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. Author manuscript; available in PMC: 2009 Jun 8.
Published in final edited form as: Tetrahedron Lett. 2007;48(4):685–687. doi: 10.1016/j.tetlet.2006.11.099

An unusual norcaradiene/tropylium rearrangement from a persistent amino-phosphonio-carbene

Joan Vignolle a,b, Bruno Donnadieu b, Didier Bourissou a,*, Guy Bertrand b,*
PMCID: PMC2692173  NIHMSID: NIHMS64244  PMID: 19513183

Abstract

An amino-phosphonio-carbene featuring a bromobiphenyl backbone was prepared and spectroscopically characterized at low temperature. This carbene was found to readily rearrange upon warm up, affording an original tricyclic phospholium derivative, presumably via a norcaradiene/tropylium isomerization.

Keywords: Carbenes, Rearrangement, Norcaradiene, Tropylium

Due to their strong σ-donor character, N-heterocyclic carbenes (NHCs) have found numerous applications as alternative ligands to phosphines for transition-metal catalysts.1 Taking advantage of the substantial stabilization brought by a single heteroatom substituent (an amino or phosphino group),2 the variety of available stable carbenes has been significantly increased over the last five years.3 Accordingly, the steric and electronic properties of amino-carbenes could be varied in a broader range, as nicely illustrated by the cyclic amino-alkyl-carbenes (CAACs).4

The spectacular achievements reported with diphosphines featuring a diaryl backbone (especially BIPHEN and BINAP ligands)5 prompted us to investigate related heteroditopic ligands combining amino-carbene and phosphine coordination sites. The synthesis of the representative biphenyl derivatives A (Scheme 1) was envisaged as a further application of the recently reported nucleophilic substitution at the carbene center of amino-phosphonio-carbenes.6 Accordingly, dications D were considered as promising precursors for A via a deprotonation/metalation sequence involving intermediates B and C. Here we report the preparation and spectroscopic characterization at low temperature of an amino-phosphonio-carbene of type C. This compound was found to readily rearrange upon warm up, affording an original tricyclic derivative, presumably via a norcaradiene/tropylium isomerization.

Scheme 1.

Scheme 1

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Proposed synthetic route to aminocarbene/phosphine derivatives A.

The phosphonio-iminium salt 1 (dication of type D) was obtained by extrapolation of the procedure described for the tricyclohexyl and triphenylphosphines.7 Accordingly, the readily available 2-bromo-2′-diphenylphosphinobiphenyl8 was treated with the C-chloroiminium chloride in the presence of 2 equiv of trimethylsilyl triflate in CH3CN (Scheme 2). Dication 1 was isolated in 82% yield as a pale yellow solid. The signals observed at low-field in the 1H (9.66 ppm, 2JHP = 23.7 Hz) and 13C NMR spectra (169.5 ppm, 1JCP = 63.5 Hz) are diagnostic of the C–H iminium moiety of such dications.9

Scheme 2.

Scheme 2

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Synthesis and rearrangement of the amino-phosphonio-carbene 2.

Deprotonation of dication 1 was achieved with sodium t-butoxide at −78 °C in THF.10 Quite surprisingly, the ensuing carbene 2 could only be spectroscopically characterized at −40 °C. The shielding of the 31P NMR signal by about 30 ppm upon deprotonation and the 13C NMR chemical shift for the carbene center (δ 291.4 ppm, 1JCP = 112.2 Hz) unambiguously establish the amino-phosphonio-carbene structure of 2.6 Upon warming to room temperature, carbene 2 cleanly rearranges within minutes into a new compound 3, which was isolated as an orange solid.11 The multi-nuclear data for 3 revealed interesting features: (i) the 31P NMR chemical shift of 3 (23.0 ppm) is very similar to that of the dication 1, (ii) the 1H and 13C NMR signals for the CH(iPr) groups (1H: 4.57 and 4.66 ppm; 13C: 57.6 and 64.0 ppm) are in the typical range for an iminium, and (iii) the related 13C NMR signal for the C=N+ moiety (166.8 ppm) is attributed to a quaternary center. Single crystals of 3 were grown from a CH2Cl2/Et2O mixture at −30 °C and the X-ray diffraction study revealed a major reorganization of the carbene (Fig. 1). On the one hand, the amino-carbene, which is no longer bonded to the phosphorus atom, has apparently contributed to the conversion of one of the phenyl rings into a tropylium moiety. On the other hand, the phosphorus atom bridges the two aromatic rings, leading to a dicationic tricyclic structure.12

Figure 1.

Figure 1

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Thermal ellipsoid diagram (50% probability) of 3, the hydrogen and counter-anions have been omitted for clarity.

Although the precise mechanism for the conversion of 2 into 3 remains rather obscure, the following hypothesis may be reasonably formulated (Scheme 3). First, nucleophilic attack of amino-carbene 2 on the biphenylbackbone would be favored by the electron-withdrawing phosphonio group. Second, the ensuing ylide 4 would attack back to the highly electron-deficient carbon center leading to the norcaradiene-type iminium 5. Third, ring enlargement13 would convert 5 into the corresponding amino-tropylium salt 6. Last, the liberated phosphine would attack at the brominated ortho position to afford the tricyclic structure 3.14

Scheme 3.

Scheme 3

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Postulated mechanism for the rearrangement of 2 into 3.

In conclusion, these results highlight the dramatic influence of the substitution pattern of amino-phosphonio-carbenes on their stability. The presence of a bromo-biphenyl moiety induced an unusual rearrangement that presumably occurred via a norcaradiene/tropylium isomerization. Alternative routes to heteroditopic amino-carbene/phosphine ligands of type A are currently under investigation.

Acknowledgments

We are grateful to the NIH (R01 GM 68825) for financial support of this work.

Footnotes

Supplementary data

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2006.11.099.

References and notes

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