- Photoinduced Fragmentation Borylation of Cyclic Alcohols and Hemiacetals
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A visible-light photoinduced fragmentation borylation of O-phthalimido cycloalkanols with bis(catecholato)diboron is described. Structurally diverse keto and formyloxy alkyl boronic esters are shown to be conveniently prepared by radical-mediated ring opening of cyclic alcohols and hemiacetals, respectively. The reactions proceed under mild conditions in the absence of additives or photocatalysts, display excellent functional group tolerance, and are shown to allow cleavage of 4-, 5-, 6-, and 7-membered ring substrates. The mechanism proceeds via sequential homolytic N-O and C-C bond cleavages, the latter of which involves β-scission of an alkoxy radical, generating a carbonyl and an alkyl radical that is trapped by the diboron reagent. Spectroscopic studies suggest direct photoexcitation of either the phthalimide or diboron substrates with blue light can initiate a radical chain mechanism.
- Aggarwal, Varinder K.,Madhavachary, Rudrakshula,Noble, Adam,Shu, Chao
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supporting information
p. 7213 - 7218
(2020/10/12)
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- Stereoselective thermal rearrangement of syn-7-(1,2-Butadienyl)-1-methylbicyclo[2.2.1]hept-2-ene [syn-7-(3-Methylallenyl)-1-methylnorbornene]
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The synthesis and separate thermal rearrangements of (±)-(1R*,4S*,7S*)-7-[(R*)-1,2-butadienyl]-1- methylbicyclo[2.2.1]hept-2-ene (8a) and (±)-(1R*,4S*,7S*)-7-[(S*)-1,2-butadienyl]-1- methylbicyclo[2.2.1]hept-2-ene (8b) are described. Both 8a and 8b are shown to rearrange to (±)-cis-1-ethylidene-3a,4,5,7a-tetrahydro-6-methylindene (9) and (±)-cis-1-ethylindene-3a,4,5,7a-tetrahydro-3a-methylindene (10) with greater than 90% stereoselectivity. Epimer 8a gives predominantly (E)-9 and (Z)-10, whereas 8b gives predominantly (Z)-9 and (E)-10, results consistent with either a six-electron [σ2s + π2s + π2s] Cope or eight-electron [σ2s + π2s + (π2s + π2a)] augmented Cope process. Stereochemical assignments (8a vs 8b, (E)-9 vs (Z)-9, and (E)-10 vs (Z)-10) are based upon experiments in nuclear Overhauser effect (NOE) difference spectroscopy.
- Duncan, James A.,Hendricks, Robert T.,Kwong, Katy S.
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p. 8433 - 8442
(2007/10/02)
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- RUTHENIUM-CATALYZED C-H BOND ACTIVATION OXIDATION OF BRIDGED BICYCLIC AND TRICYCLIC ALKANES
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Catalytic oxydation (hydroxylation) with RuO4 generated in situ occurs preferentially in tertiary position with retention of configuration on various bicyclic or tricyclic alcanes.
- Tenaglia, A.,Terranova, E.,Waegell, B.
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p. 5271 - 5274
(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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- Large Intrinsic Nuclear Magnetic Resonance Isotope Shifts Associated with Bending Motion along the Bridging Coordinate in Carbocations
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Deuterium isotope effects on the 13C NMR chemical shifts have been determined for the 2-methyl-2-bicyclooctyl and 2-methyl-2-bicycloheptyl cations.These tertiary carbocations have isotope shifts that are larger than 1 ppm per deuterium, which is an order of magnitude larger than ordinary intrinsic shifts found in nonionic model compounds and in other carbocations.For deuteriation at C3 methylene or methyl group, the same pattern occurs in both cations: isotope shifts that are large and upfield at C2, downfield at C1, and upfield at the remaining carbon directly bonded to C2.The similarity in the pattern of the isotope shifts suggests that the force field and shielding influences in both ions are similar.The specific results are interpreted as indicating the presence of a shallow potential surface for the bending motion along the direction associated with ?-bridging.Since the existence of the shallow potential does not depend on the actual extent of bridging, the magnitude of the isotope effect is not proportional to the extent of bridging.
