- Simple Access to the Heaviest Alkaline Earth Metal Hydride: A Strongly Reducing Hydrocarbon-Soluble Barium Hydride Cluster
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Reaction of Ba[N(SiMe3)2]2 with PhSiH3 in toluene gave simple access to the unique Ba hydride cluster Ba7H7[N(SiMe3)2]7 that can be described as a square pyramid spanned by five Ba2+ ions with two flanking BaH[N(SiMe3)2] units. This heptanuclear cluster is well soluble in aromatic solvents, and the hydride 1H NMR signals and coupling pattern suggests that the structure is stable in solution. At 95 °C, no coalescence of hydride signals is observed but the cluster slowly decomposes to undefined barium hydride species. The complex Ba7H7[N(SiMe3)2]7 is a very strong reducing agent that already at room temperature reacts with Me3SiCH=CH2, norbornadiene, and ethylene. The highly reactive alkyl barium intermediates cannot be observed and deprotonate the (Me3Si)2N? ion, as confirmed by the crystal structure of Ba14H12[N(SiMe3)2]12[(Me3Si)(Me2SiCH2)N]4.
- Wiesinger, Michael,Maitland, Brant,F?rber, Christian,Ballmann, Gerd,Fischer, Christian,Elsen, Holger,Harder, Sjoerd
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Read Online
- 195. Carbon Participation in the Solvolysis of 6- and 7-Substituted 2-Norbornyl p-Toluenesulfonates
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The solvolysis rates and products of several 7-anti-substituted 2-endo-norbornyl p-toluenesulfonates 11 have been determined and compared with those of the previously reported 6-exo-substituted 2-exo-norbornyl p-toluenesulfonates 1.Although the number of bonds between the substituent and the reaction site is the same in the two series, the inductive effect of the substituents is transmitted far more strongly in the 6-exo-2-exo-series 1 than in the 7-anti-2-exo-series 11; i.e. their inductivities differ widely.It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C(2) and that this involves differential bridging strain.The different reactivities of unsubstituted 2-exo- and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect.
- Flury, Peter,Grob, Cyril A.
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Read Online
- Facile Activation of Dihydrogen by Long-lived Carbonium Ions on Silica-Alumina Catalysts. An Example of a Simple -Type Reaction
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Silica-alumina catalyses the gas phase hydrogenation of norbornadiene and 1-chloroadamantane at the surprisingly moderate temperature of 90 deg C showing that when the carbonium ion intermediates are long-lived the R+ ...O- - surface species reacts with dihydrogen in a -type reaction, and does so far more readily than the corresponding H+ ...O- - cation-anion pair.
- Rooney, John J.
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Read Online
- Heterometallic Mg?Ba Hydride Clusters in Hydrogenation Catalysis
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Reaction of a MgN“2/BaN”2 mixture (N“=N(SiMe3)2) with PhSiH3 gave three unique heterometallic Mg/Ba hydride clusters: Mg5Ba4H11N”7 ? (benzene)2 (1), Mg4Ba7H13N“9 ? (toluene)2 (2) and Mg7Ba12H26N”12 (3). Product formation is controlled by the Mg/Ba ratio and temperature. Crystal structures are described. While 3 is fully insoluble, clusters 1 and 2 retain their structures in aromatic solvents. DFT calculations and AIM analyses indicate highly ionic bonding with Mg?H and Ba?H bond paths. Also unusual H????H? bond paths are observed. Catalytic hydrogenation with MgN“2, BaN”2 and the mixture MgN“2/BaN”2 has been studied. Whereas MgN“2 is only active in imine hydrogenation, alkene and alkyne hydrogenation needs the presence of Ba. The catalytic activity of the MgN”2/BaN“2 mixture lies in general between that of its individual components and strong cooperative effects are not evident.
- Wiesinger, Michael,Knüpfer, Christian,Elsen, Holger,Mai, Jonathan,Langer, Jens,Harder, Sjoerd
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p. 4567 - 4577
(2021/09/09)
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- Alkene Transfer Hydrogenation with Alkaline-Earth Metal Catalysts
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The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′2 catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe3)2] using 1,4-cyclohexadiene (1,4-CHD) as a H source. Reaction of 1,4-CHD with AeN′′2 gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1-hexene) needed a higher temperature (120 °C) but proceeded without double-bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4-CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4-CHD followed by H transfer. Convenient access to larger quantities of BaN′′2, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.
