Generation and rearrangement of spirocyclopropane‐substituted 2‐norbornyl cations

Kirmse, Wolfgang; Landscheidt, Heinz; Schleich, Annette

doi:10.1002/poc.610050104

Cited by 4 publications

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References 55 publications

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“…The tosylmini [14] . The lack of an analogous ring expansion in the case of 14 indicates that the cationic site interacts much less hydrazone 45 of 7,7-dimethylnorbornan-2-one is not suitable for comparison as the endo-selective intermediate 41 with a cyclobutane than with a cyclopropane ring.…”

Section: Methodsmentioning

confidence: 99%

“…The degenerate rearrangements of the 2-norbornyl cation [8] have often been used to probe the influence of substituents [8] [9] [10] . Dramatic changes (see below) were induced by cyclopropane rings which were spiroanellated to the 3- [11] [12] , 6- [13] [14] , and 7-positions [11] [12] of the 2-norbornyl system. We now report on analogous spiroanellations of cyclobutane, in order to compare the effects of three-and four-membered rings.…”

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Rearrangements of Spirocyclobutane-Substituted 2-Norbornyl Cations

Kirmse¹,

Landscheidt²,

Siegfried³

1998

Eur. J. Org. Chem.

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2‐Norbornyl cations with spiroannellated cyclobutane rings were generated for comparison with the previously studied cyclopropane analogues. Starting with the Diels‐Alder reaction of cyclopentadiene with methylenecyclobutane, spiro‐ [bicyclo[2.2.1]heptane‐2,1′‐cyclobutan]‐6‐one (11) was prepared. The tosylhydrazone 12 of 11 was photolyzed in NaOD/D2O to give the analogous alcohol 13 with a ca. 1:1 distribution of deuterium. Ring expansion was not observed, in contrast to the cyclopropane analogue. − The tosylhydrazone 22 of spiro[bicyclo[2.2.1]heptane‐2,1′‐cyclobutan]‐3‐one (21) and the related tosylates (28, 32) rearranged, in part, to afford derivatives of spiro[bicyclo[2.2.1]heptane‐7,1′‐cyclobutane] (29, 33, 34). In both series, ring expansion of the spiroannellated cyclobutane, by exo‐3,2‐C shift, was the major reaction, giving rise to a uniquely endo‐selective tertiary cation (36). Analogously positioned cyclopropane rings remain intact, due to stabilizing interactions with the neighboring positive charge which are lacking in the cyclobutane systems. − In CDCl3 solution, the tosylate 32 produced mixtures of isomeric tosylates by way of ion pair recombination. We observed that exo → exo shifts of the counterion proceed with little scrambling of 18O whereas complete equilibration of the tosylate oxygens is attained in exo → endo shifts.

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Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Rearrangements of Spirocyclobutane-Substituted 2-Norbornyl Cations

Kirmse¹,

Landscheidt²,

Siegfried³

1998

Eur. J. Org. Chem.

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Rearrangements of 1‐ and 2‐Cyano‐2‐norbornyl Cations

