- Thermodynamic Control of Isomerizations of Bicyclic Radicals: Interplay of Ring Strain and Radical Stabilization
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The rearrangements of 4-substituted bicyclo[2.2.2]oct-5-en-2-yl radicals, generated from the corresponding Diels-Alder adducts with phenylseleno acrylates by radical-induced reductive deselenocarbonylations, give the 2-substituted bicyclo[3.2.1]oct-6-en-2-yl radicals with some substituents, e.g., alkoxy and phenyl, but not for silyloxymethyl or benzyl substituents. Theoretical calculations with DFT give the thermodynamics of these reactions and the origins of these processes.
- Jung, Michael E.,Roberts, Courtney A.,Perez, Felix,Pham, Hung V.,Zou, Lufeng,Houk
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supporting information
p. 32 - 35
(2016/01/12)
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- Unsaturated aldehydes as alkene equivalents in the Diels-Alder reaction
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A one-pot procedure is described for using α,β-unsaturated aldehydes as olefin equivalents in the Diels-Alder reaction. The method combines the normal electron demand cycloaddition with aldehyde dienophiles and the rhodium-catalyzed decarbonylation of aldehydes to afford cyclohexenes with no electron-with-drawing substituents. In this way, the aldehyde group serves as a traceless control element to direct the cycloaddition reaction. The Diels-Alder reactions are performed in a diglyme solution in the presence of a catalytic amount of boron trifluoride etherate. Subsequent quenching of the Lewis acid, addition of 0.3% of [Rh(dppp)2Cl] and heating to reflux achieves the ensuing decarbonylation to afford the product cyclohexenes. Under these conditions, acrolein, crotonaldehyde and cinnamaldehyde have been reacted with a variety of 1,3-dienes to afford cyclohexenes in overall yields between 53 and 88%. In these transformations, the three aldehydes serve as equivalents of ethylene, propylene and styrene, respectively.
- Taarning, Esben,Madsen, Robert
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supporting information; experimental part
p. 5638 - 5644
(2009/05/30)
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- Thermal chemistry of bicyclo[4.2.0]oct-2-enes
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At 300 °C, bicyclo[4.2.0]oct-2-ene (1) isomerizes to bicyclo[2.2.2]oct-2-ene (2) via a formal [1,3] sigmatropic carbon migration. Deuterium labels at C7 and C8 were employed to probe for two-centered stereomutation resulting from C1-C6 cleavage and for one-centered Stereomutation resulting from C1-C8 cleavage, respectively. In addition, deuterium labeling allowed for the elucidation of the stereochemical preference of the [1,3] migration of 1 to 2. The two possible [1,3] carbon shift outcomes reflect a slight preference for migration with inversion rather than retention of stereochemistry; the si/sr product ratio is ～1.4. One-centered stereomutation is the dominant process in the thermal manifold of 1, with lesser amounts of fragmentation and [1,3] carbon migration processes being observed. All of these observations are consistent with a long-lived, conformationally promiscuous diradical intermediate.
- Powers, David C.,Leber, Phyllis A.,Gallagher, Sarah S.,Higgs, Andrew T.,McCullough, Lynne A.,Baldwin, John E.
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p. 187 - 194
(2007/10/03)
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- Thermal reactions of 7-d- and 8-d-bicyclo[4.2.0]oct-2-enes
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The gas phase thermal reactions exhibited by bicyclo[4.2.0]oct-2-ene and 7-d and 8-d analogues at 300 °C have been followed kinetically through GC and 2H NMR spectroscopic analyses. In contrast to the pattern of transformations exhibited by bicyclo[3.2.0]hept-2-ene and deuterium-labeled analogues, no reactions initiated by C1-C6 bond cleavage are seen, epimerization at C8 is much faster than [1,3] shifts leading to bicyclo[2.2.2]oct-2-ene, and the ratio of rate constants for [1,3] carbon migration with inversion versus migration with retention is ～1.4. Homolysis of C1-C8 to give a conformationally flexible diradical intermediate having a relatively long lifetime and multiple options for further reaction (re-formation of C1-C8 with or without net epimerization, fragmentation to 1,3-cyclohexadiene and ethylene, migration to the original C3 with inversion or retention) accords well with the observations. Clearly, orbital symmetry control does not govern stereochemistry for the [1,3] sigmatropic carbon shifts. Copyright
- Baldwin, John E.,Leber, Phyllis A.,Powers, David C.
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p. 10020 - 10021
(2007/10/03)
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- Bicyclo[2.2.2]octylidene
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Bicyclo[2.2.2]octylidene is formed in four quite different ways. Reactions of the precursors do not complicate the chemistry of the carbene. The products are tricyclo[2.2.2.02,6]octane and bicyclo[2.2.2]octene, formed in approximately a 70:30 r
- Ye, Qing,Jones Jr., Maitland,Chen, Ting,Shevlin, Philip B.