- Bauer, Heiko,Thum, Katharina,Alonso, Mercedes,Fischer, Christian,Harder, Sjoerd
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supporting information
p. 4248 - 4253
(2019/03/07)
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- Pyridyl-decorated self-folding heptaamide cavitands as ligands in the rhodium-catalyzed hydrogenation of norbornadiene
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The different binding geometries exhibited in solution by the Rh I cationic complexes of three regioisomeric self-folding heptaamide cavitands, each decorated with one pyridyl group at the upper rim, are taken into account to explain the diverse distributions of products obtained when these complexes are employed as catalysts for the hydrogenation of norbornadiene. Copyright
- Korom, Sasa,Ballester, Pablo
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p. 4276 - 4282
(2014/07/21)
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- Catalytic hydrogenation of norbornadiene by a rhodium complex in a self-folding cavitand
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It's a wrap! The inclusion of [Rh(nbd)2]BF4 (nbd=norbornadiene) in a deep-cavity cavitand produces a catalytically active species that promotes the hydrogenation of norbornadiene to norbornene (see picture). The structure of the cavitand acts as a second-sphere ligand and modifies the stability, selectivity, and reactivity observed for the free organometallic complex in solution.
- Sarmentero, Maria Angeles,Fernandez-Perez, Hector,Zuidema, Erik,Bo, Caries,Vidal-Ferran, Anton,Ballester, Pablo
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supporting information; experimental part
p. 7489 - 7492
(2010/12/20)
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- The 2-norbornyl cation via the fragmentations of exo- and endo-2-norbornyloxychlorocarbenes: Distinction without much difference
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exo- and endo-2-norbornyloxychlorocarbenes (7) were generated photochemically from the corresponding diazirines (6). Both carbenes fragmented to [2-norbornyl cation (carbon monoxide) chloride] ion pairs in MeCN or 1,2-dichloroethane solutions. Products included exo-norbornyl chloride (8), endo-norbornyl chloride (9), norbornene (10), and nortricyclene (11). Fragmentation activation energies were very low (5 s-1 in MeCN). Due to chloride return within the ion pairs, product distributions from exo- and endo-7 differed, with more endo-chloride formed from the endo-carbene: the 8/9 product ratio in MeCN was ~41 from exo-7, but only 4.6 from endo-7. Norbornene, formed by proton transfer to Cl- within the ion pairs, was a major product in both cases (44% from exo-7 and 62% from endo-7). In MeOH/MeCN, up to 28% of exo-2-norbornyl methyl ether formed at the expense of some of the norbornene, but even in 100% MeOH, the norbornyl chloride products of ion pair return still accounted for 46% and 31% of the exo-7 and endo-7 product mixtures (accompanied by 26-32% of norbornene). Electronic structure calculations on the ground states and fragmentation transition states of exo-7 and endo-7 are presented.
- Moss,Zheng,Sauers,Toscano
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p. 8109 - 8116
(2007/10/03)
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- Chemical Consequences of Long-Range Orbital Interactions in Norbornane-1,4-diol Monosulfonate Esters
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Seven monotosylated 1,4-diols with the rigid norbornane skeleton were treated with a strong base in refluxing benzene to find out whether these compounds react by initial heterolysis of the tosylate ester bond induced by long-range orbital interactions.The tosylates 1-7 were designed to investigate the influence of the ?-relay (U-, sickle-, or W-shaped) between the donor and acceptor end of the system, to check whether primary carbocationic ion pairs could act as intermediates, and to study conformational influences on reactivity and product formation.Tosylate 1 with a U-like arrangement of the ?-relay reacted relatively slowly and followed reaction pathways in which no long-range orbital interactions are involved.The reaction outcome of tosylate 2 which possesses a sickle-like arrangement of the ?-relay indicates two competitive processes with and without the involvement of long-range orbital interactions.The secondary tosylates 3, 6, and 7 which all possess a W-like arrangement reacted relatively fast and showed predominantly homofragmentation.Although an ideal W-like arrangement is present in the primary tosylates 4 and 5, no reactions in which long-range orbital interactions are involved were observed.The tosylates 6 and 7 in which the tosylate group is conformationally mobile can give rise to mixtures of products.The ratio in which these products are formed can be rationalized by using the Curtin-Hammett principle.