et al. 1993

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Rearrangements of 1-and 2-Cyano-2-norbornyl CationsSulfonates of exo-2-hydroxynorbornane-endo-2-carbonitrile (2c-e) were found to rearrange exothermally to give sulfonates of exo-2-hydroxynorbornane-1-carbonitrile (6c -e), solvolysis being a minor side reaction. In contrast, the analogous endo substrates (3c -e) afforded the rearranged alcohol 6a and tricyclo[2.2.1.02~6]heptane-l-carbonitrile (4) as the major products. We have not been able to trap 2-cyano-2-norbornyl cations by external nucleophiles or by internal 6,2-shifts of hydrogen or carbon. On the other hand, there is good evidence for the generation of 1-cyano-2-norbornyl cations from both 3d, e and 6e. The degenerate 6,2-H shift in these species has been uncovered by means of labeled or optically active precursors. The 1-CN substituent clearly promotes the 6,2-H shift relative to the parent 2-norbornyl cation, but is inferior to 1-CzFS. Ring expansion of a spiroannelated cyclopropane, involving a 6,2 shift of carbon, was also observed (25, 26 -+ 21, 28). Our data strongly suggest that 2-cyano-2-norbornyl cations are less stable than 1-cyano-2-norbornyl cations. The reaction rates of 2c versus 6c do not reflect the stability of the incipient carbocations, owing to the large difference in ground state energy.Destabilisierte Carbokationen, die elektronenziehende Substituenten am positiv geladenen C-Atom oder in dessen Nachbarschaft tragen, finden in neuerer Zeit starkes Interesse"]. Cyan-und Perfluoralkyl-Gruppen erweisen sich als besonders wirksam. In das umlagerungsfreudige und intensiv untersuchte Norbornyl-System wurde CN bereits in l -S t e l l~n g [ *~~~ und 6-Stell~ng[~.~' eingefiihrt. uber das Verhalten von 2-Cyan-2-norbornyl-Edukten war bisher nichts bekannt; diese Lucke wird nun geschlossen. Ferner berichten wir iiber 2,6-H-Verschiebungen in CN-substituierten Norbornyl-Kationen und uber analoge Kohlenstoff-Verschiebungen von Spiro(norbornan-2,1'-cyclopropyl-6-yl)-Kationen. Umlagerung und Solvolyse von Sulfonsaureestern des 2-Hydroxy-bicyclo[ 2.2.11 heptan-2-carbonitrilsDie Umsetzung von Norbornan-2-on (1) mit Cyanwasserstoff ist in der L i t e r a t~r~~, '~ erwahnt, ohne dal3 die epimeren Cyanhydrine 2a, 3a getrennt und charakterisiert wurden. Wie wir fanden, erhalt man aus 1 mit NaCN/H2S04 vorwiegend die exo-Verbindung 2 a (thermodynamische Kontrolle; 2a: 3a = 87: 13). Trimethylsilylcyanid/Zn12n liefert fast ausschlieBlich den endo-Silylether 3 b (kinetische Kontrolle; 3b:2b = 96:4), aus dem 3a durch saure Hydrolyse zuganglich ist. Die Stereoisomeren lieBen sich durch HPLC trennen bzw. reinigen. Die Konfigurationszuordnung beruht -neben der Darstellungsweise -auf den 'H-NMRSpektren. Die Signale von 3-H,,, und 3-Hend, (J3x,3n = 13.5 Hz) sind durch die Kopplungen J3xp bzw. J3n,7a leicht zu unterscheiden. Das Signal von 3-H,,, ist in 2a durch die vicinale OH-Gruppe hochfeldverschoben (6 = 1.77), in 3a durch die vicinale CN-Gruppe tieffeldverschoben (6 = 2.25).Die Signale von 3-Hend, verhalten sich umgekehrt (2a: 6 = 1.90; 3a: 6 = 1.35). 6 IAus 2 a...

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Deamination Reactions, 56. Cyclopropane Participation in Norbornyl Cations

Herrmann

Kirmse

1995

Liebigs Ann./Recl.

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Nitrous acid deamination of spiro[bicyclo[2.2.l]heptane-7,1'cyclopropan]-endo-2-amine (13) affords 74% of the analogous endo alcohol 24-OH and 26% of the rearranged exo alcohol 25-OH. The exceptional preference for endo attack of the nucleophile is attributed to intervention of the bridged norpinyl cation 21 which is stabilized by the spiroannellated cyclopropane ring. The spiro[bicyclo[2.2.1]heptane-3,1'-cyclopropanl-2-yl cations 23 arising from Wagner-Meerwein rearrangement are captured by water to give high exo:endo product ratios.The 2-norbornyl cation has been one of the most extensively explored carbocations[21. Strong evidence supports the symmetrically bridged structure 6 of the 2-norbornyl cation in non-basic mediaL3I, in the solid stateI41, and in the gas phasew. On the other hand, the origin of high exo:endo rate and/or product ratios in solvolytic systems remains debatableI6J. Recently, increasing effort has been devoted to metastable isomers of 6. The bridged norpinyl cation 4 has been assigned as a shallow minimum on the C7H: l potential surface both e~perimentally[~] and computationally [s]. The barrier calculated for the rearrangement 4 + 5 (transition state) .+ 6 (1.2 kcal/m01)[~~ is consistent with the endo:exo product ratio (7%) generated by solvolytic rearrangements of norpinyl substrates [g]. Some years ago, we reported that norbornane-endo-2-diazonium ions (1) in water produce 10% of endo alcohol 7 with complete retention of configuration and 90% of racemic ex0 alcohol 8[l0,lL]. We suggested that the extrusion of nitrogen from 1 proceeds, in part, with participation of C-7 to give 4. If this view is correct, substituents enhancing the stability of 4 should increase the fraction of endo products 7. 2 I I OR 7 8

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Generation and rearrangement of spirocyclopropane‐substituted 2‐norbornyl cations

Cited by 4 publications

References 55 publications

Rearrangements of Spirocyclobutane-Substituted 2-Norbornyl Cations

Rearrangements of Spirocyclobutane-Substituted 2-Norbornyl Cations

Rearrangements of 1‐ and 2‐Cyano‐2‐norbornyl Cations

Deamination Reactions, 56. Cyclopropane Participation in Norbornyl Cations

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