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p. 6979 - 6981
(2007/10/03)
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- Syntheses with organoboranes. IX. Vinyl- and 1-alkenyldichloroboranes as ethylene and 1-alkene equivalents for the Diels-Alder reaction
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Vinyl- and 1-alkenyldichloroboranes were used as dienophiles for the Diels-Alder reaction with representative aliphatic and cyclic 1,3-dienes. The organoborane adducts were transformed into the corresponding olefins either by protonolysis or by oxidation-mesylation-reduction. Direct protonolysis of the adducts gave in most cases mixtures of olefins whereas the reduction of mesylates with lithium triethylborohydride produced pure olefins in good yields.
- Zaidlewicz, Marek,Binkul, Jacek R.,Sokol, Wojciech
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p. 354 - 362
(2007/10/03)
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- More on adamantene
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At high dilution and temperature adamantene undergoes a retro Diels - Alder cycloaddition to a triene, as well as retro-insertion reactions to give carbenes. 1,2-Diiodoadamantane and 3-diiodomethylnoradamantane react with methyllithium in the gas phase to give adamantene, which is efficiently reduced to adamantane under these conditions. Addition of adamantene to butadiene occurs in 4 + 2 and 2 + 2 fashion.
- Bian, Nanying,Jones Jr., Maitland
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p. 8957 - 8961
(2007/10/02)
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- Cyclic Olefins by Anodic Oxidation of β-(Trimethylsilyl)carboxylic Acids. - β-(Trimethylsilyl)acrylic Acid Derivatives as Acetylene Equivalents in Diels-Alder Reactions
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Trimethylsilyl-substituted dienophiles 1, 2, and 4 react with dienes 6-14 in 66-100percent yields to give β-trimethylsilyl-substituted carboxylic acids 15-25, some of which are hydrogenated to 26-31.These are decarboxylated-desilylated to cyclic olefins 35-47 by Non-Kolbe electrolysis in 45-91percent yields.The dienophiles 1, 2, and 4 are thus suitable acetylene equivalents for Diels-Alder reactions.
- Hermeling, Dieter,Schaefer, Hans J.
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p. 1151 - 1158
(2007/10/02)
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- Diels - Alder Reactions of S-Vinyl-S-Arylsulfoximines
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Cyclopentadiene and the known S-p-tolyl-S-vinyl-N-phthalimidosulfoximine (1a) undergo Diels-Alder reaction to give a mixture of cycloadducts in excellent yield.The structures of the cycloadducts including stereochemistry are assigned by 1H NMR spectroscopy.The crystal and molecular structure of the major cycloadduct 5d, n = 1, G = 1,2 C6H4(CO)2N was determined unequivocally by X-ray crystallographic techniques.Treatment of this major cycloadduct with hydrazine in ethanol resulted in conversion of the sulfoximine to sulfoxide group concomitant with reduction of the carbon-carbon double bond.Use of allyl alcohol as solvent in this reaction allowed conversion to the corresponding unsaturated sulfoxide 8.The previously unknown S-p-tolyl-S-vinyl-N-(p-tolylsulfonyl)-sulfoximine (1b) is somewhat more reactive than phenyl vinyl sulfone and undergoes Diels-Alder reactions with cyclic and acyclic 1,3-dienes in 81-95percent yield.The endo selectivity of vinylsulfoximine 1b is 9:2 with cyclopentadiene and 93:7 with 1,3-cyclohexadiene.With 2-methyl-1,3-butadiene the para adduct is produced regioselectively in a ratio of 4:1.Although formation of the endo adducts from vinylsulfoximine 1b and cyclopentadiene and 1,3-cyclohexadiene is not diastereoselective, the diastereomers can be separated by HPLC.The cycloadducts from 1,3-cyclohexadiene and vinylsulfoximine 1b have been converted to bicyclooct-2-ene and bicyclooct-2-en-5-one.