- Bastiaansen, Petrus M. F. M.,Wijnberg, Joannes B. P. A.,Groot, Aede de
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p. 4240 - 4250
(2007/10/02)
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- Reactions of Thianthrene Cation Radical with Acyclic and Cyclic Alcohols
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Thianthrene cation radical perchlorate (Th(.+)*ClO4(-)) reacted readily with cycloalkanols (C5, C7, C8, and C12), alkan-2-ols (C3, C5, C6, and C8), 3-hexanol, neopentyl alcohol, a number of benzyl alcohols, dl- and (S)-1-phenylethanol, cyclopentyl- and cyclohexylmethanols, the exo- and endo-borneols, and norborneols.Reactions were carried out with an excess of the alcohol in acetonitrile solution containing 2,6-di-tert-butyl-4-methylpyridine.Products were alkenes, ethers, and N-substituted acetamides, depending on the structure of the alcohol.Thianthrene (Th) and its 5-oxide (ThO) were formed in equal amounts.The sum of amounts of products from the alcohol was equal to the amount of ThO.All reactions are interpretable on the basis of the ultimate formation and further reactions of a 5-alkoxythianthreniumyl ion (ROTh(+)).The predominant formation of nortricyclene from the norborneols is striking and is discussed.Swern-Moffatt-type oxidations of the alcohols were not observed.
- Shine, Henry J.,Yueh, Wang
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p. 3553 - 3559
(2007/10/02)
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- Carbenes and the O-H bond: Bicycloalkylidenes
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Photolyses of spirobicyclic diazirines in alcohols have been investigated in order to elucidate the insertion reactions of bicycloalkylidenes with O-H bonds. The formation of ethers from norbornane-derived diazirines (16 and 30) was traced to the intervention of diazo compounds which react by way of diazonium ions. Scavenging of the diazo compounds with potent dipolarophiles led to almost exclusive formation of nortricyclenes from 2-norbornylidenes (17 and 32). In contrast, 2-bieyclo[2,1.1]hexylidene (45) was found to undergo intermolecular reactions, including addition to alkenes and C-H insertion with cycloalkanes. When alcoholic solutions of bicyclo[2.1.1]hexane-derived diazirines (41, 58, 73, and 76) were irradiated, diazo scavengers did not eliminate the formation of ethers. By means of deuterium labels or methyl groups, rearrangement and fragmentation reactions were detected that are characteristic of carbocations (ion pairs). Protonation of 2-bicyclo[2.1.1]hexylidenes by ROH is therefore implied. Laser flash photolysis studies confirmed the divergent lifetimes of 17 (0.1 ns) and 45 (50-600 ns). The reaction rate of 45 was found to increase with the acidity of the alcohols.
- Kirmse, Wolfgang,Meinert, Thomas,Modarelli, David A.,Platz, Matthew S.
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p. 8918 - 8927
(2007/10/02)
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- Photochemistry of alkyl halides. 12. Bromides vs Iodides
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Conditions have been developed for optimizing ionic photobehavior material balances from alkyl bromides. Hydroxide ion as an efficient for the byproduct HBr while giving minimal competing photoreduction via electron transfer to the alkyl bomide. The photobehavior of bromides 1, 11, 25, and 40 has examined and with that of the corresponding iodides 2, 12, 26, 41 under conditions. In each case, the bromide higher yields of products derived from out of cage radical intermidiates than the corresponding iodide. However, with the 2-norbornyl bromides 11 and iodides 12 showed that, of products not formed from the out of cage 2-norbornyl radical 13, the bromides 11 gave a higher percentage of products from the ionic intermediates 15 and 16 than did the iodides. Thus, electron transfer within the radical pair 14 is apparently more rapid for bromides than iodides, as expected on the of the relative electronegativities of bromine iodine. It is that the substantially higher yields of out of radical products from alkyl bromides may be due in to formation of the radical pair with greater excess energy, which results in more rapid escape from the cage. The epimeric 2-norbornyl bromides 11x and 11n underwent no detectable interconversion and afforded somewhat different product ratios. The more hindered epimer 11n underwent conversion to products at a slower than 11x. By contrast, 12x and 12n underwent substantial interconversion via out of transfer of an iodine atom from iodide 12 to radical 13. Epimerization was significantly attenuated in the more viscous solvent tert-butyl alcohol.
