Nature of the carbonium ion. IX. 2-Oxa-6-norbornyl cation

Spurlock, Langley A.; Fayter, Richard G.

doi:10.1021/ja00763a027

Cited by 21 publications

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“…Since 2-OTs ionizes without 0 participation, the factor of 2.6. lo4 provides an approximate measure of the Z effect of the p-0-atom. The rate ratio for 8-ONs and 9-ONs of 6.8. lo6 (Table 2 ) confirms that the ionization of the former only is anchimerically assisted [9]"). The relatively low rate ratio of 235 for 10-ONs and 11-ONs is close to the value of 31 1 for 1-OTs and 2-OTs in the same solvent [lb].…”

Section: )mentioning

confidence: 62%

“…7 1 (1988) 1023 9-OTs14). The relatively low rates of 8-OTs and 9-OTs are undoubtably due to the rate retarding Z effect of the 0-atoms, as already noted by Spurlock and Fayter [9]. Since 2-OTs ionizes without 0 participation, the factor of 2.6. lo4 provides an approximate measure of the Z effect of the p-0-atom.…”

Section: )mentioning

confidence: 72%

“…-exo-and endo-2-0xabicycl0[2.2.l]heptan-6-01 (X = OH; 8-OH and 9-OH, respectively) were prepared as described in [9]. On the other hand, exo-and endo- [13].…”

Section: ')mentioning

confidence: 99%

“…exo-2-0xabicyclo[2.2.l]heptan-6-ol (X=OH, 8-OH) and the endo-epimer 9-OH were prepared according to [9]'*). The alcohol 8-OH was converted to the known p-toluenesulfonate 8-OTs [9].…”

Section: Experimental Partmentioning

confidence: 99%

“…Results. -exo-and endo-2-0xabicycl0[2.2.l]heptan-6-01 (X = OH; 8-OH and 9-OH, respectively) were prepared as described in [9]. On the other hand, exo-and endo-7-oxabicyclo[2.2.l]heptan-2-ol(lO-OH and 11-OH, respectively) were obtained by a new route, namely by condensing 1-cyanovinyl acetate with furane by the method of Black and Vogel [ 121 to give 19 which was hydrolysed and hydrogenated to 7-oxabicyclo[2.2.1]heptan-2-one (20) [13].…”

Section: ')mentioning

confidence: 99%

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Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

Flury

Grob

Wang

et al. 1988

Helvetica Chimica Acta

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(1 1. V. 88)Further evidence is presented that the 2-norbomyl cation is stabilized primarily by C(2)-C{6) bridging, and that C(2)-C(7) bridging leads to prohibitive strain. Thus, a comparison of the heats of hydrogenation of nortricyclene 17 and bicycl0[3.2.0.0~.~]heptane 18 indicates that the strain energy of the latter is ca. 21.5 kcal/mol higher than that of 17. Furthermore, 6-exo-2-oxabicyclo[2.2. ljheptyl sulfonates 8 ionize with strong O(2) participation to the bridged oxonium ion 12. In contrast, 2-endo-7-oxabicyclo[2.2.1]heptyl sulfonates 11 ionize without O(7) participation to form the unbridged carbenium ion 15.As pointed out in [la], displacement (&2) reactions of bi-and tricyclic halides and sulfonates with nucleophilic solvents are sterically hindered, if the nucleofuge is adjacent to a bridgehead atom. In these cases, solvolysis reactions occur via carbenium ions (&1) and tend to be slow unless assisted by bridging of the cationic center by neighboring atoms.In carbocyclic compounds, bridging involves weak bonding between the electrophilic cationic center and neighboring C-atoms, thereby generating some of the strain associated with the subdivision of cyclic structures into smaller rings [2]. Differential bridging of neighboring C-atoms, therefore, accounts for the frequently different rates and products of epimeric bi-and tricyclic sulfonates [lb], such as the exo-and endo-2-norbornyl p -bromobenzenesulfonates (brosylates) and p -toluenesulfonates (tosylates) 1 (X = BsO) and 2 (X = TsO), respectively, which differ by factors of more than 3002).In their original communications [3], Winstein and Trifan attributed the higher rate of 1-OBs to anchimeric assistance of exo-ionization by the antiperiplanar C( 1)-C(6) bonding electrons, i.e. by so-called CT participation3), which is absent in the endo-epimer 2-OBs. According to a modified version of this rationale [6], the transition state for exo-ionization is stabilized by C,C hyperconjugation with the strained and antiperiplanar C( 1)-C(6) CT bond. Thus, both interpretations stress the role of the C( 1)-C(6) bonding electrons in controlling the relative rates of I-OBs and 2-OBs. On the other hand, the role of the equally strained C( 1)-C(7) bond, which is somewhat deflected from the plane ') 2, 3,The IUPAC name for norbornane: 8,9,1U-trinorbornane. The rate ratio at 25"is 350 in AcOH [3], 580 in 80% EtOH [lb], and cn. 2000 in H2O [4].For a definition of u participation, see [5].

