10.1002/anie.201706848
Angewandte Chemie International Edition
COMMUNICATION
by an FLP-type mechanism to form back the hydride species.[23]
After 24 h at 60 °C, hydrogenation of E-stilbene was achieved
(Table 1, entry 4). (Triphenyl)- and (trimethyl)vinylsilane were also
hydrogenated under these reaction conditions (Table 1, entry 5
and 6).
We thank the Deutsche Forschungsgemeinschaft through the
International Research Training Group “Selectivity in Chemo-
and Biocatalysis” (IRTG 1628) for financial support.
Keywords: calcium hydride • hydrogenation • alkaline-earth
The linear alkenes 1-hexene and 1-octene were hydrogenated by
5a within 24 h at 60 °C (Table 1, entry 7 and 9). Again, trihydride
6a showed much lower activity (Table 1, entry 8) toward 1-hexene.
3-Vinylcyclohexene gave 3-ethylcyclohexene within 36 h at 1 bar
H2 and 60 °C (Table 1, entry 10). No hydrogenation of the internal
double bond such as in cyclohexene (Table 1, entry 11) was
observed under these reaction conditions. 1,5-Hexadiene was
completely hydrogenated within 36 h (Table 1, entry 12). In
contrast to transition[18a] and rare-earth[24] metal catalyzed
reactions, only traces (ca. 4%) of the ring-closed product
methylcyclopentane were detected by 1H NMR spectroscopy. No
reactivity of disubstituted 1-alkenes 3-methyl-1-butene and 3-
ethyl-1-butene was observed at higher catalyst loadings and
reaction temperatures (Table 1, entry 14).
In situ 1HNMR spectroscopy showed the hydride resonance to be
unaffected during the catalysis. Attempts to isolate the
corresponding insertion products from the reaction mixtures or
from stoichiometric reactions of 5a and 1-hexene or 3-
vinylcyclohexene failed. In contrast to activated double bonds, the
insertion of 1-alkene seems to be reversible (Scheme 4). The
unstable alkyl calcium species readily undergoes β-hydride
elimination to form back [L4CaH]+.
metal • isotope exchange •
[1]
[2]
W. M. Haynes, Handbook of Chemistry and Physics 95 ed., CRC
Press, Boca Raton, 2014, pp. 12-25.
a) E. Ronnebro, E. H. Majzoub, J. Phys. Chem. B 2007, 111, 12045-
12047; b) J. Mao, Z. Guo, X. Yu, H. Liu, J. Phys. Chem. C 2011,
115, 9283-9290; c) K. S. Alcantara, J. M. Ramallo-Lopez, U.
Boesenberg, I. Saldan, C. Pistidda, F. G. Requejo, T. Jensen, Y.
Cerenius, M. Sørby, J. Avila, J. B. von Colbe, K. Taube, T. Klassen,
M. Dornheim, J. Phys. Chem. C 2012, 116, 7207-7212; d) C.
Pistidda, F. Karimi, S. Garroni, A. Rzeszutek, C. Bonatto Minella, C.
Milanese, T. T. Le, L. H. Rude, J. Skibsted, T. R. Jensen, C.
Horstmann, C. Gundlach, M. Tolkiehn, P. K. Pranzas, A. Schreyer,
T. Klassen, M. Dornheim, J. Phys. Chem. C 2014, 118, 28409-
28417.
[3]
a) J. Spielmann, S. Harder, Eur. J. Inorg. Chem. 2008, 2008, 1480-
1486; b) S. Harder, Chem. Rev. 2010, 110, 3852-3876; c) S. Harder,
J. Spielmann, J. Organomet. Chem. 2012, 698, 7-14; d) J. Intemann,
H. Bauer, J. Pahl, L. Maron, S. Harder, Chem. Eur. J. 2015, 21,
11452-11461; e) K. Revunova, G. I. Nikonov, Dalton Trans 2015,
44, 840-866; f) M. S. Hill, D. J. Liptrot, C. Weetman, Chem. Soc.
Rev. 2016, 45, 972-988.
[4]
[5]
S. Harder, J. Brettar, Angew. Chem. Int. Ed. 2006, 45, 3474-3478.
A. Causero, G. Ballmann, J. Pahl, H. Zijlstra, C. Farber, S. Harder,
Organometallics 2016, 35, 3350-3360.
[6]
a) P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda,
Angew. Chem. Int. Ed. 2012, 51, 4452-4455; b) V. Leich, T. P.
Spaniol, J. Okuda, Inorg. Chem. 2015, 54, 4927-4933.