- Kopp, Paul J.,Adkins, Rick L.
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p. 2709 - 2717
(2007/10/02)
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- Photochemistry of phenyl thioethers and phenyl selenoethers. Radical vs ionic behavior
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In analogy with alkyl iodides and bromides, the phenyt thio- and selenoethers 2a,b, 13a, 21b,c and 35 displayed competing radical and ionic photobehavior on irradiation in solution, via a mechanism thought to involve initial homolytic cleavage of the alkyl C-S or C-Se bond followed by electron transfer within the resulting radical pair cage (Scheme I). These are the first examples of ionic photobehavior to be recognized for the C-SAr and C-SeAr chromophores. The electronegatively substituted pentafiuorophenyl analogues 2c, 13b, and 21d displayed enhanced ionic photobehavior. By contrast, the 4-methoxyphenyl derivative 21a exhibited almost exclusively radical behavior. The sulfoxide (2R*,R*s)-21f displayed principally radical behavior, accompanied by epimerization at sulfur. The quantum yields for the disappearance of the 2-norbornyl ethers 21b and 21c were 0.53-0.64 in solution and rose to 0.89-0.95 in the presence of suspended fumed silica. Irradiation of the phenyl thioether 21b on silica gel resulted in nucleophilic trapping by surface silanol groups to afford covalently bound material (33), which afforded chloride 34 on treatment with SOCl2. Irradiation of phenyl thioethers 2a and 35, phenyl selenoether 2b, or C6H5SH in allyl alcohol solution afforded acetal 11, apparently via isomerization of some of the solvent to propanal (44) followed by acetalization. Irradiation of alcoholic solutions of aldehydes containing C6H5SH is a useful means of generating acetals under neutral conditions.
- Kropp, Paul J.,Fryxell, Glen E.,Tubergen, Mark W.,Hager, Michael W.,Harris Jr., G. Davis,McDermott Jr., T. Paul,Tornero-Velez, Rogelio
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p. 7300 - 7310
(2007/10/02)
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- Stereochemistry of Base-Promoted 1,2-Elimination from exo-2-Bicycloheptyl Tosylate and Chloride
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Elimination reactions of exo-2-bicycloheptyl tosylate and chloride and their exo-3-deuterated analogues are studied in base-solvent systems that induce clean bimolecular 1,2-eliminations.Their relative propensities for competitive syn-exo and anti-endo-H elimination modes are assessed from nonkinetically determined deuterium isotope effects and the deuterium content in the bicyclohept-2-ene formed from the deuterated substrates.The competition between syn-exo and anti-endo-H elimination is influenced by base association, which stabilizes the synelimination transition state.Potential steric hindrance by oversized dissociated bases has no effect on the elimination stereochemistry.
- Bartsch, Richard A.,Lee, Jong Gun
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p. 212 - 217
(2007/10/02)
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- Base-Promoted 1,2-Eliminations from endo-2-Bicycloheptyl Halides and Arenesulfonates
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Base-solvent systems that provide clean bimolecular 1,2-elimination from endo-2-bicycloheptyl halides and arenesulfonates by suppressing competitive solvolysis and nortricyclene-forming 1,3-elimination are developed.The stereochemistries of elimination from exo-3-deuterio-endo-2-bromobicycloheptane (10) and exo-3-deuterio-endo-2-bicycloheptyl 2,4,6-triisopropylbenzenesulfonate (11) are assessed using these base-solvent systems.The competitive syn-endo and anti-exo-H elimination modes are found to be strongly influenced by base association.However, for dissociated alkoxide bases, the elimination stereochemistry is unaffected by changes in leaving group from halide to arenesulfonate, in base strength, and in base size.