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Section: )mentioning

confidence: 62%

Section: )mentioning

confidence: 72%

“…-exo-and endo-2-0xabicycl0[2.2.l]heptan-6-01 (X = OH; 8-OH and 9-OH, respectively) were prepared as described in [9]. On the other hand, exo-and endo- [13].…”

Section: ')mentioning

confidence: 99%

“…exo-2-0xabicyclo[2.2.l]heptan-6-ol (X=OH, 8-OH) and the endo-epimer 9-OH were prepared according to [9]'*). The alcohol 8-OH was converted to the known p-toluenesulfonate 8-OTs [9].…”

Section: Experimental Partmentioning

confidence: 99%

Section: ')mentioning

confidence: 99%

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Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

Flury

Grob

Wang

et al. 1988

Helvetica Chimica Acta

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The Oxidation of Alcohols by Modified Oxochromium( VI )–Amine Reagents

Luzzio

1998

Organic Reactions

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The development of oxochromium(VI)‐amine reagents as oxidants in organic synthesis has evolved since the early studies of Sisler which described the use of adducts of heterocyclic nitrogen bases and chromium(VI) oxide. The later work of Sarrett and coworkers in steroid synthesis soon established the utility of chromium(VI) oxide‐pyridine adducts as the first selective oxochromium(VI)‐amine reagents. Several modifications of the Sarrett oxidation followed. The present concern with the toxicity and environmental implications of oxochromium(VI) has provided encouragement for the study and use of catalytic oxochromium reagents in conjunction with stoichiometric co‐oxidants such as peroxides. Such technology is welcome, particularly when applied to the large‐scale preparations found in industry where the disposal of byproducts is a constant problem. Modified oxochromium(VI)‐amine reagents on polymer supports have been prepared and examined with the objective of providing a regenerable reagent system which is serviceable on a practical scale. A significant improvement in routine oxidations utilizing PCC and PDC entails the addition of adsorbents such as Celite®, alumina, silica gel, molecular sieves, or zeolites. The distinct advantages associated with the use of adsorbents in oxochromium(VI)‐amine‐mediated oxidations are discussed. The importance of buffers and desiccants in oxochromium(VI) oxidations is discussed. Virtually any number of oxochromium(VI)‐amine reagents may be prepared by varying the amine ligand and/or acid species associated with oxochromium(VI), and many of these compounds have been tested using simple substrate alcohols for oxidation to carbonyl compounds. The search for a milder and more efficient oxidant than the Sarrett‐type oxochromium(VI) reagent systems as well as the nonchromium oxidants based on dimethyl sulfoxide or dimethyl sulfide led to an examination of the utility of pyridinium chlorochromate (PCC) in selective oxidations of alcohols. PCC was superior in terms of ease of preparation, shelf stability and economy. Since the introduction of PCC in 1975 the reagent has become a commercially available “textbook” compound and is usually the reagent of choice for routine large and small scale oxidations of alcohols to carbonyl compounds. Unlike the oxochromium(VI)‐pyridine adducts, PCC is an acid salt, and its properties have proven advantageous in a number of tandem oxidative reactions such as oxidative transpositions and oxidative cationic cyclizations. Several reviews address the versatility of PCC and its extension to many different types of oxidative conversions in single and multistep organic syntheses. While PCC was gaining widespread use, the utility of pyridinium dichromate (PDC) in solvents such as dimethylformamide (DMF) and dichloromethane was reported. PDC/DMF was useful for oxidizing allylic alcohols to the corresponding, α,β‐unsaturated carbonyl compounds and non‐conjugated aldehydes and primary alcohols to the corresponding carboxylic acids. The utilization of beyond the simple oxidation of alcohols, and it is now employed for various transformations. Many other types of substituted pyridines, nitrogen heterocycles and amines form chromates, dichromates, and chlorochromates which allow many different types of oxidative processes for a wide range of substrate molecules. This chapter surveys the many applications of oxochromium(VI) complexes in oxidative conversions of alcohols to carbonyl compounds. Alternative reagent systems based on modifications involving ligands, buffering agents, or adsorbents, as well as nonchromium(VI) oxidants are discussed. A variety of substrates have been included in the Tables so as to guide the synthetic chemist in selecting reagents and conditions that will be optimal for a given transformation. Oxidative conversions of alcohols through transpositions and cationic cyclizations giving carbonyl compounds as endproducts are included.

