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

Knorr, Rudolf; Hintermeyer-Hilpert, M.; Mehlstaeubl, J.; Phung, Hoang Thi Phung Hoang Thi; Neuner, Brigitte; Boehrer, P.

doi:10.1002/chin.199320139

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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 [4] that produced comparable amounts of the 2,5-and 2,2products. 2 The earlier studied N-alkylimine model systems were reported [1][2] [5] 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%