10.1002/anie.201811873
Angewandte Chemie International Edition
COMMUNICATION
showed that the remaining alkynyl unit had become attached at
boron. We note that the Mes* substituent at phosphorus and the
C6F5 group at boron are trans-oriented at the six-membered
heterocyclic core of compound 11. Consequently, we conclude
that the addition of the components of the phenylacetylene
reagent at the cyclic P/B FLP had taken place by means of an
intermolecular reaction pathway. In solution (CD2Cl2) compound
11 shows a 1H NMR [P]-H doublet at δ 6.40 with 1JPH = 454.7 Hz
(31P: δ 18.0) and a 11B NMR feature at δ -19.0. The Δδ19Fm,p
chemical shift difference in 11 is 2.7 ppm.
Conflict of interest
The authors declare no conflict of interest.
Keywords: boranes • phosphanes • frustrated Lewis pairs •
macrocycles • self-assembly
[1]
For selected reviews, see: a) D. W. Stephan, G. Erker, Angew. Chem.,
Int. Ed. 2015, 54, 6400-6441; Angew. Chem. 2015, 127, 6498-6541; b)
D. W. Stephan, Acc. Chem. Res. 2015, 48, 306-316; c) D. W. Stephan,
J. Am. Chem. Soc. 2015, 137, 10018-10032; d) D. W. Stephan, Science
2016, 354, aaf7229.
The six-membered cyclic P/B FLP 7 is an active dihydrogen
splitting agent.[1] Heterolytic H-H cleavage was achieved by
exposure of 7 to dihydrogen in CH2Cl2 solution (2 bar H2, r.t., 5 d)
and we isolated the zwitterionic phosphonium/hydridoborate
product 12 as a white solid in 74% yield. The reaction of 7 with D2
under similar conditions gave the product 12-D2 in a similar yield.
Compound 12 was characterized by X-ray diffraction (Figure 5). It
shows a chair conformation of the six-membered heterocyclic
framework with both the bulky Mes* substitutents at phosphorus
and the C6F5 group at boron being in equatorial positions.
Consequently, the proton/hydride pair had been trans-attached at
the phosphorous Lewis base and the boron Lewis acid. Since this
cannot be achieved in an intramolecular reaction,[3,15] we
conclude that the H2 splitting reaction at the cyclic P/B FLP must
have taken place in an intermolecular fashion involving more than
one FLP molecule.[16]
[2]
a) G. C. Welch, R. R. S. Juan, J. D. Masuda, D. W. Stephan, Science
2006, 314, 1124 –1126; b) P. Spies, G. Erker, G. Kehr, K. Bergander, R.
Fröhlich, S. Grimme, D. W. Stephan, Chem. Commun. 2007, 5072-5074;
c) “Frustrated Lewis Pairs I: Uncovering and Understanding”: Topics in
Current Chemistry, Vol. 332 (Eds.: G. Erker, D. W. Stephan), Springer,
Heidelberg, 2013.
[3]
a) S. Dong, L. Wang, T. Wang, C. G. Daniliuc, M. Brinkkötter, H. Eckert,
G. Kehr, G. Erker, Dalton Trans. 2018, 47, 4449-4454; for selected
additional examples see also: b) G. Chen, L. N. Zakharov, M. E. Bowden,
A. J. Karkamkar, S. M. Whittemore, E. B. Garner, T. C. Mikulas, D. A.
Dixon, T. Autrey, S.-Y Liu, J. Am. Chem. Soc. 2015, 137, 134-137; c) F.
A. Tsao, L. Cao, S. Grimme, D. W. Stephan, J. Am. Chem. Soc. 2015,
137, 13264-13267; d) J. Chen, D. A. Murillo Parra, R. A. Lalancette, F.
Jäkle, Angew. Chem., Int. Ed. 2015, 54, 10202-10205; Angew. Chem.
2015, 127, 10340-10343; e) T. Wang, L. Liu, S. Grimme, C. G. Daniliuc,
G. Kehr, G. Erker, Chem.−Asian J. 2016, 11, 1394-1399; f) X. Liu, Y.
Zhang, B. Li, L. N. Zakharov, M. Vasiliu, D. A. Dixon, S.-Y. Liu, Angew.
Chem., Int. Ed. 2016, 55, 8333-8337; Angew. Chem. 2016, 128, 8473-
8477; g) F. A. Tsao, D. W. Stephan, Chem. Commun. 2017, 53, 6311-
6314.