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Desaminierungsreaktionen, 49. Zerfall von 2‐Oxa‐5‐ und ‐6‐norbornandiazonium‐Ionen

Kirmse

Mrotzeck

1988

Chem. Ber.

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Das Verhalten von 2-Oxad-norbornandiazonium-Ionen (19, ZO) entspricht dem der analogen Brosylate (7, 11). Belichtung des Tosylhydrazons 17 in Natmnlauge ergibt 2-Oxa-exo-6-norbornanol(15) ohne das endo-Isomere 22. Das in D20/DONa eingbaute Deuterium verteilt sich gleichmaBig auf die Positionen 1 und 6 -ein Hinweis auf das tricyclische Oxonium-Ion 8 als Zwischenstufe. -Nach vielen vergeblichen Versuchen erhielten wir 2-Oxa-exo-5-norbornawl (45a) aus cis4Hydroxy-2-cyclopenten-1-methanol (41 a), &em Prins-Produkt des Cyclopentadiens, durch. Addition von 2-Chlorethanol, Cyclisierung des primiiren Brosylats und Abspaltung der Schutzgruppe mit n-Butyl- In der Chemie der Carbokationen spielen alkylverbruckte Strukturen (eckenprotonierte Cyclopropane) eine wichtige Rolle2'. Zusammenhange zwischen Konstitution und Verbruckung bicyclischer Carbokationen sind jedoch noch wenig untersucht. Neben Ringspannung') und sterischen Effekten4' interessiert uns der EinfluB von Substituenten am potentiell pentakoordinierten Kohlenstoff '). Alkoxygruppen an C-6 des 2-Norbornyl-Kations trugen zu dieser Problematik wenig bei, da die Wagner-Meerwein-Umlagerung von 1 durch Fragmentierung (1 -+ 2 4 3) und Nachbargruppenbeteiligung des Sauerstoffs (1 + 4 + 5) uberlagert ist 'I. IOU8The reactivity of 2-oxa-6-norbornanediazonk~11 ions (19.20) conforms to that of the analogous brosylates (7, 11). Photolysis of the tasylhydrazone 17 in aqueous sodium hydroxide yields 2-0xa-exo-6-norbornanol (15) with no endo isomer 22. The deuterium incorporated from D20/DONa is distributed equally between positions l and 6 of 15, suggesting.the tricyclic oxonium ion 8 as an intermediate. -After many fortuitous attempts, 2-oxa-exo-5-norbornanol (&a) was prepared from cis4hydroxy-2-cyclopcntene-l-methanol (41 a), a hins product Df cyclopentadiene. by addition of 2chloroethanol, cyclization of the primary brosylate, and removal of the protecting group with n-butyllithium. The tosylhydrazone 47, obtained via ketone 25, gave 99% of 458 on irradiation. Deviations from equidistribution of a deuterium label were slight (5-D:4-D = 1.1 -1.2). The small effect of the internal oxygen substituent in cation 50 stands in contrast to the much stronger effect of 6-akoxy groups on 2-norbornyl cations (52)'). A rationale is provided by the conformationdependent ibteraction of akoxy groups with protonated cyclopropanes, as calculated by Schleyer et al. ihre Solvolysen sind bereits bekannt 6.n. Die Acetolysegeschwindigkeiten der endo-Brosylate 10 und 11 machen den induktiven Effekt des Sauerstoffs deutlich. Er wird in der exo-Reihe (6, 7) durch Nachbargruppenbeteiligung iiberkompensiert, so daB fur 7, 11 ein sehr hohes exo:endo-Geschwindigkeitsverhaltnis (7 . lo7) resultiert6! Die kinetischen k,,l .O 1.10-5Daten, die ausschlieBliche Bildung von 2-Oxa-exo-6-norbornyl-Produkten (9) und die gleichmal3ige Verteilung einer 6-D-Markierung in 77' und 116) auf C-1 und C-6 von 9 sprechen fur ein symrnetrisches Oxonium-Ion 8 als Zwischenstufe. Hier war lediglich zu prufen, ob 2-Oxa-6-nor...

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Nature of the carbonium ion. IX. 2-Oxa-6-norbornyl cation

Cited by 21 publications

References 0 publications

Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

The Oxidation of Alcohols by Modified Oxochromium( VI )–Amine Reagents

Desaminierungsreaktionen, 49. Zerfall von 2‐Oxa‐5‐ und ‐6‐norbornandiazonium‐Ionen

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