- Glass, Richard S.,Reineke, Karl,Shanklin, Michael
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p. 1527 - 1533
(2007/10/02)
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- DEAMINATION OF BICYCLOOCTAN-2-YL- AND BICYCLOOCTAN-2-YL-AMINES. EVIDENCE FOR CLASSICAL PRECURSORS OF NON-CLASSICAL CARBONIUM IONS
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Bicyclo octan-2-yl- and exo-bicyclooctan-2-yl-amines have been deaminated in acetic acid by nitrous acid and via their N-phenyltriazenes; their ethyl N-nitrosocarbamates have also been solvolysed in ethanol.Product distributions by a given method from the structurally isomeric starting materials are similar to each other and to common product distribution obtained from bicyclooctan-2-yl and exo-bicyclooctan-2-yl toluene-p-sulphonates.Each amine gives, however, a small but unmistakable excess of the structurally unrearranged product compared (in the case of subtitution) with the distribution obtained from the solvolysis of the corresponding bicyclo-octyl toluene-p-sulphonates. endo-Bicyclooctan-2-ylamine has also been deaminated in acetic acid by nitrous acid and via its ethyl N-nitrosocarbamate in ethanol.The prouct ratios of these reactions are characteristically different from those of the isomric amines but, as far as substitution is concerned, are similar to what is obtained from endo-bicyclooctan-2-yl toluene -p-sulphonate.A common mechanism describes all the deaminative reactions.We propose that classical carbonium ions are the initial products of fragmentation of diazo-intermediates.These are intercepted to only a small extent to give products structurally and stereochemically characteristic of the original amines; to an even smaller extent they rearrange to isomeric classical carbonium ions, which in turn may be intercepted.The predominant reaction of the initially formed classical carbonium ions is rearrangement to non-classical isomers.From both becyclooctan-2-yl- and exo-bicyclooctan-2-yl-amines, the same unsymmetrical nonclassical carbonium ion is produced as has been implicated in the solvolysis of the corresponding toluene-p-sulphonates. endo-Bicyclooctan-2-ylamine deamination gives rise to an isomeric symmetrical non-classical carbonium ion, the same one that intervenes in the solvolysis of endo-bicyclo-octan-2-yl toluene-p-sulphonate.Symmetrical and unsymmetrical non-classical carbonium ions once formed give product ratios largely independent of their origins or modes of formation although the symmetrical one appears to undergo a small extent of isomerization to the (more stable) unsymmetrical species.These results are contrasted with those obtained from simple carbocyclic systems (without branching at the β-carbon) in which deamination and toluene-p-sulphonate solvolysis give characteristically different and unrelated product distributions.
- Maskill, Howard,Wilson, Alan A.
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p. 119 - 128
(2007/10/02)
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- Dienophilic Properties of Phenyl Vinyl Sulfone and trans-1-(Phenylsulfonyl)-2-(trimethylsilyl)ethylene. Their Utilization as Synthons for Ethylene, 1-Alkenes, Acetylene, and Monosubstituted Alkynes in the Construction of Functionalized Six-Membered Rings via ? Cycloaddition Meth.
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Useful procedures for effecting the indirect capture of ethylene, acetylene, 1-alkenes, and monosubstituted alkynes in Diels-Alder cycloadditions have been developed.In the first sequence, phenyl vinyl sulfone is shown to enter into ? reactions as a moderately reactive dienophile and to do so with very good regioselectivity.The resulting adducts can be directly desulfonated or alkylated prior to such reduction.A wide range of functional groups can be appended in this fashion at a specific locus within the newly formed six-membered ring.When the analogous chemistry is applied to trans-1-(phenylsulfonyl)-2-(trimethylsilyl)ethylene (2), adducts result which undergo ready fluoride ion induced elimination with efficient introduction of a double bond.The use of 2 and its d2 derivative is highlighted by the synthesis of several functionalized dibenzobarrelenes.
- Carr, Richard V. C.,Williams, Richard V.,Paquette, Leo A.
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p. 4976 - 4986
(2007/10/02)
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- Tricyclo2,8>octan-3-one: Photochemisch hergestellte Bausteine zur enantiospezifischen Totalsynthese cyclopentanoider Naturstoffe
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Bi- und tricyclopentanoide Naturstoffe stehen seit einigen Jahren als Zielstrukturen im Vordergrund des Interesses praeparativ arbeitender Chemiker.Mangels abwandelbarer Synthesemethoden wurden bisher meist nur produktbezogene Einzelloesungen erreicht, wobei in der Regel racemische Endstufen erhalten wurden.Das hier vorgestellte neue Konzept geht von einem einfachen Grundbaustein, dem Tricyclo2,8>octan-3-on, aus und erschliesst erstmals einen enantiospezifischen Zugang zu verschiedenen cyclopentanoiden Verbindungen.Das Tricyclooctanon ist in hoher Ausbeute aus Benzol erhaeltlich; dabei ist die Triplett-sensibilisierte Oxa-di-$ p-methan-Umlagerung von Bicyclooctenon die photochemische Schluesselreaktion.Racematspaltungen koennen problemlos auf der Bicyclooctenon-Stufe vorgenommen werden.Die strukturellen Eigenschaften des Tricyclooctanons erfuellen die Anforderungen an einen zentralen Synthesebaustein: Isomerisierungs- und Aufbaureaktionen lassen sich in hohen Ausbeuten regio- und stereoselektiv ausfuehren.Erste abgeschlossene Anwendungsbeispiele, Totalsynthesen von Boschnialacton, Allodolicholacton, Irido- und Isoridomyrmecin sowie von O-Acetyl-loganinaglucon, und erfolgversprechende Ansaetze zur Synthese von Coriolin, von 6a-Carbaprostacyclin und Homologen sowie von 9,11-Dehydrooestron demonstrieren das einzigartige Synthesepotential des Tricyclooctanons fuer Totalsynthesen polycyclopentanoider Verbindungen.
- Demuth, Martin,Schaffner, Kurt
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p. 809 - 825
(2007/10/02)
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