- Forsyth, David A.,Botkin, James H.,Puckace, James S.,Servis, Kenneth L.,Domenick, Robert L.
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p. 7270 - 7276
(2007/10/02)
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- Deamination Reactions, 39. Decomposition of 1-Methylnorbornane-2-diazonium Ions
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Nitrous acid deaminations of 1-methyl-exo-2-norbornylamine (19) and of the epimeric 2-methyl-2-norbornylamines (32, 34) in water yielded endo-2-methyl-exo-2-norbornanol (42a) exclusively.In contrast, 1-methyl-endo-2-norbornylamine (23) afforded 2-methylbicycloheptan-2-ol (13a, 20percent) and 1-methyl-endo-2-norbornanol (14a, 17percent) in addition to 42a.The formation of bicycloheptyl derivatives 13 is accentuated by better nucleophiles, as shown by photolyses of 1-methyl-2-norbornanone tosylhydrazone (35) in methanol/methoxide, and in the presence of lithium azide. 14 and (in part) 13 are thought to originate from 7-bridged 1-methylnorbornyl cations (10).A substantial fraction of 13 stems from the open 2-methylbicycloheptyl cation (12).The contributions of 10 and 12 have been elucidated by means of 3-D2 labels.Introduction of an exo-3-methyl group (48, 50) enhances participation of the C-1 - C-7 bond.Cations 57 and 61 are the predominant intermediates generated from 1,exo-3-dimethylnorbornane-endo-2-diazonium ions (53).Less polar solvents (acetic acid, 2-ethylhexanoic acid) induce variations in product distribution which are attributed to increasing solvolytic displacement (ks) and ion pair collapse.
- Banert, Klaus,Kirmse, Wolfgang,Wroblowsky, Heinz-Juergen
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p. 3591 - 3610
(2007/10/02)
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- Carbon Participation in the Solvolysis of Tertiary Norbornyl Derivatives. Norbornanes, Part 15. 6-Substituted 2-Methylnorbornyl 2-exo- and 2-endo-2,4-Dinitrophenyl Ethers
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The solvolysis rates and products of the tertiary 2-methyl-2-exo- and -2-endo-norbornyl 2,4-dinitrophenyl ethers; 1 and 2, (X=2,4-(NO2)2C6H3O) have been determined.The different sensitivities of the rates of these ethers to the inductive effect of substituents at C(6) indicate that graded bridging of C(2) by C(6) occurs in the ionization of the exo-ethers 1, not, however, in the ionization of the endo ethers 2.In both cases hydrolysis leads to 2-methyl-2-exo-norbornanols only.Consequently, substitution takes place with retention at C(2) in the exo-series 1 and with inversion at C(2) in the endo-series 2.It is concluded that stereoelectronic and polar effects, rather than steric bulk effets, determine the high exo/endo rate ratios of the parent norbornyl derivatives 1a and 2a.
- Grob, Cyril A.,Waldner, Adrian
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p. 2481 - 2488
(2007/10/02)
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- Norpinyl-Norbornyl Rearrangements: 2-Methyl- and 2,exo-6-Dimethylbicycloheptane Derivatives
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Solvolyses of 2-methylbicyclohept-2-yl-4-nitrobenzoate (9), the nitrous acid deamination of the corresponding amine (4e), and acid-catalyzed rearrangements of various 2-methylbicycloheptane derivatives (4a - c) have been investigated. 2-Methylbicycloheptyl (4) and 1-methyl-endo-2-norbornyl (10) products prevailed; minor quantities of 2-methyl-exo-2-norbornyl derivatives (12) were also obtained.The product distributions were independent of the precursor but were strongly affected by the nucleophilicity of the solvent.Our observations are consistent with the 7-bridged norbornyl cation 6 as the predominant intermediate.Further stabilization of 6 by exo-3-methyl substitution is indicated by the results of the acid-catalyzed rearrangement of 2,exo-6-dimethylbicycloheptan-2-ol (17).