- Bartsch, Richard A.,Lee, Jong Gun
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p. 5247 - 5252
(2007/10/02)
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- 34. Norbornanes Part 20 Inductivity and Bridging in 2-Norbornyl Cations
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The solvolysis rates and products of several 1-substituted 2-exo- and 2-endo-norbornyl p-toluenesulfonates 7 and 8, respectively, have been determined.Hydrolyses of these epimeric tosylates yielded rearranged products in varying amounts, except when the substituent was COOCH3 or CN.The logarithms of the rate constants (log k) for the endo-series 8 correlated linearly with the corresponding inductive constants ?q1 with a reaction constant ρ1 of -1.24.On the other hand, log k values for the exo-series 7 appear to fit two regression lines,the first line (ρ1 = -1.90) defined by the tosylates that ionize, with rearrangement, to the tertiary cations 11, the second (ρ1 = -1.86) by the tosylates 7(R = H, COOCH3, and CN) that ionize to an asymetrically bridged secondary cation 19.These results confirm the unique participation of C(6) with a ρ1 of -2.00 in the ionization of 2-exo-norbornyl tosylate.
- Bielmann, Rolf,Fuso, Francesco,Grob, Cyril A.
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p. 312 - 319
(2007/10/02)
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- Pyrolysis of Bicycloheptane-2-thiols: Evidence for a Carbene Intermediate in a Thermal Hydrogen Sulphide Elimination
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Pyrolysis of endo- and exo-bicycloheptane-2-thiols yields tricyclo2,6>heptane and bicyclohept-2-ene via competing carbene and radical mechanisms.
- Johnson, Douglas E.,Dimian, Adel F.
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p. 416 - 417
(2007/10/02)
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- Hydrogenation and Dimerization of Bicyclohepta-2,5-diene Catalyzed by Cobalt(I) Complexes
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Bicyclohepta-2,5-diene 1 was hydrogenated to bicyclohept-2-ene 2 and tricyclo2,6>heptane 3 in a 70:30 ratio and dimerized to Binor-S 4 by -BF3*Et2O catalysts (X = halogen). 1 was dimerized to 4-7 having tricyclo2,6>heptane skeletons by a -ZnBr2-Zn system in THF.
- Kanai,Hiroyoshi,Watabe, Yoshiyuki,Nakayama, Taisei
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p. 1277 - 1278
(2007/10/02)
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- SELECTIVE HYDROGENATION OF CONJUGATED DIENES CATALYZED BY THIOCYANATOTRIS(TRIPHENYLPHOSPHINE)COBALT(I).
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Thiocyanatotris(triphenylphosphine)cobalt(I) prepared in situ from bis(thiocyanato)bis(triphenylphosphine)cobalt(II), triphenylphosphine, zinc bromide, selectively hydrogenated conjugated dienes to the corresponding cis-monomenes. The C//2-or/ and C//3-methyl substituted 1,3-butadienes enhance the reactivity by a factor of 2. 3 per one methyl group, while C//1- or/and C//4-methyl substituted 1,3-butadienes lower the reactivity by a factor of 1/23 per one methyl group. In the hydrogenation of cyclic conjugated dienes alpha -angle is closely related to the hydrogenation rate. The rates of hydrogenation in cyclic ethers are greater than those in acyclic ethers, but the cis-selectivity in acyclic ethers is much higher.
- Nakayama,Kanai
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- Norpinyl-Norbornyl Rearrangements: Bicycloheptane, 4-Methyl- and 6-Methylbicycloheptane Derivatives
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Norpinyl-norbornyl rearrangements have been induced by solvolysis of the 2-norpinyl nitrobenzoates 15b, c and by decomposition of norpinane-, 4-methylnorpinane-, and 6-methylnorpinane-2-diazonium ions (19, 49, 64, 65).No fragmentation to monocyclic cations was observed.The yield of norpinyl products was minimal in water (s processes).With 64 and 65, migration of the bridge trans to the leaving group predominated strongly.The rearrangements afforded exo-2- and endo-2-norbornyl products in comparable quantities.The exo/endo rates depended on the nucleophilicity of the solvent but were little effected by methyl substitution at the migrating carbon.We propose the 7-bridged norbornyl cation (21) as the endo-selective intermediate which rearranges to the exo-selective 6-bridged (or rapidly equilibrating) norbornyl cation (22, 23) in competition with solvent capture.Ion-pair collapse (cf. 15b) accentuates the endo-selectivity.However, ion pairing cannot be the only source of endo-2-norbornyl products, as shown by the deamination reactions in water.
