Stereolabile configurational units torsional and inversional stereochemistry in sulfenamides and hydroxylamines

A study of nitrogen inversion process in some camphor-based isoxazolidines

“…During the course of a structural investigation of the isomeric isoxazolidines 4c and 5c (R 1 =H, R 2 = CO 2 Me) it was interesting to observe the presence of a single invertomer for 4c, while 5c remained as an equilibrating mixture of two invertomers (cis and trans) at -40°C both in CDCl 3 and CD 3 OD (Scheme 2). This is interesting since a look at their structures seems to covey that 2,5-trans-4c and 2,5-trans-5c (or 2,5-cis-4c and 2,5-cis-5c) should have a comparable steric environments, and as such a large discrepancy in the population ratio of the invertomers in 4c and 5c is quite unexpected.…”

“…4,10 The inversion barrier decreases to some extent in hydrogen bonding solvent CD 3 OD (Table 3). Usually it is known that the inversion barriers in CD 3 OD increase in cyclic system as a result of an extra energy required for breaking of Hbonding prior to inversion. 4 However, the inversion in CDCl 3 may also require extra energy to break the intramolecular OH … N hydrogen bond prior to inversion.…”

“…1 The presence of an −N−O− moiety in an organic molecule has a distinctive place in conformational analysis, [2][3][4] oxygen being next to nitrogen raises the barrier to nitrogen inversion (N i ) to such an extent that at temperatures lower than the ambient, individual invertomers can be identified by NMR spectroscopy. 5 The asymmetric nitrone cycloaddition reactions involving camphor-derived intramolecularly H-bonded nitrone (Scheme 1) has been found to be very effective in transferring chirality to the newly created stereocenter at C(5) of the cycloadducts isoxazolidines.…”

Negative hyperconjugation in hexachloro-3-cyclopentenylaminosulphenyl halides: Preparation and X-ray structure of C5Cl6NSBr and synthesis of C5Cl6NS+AsF6−

“…The observed fluxional behaviour can be explained either by rotation about the S-N bond or inversion at the nitrogen centre. For comparison, the rotational barrier for ClSNH2 has been calculated to be 66.5 kJ mol-' (18) while experimental values for N,N-dialkyl substituted compounds are in the range 60.7-64.9 kJ mol-' (20). The inversion process would be facilitated by the negative hyperconjugative effect but it is not possible to distinguish between these two processes on the basis of the available data.…”

A study of conformational behaviour in some hexahydro-2H-isoxazolo[2,3-a]pyridines

“…3 It is often difficult to assign the configuration at various chiral centers of the cycloadducts 3 and 4 by spectroscopic analysis owing to complications arising out of slow nitrogen inversion process involving cis-and trans-fused invertomers. The presence of −N−O− moiety in an organic molecule (as in the case of isoxazolidines 3 and 4) has a distinctive place in conformational analysis [4][5][6] ; oxygen being next to nitrogen raises the barrier to nitrogen inversion to such an extent that the individual invertomers can be identified by NMR spectroscopy. 7 The previous studies 8 of nitrogen inversion have greatly contributed to the understanding of several reaction processes.…”