extent of pi-electron delocalization in the pyrene fragment. Various indices, e.g., the relative aromatic stabilization energies (DeltaASE), magnetic susceptibility exaltations (Lambda), nucleus-independent chemical shifts (NICS), and the harmonic oscillator model of aromaticity (HOMA) were used to quantify the change in aromatic character of the pyrene fragment. DeltaASE and relative Lambda values (with respect to planar pyrene) were evaluated by homodesmotic equations comparing the bent pyrene unit with its bent quinoid dimethylene-substituted analog. The bend angle, alpha, DeltaASE, and Lambda were linearly related. The aromaticity decreases smoothly and regularly over a wide range of bending, but the magnitude of the change is not large. The differences between planar pyrene (alpha = 0 degrees) and the most distorted pyrene unit (alpha = 39.7 degrees in [6](2,7)pyrenophane) are only 15.8 kcal/mol (DeltaASE) and 18.8 cgs-ppm (Lambda). Also, the geometry-based HOMA descriptor changes by only 0.07 unit. The local NICS descriptors of aromatic character also correlate very well with the global indices of aromaticity. In line with the known reactivity of pyrenophanes, the variations of NICS(1), a measure of pi-electron delocalization, were largest for the outer, biphenyl-type rings. The strain energies of the pyrene fragments were much larger and varied more than those evaluated for the bridge. Both strain energies were interrelated (correlation coefficient R = 0.979) and depend on the bend angle, alpha.
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Affiliation(s)
- Michał A Dobrowolski
- Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
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Barić D, Maksić ZB. The Baeyer strain is strongly affected by the nucleus–electron attraction—a comment on the Letter of G. Hohlneicher and L. Packschies [Tetrahedron Lett. 2007, 48, 6429–6433]. Tetrahedron Lett 2008. [DOI: 10.1016/j.tetlet.2007.12.036] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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Hohlneicher G, Packschies L. The Baeyer strain in small ring systems does not originate from a decrease in nucleus–electron attraction. Tetrahedron Lett 2007. [DOI: 10.1016/j.tetlet.2007.07.066] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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