- Kirmse, Wolfgang,Siegfried, Rainer,Wroblowsky, Heinz-Juergen
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p. 1880 - 1896
(2007/10/02)
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- A New Reducing System: Calcium Metal in Amines. Reduction of Aromatic Hydrocarbons
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A new reducing system consisting of calcium dissolved in a mixture of amines (methylamine-ethylenediamine) is described.Representative aromatic hydrocarbons have been reduced by this new reagent largely to monoalkenes.Hydrocarbons like tetralin, m- and p-xylene, and indan are reduced in excellent yields by the calcium system to a crude product containing 88percent or better of a single alkene.A new technique involving oxymercuration-demercuration is used to purify two of the monoalkene isomer mixtures obtained in these reductions.Unexpectedly, durene is reduced by the calcium reagent to 1,2,4,5-tetramethyl-1,4-cyclohexadiene in excellent yield.Likewise anthracene is reduced in one step to 1,2,3,4,5,6,7,8,9,10-decahydroanthracene.Experiments designed to elucidate why the calcium system does not reduce durene or anthracene to monoalkenes are described.Similarities and differences between the calcium-amine and the lithium-amine reducing systems are discussed.
- Benkeser, Robert A.,Belmonte, Frank G.,Kang, Jahyo
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p. 2796 - 2802
(2007/10/02)
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- Orbital Interactions. XII. Product Studies and Competition Kinetic Measurements of the Birch Reduction of a Series of Hexahydrodimethanonaphthalenes and their Interpretation in Terms of Orbital Interactions Through Space and Through Bonds
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Relative rate constants for the Birch reduction (Li/ liq.NH3/ ButOH) of the three isomeric hexahydrodimethanonaphthalenes (3)-(5) and the octahydro analogues (10)-(13) were obtained and compared with those obtained for the reduction of norbornadiene and norbornene from an earlier study.Diene (5) was reduced almost 2000 times more rapidly than norbornene and 20000 times more rapidly than the monoene (13).Rate-enhancement factors for dienes (3) and (4) were less substantial but meaningful: 19 for (3) and 35 for (4) .These rate enhancements were attributed to the operation of ?* orbital interactions through space in diene (5) and to the presence of ?* orbital interactions through four bonds in dienes (3) and (4).The existence of a linear relationship between the natural logarithm of the rate of reduction of a substrate and its LUMO energy (obtained from either gas-phase electron affinities or ab initio MO calculations) supports this conclusion.The only-fair correlation of the above relationship was attributed to the neglect of other factors, such as the electronic structure and the geometry of the anion radical, which contribute to the overall rate of the Birch reduction.These two factors were explored by using PMO theory and ab initio MO calculations.In particular, full geometry optimizations (UHF, STO-3G basis set) on the anion radicals of norbornadiene (1) (C2v symmetry constraint) and norbornene (22) (Cs symmetry constraint) were carried out, and their geometries reported.Noteworthy is the strong pyramidalization of the olefinic centres of (1) and (22) in the endo direction.These pyramidalizations explain the observed stereoselective exo protonation of the anion radical of (1), and also the much faster rate of reduction of (1) compared with (5), since the pyramidalization im the anion radical of (5) is such to hinder protonation.The geometries of anion radicals appear to have a profound effect on rates, on stereoselectivity of protonation, and on the structures of the final products, and this is discussed in detail.The synthesis of the diene (3) is also described.
- Chau, Diem Dieu,Paddon-Row, Michael N.,Patney, Harish K.
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p. 2423 - 2446
(2007/10/02)
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- Polar versus Steric Effects in the Solvolysis of 6endo-substituted 2endo-Norbornyl p-Toluenesulfonates
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The solvolysis rates and products of the 6endo-R-substituted 2endo-norbornyl toluenesulfonates 6a-6i have been determined.The rates of 6a-6g correlate with the inductive constants ?qI of the 6endo-substituents and are not related to the size of the latter.It is therefore concluded that polar rather than steric effects control the exo/endo-rate ratios of norbornyl sulfonates.Products are derived mainly from rearranged 6exo-R-norbornyl cations when the substituent is an electron donor and from unrearranged 6endo-R-substituted cations when the substituent is an electron acceptor.
