<i>Science at Oxford, 1914-1939: Transforming an Arts University</i>. Jack Morrell

Fleming, Donald

doi:10.1086/384383

Cited by 7 publications

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“…Scheme portrays our proposed and revised mechanism for the denitrogenation of pyridine and picolines, initiated by a pre-equilibrated set of steps involving precursors 2 − 4 and progressing through plausible intermediates D − F to ultimately produce the final product 8 and the corresponding arene. The earlier part of the reaction involves a series of pre-equilibrium steps such as a 1,3-shift of the α-hydrogen from the nitrogen atom in the seven-membered metallacycle to the strained metallacyclopropene α-carbon, which triggers ring expansion to generate the transient intermediate D . The formation of D represents the microscopic reverse of the last step in the formation of 2 from 1 (vide supra, Figure ) and should be an uphill reaction by at least 24.6 kcal/mol on the basis of our calculations for the ring opening of pyridine.…”

Section: Resultsmentioning

confidence: 99%

Synthetic and Mechanistic Studies of the Ring Opening and Denitrogenation of Pyridine and Picolines by Ti−C Multiple Bonds

et al. 2010

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The neopentylidene-neopentyl complex (PNP)TidCH t Bu(CH 2 t Bu) (1; PNP -= N[2-P(CHMe 2 ) 2 -4-methylphenyl] 2 ) extrudes neopentane in neat pyridine or picoline (3-or 4-picoline) under mild conditions (25°C), to generate the transient titanium alkylidyne intermediate (PNP)TitC t Bu (A), which subsequently ring-opens the pyridine by ring-opening metathesis of the aromatic NdC bond across the TitC linkage, generating the metallaazabicycles (PNP)Ti(C( t Bu)C 5 H 3 RNH) (R = H (2), 3-Me (3), 4-Me (4)). Kinetic studies suggest that the C-N activation process obeys a pseudo-first-order process in titanium, with R-hydrogen abstraction being the rate-determining step (the KIE for 1/1-d 3 conversion to 2 was 3.8(3) at 25°C). The activation parameters are ΔH q = 23(3) kcal/mol and ΔS q = -4(3) cal/(mol K). The intermolecular k H /k D ratio is close to unity, 1.07(3) at 25°C, for the conversion of 1 to 2 in pyridine versus pyridine-d 5 . Detailed theoretical studies suggest the 1 f 2 transformation proceeds in the following order: (i) formation of A in an overall endergonic step by R-hydrogen abstraction, (ii) an exergonic binding of pyridine, and (iii) concerted, exergonic [2 þ 2] cycloaddition followed by (iv) exergonic ring-opening metathesis and finally (v) a concerted hydrogen atom migration. Complexes 2-4 can denitrogenate, that is, completely remove N of the heterocycle at 65°C over 72 h, when treated with silyl chlorides such as ClSiR 3 (R = Me, i Pr, Ph) to cleanly afford the titanium silylimides (PNP)TidNSiR 3 (Cl) (R = Me (8), i Pr (9), Ph (10)) and the corresponding t Bu-arene organic byproduct. [Et 3 Si][B(C 6 F 5 ) 4 ] also promotes denitrogenation of 2 to yield t Bu-benzene, but the metal complex could not be characterized from such a reaction. The conversion 2 f 8 was found to have activation parameters ΔH q = 30(6) kcal/mol and ΔS q = 10(2) cal/(mol K), therefore yielding ΔG q ≈ 27 kcal/mol at 298.15 K. A KIE of 1.6(2) at 85°C was observed when 2/2-d 5 were denitrogenated to 8 in the presence of ClSiMe 3 , with the rate of the reaction being insensitive to both the steric nature and concentration of the trialkylsilyl chloride. Denitrogenation leading to 8-10 is proposed to occur via a series of steps including a 1,3-hydrogen migration, an electrocyclic rearrangement, a retrocycloaddition, and a Si-Cl addition. The transformations 1 f 2/3/4 and 2/3/4 f 8 can be made cyclic by a series of steps such as deimination of the imide moiety in 8 with 2 equiv of MoCl 5 , followed by reduction and transmetalation with LiCH 2 t Bu and then oxidatively induced R-hydrogen abstraction. The reactivity of 1 with other heterocycles such as THF, thiophene, and piperidine is also discussed.