1
In solution (CD2Cl2) compound 12 shows the typical broad H
NMR 1:1:1:1 quartet of the [B]-H moiety at δ 1.97 and the [P]+-H
resonance at δ 6.50 (1JPH = 456.0 Hz). The 31P NMR signal was
located at δ 18.2 and the 11B NMR resonance at δ -18.5 as a
[4]
[5]
L. Wang, S. Zhang, Y. Hasegawa, C. G. Daniliuc, G. Kehr, G. Erker,
Chem. Commun. 2017, 53, 5499-5502.
1
doublet with a typical JBH ~ 85 Hz coupling constant (for further
a) D. J. Parks, R. E. von H. Spence, W. E. Piers, Angew. Chem. Int. Ed.
Engl. 1995, 34, 809-811; Angew. Chem. 1995, 107, 895-897; b) D. J.
Parks, W. E. Piers, G. P. A. Yap, Organometallics 1998, 17, 5492–5503.
M. Erdmann, C. Rösener, T. Holtrichter-Rößmann, C. G. Daniliuc, R.
Fröhlich, W. Uhl, E.-U. Würthwein, G. Kehr, G. Erker, Dalton Trans. 2013,
42, 709-718.
details, see the SI). Both compounds 7 and 12 were used a
catalysts for an imine and an enamine hydrogenation, but they
both required an elevated reaction temperature (see the
Supporting Information for details).
[6]
Our study has shown that active cyclic six-membered P/B FLPs
can readily be prepared by means of hydroboration sequences,
combined with a specific sequence involving formation and
removal of a HB(C6F5)2 equivalent. The resulting system 7 is an
active P/B FLP. It undergoes cycloaddition-type P/B addition
reactions to π-systems[3] and it actively cleaves the dihydrogen
molecule, although this reaction deviates from the common
behavior of intramolecular P/B FLPs in the sense that it produces
the respective trans-H+/H- addition product. More importantly,
compound 7 shows a remarkable aggregation behavior. It forms
the unique macrocyclic cyclooctameric ring structure [(7)8] in the
crystal and in solution at low temperature. Cyclic frustrated Lewis
pairs have so far rarely been encountered.[3] Their aggregation
behavior might open new pathways in FLP chemistry and it might
serve to connect FLP chemistry with the important established
area of supramolecular chemistry.
[7]
[8]
E. L. Eliel, S. H. Wilen, M. P. Doyle, Basic Organic Stereochemistry,
Wiley, New York, 2001.
a) S. Ito, H. Miyake, M. Yoshifuji, Phosphorus, Sulfur Silicon Relat. Elem.
2009, 184, 917–927; b) L. Wang, S. Dong, C. G. Daniliuc, L. Liu, S.
Grimme, R. Knitsch, H. Eckert, M. R. Hansen, G. Kehr, G. Erker, Chem.
Sci. 2018, 9, 1544-1550.
[9]
T. Wiegand, M. Siedow, H. Eckert, G. Kehr, G. Erker, Isr. J. Chem. 2015,
55, 150-178.
[10] a) X. Tao, G. Kehr, C. G. Daniliuc, G. Erker, Angew. Chem. Int. Ed. 2017,
56, 1376-1380; Angew. Chem. 2017, 129, 1396-1400; b) J. Li, C. G.
Daniliuc, G. Kehr, G. Erker, Chem. Commun. 2018, 54, 6344-6347; c) A.
Ueno, J. Li, C. G. Daniliuc, G. Kehr, G. Erker, Chem.−Eur. J. 2018, 24,
10044-10048.
[11] a) A. Klamt, J. Phys. Chem. 1995, 99, 2224-2235; b) S. Grimme, C.
Bannwarth, P. Shushkov, J. Chem. Theory Comput. 2017, 13, 1989-
2009; c) J. G. Brandenburg, C. Bannwarth, A. Hansen, S. Grimme, J.
Chem. Phys. 2018, 148, 064104
[12] For rare examples of associated FLP structures see e.g. a) F. Bertini, F.
Hoffmann, C. Appelt, W. Uhl, A. W. Ehlers, J. C. Slootweg, K.
Lammertsma, Organometallics 2013, 32, 6764-6769; b) M. Erdmann, T.
Wiegand, J. Blumenberg, H. Eckert, J. Ren, C. G. Daniliuc, G. Kehr, G.
Erker, Dalton Trans. 2014, 43, 15159-215169; c) T. Wang, L. Liu, S.
Grimme, C. G. Daniliuc, G. Kehr, G. Erker, Chem. Asian J. 2016, 11,
1394-1399; d) L. Wang, S. Dong, C. G. Daniliuc, L. Liu, S. Grimme, R.
Knitsch, H. Eckert, M. R. Hansen, G. Kehr, G. Erker, Chem. Sci. 2018,
9, 1544-1550; e) see also ref. [3a].
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. RK thanks the Fonds der chemischen
Industrie for a stipend.
This article is protected by copyright. All rights reserved.