Paul Dowd and diradicals

Cramer, Christopher J.

doi:10.1039/a707644d

Cited by 21 publications

(14 citation statements)

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“…26 Theoretical studies of 5 similarly indicate the diradical description to be more accurate. [26][27][28][29] The oxyallyl diradical, which is isoelectronic with the nonKekulé hydrocarbon trimethylenemethane, 27,30,31 has substantial multireference character and as such is found to be unrealistically high in energy at the MP2 level. A more accurate estimate of the energy difference between the cyclopropanone and oxyallyl tautomers of 5 has been provided by Lim et al, 26 who computed the difference to be 32.4 kcal mol À1 (1 kcal = 4.184 kJ) at the multireference CASPT2N/6-31G*//CASSCF/6-31G* level using a four-electron in four-orbital active space.…”

Section: Oxyallyl Vs Cyclopropanone Geometriesmentioning

confidence: 98%

Quantum chemical characterization of cycloaddition reactions between 1,3-butadiene and oxyallyl cations of varying electrophilicity

Cramer

Barrows

2000

J. Phys. Org. Chem.

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Hydroxyallyl cation and lithium and sodium oxyallyl cations are predicted to react with 1,3-butadiene both in a stepwise fashion and via concerted [4 3] cycloaddition with so-called extended stereochemistry. With hydroxyallyl cation, the stepwise process is preferred and subsequent second bond closures generate products equivalent to those that would arise from concerted [4 3] or [3 2] cycloadditions. For lithium and sodium oxyallyl cations, concerted, asynchronous processes are predicted to be preferred over stepwise processes, with [3 2] cycloaddition to generate a 3H-dihydrofuran followed by Claisen rearrangement of that intermediate being the lowest energy pathway for formation of a seven-membered ring. In the case of uncharged 2-oxyallyl, only transition state structures for concerted cycloadditions appear to exist. We infer that for [4 3] cycloadditions, concerted pathways are preferred over stepwise pathways provided that the separation between the electrophilicity of the allyl component and the electrofugacity of the 4p component is not too large. The Hammond postulate is shown to rationalize variations in free energies of activation for different processes as a function of allyl electrophilicity. Factors influencing the stereochemical outcome of different cycloadditions are discussed.

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Section: Oxyallyl Vs Cyclopropanone Geometriesmentioning

confidence: 98%

Quantum chemical characterization of cycloaddition reactions between 1,3-butadiene and oxyallyl cations of varying electrophilicity

Cramer

Barrows

2000

J. Phys. Org. Chem.

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“…[4] Die schwierige Nachweisbarkeit von 1 ist umso bemerkenswerter, als der verwandte Nicht-KekulØ-Kohlenwasserstoff Trimethylenmethan (TMM, 2) schon vor mehr als 40 Jahren in Dowds grundlegender Arbeit hergestellt und EPR-spektroskopisch in seinem Triplettgrundzustand charakterisiert worden ist. [5] Die bisher nicht geglückte direkte Beobachtung von 1 liegt nicht am halbherzigen experimentellen Eifer. Tatsäch-lich ist eine Reihe nicht erfolgreicher Herstellungsversuche bekannt, darunter auch die Photolyse von 1,3-Diiodaceton.…”

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“…[6] Statt 1 können unter anderem zweifelsfrei Cyclopropanon (3) und Allenoxid (4) [7] In 2 gibt es zwei entartete nichtbindende Molekülorbitale (NBMOs), und da diese nicht disjunkt (nondisjoint) sind, resultiert daraus ein Triplettgrundzustand. [5] Die Gegenwart Noch wichtiger ist, dass das Singulett-Oxyallyl leicht eine Ringschlussreaktion eingehen kann. Es ist allerdings noch nicht einmal klar, ob der Singulettzustand dieser Verbindung einem Minimum auf der C 3 H 4 O-Potentialhyperfläche entspricht.…”

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“…Borden et al legten die Grundlagen zum Verständnis des Oxyallyl-Diradikals: [7] In 2 gibt es zwei entartete nichtbindende Molekülorbitale (NBMOs), und da diese nicht disjunkt (nondisjoint) sind, resultiert daraus ein Triplettgrundzustand. [5] Die Gegenwart eines Sauerstoffatoms in 1 hebt die Entartung dieser NBMOs auf; als Konsequenz werden die Singulett-und Triplettzustände ( 1 A 1 bzw. 3 B 2 ; 1 1 bzw.…”