- Banert, Klaus,Kirmse, Wolfgang,Wroblowsky, Heinz-Juergen
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p. 2474 - 2485
(2007/10/02)
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- The Synthesis and hydrolysis of 6-exo-Substituted 2-Methyl-2-exo-norbornyl and 2-Methyl-2-endo-norbornyl 2,4-Dinitrophenyl Ethers
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The synthesis of the title compounds and their hydrolysis products in aqueous dioxane are described.Upon hydrolysis, the 2-exo-ethers 1 (X=N2phO) as well as the 2-endo-ethers 2 (X=N2phO) yield the corresponding 2-methyl-2-exo-norbornanols 3 only.Therefore
- Grob, Cyril A.,Sprecher, Georg von,Waldner, Adrian
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p. 2656 - 2665
(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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- Synthese und Hydrolyse von 6endo-substituierten p-Toluolsulfonsaeure-estern.
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The Synthesis and Hydrolysis of 6endo-Substituted 2endo-Norbornyl p-Toluenesulfonates; The hydrolysis products of the p-toluenesulfonates of several hitherto unknown 6endo-substituted 2endo-norbornanols have been determined.
- Grob, Cyril A.,Guenther, Bettina,Hanreich, Reinhard
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p. 2288 - 2298
(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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- EPIMERIC 2-NORBORNYL CATIONS IN THE SOLVOLYSIS OF 6-exo- AND 6-endo-SUBSTITUTED 2-exo-NORBORNYL p-TOLUENESULFONATES
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Solvolysis of 6-exo- and 6-endo-substituted 2-exo-norbornyl tosylates 1 and 3, respectively, yields identical or different product mixtures depending on the inductive effect of the substituent, indicating that induction involves graded ?-participation.
- Grob, Cyril A.,Guenther, Bettina,Hanreich, Reinhard,Walder, Adrian
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p. 835 - 838
(2007/10/02)
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- Carbon Participation in the Solvolysis of 6-endo-substituted 2-exo-Norbornyl Toluenesulfonates. Norbornanes Part 6
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The solvolysis rate constants k for the 6-endo-substituted 2-exo-norbornyl toluenesulfonates 7 have been determined.Values of logk correlate well with the respective inductive constants of the substituents except when the latter are nucleophilic and therefore lead to endo-cyclization, or when they are n-electron donors and cause concerted fragmentation.In general 6-endo-substituted tosylates 7 react somewhat more slowly than their 6-exo-epimers. Identical or different mixtures were obtained from the C(6)-epimers 7 and 1 depending on whether the substituent was an electron donor or acceptor.It is concluded that donor substituents at C(6) enhance 1,3-bridging in the intermediate epimeric cations and lead to their rapid and complete equilibration, and that electron acceptors reduce bridging and hence their equilibration rates.
- Grob, Cyril A.,Guenther, Bettina,Hanreich, Reinhard
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p. 2312 - 2321
(2007/10/02)
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- POLAR AND ANCHIMERIC EFFECTS IN THE SOLVOLYSIS OF 6-endo-SUBSTITUTED 2-NORBORNYL-p-TOLUENE-SULFONATES
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A study of the influence of 6-endo substituents on the reactivity of 2-exo- and 2-endo-norbornyl p-toluenesulfonates 3 and 4, respectively, confirms that polar rather than steric effects control relative rates.
- Grob, Cyril A.,Guenther, Bettina,Hanreich, Reinhard
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p. 1211 - 1214
(2007/10/02)
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- The Synthesis of 6-substituted 2-Norbornanols
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The preparation of several 6-exo- and 6-endo-substituted 2-exo- and 2-endo-norbornanols and their p-toluenesulfonates is described.
- Grob, Cyril A.,Guenther, Bettina,Waldner, Adrian
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p. 2709 - 2720
(2007/10/02)
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- REMOTE INDUCTIVE EFFECTS ON SECONDARY AND TERTIARY 2-NORBORNYL SOLVOLYSIS RATES
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A 5-cyano substituent decelerates the solvolysis rate of exo-2-norbornyl brosylate (1-H) by a factor of 1790.A much smaller deceleration (24-90 fold) is observed for the secondary endo and for both exo and endo tertiary 2-norbornyl derivatives.These results support the occurrence of ?-participation in the solvolysis of 1-H.
- Apeloig, Yitzhak,Arad, Dorit,Lenoir, Dieter,Schleyer, Paul von Rague
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p. 879 - 882
(2007/10/02)
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