- Grob, Cyril A.,Guenther, Bettina,Hanreich, Reinhard
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p. 2110 - 2117
(2007/10/02)
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- DIRECT PHOTOLYSIS AT 185 NM OF CYCLOPENTENE AND 2-NORBORNENE. A NOVEL REACTION CHANNEL FOR ?, ?* EXCITED SINGLET ALKENE
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Upon direct irradiation at 185 nm, the title compounds undergo hydrogen abstraction from the solvent via a ?, ?* excited singlet state as well as skeletal rearrangement via a ?,R(3s) Rydberg excited state.
- Inoue, Yoshihisa,Mukai, Toshio,Hakushi,Tadao
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p. 1045 - 1048
(2007/10/02)
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- PHOTOCATALYTIC HYDROGENATION OF DIENES WITH CHROMIUM CARBONYLS
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The photochemical hydrogenation of norbornadiene (NBD) in the presence of Cr(CO)6 or Cr(CO)4NBD at normal pressure yields nortricyclene (NTC) and norbornene (NBN) in a ratio of 3/1.With increasing hydrogen pressure the NTC/NBN ratio changes to 0.8/1 at 100 bar H2, due to a faster formation of norbornene.The formation of NBN is more strongly inhibited than that of NTC by 50 bar of CO.Other conjugated dienes such as cyclohexadiene, isoprene, or 1,3-pentadiene give exclusively 1.4-hydrogenation products.The results are rationalized by a mechanism in which the 1.4-addition and the NTC formation are initiated by a CO dissociation, whereas the norbornene is formed via initial cleavage of a Cr-diene bond in the Cr(CO)4 diene catalyst.
- Mirbach, Manfred J.,Phu, Tuyet Nhu,Saus, Alfons
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p. 309 - 320
(2007/10/02)
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- Hindered Nucleophilic Displacement (SN2) Reactions of 2exo- and 2endo-Norbornyl Derivatives. Norbornane Series 4.
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The reactions of 2exo- and 2endo-norbornyl bromide (1 e and 2 e, respectively) in 90 percent ethanol with a large excess of potassium hydroxide, and 2exo-norbornyl-p-toluenesulfonate (1 g) with excess sodium thiophenolate in methyl cellosolve, have been studied.They obey the first order rate law and are zero order with respect to the base-nucleophile.However, the ratio of 1,2- and 1,3-elimination to exo-substitution products depends strongly on the base-nucleophile concentration.The extreme inertness of 2-norbornyl derivatives in displacement reactions is due to severe steric hindrance of rearside nucleophilic attack, a feature they have in common with other bi- and tricyclic molecules bearing nucleofugal groups adjacent to one or two bridgehead atoms.
- Grob, Cyril A.,Lutz, Erich
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p. 153 - 160
(2007/10/02)
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- Free-Radical and Anion Intermediates in the Reactions of 5-Halo-2-norbornenes and 3-Halonotricyclenes with (Trimethyltin)sodium
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The reaction of (trimethyltin)sodium with exo-5-chloro- and 5-bromo-2-norbornenes, endo-5-bromo-2-norbornene, and 3-bromo- and 3-iodonortricyclene in tetrahydrofuran has been examined.Use of dicyclohexylphosphine as a trap for intermediate radicals by hydrogen atom transfer showed that the bromides and chloride reacted predominantly by way of free-radical intermediates and by geminate reactions.The geminate products showed complete equilibration of the intermediate radicals. 3-Iodonorbornene reacted predominantly by an anionic intermediate mechanism as shown by trapping of the anions with tert-butylamine.
- Alnajjar, Mikhail S.,Kuivila, Henry G.
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p. 1053 - 1057
(2007/10/02)
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- REACTION OF exo-2-NORBORNYL AND 1-ADAMANTYL p-TOLUENESULFONATES WITH SODIUM PHENOXIDE IN TETRAHYDROFORAN. IONIZATION OF THE SUBSTRATES UNDER "APPARENTLY-SN2" CONDITIONS
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exo-2-Norbornyl and 1-adamantyl tosylates react with sodium phenoxide in tetrahydrofuran via ionization, most probably through a cyclic transition state.The results call for attention in interpreting substitution reactions under "apperently-SN2" conditions.
