(<i>E/Z</i>) Equilibria, 16. Lone Electron Pair Donor Quality of the Imino Function: Synthesis and Reactivity of Sterically Strongly Congested Iminocyclopentanes

Knorr, Rudolf; Hintermeyer, Monika; Hilpert,; Mehlstäubl, Johann; Hoang, Thi Phung; Neuner, Brigitte; Bührer, Petra

doi:10.1002/cber.19931260129

Cited by 7 publications

(1 citation statement)

References 21 publications

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“…The possibility to use the =N– t Bu orientation as a control instrument depends on the direction and velocity of the ( Z , E ) re‐orientations. These ( Z , E ) stereoinversion processes were too fast for controlling purposes in the case of an N ‐aryl model that produced comparable amounts of the 2,5‐ and 2,2‐products . The earlier studied N ‐alkylimine model systems were reported [ ][ ][ ] to depend on further factors; but they suggested to us that an N– t Bu moiety might favor the anti over the alternative syn deprotonation sufficiently to control the regioselectivity.…”

Section: Introductionmentioning

confidence: 99%

Regioselective Dialkylations of N‐(tert‐Butyl)iminocyclopentane via Deprotonating One‐Pot Procedures

Knorr

Neuner

2018

Helvetica Chimica Acta

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The title compound (1) was chosen as a model for the α/α′‐regioselectivity of deprotonation and subsequent alkylation adjacent to the C=N bond. With the bulky base lithium N,N‐diisopropylamide (LDA) as a catalyst, the one‐pot deprotonation steps can be performed through titration with methyllithium, using gas‐volumetric observation of the liberated methane. In the first step with ensuing methylation by iodomethane, the primary product is born at −40 °C in its metastable (Z) configuration (kinetic control) and may be either isolated or converted in situ at 30 °C into its thermodynamically favored (E)‐isomer via cis to trans stereoinversion at the N‐atom. Being slow enough on the laboratory time scale, this stereoinversion process can serve to control the regioselectivity of the second deprotonation/alkylation sequence as follows. The α,α′‐products are formed from the intermediate (Z)‐imine, whereas α,α‐products result from the intermediate (E)‐imine; in either case, syn deprotonation (cis to tBu at nitrogen) by LDA is apparently disfavored by the tBu group, so that anti deprotonation becomes obligatory. If a third one‐pot deprotonation step is too slow with LDA, it may be performed with the stronger base butyllithium/HMPA which, however, reacts regio‐unselectively. Regioselective one‐pot, LDA‐catalyzed deprotonation with alkylation by oxiranes (alone, or alternatingly with iodomethane) opens a short access to spiro‐[2.4]heptan‐4‐ones.

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Section: Introductionmentioning

confidence: 99%

Regioselective Dialkylations of N‐(tert‐Butyl)iminocyclopentane via Deprotonating One‐Pot Procedures

Knorr

Neuner

2018

Helvetica Chimica Acta

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Sterically Congested Molecules, 6. Lone Electron Pair Donor Quality of the Imino Function: Increased Front Strain and Electronic Substituent Effects on Sterically Accelerated Nitrogen Inversion in Iminocyclopentanes

et al. 1993

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The p-substituents of 2,6-dimethyl-N-(2,2,5,5-tetramethylcyclopenty1idene)anilines are modified without interfering reactions at the CN double bond. The resultant series (5-8, lO -19) shows a strong (ca. -4.7 kcal/mol) steric acceleration of (EIZ) diastereotopomerization by front strain along the CN double bond but also the usual electronic substituent dependence, characterized by a Hammett op correlation (e = + 2.7). Conversely, the substituent constant for lithium at the p-position of 7 may be estimated. The volume of activation is 1.5(8) cm3 rno1-l for 5. The x donor quality of the imino group corresponds to ca. 55 ( f 8)% of pn character as evaluated by spectral (13C NMR of 5 and 13, IR of 13) and reactivity data (of 13).

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

et al. 1993

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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...

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(E/Z) Equilibria, 16. Lone Electron Pair Donor Quality of the Imino Function: Synthesis and Reactivity of Sterically Strongly Congested Iminocyclopentanes

Cited by 7 publications

References 21 publications

Regioselective Dialkylations of N‐(tert‐Butyl)iminocyclopentane via Deprotonating One‐Pot Procedures

Regioselective Dialkylations of N‐(tert‐Butyl)iminocyclopentane via Deprotonating One‐Pot Procedures

Sterically Congested Molecules, 6. Lone Electron Pair Donor Quality of the Imino Function: Increased Front Strain and Electronic Substituent Effects on Sterically Accelerated Nitrogen Inversion in Iminocyclopentanes

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

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