Organic Letters
Letter
5, showing additional helical chirality, as a result of a high
distortion of the planarity provoking a significant transfer of
chirality from the planar-chiral ferrocene moiety to the helical
ortho-condensed aromatic system.
Scheme 5. Asymmetric Synthesis of Enantiopure
Pentacyclic Helical Ferrocene (Rp)-5
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
Synthetic procedures, characterization data, copies of 1H
and 13C NMR spectra, and X-ray data for (Rp)-1c (PDF)
AUTHOR INFORMATION
■
Corresponding Author
ORCID
followed by treatment of the dibromoalkene intermediate with
n-BuLi afforded alkyne (Rp)-6f, in 52% yield. Finally, the
cycloisomerization of (Rp)-6f with PtCl2 (10%) led to
enantiopure helical ferrocene (Rp)-5, in 93% yield, showing a
́
̃
Notes
very high optical rotation value {[α]20 = +3240 (c 0.0025,
D
CH2Cl2)}.15
The authors declare no competing financial interest.
The CD spectrum of enantiopure pentacyclic helical
ferrocene (Rp)-5 is shown in Figure 3, together with those of
ACKNOWLEDGMENTS
■
́
We thank Ministerio de Economıa y Competitividad (Grants
CTQ2017-83309-P and CTQ2014- 53894-R) for financial
support.
REFERENCES
■
(1) (a) Chirality at the Nanoscale, Nanoparticles, Surfaces, Materials
and More; Amabilino, D., Ed.; Wiley-VCH: Weinheim, 2009.
(b) Molecular Switches; Feringa, B. L., Browne, W. R., Eds.; Wiley-
VCH: Weinheim, 2001.
(2) Selected reviews: (a) Link, A.; Sparr, C. Chem. Soc. Rev. 2018,
47, 3804. (b) Li, C.; Yang, Y.; Miao, Q. Chem. - Asian J. 2018, 13, 884.
(c) Chen, C.-F.; Shen, Y. Helicene Chemistry: From Synthesis to
Applications; Springer-Verlag: Berlin, 2017. (d) Wang, K. K. Top.
Curr. Chem. 2014, 349, 31. (e) Gingras, M. Chem. Soc. Rev. 2013, 42,
Figure 3. CD spectra of enantiopure tricyclic ferrocene (Rp)-1a,
tetracyclic (Rp)-12, and pentacyclic (Rp)-5.
968. (f) Urbano, A.; Carreno, M. C. Org. Biomol. Chem. 2013, 11, 699.
̃
(g) Shen, Y.; Chen, C.-F. Chem. Rev. 2012, 112, 1463. (h) Urbano, A.
Angew. Chem., Int. Ed. 2003, 42, 3986. (i) Katz, T. J. Angew. Chem.,
Int. Ed. 2000, 39, 1921.
tricyclic and tetracyclic derivatives, (Rp)-1a and (Rp)-12, for
comparison. As can be seen, the pentacyclic helical ferrocene
(Rp)-5 showed a very intense circular dichroism response with
a band at 317 nm (Δε = +160) with a 5-fold intensity with
respect to that presented by the tetracyclic helical derivative
(Rp)-12 and near 30-fold intensity related to the unsubstituted
planar tricyclic compound (Rp)-1a.22 This strong CD signal
could be attributed to the presence of additional helical
chirality in (Rp)-12 due to a high distortion of the planarity by
the presence of five ortho-condensed aromatic rings.
In summary, we have described the enantioselective
synthesis of planar-chiral enantiopure ferrocenes, with three,
four, and five ortho-condensed aromatic rings. The helicenic
structure was generated at the end of the synthetic sequence
featuring a PtCl2-catalyzed cycloisomerization process of
suitable planar-chiral ferrocenes previously formed from
enantiopure (SS,Sp)-p-tolylsulfinyl ferrocenyl boronic acid, as
the source of planar chirality. The study of their chiroptical
properties evidenced a very high optical value and a huge
intensity of the CD signals for the pentacyclic derivative (Rp)-
(3) Selected reviews: (a) Zhu, J.-C.; Cui, D.-X.; Li, Y.-D.; Jiang, R.;
Chen, W.-P.; Wang, P.-A. ChemCatChem 2018, 10, 907. (b) Gao, D.-
W.; Gu, Q.; Zheng, C.; You, S.-L. Acc. Chem. Res. 2017, 50, 351.
(c) Zhu, D.-Y.; Chen, P.; Xia, J.-B. ChemCatChem 2016, 8, 68.
(d) Arae, S.; Ogasawara, M. Tetrahedron Lett. 2015, 56, 1751.
(e) Toma, S.; Csizmadiova, J.; Meciarova, M.; Sebesta, R. Dalton
Trans 2014, 43, 16557. (f) Butt, N. A.; Liu, D.; Zhang, W. Synlett
2014, 25, 615. (g) Drusan, M.; Sebesta, R. Tetrahedron 2014, 70, 759.
(h) Schaarschmidt, D.; Lang, H. Organometallics 2013, 32, 5668.
(4) (a) OuYang, J.; Crassous, J. Coord. Chem. Rev. 2018, 376, 533.
(b) Isla, H.; Crassous, J. C. R. Chim. 2016, 19, 39. (c) Saleh, N.; Shen,
C.; Crassous, J. Chem. Sci. 2014, 5, 3680.
(5) Crassous, J. Chem. Commun. 2012, 48, 9684.
(6) (a) Katz, T. J.; Pesti, J. J. Am. Chem. Soc. 1982, 104, 346.
(b) Sudhakar, A.; Katz, T. J. J. Am. Chem. Soc. 1986, 108, 179.
(7) (a) Pammer, F.; Sun, Y.; May, C.; Wolmershauser, G.; Kelm, H.;
Kruger, H.-J.; Thiel, W. R. Angew. Chem., Int. Ed. 2007, 46, 1270.
(b) Pammer, F.; Sun, Y.; Pagels, M.; Weismann, D.; Sitzmann, H.;
Thiel, W. R. Angew. Chem., Int. Ed. 2008, 47, 3271. (c) Pammer, F.;
Sun, Y.; Weismann, D.; Sitzmann, H.; Thiel, W. R. Chem. - Eur. J.
D
Org. Lett. XXXX, XXX, XXX−XXX