(E, Z)1‐Equilibria, 17 Demonstration of the Nitrogen Inversion Mechanism of Imines in a Schiff Base Model

Abstract

Experimental differentiation between pure C = N double bond rotation and nitrogen inversion in N-arylimines is possible with a single compound (13b) under the proviso of slow rotation about the N-aryl single bond. Labelling by 'H and 13C nuclei at the diastereotopic faces of the C = N moiety as well as of the N-aryl group is the clue to a successful stereodynamic analysis, as performed by variable-temperature NMR spectroscopy of 13b, a sterically congested and chiral model compound. Interpretation of similar measurements on a second model (13d) is less straightforward. The experimental observation of time-averaged C , symmetry by NMR coalescences is only compatible with a mechanism of ( E / Z ) stereomutation either by pure inversion at sp2 nitrogen or by a contribution from C = N rotation together with a synchronized (geared) contrarotation about the N-aryl single bond. However, the latter combination is concluded to be predominantly inversion-like by comparisons with related imines.The conformational lability at a C =X double bond in 1 and 2 is measured by the rate of (E,Z) diastereoisomerization, or of (E,Z) diastereotopomerization if the substituents e and .f are constitutionally equal. This stereomutation might occur by a 180"-rotation about the C = X double bond via a transition state 3 but if the key atom X carries a lone electron pair (like X = N or C-Li+), the alternative mechanism of "planar" inversion (or "lateral shift"[21) is more probable with linear coordination of X in a transition state 4. Quantum-mechanical calculations on imines (X = N) indicate a much higher energetic barrier of rotation (3) than of inversion (4) for iminomethane (1, R 5Definite experimental evidence to differentiate between these mechanisms has apparently not been published. The methyl groups within each one of the o-isopropyl substituents are diastereotopic (inequivalent) in the ground state of the quinone imine 6 a and should become enantiotopic (equivalent by mirror symmetry) during inversion (4); but the attempt to demonstrate this was thwarted by faster rotation about the N-aryl single bond [","] which leads to premature positional equilibration of these methyl groups. Kessler and Leibfritz ["] had to use guanidines to show that the diastereotopomerization of the groups e and f in 6 b occurred as fast as isopropyl enantiotopomerization; but comparisons with 6c and further derivatives were required to exclude contributions from N -aryl single-bond rotation in 6b. c I MQN--NMeCH2PhThus all available evidence accumulates to a "consen-S U S " [~~] with "many arguments"['31 for the inversion mechanism of imine stereomutation. However, comparisons within a series of related compounds would be time-consuming in other systems["] (e. g., X = C-Li+) and may be quantitatively questionable for reasons of possible electronic[31 and c~nformational[~~ differences in the ground states. Guided by synthetic and kinetic experien~e ['~~'~] with the conformationally quite rigid imine 7, we therefore considered the construction of a mode...

“Conducted Tour” Migration of Li+ during the cis/trans Stereoinversion of α‐Arylvinyllithiums

“…Groundstates of the alkenyllithiums The carboxylic acid 10 was prepared from 4-bromo-2,2-dimethylbenz[f]indane [8] (8)v ia the lithio derivative 9 and its carboxylation (Scheme 3);i tw as important to realize that 10 was weaklys oluble in both Et 2 Oa nd aqueous NaOH. The ensuing formation of the 4-acetyl derivative 11 from 10 with methyllithium (3 equiv) wast erminated through quenching with ClSiMe 3 before aqueous workup,s ot hat remnant methyllithium was destroyed before the ketone 11 was liberated through hydrolysis.…”

“…2,2-Dimethylbenz[f]indan-4-carboxylic acid (10): nBuLi (12.2 mmol) in hexane (4.4 mL) was added dropwise to astirred solution of 4-bromo-2,2-dimethylbenz[f]indane [8] (8;3 .28 g, 11.9 mmol) in anhydrous Et 2 O( 20 mL) at À30 8Cu nder inert gas cover.T he mixture changed color from green to yellow,t hen orange-red during the addition. After 10 min at RT,t he solution was poured onto solid CO 2 and warmed up to RT.T he yellow mixture was completely dissolved in Et 2 Oa nd as ufficient amount of conc.…”

“…2017, 23,12861 -12869 www.chemeurj.org of nBu 6 Li (22.9 mmol) in pentane (18.4 mL). The Br/Li interchange reaction was almost complete after 25 min at RT,w hereupon the product 2 began to precipitate completely within 4h.T he supernatant was withdrawn by pipette, and the precipitate was washed with anhydrous pentane (2 ), then evaporated to dryness at RT in vacuo:Y ield:2 .0 g( 66 %); 1 HNMR (THF,8 [8] (8;3 .28 g, 11.9 mmol) in anhydrous Et 2 O( 20 mL) at À30 8Cu nder inert gas cover.T he mixture changed color from green to yellow,t hen orange-red during the addition. After 10 min at RT,t he solution was poured onto solid CO 2 and warmed up to RT.T he yellow mixture was completely dissolved in Et 2 Oa nd as ufficient amount of conc.…”

New molecular switch architectures

“…Increasing the ring inversion barrier was achieved by expanding the dibenzo[a,d]cycloheptene ring, effectively pushing the C-H bonds of the peripheral phenyl groups closer to the rotor, and using the bulky t Bu group on the rotor. While in-plane imine inversion has been wellestablished (69), the out-of-plane rotation was only assumed, based on computation (70). Lehn and coworkers (21) later established, via the products of the photochemical ring expansion of cyclopropane rings in a simple camphor-based imine motor, that the rotation is indeed out-of-plane, and is biased to one direction at a 4:1 ratio.…”

Demonstration of NC(sp2)‐centered stereofluctuations in a guanidine derivative and its cation

“…One half‐rotation (by 180°) about the N —aryl single bond, however, will interchange the inner and outer sites and their chemical shifts; with increasing temperatures, this “diastereotopomerization” leads first to NMR line broadening and then to pairwise averaging (“coalescence”) of the NMR absorption lines when the stereofluctuation rate constant begins to exceed the frequency distance of the pair under consideration. The N ‐aryl tool in 6 and 7 had been used for a similar demonstration of the N‐stereoinversion mode in a ketimine.…”