- Takeuchi, Ken'ichi,Kato,Yukari,Moriyama, Takamasa,Okamoto, Kunio
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p. 935 - 938
(2007/10/02)
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- Carbon Participation in the Solvolysis of 6-exo-substituted 2-exo- and 2-endo-Norbornyl p-Toluenesulfonates. Norbornanes Part 5
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The solvolysis rates and products of the 6-exo-substituted 2-exo- 1a-1u, and 2-endo-norbornyl p-toluenesulfonates 2a-2u, have been determined.In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants ?qI of the substituents at C(6); however, their sensitivity to ?qI is much larger in the 2-exo-series 1 than in the 2-endo-series 2.This differential transmission of polar effects is the cause of decreasing 2-exo/2-endo rate ratios from 2388 for R=t-C4H9 to 0.37 for R=Br, i.e., with increasing electron attraction by the substituent.The high sensitivity of the rate constants for the 2-exo-p-toluenesulfonates 1 to ?qI indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2).This interaction is ascribed to the participation of the pentacoordinate C(6)-atom, i.e. to 1,3-bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes.Donor substituents enhanche 1,3-bridging, lead to faster reactions and to the formation of 2-exo substitution products.Conversely, acceptor substituents reduce 1,3-bridging, decrease rates and facilitate the formation of 2-endo substitution products.Graded 1,3-bridging is discussed in the light of Winstein's nonclassical ion concept.
- Fischer, Walter,Grob, Cyril A.,Hanreich, Reinhard,Sprecher, Georg von,Waldner, Adrian
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p. 2298 - 2311
(2007/10/02)
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- Orbital Interactions. VIII. Competition Kinetic Measurements of the Birch Reduction of Some Compounds Having the Bicycloheptadiene Framework as a Means of Investigating Orbital Interactions Through Space in These Systems
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Relative rate constants for the Birch reduction (Li liq.NH3/t-butyl alcohol) of norbornadiene (1), 1,4-dihydro-1,4-methanonaphthalene (3), 9-(1-Methylethylidene)-1,4-dihydro-1,4-methanonaphthalene (5), 1,4-dihydro-1,4-epoxynaphthalene (7), lithium benzoate, α-methylstyrene, and the hexahydrobenzenomethanoanthracene (20a), were obtained and compared with that obtained for the reduction of norbornene from an earlier study.Compounds (1), (3) and (5) were some 105 times more readily reduced than norbornene.These results are best explained in terms of the presence of trough-space interactions operating between the two lowest unoccupied molecular orbitals in these molecules.The Birch reduction of (7) gave mainly 1,4-dihydronaphthalene (17).A mechanism for this reaction is presented.
- Paddon-Row, Michael N.,Hartcher, Robert
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p. 785 - 794
(2007/10/02)
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- Die Hydrolyse von 6exo-substituierten 2exo- und 2endo-Norbornylestern der p-Toluolsulfonsaeure
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Hydrolysis of the 6exo-substituted 2exo- and 2endo-norbornyl p-toluenesulfonates 1b-l and 2b-l, respectively, in 70percent dioxane led to different amounts of the following products: Unrearranged 2exo-norbornanols 3 and norbornenes 5, accompained in somes cases by small amounts of the rearranged Rendo-epimers 4 and 6 and by nortricyclenes 7.When the 6exo-substituent was a nucleophile group as in 1e-l and 2e-l, various amounts of tricyclic products were also formed by endo-cyclozation.These results show that the 2exo- and 2endo-esters 1 and 2, respectively, react by way of different intermediates.In cases where the 6exo-substituent was an n-electron donor, as in 1m-r and 2m-r, quantitative fragmentation to (3-cyclopentenyl)acetaldehyde (13) occurred.
- Fischer, Walter,Grob, Cyril A.,Sprecher, Georg von,Waldner, Adrian
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p. 928 - 937
(2007/10/02)
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- THE EFFECT OF REPLACING CARBONYL GROUPS BY OTHER LIGANDS ON THE CATALYTIC PROPERTIES OF ARENECHROMIUM CARBONYL COMPLEXES
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The activities of a series of areneCr(CO)2L complexes in catalytic hydrogenation has been studied and found to be poorer than those of the Cr(CO)3 analogs.The IR ν(CO) changes of these complexes in THF have been shown to have value in predicting catalytic properties.
- Dabard, R.,Jaouen, G.,Simonneaux, G.,Cais, Michael,Kohn, D.H.,et al.
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