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Das Oxyallyl‐Diradikal: Beobachtung des Singulett‐ und Triplettzustands mit Negativionen‐Photoelektronenspektroskopie

Bettinger

2010

Angewandte Chemie

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[3+2] Cycloaddition of Trimethylenemethane and its Synthetic Equivalents

Yamago

Nakamura

2002

Organic Reactions

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Trimethylenemethane (TMM) is a non‐Kekulé molecule composed of four carbons, six hydrogens, and four π‐orbitals. Therefore, TMM can only be expressed as resonance structures involving 1,3‐diyls and zwitterions. The two degenerate nonbonding molecular orbitals in the Hückel molecular orbital of TTM indicate the existence of singlet and triplet electronic configurations, which play significant roles in the TMM chemistry. TMMs were imaginary molecules until the late 1960's, and parent TMM was first isolated in a low‐temperature matrix. It is stable for several weeks at 77 K, and is a ground‐state triplet with a D 3 h ‐planar structure as assigned by ESR spectroscopy. The unique structures coupled with the complex electronic states of TMM derivatives have been topics of theoretical, mechanistic, and synthetic studies. By analogy with the Diels‐Alder reaction for the synthesis of six‐membered rings through [4 + 2] cycloadditions, the use of TMM as a three‐carbon unit for [3 + 2] cycloadditions appears to represent a viable synthetic possibility, because the four π‐electron orbital system of TMM is theoretically suitable for the [4π s + 2π s ] cycloaddition. As expected, however, rapid ring closure to methylenecyclopropane (MCP) is preferred to cycloaddition with a 2π acceptor. Therefore, for synthetic use, tailor‐made derivatives of TMM and its equivalents need to be designed to achieve [3 + 2] cycloaddition reactions. The first breakthrough in the use of TMMs for [3 + 2] cycloaddition reactions was reported in the reaction of isopropylidenecyclopentane‐1,3‐diyl, which has a considerable lifetime and reacts with electron‐deficient alkenes. Because the parent TMM gives methylenecyclopropane and many MCP derivatives undergo thermal isomerization, MCPs have been considered to serve as precursors for TMMs. However, direct evidence for the formation of TMMs from MCPs and their synthetic use as TMM precursors was rather scant until recently. Since TMM species are generally too short‐lived and too reactive to be used for organic synthesis, various attempts have been made to stabilize them with transition metal templates. Efficient transition metal catalyzed TMM cycloaddition is achieved using MCPs and Ni(0) or Pd(0) catalysts. The reaction proceeds via ring opening of the MCP to form (methylene)metallacyclobutane‐type intermediates. More recently, Pd(0)‐catalyzed reactions of [2‐(acetoxymethyl)allyl]trimethyl‐silane with 2π acceptors have emerged as a powerful method for cyclopentane synthesis. This reaction is considered to involve the zwitterionic TMM‐Pd intermediate. The reaction serves well for the [3 + 2] cycloaddition to electron‐deficient acceptors, and it has been applied to the synthesis of several cyclopentanoid natural products. This review surveys the [3 + 2] cycloaddition reactions of TMMs and their metal complexes to 2π acceptors. The reactions are divided into five categories, defined by reaction types and precursors: (1) cycloadditions of free TMMs generated from diazenes, (2) cycloadditions of free TMMs generated from MCPs, (3) transition metal catalyzed reactions of MCPs, (4) transition metal catalyzed reactions of silylated allylic acetates, and (5) cycloaddition of stable TMM‐metal complexes. Transformations that accomplish a net cycloaddition but require several steps using TMM equivalents are not covered. Various aspects of TMMs, including physical and synthetic chemistry, have already been reviewed.

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Paul Dowd and diradicals

Cited by 21 publications

References 61 publications

Quantum chemical characterization of cycloaddition reactions between 1,3-butadiene and oxyallyl cations of varying electrophilicity

Quantum chemical characterization of cycloaddition reactions between 1,3-butadiene and oxyallyl cations of varying electrophilicity

Das Oxyallyl‐Diradikal: Beobachtung des Singulett‐ und Triplettzustands mit Negativionen‐Photoelektronenspektroskopie

[3+2] Cycloaddition of Trimethylenemethane and its Synthetic Equivalents

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