V. Leich, T. P. Spaniol, L. Maron, J. Okuda, Chem. Commun. 2014,
50, 2311-2314.
a) W. Fegler, A. Venugopal, M. Kramer, J. Okuda, Angew. Chem.
Int. Ed. 2015, 54, 1724-1736; b) J. Okuda, Coord. Chem. Rev. 2017,
340, 2-9.
[7]
[8]
[9]
V. Leich, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int. Ed.
2016, 55, 4794-4797.
[10]
a) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134-
2143; b) J. Halpern, D. P. Riley, A. S. C. Chan, J. J. Pluth, J. Am.
Chem. Soc. 1977, 99, 8055-8057; c) J. Halpern, Science 1982, 217,
401-407; d) J. G. de Vries, C. J. Elsevier, The Handbook of
Homogeneous Hydrogenation, Wiley-VCH, Weinheim, 2007
a) L. J. Hounjet, C. Bannwarth, C. N. Garon, C. B. Caputo, S.
Grimme, D. W. Stephan, Angew. Chem. Int. Ed. 2013, 52, 7492-
7495; b) D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2015, 54,
[11]
[12]
6400-6441; c) D. W. Stephan, Acc. Chem. Res. 2015, 48, 306-316.
a) J. Spielmann, F. Buch, S. Harder, Angew. Chem. Int. Ed. 2008,
Scheme 4. Proposed catalytic cycle for the hydrogenation of terminal alkenes
with calcium hydride 5a.
47, 9434-9438; b) S. Harder, Chem. Commun. 2012, 48, 11165-
11177.
In conclusion, the reactive fragment [CaH]+ has been stabilized
by the macrocyclic NNNN-type ligand Me4TACD as a dimer with
weakly coordinating borate anions and shows high activity in the
isotope exchange of H2 and D2 and more remarkably, in the
hydrogenation of unactivated 1-alkenes. This activity can be
ascribed to the cationic charge of the formally five-coordinate
calcium center in [CaH(Me4TACD)]+ that can activate both H2 and
unactivated double bonds. The latter appears to be in equilibrium
with the insertion product that, in contrast to FLP systems,
undergoes σ-bond metathesis with H2 in the final reaction step.[11c,
13, 25] As in other cases,[8, 19] it appears that the cationic charge is
critical in imparting sufficient electrophilicity to the electropositive
calcium center and to hydrogenate an unactivated olefinic bond.
[13]
[14]
Y. Wang, W. Chen, Z. Lu, Z. H. Li, H. Wang, Angew. Chem. Int. Ed.
2013, 52, 7496-7499.
a) J. B. Schilling, W. A. Goddard, J. L. Beauchamp, J. Am. Chem.
Soc. 1986, 108, 582-584; b) A. Boutalib, J. P. Daudey, M. El
Mouhtadi, Chem. Phys. 1992, 167, 111-120; c) S. Canuto, M. A.
Castro, K. Sinha, Phys. Rev. A 1993, 48, 2461-2463; d) M. Aymar,
O. Dulieu, J. Phys. B 2012, 45, 215103.
[15]
[16]
S. Harder, S. Müller, E. Hübner, Organometallics 2004, 23, 178-183.
Crystallization of 2 is driven by the high lattice energy of this AX2
pair. Use of Ph2SiH2, PhSiH3 or Ph2MeSiH also generated
[Ca2H2(Me4TACD)2]2+ with decreasing selectivity. Further attempts
to isolate those species by crystallization were not successful.
A. Causero, G. Ballmann, J. Pahl, C. Farber, J. Intemann, S. Harder,
Dalton Trans 2017, 46, 1822-1831.
a) J. Gavenonis, T. D. Tilley, J. Am. Chem. Soc. 2002, 124, 8536-
8537; b) R. Bhattacharjee, A. Nijamudheen, S. Karmakar, A. Datta,
Inorg. Chem. 2016, 55, 3023-3029.
a) W. Fegler, A. Venugopal, T. P. Spaniol, L. Maron, J. Okuda,
Angew. Chem. Int. Ed. 2013, 52, 7976-7980; b) W. Fegler, A.
Venugopal, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. 2013,
125, 8134-8138.
[17]
[18]
[19]
[20]
Acknowledgements
a) H. Hagemann, V. D’Anna, J.-P. Rapin, K. Yvon, J. Phys. Chem.
C 2010, 114, 10045-10047; b) M. Sharma, D. Sethio, V. D’Anna, J.
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