Chiral Concave Imidazolinium Salts as Precursors to Chiral Concave N‐Heterocyclic Carbenes

Abstract

Imidazolinium moieties have been incorporated into bimacrocycles to generate precursors for concave N‐heterocyclic carbenes (NHCs). By using one symmetrically substituted benzene bridgehead and one naphthalene bridgehead devoid of local C2‐symmetry, axially chiral concave imidazolinium ions have been obtained. Starting from 2,7‐dihydroxy‐1‐nitronaphthalene (7), the phenol groups have been transformed to 4‐pentenyl ethers 8, and the nitro group was then reduced to the corresponding amine 9. Next, 9 and a 2,6‐bis(alkenyloxy)aniline 10 were connected by an oxalic acid linker. After reduction of the diamide 13 to diamine 14, a bridge was installed with triethyl orthoformate to give a tetraalkenyl‐substituted imidazolinium salt 15. Ring‐closing metathesis of 15 followed by hydrogenation of the products 16 yielded the bimacrocyclic salts 17 in 9–18 % overall yield (based on 7), giving amounts up to >300 mg. The configurational stability of 17 was investigated by NMR using chiral enantiopure anions TRISPHAT 18 and BINPHAT 19 as stereodynamic probes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A Configurationally Stable Chiral Concave Imidazolinium Salt

“…We showed that in case of the 4,5-nonsubstituted chiral imidazolinium salt 10, the use of enantiopure Λ-BINPHAT (20) as a counterion led to NMR spectra with two sets of signals with different intensities. [25] The rational is that one enantiomeric imidazolium ion interacts more strongly with the chiral counterion than the other and is stabilized by this ion pair formation. Noteworthy is that the amplification of one species can only happen by rotation along the N-C Ar bonds (Scheme 1) and only if the energy of the rotation barrier is low.…”

“…In solution, the molecules are flexible, and 10 inverts to its enantiomer quickly. [25] For investigation of the configurational stability of new dimethylated axially chiral concave bimacrocycle 19, two NMR experiments were carried out: First, racemic imidazolinium chloride 19 was converted into a diastereomeric salt with enantiopure Λ-BINPHAT [27] ( Figure 5). We showed that in case of the 4,5-nonsubstituted chiral imidazolinium salt 10, the use of enantiopure Λ-BINPHAT (20) as a counterion led to NMR spectra with two sets of signals with different intensities.…”

“…The naphthyl ring in 19 is as orthogonal to the imidazolin- [25] (bottom; n = 4, see Scheme 1). For clarity, only carbon, nitrogen, and oxygen atoms are shown.…”

“…2,6-Bis(pent-4-enyloxy)aniline [23] and 2,7-bis(pent-4-enyloxy)naphthylamine [25] were synthesized according to literature procedures. Tetrahydrofuran was dried by heating at reflux with lithium aluminum hydride.…”

“…[25] However, this NHC precursor proved to be configurationally unstable. At room temperature, rotation along the N-C Ar bonds was observed by NMR spectroscopy, which resulted in interconversion of the enantiomers.…”