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Section: Resultsmentioning

confidence: 99%

Synthetic and Mechanistic Studies of the Ring Opening and Denitrogenation of Pyridine and Picolines by Ti−C Multiple Bonds

et al. 2010

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“…Whether these rearrangements occur by stepwise or concerted processes has been a matter of some debate. For first-row elements lacking low energy vacant orbitals (C, N, O, F), orbital symmetry prohibits concerted migration in a suprafacial−suprafacial fashion and geometric constraints render an antarafacial−suprafacial process unlikely . On the other hand, unsaturated groups such as sp 2 - or sp-hybridized carbons or main group elements that can expand their octets may rearrange in a concerted manner …”

Section: Introductionmentioning

confidence: 99%

Carbanionic Rearrangements of Halomethylenecyclobutanes. Stereochemistry of the Migrating Group

Erickson

2010

J. Org. Chem.

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The unusual base-induced ring-enlargement of halomethylenecyclobutanes to 1-halocyclopentenes was examined with unsymmetrical and (13)C-labeled substrates to study regio- and stereochemical characteristics. Migration of a ring carbon atom (single migration) or simultaneous migration of a ring carbon atom and the halide (double migration) gives the ring-enlarged products. (13)C-labeling experiments established that both rearrangements occur with retention of configuration at the migrating center. These systems are suggested as models for the Fritsch-Buttenberg-Wiechell (FBW) rearrangement.

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“…The reaction between dicyclopentadienones, 3a – 3d , and phenylacetylene can lead to the formation of two types of regioisomers, which were identified as cis and trans depending on the relative orientation of phenyl rings highlighted in blue color (Figure ). It is well established that the presence of the electron-withdrawing substituents on the diene has a strong effect on the regioselectivity of the Diels–Alder reaction . As such, we varied the substituents of dicyclopentadienones in the order of electron donating to electron-withdrawing groups (R = OMe, H, CF 3 , and F).…”

Section: Resultsmentioning

confidence: 99%

Edge-Functionalized Graphene Nanoribbon Frameworks for the Capture and Separation of Greenhouse Gases

et al. 2017

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We demonstrate a bottom-up synthetic approach for the synthesis of graphene nanoribbon frameworks (GNFs) incorporating edge-functionalized graphene nanoribbons via the Diels−Alder cycloaddition polymerization and a subsequent FeCl 3 -catalyzed cyclodehydrogenation reactions. This approach not only allowed us to precisely position substituents, namely, −OMe (GNF-0), −H (GNF-1), −CF 3 (GNF-2), and −F (GNF-3), but also enabled to tune textural properties and gas affinity of resulting frameworks. GNFs exhibited promising physical properties such as high surface areas (up to 755 m 2 g −1 ) and excellent physicochemical and thermal stabilities (up to 400 °C). Narrow pore size distribution and the presence of large aromatic units led to high affinity toward gases such as CO 2 (27.4−30.9 kJ mol −1 at 1 bar), CH 4 (21.3−26.0 kJ mol −1 at 1 bar), and H 2 (6.5−8.2 kJ mol −1 at 1 bar). Notably, GNFs also showed promising CO 2 /CH 4 breakthrough separation performance for natural gas sweetening and landfill gas separations at 298 K. The edge-functionalization of GNFs with −CF 3 and −F significantly improved their affinity toward perfluorocarbons and CFCs, which are classified as potent greenhouse gases. Compared to GNF-3, GNF-2 containing −CF 3 moieties showed much higher uptake capacity toward CFC-113 (67 wt % at 298 K).

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Science at Oxford, 1914-1939: Transforming an Arts University. Jack Morrell

Cited by 7 publications

References 0 publications

Synthetic and Mechanistic Studies of the Ring Opening and Denitrogenation of Pyridine and Picolines by Ti−C Multiple Bonds

Synthetic and Mechanistic Studies of the Ring Opening and Denitrogenation of Pyridine and Picolines by Ti−C Multiple Bonds

Carbanionic Rearrangements of Halomethylenecyclobutanes. Stereochemistry of the Migrating Group

Edge-Functionalized Graphene Nanoribbon Frameworks for the Capture and Separation of Greenhouse Gases

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