ChemInform Abstract: Sterically Crowded Five‐membered Heterocyclic Systems. Part 2.

Cygler, M.; Dobrynin, K.; Grabowski, M. J.; Nazarski, Ryszard B.; Skowroński, R.

doi:10.1002/chin.198602059

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“…423 sh * nm (ca. 5 *) (see text); 6,(80 MHz, after reduction) 0.7-0.9 (-9 H, m, Me), 1.03-1.35 (-15 H, 6 x s, Me), and 1.4-2.2 (8 H, m, 3-H, 4-H, and bridgehead CH,); 6,(20.0 MHz, after reduction) 13.7, 14.0, 24.8, 15.5, 15.8,23.1,24.5,24.75, 26.4, 26.7, and 27.9 (off res., 11 x q, 11 x Me), 37.9, 38.1, and 39.5 (3 (18), 126 (27), 124 (26), 83 (37), 70 (39), 69 (40), 55 (96), 43 (34), 42 (33), 41 (loo), 39 (24), and 29 (23). An X-ray crystallographic analysis confirmed the structure assigned to compound (5b) (see text).…”

Section: Methodsmentioning

confidence: 99%

Sterically crowded five-membered heterocyclic systems. Part 3. Unexpected formation of stable flexible pyrrolidinoxyl biradicals via nitrone aldol dimers: a spectroscopic and mechanistic study

Nazarski¹,

Skowroński²

1989

J. Chem. Soc., Perkin Trans. 1

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wicza 68, 90-136 kodz, Poland -~~ Addition of methylmagnesium iodide t o various substituted cyclic nitrones (1 ) and subsequent aerial oxidation gave new pyrrolidinoxyl radicals (4) and the corresponding crystalline biradical species, to which the structure (5) was assigned unequivocally from spectroscopic (e.s.r., u.v.-visible, mass, and H and 13C n.m.r. spectra) and mechanistic considerations. Base-catalysed self -condensation of the nitrones ( I ) , to give the nitrone hydroxylamines (18), is followed by Grignard addition to give the crowded bis-N-hydroxy compounds (3); these are then oxidised to the biradicals (5). The possibility of intramolecular charge transfer interaction between the two aminoxyl groups of the biradicals (5) (in order to explain the observed U.V. absorption at ca. 328 nm), and also the dimerisation of 1 -pyrroline 1 -oxides (particularly those that are sterically crowded), are discussed briefly.Sterically crowded carbo-and hetero-cyclic systems have receit ed considerable attention in recent years, mostly in connection with their usually difficult, multi-step syntheses and often unique properties.2 Of such compounds, stable aminoxyl 1 free radicals are of particular interest.In continuation of our previous work on crowded fivemembered rings,' ,5 we focussed our attention on the synthesis of 2,2,5,5-tetramethylpyrrolidin-l-oxyl radicals (4) bearing an alkyl or aryl substituent R1 at C-3. These compounds, with hydroxylamines (2) and amines (6), served as model compounds in O U T studies on the oxidation of pyrrolidine derivatives6 and in the conformational analysis of saturated five-membered rings.6p8 We now report the somewhat surprising results obtained during the synthesis of compounds (4b, c) by addition of methylmagnesium iodide to l-pyrroline l-oxides (lb, c) § (see Scheme 1). As expected, Grignard reactions in ethereal solution, followed by aqueous work-up in air, afforded a mixture of diaand para-magnetic compounds. The proportion of the latter increased with time as monitored by ' H n.m.r. spectroscopy. However, when the crude products were subjected to Cut'catalyzed aerial oxidation, mixtures of two paramagnetic specits (separable by column chromatography) were obtained.Thest: were characterised as the desired monoradicals (4b, c) and the corresponding solid biradicals (e.s.r., u.v., and mass spectral evidence) C, 7H,,N,02 (5b) and C27H,6N202 (5c), which had much lower RF values. Biradicals of this type have not previously been prepared in this way.These results led us to consider the processes affording the title biradicals ( 5 ) and their air-sensitive N-hydroxy precursors (3). Results and DiscussionSjnthesis of the Radicals (4).-The 2,2,5,5-tetramethylpyrrolidin-l-oxyl radicals (4) may, in principle, be prepared by oxid;ition of the amine (6) or the hydroxylamine (2); both methods have been described for the preparation of the parent radical (4a).9 As the pyrrolidines (6b, c) were not available [though they can be obtained" from (2b, c)], the general nitrone method developed...

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